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CNC Machining

NM09/2Second edition, ©2002

NSW TAFE Commission

PO Box 218 Bankstown NSW 2200

Table of contents

Section 1: Industrial applications of CNC machines 6

Objectives 6

1.1 Numerical control 6

1.2 CNC operation 8

1.3 Computer numerical control (CNC) applications 9

1.4 Economics of numerical control 10

1.5 Numerical control (NC) advantages 11

1.6 Numerical control (NC) disadvantages 12

1.7 Features of numerical control machines 12

1.8 CNC machine features 13

1.9 Types of machine control units (MCU) 19

1.10 Machine control unit (MCU) development 21

1.11 Computer numerical control machine axes 26

1.12 CNC lathes 31

Review questions — section 1 32

Section 2: Basic operation of CNC machines 34

2.1 Machine positioning system 34

2.2 Motion control systems 38

2.3 Stored stroke limit 41

2.4 Buffer storage 42

2.5 Work handling 43

2.6 Tool changers 44

2.7 Work holding 46

2.8 Flexible manufacturing systems 47

2.9 Types of manufacturing systems 49

2.10 Special manufacturing systems 49

2.11 Manufacturing cells 50

2.12 Flexible manufacturing systems 51

2.13 Components of a CIM system 53

CNC Machining © NSW DET 2010


Section 3: Job planning 59

3.1 The NC procedure 59


Section 4: Write and edit basic CNC programs 70

4.1 Cartesian coordinate system 70

4.2 Program zero points 73

4.3 Absolute and incremental co-ordinate programming 76

4.4 Machine home and work zero 77

4.5 CNC calculations — Basic trigonometry 77

Review questions — Section 4 85

Practical exercises — section 4 87

4.6 Preparing NC programs 93

4.7 Program validation 94

4.8 The computer 97

4.9 Review of computer terms 98

4.10 Computer hardware 98

4.11 Punched tape 99

4.12 Computer software 102

4.13 CAD/CAM software 103

4.14 Writing simple programs 104

4.15 Program formats 104

4.16 Modal codes 108

4.17 Contour machining using circular interpolation 114

4.18 Programming examples 119

4.19 Writing a program for a machining centre 122

4.20 CNC Machining centre program format 126

4.21 Writing a program for the 0kuma LB15 CNC lathe 135

4.22 OKUMA LBt5 CNC lathe program format 138

4.23 Points on CNC lathe programming 142

4.24 Tool nose radius compensation 149

4.25 How to call up and use tool nose radius compensation 152

Exercise 9 — section 4 — lathe programming 154

4.26 Quality control 156

4.27 Canned cycles 157

CNC Machining© NSW DET 2010


Section 5: Transfer programs to CNC machines 159

5.1 Setting up a CNC Milling Machine 159

5.2 Program entering and checking 160

5.3 Safety 160

Exercise 10 — section 5 — program transfer 161

Section 6: CNC machine preparation 162

6.1 Work holding devices 163

6.2 Mounting tools in the tool changer 163

6.3 Offsets 164

6.4 Tool change position 164

6.5 Workpiece datum 164

6.6 Workpiece and machine checks before and after machining 165

6.7 Dry running program 166

Practical exercise 11 — section 6 166

Section 7: CNC machine operation 169

7.1 Quality Assurance 170

7.2 Machine operations 171

7.3 Control Panel 171

7.4 Machining 172

7.5 Proving 172

7.6 Editing 173

7.7 Machining 173

Practical exercise 12 — section 7 174

Answers to questions 177


Answers to exercises — section 4 — exercise 1 185








Introduction

CNC machines are now common place in the manufacturing industry, and as a trades person in that industry you will be required to have skills and knowledge related to CNC applications, CNC machine setting, programming and operation.

This module is designed to give you the skills and knowledge related to computer numerical control machining, its' applications, programming and machine operation.

This module provides underpinning knowledge and skills for three National Metals and Engineering Competency Standards Units:

7.l5A -Set numerical control computer numerical control machines (basic).

7.18A -Basic numerical control/computer numerical control programming.

7.28A -Operate numerical control /computer numerical control machines (basic).

Review questions At the end of each of each section there are some review questions. Doing the questions will help learn the TECHNICAL INFORMATION in the module.

References • Gain, J., 1996, Engineering Workshop Practices, Thomas Nelson

Australia Ltd., South Melbourne. • Culley, R., (ed) 1989, Fitting and Machining, TAFE Publications Unit

RMIT Ltd., Collingwood, Victoria.


Section 1: Industrial applications of CNC machines

Objectives At the end of this section, you will be able to:

• explain numerical control • list industrial applications of NC and CNC machines• identify the features of a CNC machine• identify the axes of CNC machines

Safety reminders • In the workshop, always wear safety glasses, safety boots, hair

protection and suitable clothing. • Avoid back injuries -lift the correct way. • Do not use a machine fitted with a Danger Tag. • Know where the first aid station is. • Don't run or play in the workshop. • Use ear muffs or plugs to protect your hearing.

1.1 Numerical control Numerical Control is a system where machine action is generated from the input of numeric data.

The numeric data is, in the beginning, written words in an easily understood code of letters and numbers (alphanumeric characters) known as a program, which in tum is converted by the machine control unit (MCU) into the electrical signals used to control the machine movements.

The relationship between the words 'numerical' and 'control’ is shown below.

Numerical: An instructional expression, in a language of numbers, which represents a series of commands for specific machine tool movements.


Control: To control such machine actions as:

• Directing• Commanding• Prescribing• Sequencing• Initiating• Altering• Timing• Ceasing• Guiding.

An operational numerical control system consists of the following three basic components:

1. Program of instructions. 2. Controller unit, also called a Machine Control Unit (MCU). 3. Machine tool or other controlled process.

The general relationship between the three components is illustrated in figure 1.1. The program of instructions serves as the input to the controller unit, which in tum commands the machine tool or other processes to be controlled.

Figure 1.1

When considering the applications and general characteristics of NC machines it is important that two points be kept in mind.

Point 1 An NC machine tool can do more than it was capable of doing before a control unit was joined to it. There are no new metal removing principles involved. NC machine controls simply position and drive the cutting tools, but the same milling cutters, drills, taps and other tools still perform the cutting operations. cutting speeds, feeds, and tooling principles must be adhered to.


Point 2 Contrary to what some people think, numerical control machines can not initiate anything on their own. The machine accepts and responds to commands from the control unit. Even the control unit cannot think, judge or reason. Without some input medium, ego punched tape or direct computer link, the machine and control unit will do nothing.

1.2 CNC operation CNC stands for Computer Numerical Control. It is a N.C system in which a dedicated stored program computer is used to perform basic control functions.

The functions of CNC controllers are:

1. To read and store program information 2. To interpret the information in a logical command sequence. 3. To control the motion of the machines mechanical members. 4. To monitor the status of the machine.

The interpretation of program commands by a machine control unit and its conversion of those commands into machine motion is complex.

Some of the features and functions are discussed later in this unit, but a simplified diagram of the basic elements of a CNC machine are shown in the schematic below.


Figure 1.2 Basic elements of an NC system

1.3 Computer numerical control (CNC) applications Most trades persons are usually aware of numerical control metal cutting machines such as CNC mills and CNC lathes. However the scope for NC application extends much further than these applications. Generally NC applications may be considered under the following headings.

1.3.1 Metal machining • lathes • milling machines and machining centres • drilling machines


• electric discharge machines(EDM) • tool and cutter grinders • surface and cylindrical grinders.

1.3.2 Metal forming • punches and nibblers • guillotines • pipe benders • flame cutters.

1.3.3 Metal finishing • painting • plating.

1.3.4 Component assembly/inspection • pick and place robots • spot and seam welding machines and robots • assembly of components into printed circuit boards.

1.3.5 Design • computer aided drafting machines • computer controlled plotting machines.

Other industries use NC systems for a wide range of activities such as:

• composite materials laying such as kevlar, carbon fibre etc. • carpet dyeing and weaving • wood machining and furniture manufacture • cloth cutting in the clothing trade • warehouse control and materials handling • control of testing and measuring machines.

1.4 Economics of numerical control Within the machining category, NC machine tools are appropriate for certain jobs and inappropriate for others. Following are the general characteristics in metal machining for which numerical control would be most appropriate.

• parts are processed frequently and in small batches • the part geometry is complex • many operations must be performed on the part in its processing • much metal needs to be removed


• engineering design changes are likely • close tolerances must be held on the workpiece • high cost part where mistakes in processing would be costly.

It has been estimated that a large proportion of manufactured parts are produced in lot sizes of 50 or fewer. Small lot and batch production jobs represent the ideal situations for the application of NC. This is made possible by the capability to program the NC machine and to save that program for subsequent use in future orders. If the NC programs are long and complicated (complex part geometry, many operations, much metal removed), this makes NC all the more appropriate when compared to manual methods of production. If engineering design changes or shifts in the production schedule are likely, the use of tape control provides the flexibility needed to adapt to these changes. Finally, if quality and inspection are important issues (close tolerances, high part cost, 100% inspection required), NC would be most suitable, owing to its high accuracy and repeatability.

In order to justify that a job be processed by numerical control methods, it is not necessary that the job possess every one of these attributes. However, the more of these attributes that are present, the more likely it is that the part is a good candidate for NC.

1.5 Numerical control (NC) advantages The great variety of numerical control applications were introduced in the preceding pages. We also examined the general characteristics of production jobs for which NC seems to be particularly well suited. When properly applied, numerical control provides the user with a significant number of economic advantages.

In this section the advantages and disadvantages of NC are discussed and compared with conventional manual methods of production.

1. Reduced non productive time. Numerical control has little or no effect on the basic metal cutting process. However, NC can increase the proportion of time the machine is engaged in the actual metal cutting (or other manufacturing) process. It accomplishes this by means of fewer setups, less time setting up, reduced workpiece handling time, automatic tool changes on some machines and so on.

2. Reduced fixture cost. Numerical control requires fixtures that are simpler and less costly to fabricate because the positioning is done by the NC program rather than the jig or fixture.

3. Reduced manufacturing lead time. Because jobs can be set up more quickly with NC and fewer setups are generally required with NC, the lead time to deliver a job to the customer is reduced.


4. Greater manufacturing flexibility. With numerical control it is easier to adapt to engineering design changes, alterations of the production schedule, changeovers in jobs for rush orders and so on.

5. Improved quality control. NC is ideal for complicated workpieces where the chances of human mistakes are high. Numerical control produces parts with greater accuracy, reduced scrap and lower inspection requirements.

1.6 Numerical control (NC) disadvantages Along with the advantages of NC, there are several features about NC which must be considered as disadvantages.

1. Higher investment cost. Numerical control machine tools represent a more sophisticated and complex technology. This technology costs more to buy than its non-NC counterpart. The higher cost requires manufacturing managements to use these machines more aggressively than ordinary equipment. Machine shops must ideally operate their NC machines two or three shifts per day to achieve this high machine utilisation.

2. Higher maintenance cost. Because NC machines are more complex technology and because NC machines are used harder, the maintenance problem becomes more acute. Although the reliability of NC machines will generally be higher than conventional machines the overall cost of maintaining them is greater.

3. Finding and/or training NC personnel. Certain aspects of NC shop operations require a higher skill level than conventional operations. Part programmers and NC maintenance personnel are two skill areas where available personnel are in short supply. The problems of finding, hiring and training these people must be considered a disadvantage to the NC shop.

1.7 Features of numerical control machines The term Numerical Control refers to the ‘encoding’ of information in a way so as to drive machine tools and slides. You will understand that, regardless oft heir application, most NC machines have three basic sub units:

• The machine tool itself • The control unit • The machine positioning system.


On a conventional machine an operator controls these functions and sets or alters them when the operator considers it necessary, the decision resulting from his/her training, skill and experience.

Obviously, the machine settings may differ between operators as will the time taken to read scales, set positions, change tools, alter speeds and feeds, engage drives and set up the work piece etc.

CNC automatic control can be applied to these functions and so result in consistent and reduced machining times through optimised cutting data, fast accurate positioning between cuts and fast automatic tool changing.

1.7.1 CNC Machining centre

Axis designation Like conventional milling machines a CNC machining centre has three basic axis of motion which are driven by the part program in either a pre-selected feed rate or in rapid traverse which is generally in the range of between 10 to 100 metres/minute. The tool slides can be programmed to move independently or as a combined movement of any two or three axis.

Figure 1.3 Machining centre axes

1.8 CNC machine features CNC Machines differ in construction to conventional machines in many areas other than their method of control.


It is essential to accelerate CNC machine slides quickly and also to bring them to rest quickly, so the design must offer as little friction as possible and be rigid without excess weight. If weight and friction can be reduced then so can the size and weight of the drive motor and gearing thereby improving performance.

Slide friction can be reduced with the use of materials offering low co-efficient of friction as well as attention to details such as surface finish and lubrication.

Flat slides also can be 'floated' on a high pressure film of lubrication oil to virtually eliminate normal slide friction. This design is known as hydrostatic sideways, and is complex and costly compared to other systems. Roller or linear ball bearing slides offer low friction also but usually have a trade-off in load capacity.

The main structure of the machine, on which the slides are positioned, must be rigid and stable under conditions such as:

• Heavy loads (static load) • Heavy cutting conditions (dynamic load) • Reaction forces from rapid acceleration of slides (dynamic load) • When slides over hang at the extremes of their travel (static load) • Heat build up after prolonged use (thermal source).

Because component sizes are produced by machine motions that are controlled by unchanging numeric data, it is important that the last part produced at the end of the day will not be altered from the first by thermal instabilities.

On conventional machines, sizes and positions are controlled manually, and it is quite usual for an operator to be constantly altering positions during the machining operations or throughout the day, so thermal stability is not as critical as for CNC machines.

Thermal instability falls into three basic groups:

• Machine design • Machining processes • Machine siting • Machine design.

In the first, the greatest source of heat is from the spindle or geared head, but localised heating of slides and lead screws as well as heat transmitted from drive motors can also affect accuracy. Therefore, the machine tool manufacturer must take thermal effects into consideration at the design stage.

Machining processes can result in a great deal of heat. For example, heavy cutting of large work pieces on milling machines can result in heat being conducted readily into the machine table and slides.

Machines must be sited away from or screened from sources of heat such as afternoon sun through a window, heaters, hydraulic power packs, ovens etc.


Remember a temperature difference of only 1°C over 1000 mm can cause an error of 0.01 mm which may be within the required machine accuracy, but outside the tolerance for the job.

It is usual to build conventional machines from cast iron -a material that offers rigidity and vibration damping, however for a given weight a fabricated (welded) steel structure offers greater rigidity and strength, and it is this construction method that is commonly used for CNC machines.

CNC machines using chip producing machining methods also commonly have tool holding and automatic changing devices in order to maximise production by reducing tool changing times to a few seconds at most.

1.8.1 The control unit The CNC Machine Control Unit (MCU) has to read and decode the part program, and to provide the decoded instructions to the control loops of the machine axes of motion, and to control the machine tool operations.

The main grouping of parts of a control could be considered as:

• The control panel • The tape reader • The processors.

1.8.2 Control panel This is the human interface that allows various modes of machine or control operation to be initiated, from switching on and homing, to program loading and editing, to setting work positions and tool offsets, manually controlled movements and commencing the automatic cycling of a program. Information about machine status and condition is available to the operator via VDU screens, gauges, meters, indicator lights and readouts.

A typical control panel is shown on the Sinumerik 820 Machine Control Unit. This panel divides into two broad functional areas.

1 An interface which relates to loading editing and validating the program and,

2 An interface which relates to the manual control of the machine including program over-ride.

1.8.3 Program interface This part of the control panel allows the operator to communicate with the program and any supporting software which is part of the Read Only Memory (ROM). This interface also has ‘Keyboard’ facilities which allows for Manual Data


Input as well as editing and validation of programs. The units video monitor provides a visual display of the programs either as readable data or animated graphics.

Figure 1.4 CNC machine control panel

A. Graphics display with softkey inputB. Display panelC. Address keysD. Symbol keysE. Calculation keysF. Numerical keysH. Control keysI. User defined keys

1.8.4 Machine control interface Apart from such basic controls as stop and start, this aspect of the control panel also provides the means of manual control and program over-ride. This manual control is needed for tasks such as setting zero and tool offsets.


Figure 1.5 CNC control interface

1. Emergency stop button 2. Mode selection switch 3. Single block switch 4. Spindle over-ride switch 5. Feed rate over-ride switch 6. Machine ON switch 7. Key locked switch 8. RESET key 9. NC stop key 10. NC start key 11. Spindle stop key 12. Spindle start key 13. Feed stop key 14. Feed start key 15. Axis selector switch 16. Direction key 17. Aux. axis key 18. Serial interface

1.8.5 Tape reader The tape reader, where fitted, is used to transfer the program information contained on a program tape into the control unit. Most tape readers are of the photo-electric type which offer high speed reading with reliability and accuracy providing the tape is in good condition and the reader is kept clean and free of paper dust particles.

Figure 1.6 Tape reader


The tape reader reads NC tape coding and passes the information to registers within the machine control unit (MCU). The coded information then passes electronically to the machine tool where appropriate action or movement occurs.

Punched tape may be read:

1 Mechanically, via sensors that pass through the holes and operate electrically.

2 Photo-optically, using light beams and photo cells to provide electrical signals.

3 Pneumatically.

1.8.6 The data processing unit and the axis control processor

The processor The processes within a control are the electronic circuits that permit conversion ofpart program data into the machine motions and they may be classified into two main sections:

The data processing unit The prime function of a data processing unit is to receive and decode the commands detailed in a part program. Additional functions include:

• The input device, such as a tape reader, keyboard or memory. • Reading circuits and parity checking logic. • Decoding circuits for distributing data to the controlled axes. • An interpolator to supply velocity commands to the axis, either singly

or in combination.

The axis control processor This part of the control unit circuitry receives the decoded signals from the data processing unit and in tum operates the slide drives. The axis control processor also receives and interprets feed back signals on the actual position and velocity of each action.

The axis control processor consists of the following circuits:

• Position control loops for each and all axis. • Velocity control loops • Deceleration and backlash take up circuits.

An MCU is adaptable to virtually any machine, the differing control motions and codes being a result of the way the control has been programmed. This


permanent resident program is known as an executive program and resides in the read only memory (ROM) of the control, whereas the N.C program resides in the Random Access Memory (RAM). RAM allows external access and alteration if necessary, while ROM is programmed by the manufacturer and cannot be accessed through the control keyboard.

1.9 Types of machine control units (MCU) Over the last twenty years or so, Machine Control Units have progressively evolved from simple units which could control motion on a point basis to the sophisticated control units which can control a number of axis movements simultaneously to produce components with complex geometry. This evolution of MCU development can be summarised as:

1.9.1 Point to point Point to point control of motion as shown below was a feature of early NC machines where the NC control was often limited to 2 axis table motion only. Such controls were usually to drilling and tapping operations.

Figure 1.7 Point to point control

1.9.2 Continuous path As multi axis control became more developed, straight line continuous path cutting ability soon made simple point to point machines obsolete. The ‘continuous path’ form of motion control as shown is referred to as ‘Linear Interpolation’ which depending on the machine, can involve all three axis being driven simultaneously.


Figure 1.8 Continuous or linear interpolation

1. 9.3 Contour path With the development of computer numerical control (CNC) machines evolved the ability to cut true true geometric arcs and circles. This motion control referred to as circular interpolation allows cutting motion to blend from straight line to curves in one continuous motion.

Figure 1.9 Contour path programming


Figure 1.10 Path of linear interpolation

Although linear interpolation is essentially a straight line motion control, circles and arcs can be cut, but programming these is a series of short straight line cuts. The accuracy of the arc is dependent on the high and low limits of size within the arc.

1.10 Machine control unit (MCU) development

1.10.1 NC machines Numerical Control is the term used to describe a process of sending program commands to the machine block by block. Put another way, the part program data is fed to the MCU via a tape reader in such a way that the MCU receives a block of information, processes it and then performs that step. Once completed, the tape is advanced and the next block of data is sent to the MCU for action. This step by step process repeats itself until all blocks have been executed.

With NC machining the program punched tape is the source of program data and, as such, any changes or edits will require a new tape to be punched.


1.10.2 NC control limitations Conventional NC control units are either simple point to point or linear interpolation controls and as such have certain limitations which include:

Part programming mistakes In preparing the punched tape, part programming mistakes are common. The mistakes can be either syntax or numerical errors, and it is not uncommon for three or more passes to be required before the NC tape is correct. Although there are tape editing facilities available they are generally clumsy and for this reason any changes to a part program will normally require a new tape to be punched.

Non-optional speeds and feeds In conventional numerical control, the control system does not provide the opportunity to make changes in speeds and feeds during the cutting process. As a consequence, the programmer must set the speeds and feeds for worst-case conditions. The result is lower productivity.

Tool ringing The fact that motion is controlled as single block steps leads to cutting tools dwelling in the same spot waiting for the next command this dwell leads to tool ringing and poor finish.

Punched tape Another problem related to programming is the tape itself. Paper tape is especially fragile, and its susceptibility to wear and tear causes it to be an unreliable NC component for repeated use in the shop. More durable tape materials, such as, mylar are utilised to help overcome this difficulty. However, these materials are relatively expensive.

Tape reader The tape reader that interprets the punched tape is generally acknowledged among NC users to be the least reliable hardware component of the machine. When a breakdown is encountered on an NC machine, the maintenance personnel usually begin their search for the problem with the tape reader.

1.10.3 Computer numerical control Computer Numerical Control is an NC system that utilises a dedicated computer to perform some or all of the basic numerical control functions. Because of the trend toward downsizing in computers, most of the CNC systems sold today use a microcomputer-based controller unit. Over the years, minicomputers have also been used in CNC controls.


The external appearance of a CNC machine is very similar to that of a conventional NC machine. Part programs are initially entered in a similar manner. Punched tape readers are still used to input the part program into the system. However, unlike conventional NC, where the punched tape is cycled through the reader for every workpiece, CNC, programs are only read once and then stored in the computer memory. Thus the tape reader is used only for the original loading of the part program and data. Compared to conventional NC, CNC offers additional flexibility and computational capability. New system options can be incorporated into the CNC controller simply by re-programming the unit. Because of this re-programming capacity, both in terms of part programs ands system control options, CNC is often referred to by the term ‘soft wired’ NC.

Figure 1.11 CNC system configuration

Computer Numerical Control has a number of advantages over conventional NC. preceding discussion:

1.10.4 Advantages of CNC 1 The part program tape and tape reader are used only once to enter

the program into computer memory. This results in improved reliability, since the tape reader is commonly considered the least reliable component of a conventional NC system.

2 Tape editing at the machine site. The NC tape can be corrected and even optimised (Example, tool path, speeds, and feeds) during tape tryout at the site of the machine tool.

3 Metric conversion. CNC can accommodate conversion of tapes prepared in units of inches into the international system of units.

4 Great flexibility. One of the more important advantages of a CNC control unit is its ability to ‘read ahead’, known as buffer storage. This feature allows the control unit to read two or more blocks ahead and perform the processing and calculations for the up and coming blocks


while machining or executing the current block of data. This feature offers, in: o Continuous motion without stops and so eliminating tool dwell

which causes ‘ringing’. o Circular interpolation control by the use of specific ‘G’ codes. o Tool nose and cutter radius compensation which can be

automatically applied. o Special machining cycles such as threading cycles, sub routines

and canned cycles.

1.10.5 Direct numerical control Direct numerical control can be defined as a manufacturing system in which a number of machines are controlled by a computer through direct connection and in real time. The tape reader is omitted in DNC, thus relieving the system of its least reliable component. Instead of using the tape reader, the part program is transmitted to the machine tool directly from the computer memory. In principle, one larger computer can be used to control more than 100 separate machines. The DNC computer is designed to provide instructions to each machine tool demand. When the machine needs control commands, they are communicated to it immediately. DNC also involves data collection and processing from the machine tool back to the computer.

1.10.6 Components of a DNC system 1 A direct control system consists of four basic components: 1 Central computer 2 Bulk memory 3 Telecommunication lines 4 Machine tools.

A basic overview of a DNC system us shown below:


Figure 1.12 DNC system configuration

In the DNC system configuration the computer calls the part program instructions from bulk storage and sends them to the individual machines as needed. It also receives data back from the machines. This two way information flow occurs in real time, which means that each machine's requests for instructions must be satisfied almost immediately. Similarly, the computer must always be ready to receive information from the machines and to respond accordingly. The remarkable feature of the DNC system is that the computer is servicing a large number of separate machine tools all in real time.

Just as CNC had certain advantages over a conventional NC system, there are also advantages associated with the use of direct numerical control. The following list will recapitulate much of our previous discussion of DNC.

1.10.7 Advantages of DNC 1 Elimination of punched tapes and tape readers. Direct numerical

control eliminates the least reliable element in the conventional NC system. In some DNC systems, the hard-wired control unit is also eliminated, and replaced by a special machine control unit designed to be more compatible with DNC operation.

2 Greater computational capability and flexibility. The large DNC computer provides the opportunity to perform the computational and data processing functions more effectively than traditional NC. Because these functions are implemented with software rather than with hard-wired devices, there exists the flexibility to alter and improve the method by which these functions are carried out. Examples of these functions include circular interpolation and part programming packages with convenient editing diagnostics features.


3 Convenient storage of NC part programs in computer files. This compares with the more manually oriented storage of punched tapes in conventional NC.

4 Reporting of shop performance. One of the important features in DNC involves the collection, processing, and reporting of production performance data from the NC machines.

4. Establishes the framework for the evolution of the computer automated factory. The direct numerical control concept represents a first step in the development of production plants which will be managed by computer systems.

1.11 Computer numerical control machine axes Most machines have two or three slideways placed at right angles to one another. On CNC machines each slide is fitted with a control system, and is identified with either the letter X, Y or Z. conventions have been adopted as to the naming of each axis.

The X axis is the motion of the largest travel of the primary movement.

The Y axis then makes the third motion and is the shorter primary movement.

The Z axis is the axis of the main spindle, whether it is the tool spindle or the axis about which the work piece rotates.

The following overview will show some of the more conventional axis designation found on NC machines. The table below shows the relationship between each of the axis designations to be considered.

Linear axis X Y Z

Rotary axis A B C

Secondary linear U V W

Interpolation I J K


1.11.1 Linear axis

Figure 1.13 Horizontal spindle machine centres

X = table feed

Y = Knee feed up

Z = Cross slide feed

Figure 1.14 Two axis lathe

X = Work diameter

Z = Length feed


Figure 1.15 Vertical spindle machining centre

X = Table

Y = Cross slide feed

Z = Tool feed

Figure 1.16

A technique which will assist you to remember the three axes and which is the positive direction for these axes, is the right hand rule


1.11.2 Rotary axis

Figure 1.17

Where fitted, such features as indexing chucks, rotary tables and rotary tool or work heads are given the following designation. Note the relationship between

‘A’ relates to ‘X’

‘B’ relates to ‘Y’

‘C’ relates to ‘Z’

Figure 1.18

Another technique to help you determine which is the positive direction for a rotary axis also uses the right hand.

Point your thumb in the positive direction of the main linear axis and the curl in your fingers will be pointing in the direction of the rotary axis.

Figure 1.19 Three axis lathe with indexing chuck

X = work diameter

Z = feed length

C = indexing


Figure 1.20 Vertical spindle machining centre with two rotary axis features

‘A’ axis is to ‘X’ axis

‘C’ axis is to ‘Z’ axis

1.11.3 Secondary linear axis Where machines are fitted with linear axis accessories such as tool turrets tailstocks, auxiliary tools slides and steady the axis designation of ’U’ ‘V’ and ‘W’ are assigned. Because these are relative to linear axis they can be considered as:

Figure 1.21 CNC lathe with secondary linear axis

‘U’ axis relates to X

‘V’ axis relates to Y

‘W’ axis relates to Z

In figure 1.21 the turret has a secondary slide movement designated ‘W’ which is on the same plane as the Z axis.


Figure 1.22 Multi axis machining centre

‘W’ axis is relative to the Z axis

‘U’ is relative to the X axis

Interpolation axisInterpolation axis using the designation ‘I’ ‘J’ and ‘K’ are used to establish arc centre off sets when programming for centre and circular geometry. This is covered in section four of this module.

1.12 CNC lathes The construction of CNC lathes can vary from a simple two axis machine resembling a conventional engine lathe to multi-axis multi-spindle machines often found in the manufacturing industry. Regardless of the physical shape or configuration of each machine the fundamentals of programming remain the same.

Axis designation

Figure 1.23

On a simple two axis lathe as shown in figure 1.23 the following rules apply:

• The Z axis refers to the longitudinal travel of the tool.

• The X axis refers to tool motion in relation to the diameter of the workpiece.


Figure 1.24 Two sliding turning centre

Not all CNC lathes are simple two axis machines. The lathe shown in Figure 1.24 has two tool slides each of which are controlled by the part program. In this example:

X axis = Diameter U axis = Diameter Z axis = Length W axis= Length

Review questions — section 1

Industrial applications of CNC machines •

1. What does ‘numerical control’ mean? 2. List the three basic components that make up and operational control

system. 3. What does M. C. U. stand for? 4. What does CNC stand for? 5. What are four functions of a CNC control? 6. Give three examples of CNC metal cutting machines. 7. Give two examples of where CNC is used in the component assembly

area. 8. Give three examples of other industries using NC systems. 9. Why is a small batch ideal for manufacture on a NC machine? 10. Give three advantages of utilising NC for production jobs.


11. Give three disadvantages of utilising NC for production jobs. 12. What are the three basic sub-units of a NC machine? 13. What is meant by ‘thermal stability’? 14. On a CNC machining centre, which is the X axis? 15. What is one advantage and one disadvantage of linear ball bearing

slides? 16. What is the purpose of the control panel on the MCU? 17. If a machine has a tape reader, what type of tape reader would it

most likely be fitted? 18. What does ROM stand for? 19. What is meant by the term ‘tool ring’? 20. List four advantages of CNC as opposed to NC. 21. What does DNC mean? 22. What letters are used to describe the rotary axes on a CNC machine? 23. How can you determine what should be the positive direction of a

rotary axis without accessing the machine? 24. What are the two main axes on a CNC lathe?

Answers are at the end of CNC Machining text


Section 2: Basic operation of CNC machines

This section covers the basic operation of CNC machines with regard to axes movement, stored stroke limits and buffer storage, work handling methods and the role of CNC in flexible manufacturing systems.


• describe the methods of moving CNC axes • describe stored stroke limits • state the purpose of buffer storage • list methods of work holding • outline the role of CNC in flexible manufacturing systems.



2.1 Machine positioning system Once the Machine Control Unit has decoded the part program it can now send the signals required to drive and position the table and tool slides as required by the program. This chain of event normally involves three specific sub units.

1. Slide driving motors 2. Slide positioning devices


3. Feed back controls.

The figure below shows the general flow path of signals which drive, position and control all aspects of slide motion.

Figure 2.1 Machine motion drives and controls

2.1.1 Maw spindles and slide drive motors In general, the multi-change gear boxes with fixed ratios driven by constant speed electric motors, as commonly used on conventional machine tools, are not always entirely suitable for NC machines. Variable speed drives enable cutting speeds and feeds to be maintained at optimum values, thus utilising a machines cutting capacity to the full. The extra cost of providing variable speed drives and the somewhat lower efficiency of the units (disadvantages in relation to conventional machines) are less significant on NC machine tools because the higher cost and depreciation rates of NC machines make it essential to fully exploit their potential.

On NC machine tools, speed changes are carried out in response to instructions written into the tape input data according to the machining program. Selection of the most suitable ratio is readily carried out ifvariab1e speed systems are used in the transmission. Some of the recognised methods of obtaining speed ratios are considered in the following paragraphs.

Mechanical drives For reasons mentioned above, the conventional fixed ratio gearbox is used less often in NC applications, but when the system is used, the gears providing the various ratios are usually in constant mesh and remotely controlled electro-magnetic or hydraulic clutches are employed to engage the selected ratio. Variable speed drives based on mechanical principles have limitations in respect of the rate at which a change can be carried out, and of the relatively high forces which are needed to operate the change.


Electrical drives Drives in which speed variation is obtained entirely by electrical means are of two main types, ie. systems utilising either an AC or a DC final drive motor. The AC motor achieves speed variation by changing the frequency of the supply through a frequency controller. The DC motor speed variation is obtained by altering the DC supply voltage, which in tum has been converted from normal AC supply. Both the AC and DC motors offer similar speed/power performance, with the AC motor being smaller and lighter and therefore giving superior acceleration and deceleration. Because the AC motor is an induction motor, it does not have brushes to wear and to replace as does the DC motor, but the DC motor is less expensive to produce and fit to the machines.

Hydraulic drives Hydrostatic hydraulic drives are being used increasingly for NC applications. The usual system spindle drives is based on a constant speed electric motor driving and hydraulic pump which then supplies hydraulic oil under pressure to drive an hydraulic motor. The pumps and motors may be of either the fixed or the variable displacement type, and the choice will often depend on the characteristics required from the output, ie. high efficiency, rapid response, constant torque, constant power, wide speed range, etc.

Advantages of hydraulic systems, especially those incorporating both variable delivery pumps and variable displacement motors, are:

a. Wide range of stepless variable speeds b. High torsional stiffness c. Relief valves ensure machine can stall without damage if an

overload occurs d. No backlash.

Disadvantages

a. There is a limitation on minimum motor speeds needed for smooth operation

b. Heat generated in the system may have to be dissipated by a heat exchanger

c. Oil leakage may occur at higher loads (arrangement to compensate for oil losses can be made)

d. Special oils are needed to reduce hazards due to fire risks.

2.1.2 Slide positioning devices Many of the foregoing comments relating to spindle drives will also apply to drives operating feed mechanisms, ego drives to tables and slides, whereas rotary drives are required for spindles, linear motion is usually required for feed mechanisms. The choice of mechanism is largely confined to:


a. Hydraulic ram b. Rack and pinion, or c. Leadscrew and nut

Each of these mechanisms have their place, and all can be readily controlled for NC purposes. The choice is usually influenced by two factors, the length of stroke and the mass to be displaced.

Hydraulic rams are suitable for relatively short strokes and are particularly attractive for low and medium power ranges. Hydraulic actuators are economic and provide a smooth, still transmission in the smaller range, but as size increases, compliance becomes something of a problem. The column of oil in a cylinder is subjected to slight compression and acts as a liquid spring. In addition, slight elastic deflection of mechanical elements, ego cylinder walls, may occur. When such considerations become noticeable, a change is frequently made to a leadscrew drive.

Rack and pinion drives are particularly suitable for moving the slides of very large machines mainly because the range of stroke is not limited as is the case for machines relying on a leadscrew. Very long leadscrews need to be well supported and are generally of large diameter to minimise linear and torsional deflection over their length. Machine slides operates by rack and pinion have the advantage of high rigidity regardless of stroke length although the positional accuracy of rack and pinion systems is not as high as other methods.

Screw thread drives used on conventional machine tools, usually employ trapezoidal threads, ego Acme. These thread have several disadvantages. They are very inefficient (often less than 25% efficiency is obtained form Acme thread), due to high frictional resistance between the flanks of the screw and nut -and with the increased diameter of leadscrews used on NC machines this friction increases the torque requirements.

The friction gives rise to local heat, and inaccuracy results from this cause. Backlash of the magnitude met with in normal screw drives is quite unacceptable for many NC applications; it can be removed by using a backlash eliminator, but this device introduces further frictional resistance and accentuates the problems referred to above.

The use of recirculating ball leadscrew and nut, ensures which the connection between the screw and nut is achieved by an endless stream of recirculating steel balls.


Figure 2.2 Recirculating ball leadscrew

The efficiency of the recirculating ball screw is very high, often 90%, and even when subject to pre-loading to eliminate backlash, the frictional resistance is still not objectionable and the efficiency remains remarkably high.

2.2 Motion control systems

2.2.1 Open loop system In the open loop system, shown below, the tape is fed into a tape reader which decodes the information punched on the tape and stores it briefly until the machine is ready to use it. The tape reader then coverts the information into electrical pulses or signals. These signals are sent to the control unit, which energises the servo control units. The servo control units direct the servomotors to perform certain functions according to the information supplied. The amount each servomotor will move depends upon the number of electrical pulses it receives from the servo control unit.


Figure 2.3 Open loop control

Precision leadscrews, usually having 10 threads per inch (tpi), are used on NC machines. If the servomotor connected to the leadscrew receives 1000 electrical pulses, the machine slide will move 1 in. (25.4 mm). Therefore, one pulse will cause the machine slide to move 0.001 in. (0.0254 mm). The open loop system is fairly simple, however, since there is no means of checking whether the servomotor has performed its function correctly, that is, no means of feedback, this system is not generally used where an accuracy greater than 0.001 in. (0.025 mm) is required.

2.2.2 Closed loop control systemThe closed loop system is similar to the open loop system with the exception that a feedback unit is introduced into the electrical circuit. This feedback unit, often called a transducer, converts the movement by the servomotor to an electrical signal. The control compares the signal received from the transducer to the signal that was sent to the servomotor and will instruct the servomotor to make whatever adjustments are necessary until both the signal sent from the control unit and the one received from the servo unit are equal.

Figure 2.4 Closed loop feedback control

Closed loop NC systems are very accurate because the command signal is recorded, and there is an automatic compensation error. If the machine slide is forced out of position due to cutting forces, the feedback unit indicates this


movement and the machine control unit (MCU) automatically makes the necessary adjustments to bring the machine slide back to position.

‘A transducer is a device that converts one form of energy into another form of energy in such a way that the output is a known function of the input’.

2.2.3 Transducers Transducers used on most modem day NC/CNC machines are either analog or digital systems.

Analog Analog transducers, such as potentiometers and synchros, produce an electrical voltage which varies as the input shaft is turned or rotated. This voltage is in proportion to the rotation of the input shaft which can be converted into very accurate machine table positions.

Figure 2.5 Analog transducer system

Digital Digital feedback units, attached to the leadscrew of a machine tool change the rotary motion of the machine screws to individual or discrete electrical pulses. This series of pulses can be counted to indicate exactly how much the leadscrew shaft has turned, which indicates the amount the machine table has moved.

Figure 2.6 Digital transducer system


Transducers used in closed loop system may be divided into two broad groups.

1 Rotary transducers 2 Linear transducers.

2.2.4 Rotary transducers

Resolvers Resolvers generate their signal directly by having windings at 90 0 on the rotor and two windings at 90 0 in the stator. An AC voltage is applied to a stator winding, and as a rotor winding passes it a voltage is generated on that winding. This voltage is treated in such a way as to provide an electrical pulse that can be used for positional information without the necessity of photo electric devices or brushes.

Rotary transducers are usually fitted at the opposite end of the ballscrew to the motor as shown in the preceding sketches, or they are fitted to the end of the motor. This means they actually determine the motion of the ballscrew or the motor and not necessarily the machine slideway itself.

Tachometers Tachometers are used to sense velocity and are small DC generators coupled to the leadscrew or motor shaft and supply a voltage that increases in direct proportion to its speed of rotation. This voltage represents the actual speed and can be used in comparison to the voltage supplied for the desired speed as a method of reducing the velocity error to a minimum.

2.2.5 Linear transducers There are also linear devices, the most well known being the inductosyn. These transducers work by having two grating combs mounted closely over one another, one fixed, one moving with the slide. The gratings are pitched differently, giving an effect like an electronic version of a vernier scale. Signals are usually generated electrically, but can also be generated by photo electric devices.

Linear transducers are similar in construction to the digital readout scales fitted to manual machines. These linear scales are fitted to the actual slideway so they determine the motion of the slideway.

2.3 Stored stroke limit Limit switches are used to control motion at the extreme position of each slide and may take the form of physical (hard wired) switches, or a stored (soft wired) position within the control. Depending on the machine, there may be two or three


limit switches per slide with the outermost switch (usually hard wired) being the emergency limit position. Once the machine is in normal operation this switch is not activated. If, due to some error this switch is activated, an alarm condition will exist with power isolated from the servo motors and only resetable through the emergency over travel release sequence. This will prevent in many cases any mechanical damage to the axis drive assembly.

Just inside the emergency limit position switch is the stroke end limit position (usually soft wired). If this position is exceeded during manual (JOG) operation an alarm will exist and can usually be cleared simply by moving the slide away in the opposite direction.

On some machines (particularly lathes) there can be a third limit position -this is a soft wired variable limit and the position will be set as desired to suit the size of the workpiece. Turret indexing is executed at this position. The variable limit can be set to the same position as the stroke end limit position.

Variable limits can also be set on some machines to create an area that is forbidden to the tool. This 'forbidden area' or 'barrier' is activated only when an appropriate code is programmed, or entered into the MCU from the keyboard.

The function of this feature is to reduce the possibility of tool collisions. As there are variations between machines, it will be worthwhile conferring with your teacher to find out how the machine you are operating is configured.

2.4 Buffer storage

Primarily, buffer storage s used in N.C. and CNC controls to permit smooth continuous motion from block to block.


Figure 2.7

Most modem NC and CNC controls are equipped with buffer storage. As shown above, this feature allows the control to accept information into a buffer register while an operation is being performed from the active machine registers. When that operation is completed, the information is transferred from buffer storage to the machine actuation registers. This transfer of information is instantaneous, thereby reducing the time between tape reading and machine performance.

Buffer storage reduces the amount of dwell time between machine operations because the next block of tape is read and stored while the machine is executing the previous block. Part finish is also better because the cutter does not come to a halt to process the next block of information in the middle of curves, angles, or other part configurations.

Buffer storage may differ from one control to another, some CNC controls being capable of decoding and buffering 200 blocks of program information ahead of machine motion. In this situation program errors can be detected and corrected during program trialing without necessarily having to restart the program from the beginning.

2.5 Work handling Raw materials and components must be handled at various stages of manufacture, also tooling and fixturing will require handling at some time.


The more automatic any of these processes can be made, the more efficient a system will become.

The trade-off against the shortest time it takes to convert raw materials into finished assemblies will be the cost of building or installing automated handling equipment.

Obviously, mass production lends itself to automation at all levels, but efficiencies can be improved in jobbing shops by reducing manual handling as much as possible.

Work handling methods that can be used singly or jointly commonly in use are:

• Bar feeding mechanisms • Parts catcher • Robots and robotic arms • Hopper feed • Feeder tracks • Conveyor belts • Transfer lines • AGV's (automatic guided vehicles) • Pallet changers.

2.6 Tool changers Most modem CNC Machining Centres have some form of automated tool change system which can select and load tools as they are required. Both the process of selection and loading are controlled by the Machine Control Unit which responds to program instructions by activating the tool change routine.

Turret type tool changers are often 6, 8 or 12 station turrets designed to hold stub arbor or collet mounted milling cutters.


Figure 2.8

Magazine type tool changers have the program align the required tool with the tool change arm. When instructed the grip arm rotates and grips both the new and old tool in the spindle. Both tools are then withdrawn and rotated 180 °. The grip arm then retracts and loads the new tool and replaces the old tool in the magazine.

Figure 2.9


2.7 Work holding

CNC machining centres come equipped with a ‘tee slot’ table which can be used as a means of supporting the work or a holding device. In practice, clamping the work directly to the machine table is not a preferred option because of the problems of ensuring each successive workpiece is clamped in exactly the same position.

The more preferred option is to use some from of holding device which has the ability to not only hold the work but also accurately locating the job. Such devices are normally set up and fixed to the table. Examples of work holding devices include:

Figure 2.10 Machine vice

Work held in a machine vice is normally located to a fix reference. For example: Each workpiece is held with the left hand edge set 25 mm in from the fixed jaw of the vice.

Figure 2.11 Fixtures


Fixtures are used to not only hold the workpiece but just as importantly they are used to locate each successive part in exactly the same position.

Setting blocks or ‘dummy tables’ are normally and accessory which is clamped to the machine table. These blocks may vary in size and design however, they do generally include:

1 Tee slots for clamping work or location blocks. 2 Precision drilled holes used for location dowels. 3 An engraved grid to aid in setting up work.

Figure 2.12 Setting blocks

2.8 Flexible manufacturing systems

2.8.1 Computer integrated manufacturing systems Computer integrated manufacturing systems (CIMS) are designed to fill the gap between high volume production transfer lines and low production NC machines.


The relative position of the CIMS concept is illustrated in figure 2.13.

Figure 2.13 Relative position of the CIMS concept

Transfer linesTransfer lines Transfer lines are very efficient when producing parts in large volumes at high output rates. The limitation on this mode of production is that the parts must be identical. These highly mechanised lines are inflexible and cannot tolerate variations in part design. A change over in part design requires the line to be shut down and re-tooled. If the design changes are extensive, the line may be rendered obsolete.

NC machines Stand-alone numerically controlled machines are ideally suited for variations in work part configuration. Numerically controlled machine tools are appropriate for jobbing shop and small batch manufacturing because they can be conveniently reprogrammed to deal with product changeovers and part design changes.

CIMS In terms of manufacturing efficiency and productivity, a gap exists between the high production rate transfer lines and the highly flexible NC machines. This gap includes parts produced in mid range volumes. These parts are of fairly complex geometry. The production equipment must be flexible enough to handle a variety of part designs. Transfer lines are not suited to this application because they are inflexible. NC machines are not suited to this application because their production rates are too slow. The solution to this mid volume production problem is the Computer Integrated Manufacturing System.


2.9 Types of manufacturing systems The mid range shown, below as a Computer Integrated Manufacturing System, (CIMS) can be further divided into finer categories. These categories represent different levels of compromise between the objective versus production capacity.

Generally these are referred to as:

1 Special manufacturing systems 2 Flexible manufacturing systems (FMS) 3 Manufacturing cells.

Figure 2.14 Finer categories of CIMS

2.10 Special manufacturing systems The special manufacturing system is the least flexible Computer Integrated Manufacturing System. It is designed to produce a very limited number of different parts (perhaps two to eight) in the same manufacturing family. The annual production rate per part would typically lie between 1500 and 15,000 pieces. The configuration of the special system would be similar to the high production transfer line. The variety of processes would be limited, and specialised machine tools would not be uncommon.


Figure 2.15 Special manufacturing system line

2.11 Manufacturing cells At the opposite end of the mid volume range is the manufacturing cell. It is the most flexible but generally has the lowest production rate of the three types. The number of different parts manufactured in the cell might be between 40 and 800 and annual production levels for these parts would be between 15 and 500. The highly integrated and in line flow is evident in the work part handling system shown below. This diagram also shows how the manufacturing cell might consist


of several separate NC machines without an interconnecting materials handling system.

Figure 2.16 Manufacturing cells

2.12 Flexible manufacturing systems The Flexible Manufacturing System covers the wide middle territory within the mid volume, mid variety production range. A typical FMS will be used to process several part families, with 4 to 100 different part numbers being the usual case. Production rates per part would vary between 40 and 2000 per year. A representative layout for a flexible manufacturing system is shown on the following page.

Work parts are loaded and unloaded at a central location in the FMS. Pallets are used to transfer work parts between machines. Once a part is loaded onto the handling system it is automatically routed to the particular work stations required in its processing. For each different work part type, the routing may be different and the operations and tooling required at each work station will also differ. The coordination and control of the parts handling and processing activities is accomplished under command of the computer. One or more computers can be used to control a single FMS.


Figure 2.18 Representative layout for a flexible manufacturing system


2.13 Components of a CIM system A computer integrated manufacturing system consists of the following basic components:

1 Machine tools and related equipment 2 Materials handling system 3 Computer system 4 Human labour.

2.13.1 Machine tools and related equipment The machine tools and other equipment that comprise a computer integrated manufacturing system include the following:

• Standard CNC machine tools • Special purpose machine tools • Tooling for these machines • Inspection stations or special inspection probes used with the

machine tools.

The selection of the particular machines that make up a CIMS depend on the processing requirements to be accomplished by the system. These processing needs also influence the design of the parts handling system. Some of the factors that define the processing requirements are the following:

1 Part size. The size of the work parts to be processed on the CII\1S will influence the size and construction of the machines. Larger parts require larger machines.

2 Part shape. Machine work parts usually divide themselves naturally into two types according to shape, round and prismatic. Round parts, such as gears, disks, shafts, requiring boring operations.

3 Part variety. If the part variety is limited, the machine tools would be more specialised for higher production. The CIMS would be designed as a special system. If a wide variety of parts are to be processed, standard machine tools which are more versatile would be selected.

4 Operations other than machining. Most computer integrated manufacturing systems are designed for machining exclusively. In some cases the processing requirements include other operations, such as assembly or inspection.

2.13.2 Material handling systems The material handling system in a CIMS must be designed to serve two functions. The first function is to move work parts between machines. The second function is to orient and locate the work parts for processing at the machines. These two functions are often accomplished by means of two different but connected


handling systems referred to as the primary handling system and the secondary handling system.

Primary handling system The primary work handling system is used to move parts between machine tools or cells by way of an Automated Guided Vehicle CAGV). The requirements usually placed on the primary material handling system are:

• It must be compatible with the computer control • It must provide random independent movement of palletised work

parts between machine tools in the system. • It must permit temporary storage or banking of work parts. • It should allow access to the machine tools for maintenance, tool

changing etc. • It must interface with the secondary work handling system.

Secondary handling system The secondary parts handling system must present parts to the individual machine tools in the CIMS. The secondary system generally consists of one transport mechanism for each machine. The specifications placed on a secondary materials handling system are:

• It must be compatible with the computer control. • It must provide parts orientation and location at each work station. • It must permit temporary storage of work parts • It should allow access to the machine tools for maintenance, tool

changing etc. • It must interface with the primary work handling system, parts must

be able to be transferred automatically between the primary and secondary system.

Robot — characteristics and applications

General application characteristics There are certain general characteristics of an individual situation which tend to make the installation of a robot economical and practical. These general characteristics include the following:

1 Hazardous or uncomfortable working conditions. In job situations where there are potential dangers or health hazards due to heat, radiations, or toxicity, or where the work place is uncomfortable and unpleasant, a robot should be considered as a substitute for the human worker. This sort of application has a high probability for worker acceptance oft he


robot. Examples of these job situations include job forging, die casting, spray painting, and foundry operations.

2 Repetitive tasks. If the work cycle consists of a sequence of elements which do not vary from cycle to cycle, it is possible that a robot could be programmed within a limited work space. Pick and place operations and machine loading are obvious examples of repetitive tasks.

3 Difficult handling. If the work part or tool involved in the operation is awkward or heavy, it might be possible for a robot to perform the task. Operations involving the handling of heavy work parts are a good example of this case. A human worker would need some form of mechanical assistance to lift the part, which would add to the production cycle time. Some industrial robots are capable of lifting payloads weighing several tonnes.

4 Multishlft operation. If the initial investment cost of the robot can be spread over two or three shifts, the labour saving will result in a quicker payback. This could mean the difference between whether or not the investment can be justified. Plastic injection moulding and other processes which must be operated continuously are examples of multi shift robot applications.

Application areas for industrial robots Industrial robots have been applied to a great variety of production situations. Robot applications include:

• Material transfer • Machine loading • Welding • Spray coating • Processing operations • Assembly • Inspection.

Tools end effectors There are a limited number of applications in which a gripper is used to grasp a tool and use it during the work cycle. In most applications where the robot manipulates a tool during the cycle, the tool is fastened directly to the robot wrist and becomes the end effector. A few examples of tools used with robots are the following:

• Spot welding gun • Arc welding tools (and wire feed mechanisms) • Spray painting gun • Drilling spindle • Routers, grinders, wire brushes.


Robotic sensors For certain applications robots require more human like senses and capabilities in order to perform their functions in the most effective and efficient way. These senses and capabilities include abilities such as vision, hand/eye coordination and bearing. The figure below shows an adaptable programmable assembly system using robots and humans. This type of integration may lead to an assumption that robots can have some form of intelligence, this is not so. In this example we can assume that humans are capable of making decisions based on intelligent observations and reasoning, on the other hand, a robot is limited to the use of sensors which trigger a response when an object moves into the proximity field of the robots sensor.

Robotic sensors are generally considered in three broad groups:

• Vision sensors • Tactile sensors • Proximity sensors.

Figure 2.19 Robots on line

2.13.3 Computer control system The functions accomplished by the computer control system can be divided into seven categories. The following descriptions apply best to the case of the flexible manufacturing system. To a slightly lesser extent, they also apply to the Special System and the Manufacturing Cell.

1 Machine control. This is usually accomplished by Computer Numerical Control (CNC). The advantage of CNC is that it can be conveniently interfaced with the other elements of the computer control system. In some of the special systems which are dedicated to a limited part variety, CNC may be a sufficient control method for the system.


2 Direct numerical control (DNC). Most CIMS operate under DNC mainly because of its flexibility of functions, functions which include NC part program storage, distribution of programs to the individual machines in the system, post processing, and so on.

3 Production control. This function includes decisions on part mix and rate of input of the various parts onto the system. These decisions are based on data entered into the computer, such as desired production rate per day for the various parts, numbers of raw work parts available, and number of available pallets.

4 Traffic control. This term refers to the regulation of the primary workpiece transport system which moves parts between work stations. This control can be accomplished by dividing the transport system into zones. A zone is a section of the primary transport system (towline chain, conveyor, etc) which is individually controlled by the computer.

5 Work handling system monitoring. The computer must monitor the status of each cart and/or pallet in the primary and secondary handling systems as well as the status of each of the various work part types in the system.

6 Tool control. Monitoring and control of cutting tool status is an important feature of the computer system. There are two aspects to tools control:

• accounting for the location of each tool in the CIMS and • tool life monitoring

7 System performance and reporting. The system computer can be programmed to generate various reports desired by management on system performance.

2.13.4 Human labour in the manufacturing system The Computer Integrated Manufacturing System is a highly automated production facility, however, human resources are required to operate the system. In the majority of CIMS installations, the individual machines are operated under CNC or DNC control (or a combination of these). The machines are not manually operated except in certain

special operations, such as assembly. Personnel are required principally to manage, maintain, and service the CIMS. Personnel such as:

1. Systems manager. This person has overall responsibility for the operation of the CIMS. The functions include production planning, responding to deviations and exceptions to normal operations and supervision of the other human resources which support the system.

2. Electrical technician. This person is often a member of the plants electrical maintenance crew. Duties performed include maintenance and


repair services on the electrical components of the machine tools and materials handling system.

3. Mechanical hydraulic technician. Again, this person is likely to be a regular member of the plant maintenance department. Technical services consist of maintenance and repair of the mechanical and hydraulic components of the CIMS.

4. Tool setter. The tool setter is responsible for the tooling inventory and making the tools ready for production.

5. Fixture setup and lead person. The person is responsible for setting up the fixtures, pallets, and tools for the system.

6. Load/unload person. This person is responsible for loading raw work parts and unloading finished parts. This is typically done according to instructions and schedules generated by the computer. The load/unload area is at a convenient central location in the manufacturing system.

7. Rover operator. The duties of the rover operator include reacting to unscheduled machine stops, identifying broken tools or tools in need of immediate replacement, tool adjustments and so forth. This person may also be responsible for certain manual production tasks or inspection operations.


Basic operation of CNC machines 1. What is the technical name given to ‘feedback controls’? 2. What other type of motor can be used, besides electric, to drive the main

spindle? 3. List three devices used for slide positioning. 4. Why are recirculating ball screws pre-loaded? 5. What sort of efficiency is typical for recirculating ball screws? 6. What is missing from an open loop control system when compared to a

closed loop control system? 7. What is the difference in signal between an analog transducer and a

digital transducer? 8. List two types of rotary transducer. 9. What are ‘stored stroke limits’? 10. What is the primary function of buffer storage? 11. Give three examples of work holding used on CNC machining centres. 12. What is meant by the abbreviation CIMS? 13. What volume of work is best suited to special manufacturing systems? 14. What is meant by the abbreviation FMS?



Section 3: Job planning

This section covers the sequencing of operations for efficient job completion, the selection of tooling, the concepts of radius compensation and workpiece and machine checks before and after machining.


• document a logical sequence of operations which will enable successful job completion

• indicate suitable tooling for each task.



The planning for NC machining operation is an essential part in the whole process of producing a component on NC machine. Planning may be defined as the step by step process for successful completion of the job. Good, thoughtful planning will result in efficient machine times, reduced waste, effective use of tooling and efficient operator time utilisation. All of these points are necessary to make the most effective use of expensive machine tools and tooling accessories.

3.1 The NC procedure To utilise numerical control in manufacturing, the following steps must be accomplished.


1. Process planning The engineering drawing of the workpiece must be interpreted in terms of the manufacturing processes to be used. This step is referred to as process planning and it is concerned with the preparation of a route sheet. The route sheet is a listing of the sequence of operations which must be performed on the workpiece. It is called a route sheet because it also lists the machines through which the part must be routed in order to accomplish the sequence of operations. We assume that most of the operations will be performed on one or more NC machines.

2. Part programming A part programmer plans the process for the portions of the job to be accomplished by NC. Part programmers are knowledgeable about the machining process and they have been trained to program numerical control. They are responsible for planning the sequence of machining steps to be performed by NC and to document these in a special format

There are two ways to program for NC:

• Manual part programming, • Computer-assisted part programming.

In manual part programming, the machining instructions are prepared on a form called a part program manuscript. The manuscript is a listing of the relative cutter/workpiece positions which must be followed to machine the part. In computer-assisted part programming, much of the tedious computational work required in manual part programming is transferred to the computer. This is especially appropriate for complex workpiece geometry and jobs with many machining steps. Use of the computer in these situations results in significant savings in part programming time.

3. Program preparation A program is prepared from the part programmer's NC process plan. In manual part programming, the punched tape can be prepared directly from the part program manuscript on a typewriter like device equipped with tape punching capability. In computer-assisted part programming, the computer interprets the list of part programming instructions, performs the necessary calculations to convert this into a detailed set of machine tool motion commands, and then controls a tape punch device to prepare the tape for the specific NC machine.

4. Program verification After the program has been prepared, a method is usually provided for checking the accuracy of the program. Sometimes the program is checked by running it through a computer program which plots the various tool movements (or table movements) on paper. In this way, major errors in the program can be discovered. The ‘acid test’ of the program involves trying it out on the machine tool to make


the part. A foam machinable wax or plastic material is sometimes used for this tryout. Programming errors are not uncommon, and it may require several attempts before the program is correct and ready for use.

5. Direct link When a program has been written on a CAM system, it can be fed to the CNC machine directly from the computer on which it was generated. In this case there is no facility for the program to be proved on the machine and the operator must rely on the proving of the program in the CAM system before it goes directly to the machine. Using the direct link is usually done when the program is too large for the machines memory.

6. Transfer to machine memory This can be done by sending the program via a direct link to the machines memory or by saving on a disk and then loading the machine from a computer next to the machine or from a disk drive built into the machine. Once the program has been loaded it can be proved and edited on the machine.

7. Production The final step in the NC procedure is to use the NC program in production. This involves ordering the raw workpieces, specifying and preparing the tooling and any special fixtures that may be required, and setting up the NC machine tool for the job. The machine tool operator's function during production is to load the raw workpiece in the machine and establish the starting position of the cutting tool relative to the workpiece. The NC system then takes over and machines the part according to the instructions in the program. When the part is completed, the operator removes it from the machine and loads the next part. It is also part of the operators job to monitor the size of the parts coming off the machine and to make alterations to the wear offsets in the machine control to compensate for the small amounts of wear that occur to the cutting tools during machining.


Figure 3.1 Process planning for NC machining

3.2 Planning steps A list of points that should be considered when developing the job plan are as follows:

• raw material preparation


• process selection • process sequencing • machining parameter selection • tool path planning • machine selection • tool selection • fixture or work holding method.

1 Sketch the part. Add incremental or absolute dimensions to the sketch or drawing

2 Select work holding. Select fixtures which have minimal projections above the part.

3 Select datum. Locate the set-up point near a corner of the part or a spot above the fixture. Consider space requirements for part loading and unloading, tool changing

4 Plan operation sequence. Mark sequence pattern on the sketch. Test program data for accuracy through plot program or MDI.

5 Record necessary data for each movement of the table and tool on the program sheet.

6 Operators instructions Record instructions for the machine operator, including: Tools needed Speed and feed data Tool change points Console switch setting.

3.3 Operation sheets An operation sheet is a document that presents the order of machine operations and tooling required to produce the finished part.

The operation sheet or sheets should contain information on:

• Machine tool (specific) • Tooling (description, identification and offsets) • Fixturing (identification, positioning, datum, clamping) • Sequence of operations (order of tool use and cutting sequence for

each) • Inspection requirements • Program notes.

Example: The sketch below is to have the holes drilled and tapped. The operators sheet shows the relevant details to allow setting the fixtures, clamps, part and tools correctly.


Figure 3.2 Threaded plate


Operators sheet


3.3 Tool radius compensation

Definition Tool radius compensation is the alteration of program co-ordinates to allow for the size or shape of the cutting tool.

Purpose The purpose of tool radius compensation is to achieve correctly sized and shaped parts.

Methods 1 By determining where errors will be produced and compensation for

those errors by calculating different program co-ordinates. 2 By using an automatic compensating function that most controls have.

Advantages of each method Method I: At all times the exact position of the cutting tool will be known.

Method 2: Less time is usually consumed in preparing a program. Different size cutters can be used easily.

Disadvantages of each method Method I: Time consuming to determine calculations. Only one sized cutter can be used for those co-ordinates.

Method 2: Thorough familiarity of the automatic system must be obtained by theoretical study if profile errors are not to be introduced to the part due to incorrect use of the system. When die sinking profiled surfaces, automatic compensation is not available with all controls, therefore method one must be used, for these circumstances.

The coordinates in a NC program create machine movements following those coordinates. This is called the program path. It stands to reason that a cutting tool (eg. milling cutter) will reduce the sizes of the part by an amount related to the cutter radius.

Two examples

1. A simple block would be reduced in size by an amount exactly equal to the size of the cutter radius.


Figure: Example 1

2. A lathe tool facing to the work centre will leave a tit.

Solution-Program to move the tool past the centre to a diameter equal to twice the tool nose radius.

Figure: Example 2

This concept is explained in detail in Section 4.



Job planning

1 What are the seven steps that make up the procedure to utilise numerical in manufacturing?

2 What are the steps required for good job planning? 3 What information is contained on an operators set up? 4 What is the purpose of tool radius compensation? 5 What are two methods of compensating for the tool's radius? 6 How do you manually compensate for the tool nose radius when facing a

job in the lathe? 7 Complete the operators sheet for the job below:


Operators sheet

8 List eight points to be considered when planning for NC manufacturing of a job.



Section 4: Write and edit basic CNC programs

This section covers the writing and editing of basic CNC programs, the selection of machine and work piece zeros' and basic trigonometry to establish co ordinate positions.


• differentiate between absolute and incremental programming • define machine home and select an appropriate work piece zero • perform basic trigonometry operations to establish co ordinate

positions D list methods of preparing a program • write a program to produce straight and circular tool movements to

standard code format • enter and edit a simple program • explain the concept of canned cycles.



4.1 Cartesian coordinate system The Cartesian, or rectangular, coordinate system was devised by the French mathematician and philosopher Rene Descartes. With this system, any specific point can be described in mathematical terms from any other point along three perpendicular axis. This concept fits machine tools perfectly since their construction is generally based on three axis of motion (X, Y, and Z), plus an axis of


rotation. On a plain vertical milling machine, the axis is the horizontal movement (right or left) of the table, the Y axis is the table cross movement (toward or away from the column), and the Z axis is the vertical movement of the knee or the spindle. NC systems rely heavily on the use of rectangular coordinates because the programmer can locate every point on a job precisely.

When points are located on a workpiece, two straight intersection lines, one vertical and one horizontal, are used. These lines must be at right angles to each other, and the point where they cross is called to origin, or zero point.

Figure 4.1 Origin or zero point

4.1.1 Three dimensional coordinates planes (axis) used in NC The three dimensional coordinate planes are shown in opposite, X and Y planes (axis) are horizontal and represent horizontal machine table motions. The Z plane or axis represents the vertical tool motion. (The plus (+) and minus (-) signs indicate the direction from the zero point (origin) along the axis of movement.


Figure 4.2 3D co-ordinate planes

The four quadrants formed when the X Y axis cross are numbered in a counter clockwise direction. All positions located in quadrant 1 would be positive X (+ X) and positive Y (+ Y). In the second quadrant, all positions would be negative X (-X) and positive Y (+ Y). In the third quadrant, all locations would be negative X and Y (-X-Y). In the fourth quadrant, all locations would be positive X (+ X) and negative Y (-Y).

Figure 4.3 Two axis quadrants

Point ‘A’ in figure 4.4, would be 2 units to the right of the axis and 2 units above the Y axis. Assume that each unit equals 10 mm. The location of point A would be X (+) 20.0 and Y (+) 20.0. For point B, the location would be X (+) 10.0 and y (-) 20.0. In NC programming it is not necessary to indicate plus (+) values since these are assumed. However, the minus (-) values mus be indicated.

A=X2.00 Y2.00


B=X1.00 Y-2.00

Figure 4.4

4.1.2 Guidelines Since NC is so dependent upon the system of rectangular coordinates, it is important to follow some guidelines. In this way, everyone involved in the manufacture of a part, the engineer, draftsperson, programmer, and machine operator, will understand exactly what is required.

1 Use reference points on the part itself, if possible. This makes it easier for quality control to check the accuracy of the part later.

2 Use Cartesian coordinates -specifying X, Y and Z planes -to define all part surfaces.

3 Establish reference planes along part surfaces which are parallel to the machine axis.

4 Establish the allowable tolerances at the design stage. 5 Describe the part so that the cutter path maybe easily determined and

programmed. 6 Dimension the part so that it is easy to determine the shape of the part

without calculations or guessing.

4.2 Program zero points A program zero is simply the point which a programmer uses as a datum or reference point from which movement instructions originate. These points can be anywhere on the workpiece or in fact off the workpiece. The figures below show some of the options which can be used.


Figure 4.5 Figure 4.6

Figure 4.7Figure 4.8

Once a program zero reference has been established and Cartesian coordinates applied the tool path instruction will adopt sense of direction based on positive and negative X, Y and Z coordinates. Generally the program zero selected for a milling job is the bottom left hand corner of the work so allowing tool movement across the work to be in the (+) X, (+) Y coordinates quadrant.

Figure 4.9


For programming on a CNC lathe the program zero can be set either at the face of the chuck jaws, or the end of the workpiece. Once again to allow programming in the (+) X +Z coordinates the preferred program zero point is the face of the chuck jaws.


Note

In programming tool paths for both lathes and machining centres the programmer calculates the coordinates on the assumption that a point on the tool will move from one coordinate to another. This is done even through on a machining centre X and Y movement is created by table movement.

4.3 Absolute and incremental co-ordinate programming

4.3.1 Incremental (G91) The word ‘incremental’ may be defined as a dimension or a movement with respect to the preceding point in a prescribed sequence of points. Each positioning move is described quantitatively in distance and in direction from a previous point rather than from a fixed zero reference point. In incremental mode all moves are with respect to the last position reached.


Figure 4.12 Tool path

4.3.2 Absolute (G90) The data in the absolute system describes the next location always in terms of its relationship to the fixed zero point (0,0). The zero point when used as a program datum is known as the program origin. The G90 code sets the control up in ABSOLUTE mode. All moves are performed with respect to the axes zero.

Figure 4.13 Tool path

4.4 Machine home and work zero

4.4.1 Principal programming points 1. Machine reference or home position is the datum from which all machine motions are taken. Most machines require movement to this position during the power-up procedure so that this datum is established. This is known variously as homing, machine referencing, zero return etc.


2. Program origin is a datum position on the workpiece or the fixture holding it. Basing program co-ordinates from a Datum on the workpiece is far more convenient than trying to base part co-ordinates on the machine home position, so a variety of techniques have been adopted by control manufacturers to allow setting program origins at any position within the machine movements.

Three systems are in general use, some controls are capable of all three, some only two of them. They are:

• By manually zeroing each axis via a button on the operating panel while the machine is at the appropriate position.

• By achieving the same result as above but via a program code. • By offsetting (or shifting) the machine datum to the required

workpiece position via program codes, or keyboard entry on the machine control unit (MCU).

Once the program origin has been established by one of these methods, all machine motions are referenced to the program origin and will remain so until cancelled.

4.5 CNC calculations — Basic trigonometry The assumption is made that students have the required maths competence for this unit.

The following section is designed to re-aquaint all CNC Machining students with the geometry and trigonometry which are relevant to this field of technology.


4.5.1 Basic geometry Example Exercise

Answers:

A = 130°

B = 120°

C = 50°

4.5.2 Circles, radii and tangents


A line drawn from the centre of an arc or circle to the circumference will strike any tangent line at 90°

Similar triangles

Any triangle drawn within a circle using the diameter as the hypotenuse with the points of intersection at the circumstances will be a right angled triangle.


4.5.3 Laws of triangles An Isosceles triangle is one which has two lengths and two adjacent angles equal.

Isosceles triangles are most common in any calculation which involves circles, radii and chords.

Any line drawn from the vertex of an isosceles to strike the base at 90° will:

a) Bisect the base b) Bisect the vertex

angle and, c) Form two right

angled triangles

An equilateral triangle is one which has all sides of equal length and all angles equal to 60°.

A line which bisect the angles of an equilateral triangle pass through the centre of the triangle.

The centre of an equilateral triangle is also the centre for any circle to be drawn inside or outside of the triangle.


Congruent triangles are those which are the same in all respects.

Note: where two radii blend it is often a requirement that you must first solve for triangles not directly associated with the problem in order to get relevant information.

4.5.4 Right angled triangles In a right angled triangle the hypotenuse is always the longest side and is always opposite the right angle.

The adjacent and opposite sides are always relative to a given angle of reference.

4.5.5 Pythagoras' theorum In a right angle triangle the square of the hypotenuse is equal to the sum of the squares of the other two sides.


4.5.6 Trigonometry — Tangent Sine Opposite side

Hypotenuse

Cosine Adjacent side Hypotenuse

Tangent Opposite side Adjacent side

A handy way to remember these ratios is to use a simple phrase, the first letter in each word of the phrase relates to a side of the triangle.

Sine Only Half

Cosine An Hour

Tangent Of Arithmetic


Example 1

Exercise 1 -Calculate the value of X

Exercise 2 - Calculate the value of Y

Answers: X = 20. Y = 30

4.5.7 Trigonometry — Sine Examples


Exercise 3 - Calculate the value of angle 0

Exercise 4 -Calculate the value of the opposite side

Answers: Ø = 45.58° , opposite = 26.31

4.5.8 Trigonometry — Cosine Examples

Exercise 5 - Calculate the value of angle 0


Exercise 6 - Calculate the length of the adjacent side.

Exercise 7 - Calculate the length of the hypotenuse.

Answers: Ø = 36.87°, adjacent = 21.67, hyp. = 39.96

Review questions — Section 4 Note: Show all workings

Question 1

Given that the circle diameter is 60 mm and the width across the flats is 40 mm. Calculate the length of ’W’.


Question 2

Calculate the chordal distance between points ‘A’ and ‘B’.

Question 3

From the information given, calculate the value of radius.

Question 4

Calculate the diameter of the largest circle which can be drawn inside the triangle shown.


Practical exercises — section 4

CalculationsCalculations Student instructions

1 The exercises which follow represent the type of geometry associated with the plotting of tool path co-ordinate for CNC machining.

2 To make the calculations easier always attempt to develop right angled triangle at point where lines are tangent to curves or where line from a common link between circle or arc centres.

3 Make your calculations accurate to two decimal places as most Machine Control Units require each tool path plot to be within an accuracy of 0.02 mm.

4 Calculators should be used. 5 Do not be tempted to look to the solution sheets before you have

completed your own effort. This may cause you some difficulty come the time you will be required to apply your skills under assessment conditions.

Linear programming — exercise sheets 1 -3 Work out the values for ‘x’ and ‘y’ for the points shown on exercise sheets 1 -3. Fill in your answers in the coordinates box on each sheet.





Circular interpolation — exercise sheet 4 Work out the values for ‘x’ and ‘y’ for points 1 -7 in exercise 4 and write your answers in the coordinate box on the sheet.


Exercise 5

An NC controlled boring machine is to be programmed to drill a number of gear boxes as shown below. In the co-ordinate tables provided, complete the drilling tool path values starting and finishing at point ‘A’ in both Incremental and Absolute terms.

Absolute values Incremental valuesX values Y values X values Y values


4.6 Preparing NC programs

4.6.1 Manual systems In any manual programming system, the programmer is required to write a complete manuscript which must include all tool co-ordinates as well all the appropriate machining codes.

Earlier systems used a Buffer Teletype which was like a large typewriter with a chip set and buffer memory that could encode man readable data into a numeric code. With the evolution of micro computers, Buffer Teletypes are rarely used simply because computers now perform the encoding process much faster as well as offering many other advantages. Once encoded, data is normally down loaded to a tape punch which stores the program in the form of a punched tape.

4.6.2 Direct input CNC machines have a Machine Control Unit (MCU) which can accept part programs keyed directly into the control unit via an alpha-numeric keyboard.

This direct input can be via:

1 Edit function —By selecting the edit function an MCU program data can be entered, edited and stored.

2 MDI facility —MDI (Manual data input) is another facility most modem machine control units have, generally this facility is not used to insert long part programs instead it is used to write short statements such as the command to start spindle rotating when using an edge finder to set a program zero.

3 IGF facility — As an accessory many MCU's have available a computer assisted chip set known as an Interactive Graphical Function. This feature allows a conversational type input resembling APT programming language. The computer asks the programmer to respond to prompts (conversion) and once all the data is entered the computer will complete all of the calculations and assign the appropriate machining codes to machine the part. The computer will also provide a program output in the word address format.

4.6.3 Computer aided programming

This aspect of NC part programming uses a stand alone computer which has been loaded with a Computer Aided Manufacturing (CAM) software package. Depending on the nature of the industry the computer can be a mainframe, a mini


or a micro computer such as is found in personal computers. Like in any computer, the heart of the system is the software.

Software such as SmartCAM, MasterCAM and GeoPath are all examples of CAM software. These brand names like many others have the facilities to:

a) Draw the part b) Automatically calculate and define all tool path coordinates. c) Define appropriate post processing functions such as M Codes

etc. d) Validate the program by an animated graphical display of Tool

Path. e) Edit the program as required. f) Output the program data to the MCU.

4.6.4 Voice numerical control Voice programming of NC machines (abbreviate VNC) involves vocal communication of the machine procedure to a voice-input NC tape preparation system. VNC allows the programmer to avoid steps such as writing the program by hand, key punching or typing, and manual verification. One of the principal companies specialising in voice input systems is Threshold Technology, Inc., of Delran, New Jersey.

To perform the part programming with VNC, the operator speaks into a headband microphone designed to reduce background acoustical noise. Communication of the programming instructions is in shop language with such terms as ‘tum’, ‘thread’, and ‘mill line’, together with numbers to provide dimensional and coordinate data. Before the voice input system can be used, it must be ‘trained’ to recognise and accept the individual programmers voice pattern. This is accomplished by repeating each word of the vocabulary about five times to provide a reference set which can subsequently be compared to voice commands given actual programming. The entire vocabulary for the Threshold system contains about 100 words. Most NC programming jobs can be completed by using about 30 of these vocabulary words.

The advantages of VNC lie principally in the savings in programming time and resulting improvements in manufacturing lead time. Savings in programming time up to 50% are claimed. Improvements in accuracy and lower computer skill requirements for the programmer are also given as benefits of VNC.

4.7 Program validation Very few CNC programmers have the sort of confidence which allow them to run an NC program without first doing some sort of a validation check. The more common methods of proving and NC program are listed below.


4.7.1 Computer graphics Most CAM software have an animated graphical feature which will trace this tool path relative to the data input. In most cases rapid traverse is shown as dotted lines while feed rates are displayed as solid lines.

Advantages 1 Visual of Tool path geometry. 2 3D graphics will also show Z AXIS motion of milling cutters.

Limitations 1 Graphical resolution is often limited to the resolution of the CRT

(monitor). 2 Graphical tolerances are often greater than what the MCU will permit.

Often what works on a computer graphics display will cause error messages from the MCU.

4.7.2 MCV graphics Most modem Machine Control Units have some form of animated graphical display which can be used to validate and monitor the program.

Advantages 1 MCU graphics are relative to the machine slide controls so that what the

graphics accept is what the machine accepts. 2 MCU graphics often allow ‘dry run’ validation at a rapid traverse speed.

This saves time in proving long programs. 3 MCU graphic often include a clock which will time the process as if it were

actually cutting.

Limitations 1 MCU graphics can not detect other problems such as incorrect tool

offsets, incorrect program zero or any errors in the physical dimensions of the workpiece.

2 MCU graphics are often limited to two axis displays which, in the case of machining centres, does not allow the checking of Z axis motion.

4.7.3 Single block machining Single block validation requires the machine to be manually switched to single block mode via the MCU soft keys. With this feature set each block is first read and then executed by the operator pressing the cycle start button. During all rapid traverse motions it is good practice to manually override the feed rate to about


10-20% of the feed rate. This will slow the tool traverse to a speed which allows the operator to make any emergency stops well before a crash.

Advantages 1 All axis motion are seen in relation to the size and location of the

workpiece. 2 Feeds and speeds can, be validated and also fine tuned during the cutting

cycle using the manual override.

Limitations 1 Can be time consuming in the first instance. 2 Error messages will often ‘lock up’ the program until the edits are made. 3 Because the tool dwells in the same position between each block ‘ringing’

will occur and surface finish may be impaired.

4.7.4 Printer/Plotter path Normally you would find that any computer terminal would be connected to a printer or a plotter which could be used to generate the tool path of a part program. Depending on the required accuracy and resolution of the printout the printer used could be something as simple as a nine pin dot matrix printer or something as sophisticated as an ink jet drum plotter.

Advantages 1 A visual full size plot of the tool path is produced as a permanent record.

Limitations 2 Plots are only two axis therefore other axis such as the Z axis are not

shown 3 Like computer graphics the tolerance for plotting are quite large which

again means that what ‘appears’ to be correct may call up MCU error messages.

4.7.5 Pen plotting program path This method is generally used to plot a part program for a Machining Centre. The process requires a spring loaded pen/marker to be manually driven down to the Z zero reference face of the workpiece. Once at Z zero, the Z axis is locked using the MCU soft key/switch (this has the effect of ignoring all Z axis commands) the table is then driven back to the ‘machine home’ in the X and Y axis and the program started either in ‘feed rate’ or ‘dry run’ mode.


Advantages 1 Tool path is relative to the workpiece. 2 Accuracy is relative to the program data processed by the MCU. 3 Miscellaneous codes can be proved.

Limitations 1 This method can only validate X and Y axis coordinates. 2 MCU will often ‘lock up’ the program at errors requiring editing. This in

tum often requires the program to be restarted from the beginning if a sequence restart facility is not available.

4.8 The computer The following items are designed to visit some key points, terms and definitions.

Types of computers

4.8.1 Mainframes and supercomputers In computer jargon, large computers are called mainframes. Mainframes have assess to billions of characters of data and can process data very quickly -millions of instructions per second. The price of a mainframe varies from several hundred thousand dollars to over $10 million. Clearly with that kind of price tag, you will not buy a mainframe to use as an electronic diary. Generally they are used by governments, big business and industry.

4.8.2 Minicomputers Computers with less storage than mainframes are called minicomputers. They are slower but less costly than mainframes, around $10,000 to $50,000. Minicomputers were intended to be small and serve some special purpose. Improvements quickly turned them into a very versatile machine. Today the line between minicomputers and mainframes is somewhat blurred, to a point where the name, mini, no longer seems to fit. The term super mini has been coined to describe minis at the top end of the spectrum —up to $500,000. These are widely used by government and business.

4.8.3 Microcomputers Small computers such as desk top, personal or home computers are called microcomputers. For many years the computer industry put much, if not all, of its research effort into big computers. Small computers, and those suggesting a niche for them, were scoffed at. Time has proved them right however. Relatively inexpensive, small computers are now readily available.


4.8.4 Mini-microcomputers The trend to smaller computers continues. Small mini-computers better known as Laptops are now part of the computer market. Many are as powerful and have the same capabilities as a PC. The advantage to Laptops with their size, weight and the fact they can run on battery power, is they can be taken anywhere. Many sales engineers do calculations and enter orders on their Laptops on-site and transmit the information to their office over a telephone using a modem.

4.9 Review of computer terms

Figure 4.14 Desk top computer system

The subject of computers generally focuses on two broad aspects, computer hardware and computer software

4.10 Computer hardware The term computer hardware is a general term used to describe the computer and the peripheral devices which aid in the use of the unit. These devices are often considered under the following headings:

1 Input devices 2 Central processing unit 3 Storage devices 4 Output devices.


4.10.1 Input devices As the name implies, input devices are used to enter data into the computer. Among the more common of such devices are:

• Keyboards • Mice (mouse) • Light pens • Digitisers • Scanners • Punched tape readers.

4.10.2 Central processing unit (CPU) The central processing unit (CPU) is the computers centre of activity. This is made up of electronics circuits that interpret and execute program instruction and communicates with the input, output and storage devices.

4.10.3 Storage devices Because RAM (Random Access Memory) is only a temporary method of storing data, more permanent storage methods must be used to save NC data. This can be done by:

• Storing data on the computer hard drive. • Storing data on the floppy disc • Storing data on magnetic tape cassettes. • Storing data as punched tape.

4.10.4 Output devices Once the CPU has processed the data some form of output is requires. This output can be in the form of man readable information or coded signals which only decoding devices can interpret. More common output devices include:

• Dot matrix and laser printers. • Flat bed and drum plotters. • Video display units (VDU) and cathode ray tubes (CRT). • Punched tape • Direct link cables connecting the computer to a CNC machine control

unit.


4.11 Punched tape Punched paper tape and tape punchers were among the first input/output devices to be used with computer. Compared with printing speeds today, they are somewhat slow —with tape readers operating at speeds of 300 to 1000 characters a second, and tape punchers at speeds slower than that. The slow reading and punching speeds, poor durability and the difficulty of adding or deleting data has led to punched tape being gradually superseded by other technology. The sketch below shows a typical tape reader.

Figure 4.15 tape reader

4.11.1 What the columns mean Each column of holes on the tape either represents a numerical value or has some special meaning. The sketch shown below describes what each of these eight columns means. The columns are numbered from right to left, which follows the standards and also the binary coded system on which the standard is based.

Figure 4.16


4.11.2 Tape material While the maximum thickness and other dimensions of punch tape has been standardised, the materials from which tape is made has not.

The four types in general use are:

1 Paper tape. This is the least expensive and is popular for short runs or trial runs. Also, this tape is often used as a master and kept in the tool crib so that production tapes may be duplicated from it.

2 Laminated paper tape with reinforced mylar plastic. This looks very much like the paper tape. However, it has a thin sheet of mylar plastic between two paper strips, it is much stronger than the paper tape. It is also more expensive. This type of tape material is used when higher production runs are required than those for which paper tape is used.

3 Aluminium tape with a clear mylar plastic coating. This is one of the most durable and strongest tapes and is also one of the most expensive.

4 Plastic tape. This is also a very strong tape and compares with the aluminium mylar tape for toughness, durability, and cost.

4.11.3 Tape terminology The following terminology applies to punched tape used in programming.

A character. Any number, letter or symbol which can be seen on the keyboard of a type writer or computer is referred to as a character. Some characters such as G and M are very commonly used while others such as % or / are special characters having meanings specific to a particular MCU.

Row. A row is a path of holes perpendicular to the edge of the tape where the holes for one character are located.

Channel. A channel is a path parallel to the edge of the tape, that can contain a hole or no hole, depending on the characters. There are eight channels on the tape used.

Sprocket. The sprocket is the smaller hole or channel between channels three and four and is used to propel the tape through the reader and for timing control within the CNC system.

Word. A word is a combination of characters that represent an axis command, a sequence number, or a function. The first character in each word must be a latter address. For example: N49 X13 Y001 M05. The first word is the sequence number (N49), the next two words are axis commands, (X123 Y001) while the last word (M05) is a miscellaneous function command.

Block. A block is a combination of words that represents one complete sequence of commands. The block length is variable, since the blocks need to contain only


those command words that have changed from the previous block. If only one axis or only one function is changed from the previous block, the new block only needs to have the words which have changed from the preceding block.

For example: N50 G01 X25 Y50 M03

N51 X50

N52 Y40 M09

Parity. The term parity refers to a system used to identify and proofread punch tapes which have been produced under the EIA or ACSIIIISO standards.

EIA standard. Tapes cut under the EIA standards have what is referred to as ‘odd number parity’. Put simply this means that each character is represented as an odd number of holes across the tape. The example below shows the odd number parity format for the EIA standard codes.

Figure 4.18 EIA

ASCII/ISO standard. To distinguish between this and the EIA standard ASCIIIISO tapes have even number parity. The example below shows the even number parity format of ASCIIIISO standard codes.

Figure 4.19 ISO/ACCII


4.12 Computer software No matter how sophisticated input and output devices may be and, no matter how powerful the CPU may be, a computer is useless without software. The software used depends on what you are trying to achieve, for example word processing software for written communication and CAD (Computer Aided Drafting) for graphical communication.

4.13 CAD/CAM software The computers you will be using for CNC programming are loaded with a CAD/CAM (Computer Aided Drafting /Computer Aided Manufacturing) program for which the operator instructions are included in the following section.

Advantages of computer aided drafting

Saving in geometry definition Since the part geometry data has already been created during the design phase using the CAD/CAM graphics system, the part programmer is not required to redefine the geometry of the part.

Immediate visual verification The graphics terminal provides a display of the tool path for immediate verification by the part programmer. Most programming errors can be detected by the user and corrected at the time the error is made. with conventional APT or other NC languages, there is a delay between writing the program and the verification/correction process.

Use of automatic programming routines For common part programming situation such as profiling and pocketing, the use of automatic macro type routines (sub routines) yields a significant reduction in part programming time.

One of a kind jobs Because the part programming time is significantly reduced when using a CAD/CAM system, numerical control becomes an economically attractive method for producing one of a kind jobs. without CAD/CAM, the time required to prepare the part program represents a significant obstacle which often precludes the use of NC for one/off production.


Ease of data storage and management Computer hard drives and floppy disks provide a means of storing the equivalent of thousands of drawings and tool path geometry. Apart from the security and conveniences of electronic storage changes, edits and revisions of data is far easier through computing than any other method.

4.14 Writing simple programs In manual CNC part programming, the process of writing any program starts with the programmer interpreting the drawing and producing a job plan which details such things as dimensions, types of operations, tools required, feed and speeds. Once this data has been established the process of writing a program manuscript can begin.

4.15 Program formats Over the years a number of programming formats have been developed. However, the one which has dominated is ISO word address

4.15.1 Word address format The term word address means that each word in a block is pre-fixed by a letter. The letter acts as an address and directs the particular word to are addressed to their respective storage registers. The N word would be directed to the N word register, the word X register, and so on. If there is no requirement for a word in a particular block, the word need not be shown on the tape.

For example: If there were no z motion required, there would be no need to show a z word on the tape.

Figure 4.20 Word address format


Technically the words may be in any order. However, a standard has been developed which specifies that they be arranged with the sequence number word being first, and the other words in the following order.

N Sequence number word

G Preparatory word

X } Y} Dimension words (straight line) Z}

A} B } Dimension words (rotary) C}Other programming formats are:

4.15.2 Tab sequential format This format is very helpful when programming for point-to-point equipment since the majority of tapes for point-to-point machines are prepared on a device that resembles a typewriter. As far as the tape format is concerned, it may be noted that the tab character is used to separate the words in a block. See below. Also, if the words are kept in a fixed order, such as the sequence number word coming first, the G word next, etc., then it is not necessary to note the letter addresses, since the words will be directed automatically to their proper registers in the control unit.

In some point-to-point machines the trailing zeros as well as the plus (+) sign before the X and Y words need not be written. While there is no harm in showing the plus (+) sign, it is not necessary to do so with most NC systems since the system interprets the absence of a sign as a plus (+) sign. The minus (-) sign, however, must be shown.


Figure 4.21

4.15.3 The APT programming system The APT or Automatically Programmed Tool system, was one of the first computer part programming systems developed. Although APT is not the simplest computer programming system, it has been chosen for description in this text because it is the system having the widest use and the greatest range of capabilities. It can be used with more types of numerical control machines than any other programming system.

A complete part program, which would be suitable for preparing a tape via the computer, consists of four types of statements. These are:

1 Auxiliary statements 2 Post processor statements 3 Geometry statements and, 4 Motion statements.

Usually the first statement of a part program are auxiliary and post processor statements, then follow geometry statements, and last motion statements. There is no fixed rule covering this, and auxiliary and post processor statements may also be scattered throughout the program. Where necessary, geometry statements may be mixed in with motion statements providing any symbol used in a geometry or motion statement has been described prior to its use. Shown below is a typical APT language program for the part out line shown below.


Figure 4.22


4.15.4 APT Program example

Unlike the WORD ADDRESS and TAB Sequential formats, APT language is not a manual programming format, it requires a computer with the appropriate post processing software to encode such statements as ‘GOTO’ (GOTO) ‘GORIGHT’ (GORGT) and ‘GOLEFT’ (GOLFT) as signals which the MCU will accept.

ISO Word address By far the most accepted program format used through out the world is the ‘ISO WORD ADDRESS —variable length’ format. This format is in accordance with the following international standards.


EIA RS 224 ISO IS0840 The addresses and their numerical values are normally grouped into a block in the following order:

N G X Y Z I J K F T S M

Each block does not contain all addresses as their value may not always change from block to block. Normally only those values that are changing need to be entered, exceptions to this will be noted in the following descriptions.

Description of letter codes

‘N’ sequence number The sequence number is employed to allow the operator or programmer to identify that portion of the program. The information is the four digits after N. The sequence number is not a counter, therefore the sequence count (NI, N2, N3 ...... N9999), is the responsibility of the programmer. It must be changed for a given block, in order to be able to use the sequence number search facility to find that block.

‘G’ preparatory functions The two digit code preparatory word designates to the control the mode of operations and the means by which tools should move from one point to another. G codes for example, can change the control to an inch mode or specify rapid traverse. These will be dealt with in more detail shortly.

‘G Codes’ G codes can be split into groups on their function. Any number of G codes may appear in the same block, but the first ‘G’ code which requires a second letter address (GOI XIOO) will terminate the scanning of the block.

Example: N12G91G17G01X100

4.16 Modal codes Codes may be modal, that is, they remain in affect until cancelled or cleared by another code from the same ‘modal family’ in another block. Non modal codes apply only in the block which they are programmed. Within each ‘family’ group of G codes, the execution of any G code will cancel the other G codes inside that group.

Example: G01 will cancel G00, G02, G03 or G33


The examples of modal ‘G’ codes shown in the chart below are a general list based on ISO standard ‘G’ codes. For more specific modal codes you should refer to the manufacturers specifications supplied with the MCU.

Modal G code groups

4.16.1 G codes for milling

Sinumeric 820 M control unit The following list of codes are presented in their ‘family groups’ with a brief description of each code. These groups are significant in that codes from the same group may not appear in the same block in a program simply because by being modal they cancel each other.

Code Function

I GOO Rapid positioning * Commands rapid movement in an appropriate straight line to the commanded position.

I GOI Linear interpolation * Movement at the feed rate to the commanded position. 1 G02 Circular interpolation clockwise * Movement in an arc at the feed rate to the required

position I G03 Circular interpolation counter clockwise * Movement in an arc at the feed rate to the

required position 2 G04 Dwell, duration under address X, F or S

G04 Xl.O = one second dwell G04 FIOO = one fifth of a minute if the feed rate is 500 mm/minute G04 S I 00 = one sixth of a minute if the speed is 600 revolutions/minute

4 Gl7 Plane selection X, Y * Circular interpolation is calculated in the X/Y plane. Arc centre values to be in I and/or J, or radius if available


4 GIS Plane selection X, Z * Circular interpolation is calculated in the X/Z plane. Arc centre values to be in I and/or K, or radius if available.

4 Gl9 Plane selection Y, Z * Circular interpolation is calculated in the Y/Z plane. Arc centre values to be in J and/or K, radius if available.

55 G25 G26

Minimum working area limitation Maximum working area limitation * The G25/G26 codes may be used to define in absolute, the maximum and minium co-ordinate values for an area into which the tool must not travel

7 G40 Cutter radius compensation cancel 7 G41 Cutter radius compensation left 7 G42

Cutter radius compensation right * If accurate contours are to be produced and tool wear is to be taken into consideration, then some means must be available whereby an adjustment to the ‘radius value’ in the offset register for a cutter will cause an adjustment to the cutter path. Job size and shape can be controlled without changing the program by using cutter radius compensation. For a full explanation refer to the cutter radius compensation section of this book.

S G53 Suppression of zero offset * Used to suppress G54 to G57 codes 9 G54 Zero offset No. I 9 G55 Zero offset No.2 9 G56 Zero offset No.3

G57 Zero offset No.4 * Machine zero is a position set by the manufacturer and doesn't vary from day to day, but the position of the job on the machine table will vary from job to job and a code must be available by which this ‘job zero’ position may be identified in the program. G54 to G57 may be used to drive the tool to the job zero position. ego G55 XO YO For a full explanation see the zero offset section in this book

1010 G58 G59

Programmable additive zero offset No. 1 Programmable additive zero offset No.2 * The G58 and G59 codes are used to add to or subtract from already programmed values in the G54 to G57 register. ego G58 X100.0 will add 100 mm to the zero offset being used.

11

G60 Speed reduction, fine exact positioning. * The group 11 codes are used to control deceleration and transition to the next block to prevent movement in the new block commencing before the tool has completed the current move. G60 provides the maximum deceleration zone at the end of each block. Sometimes called square comer mode.

11 G62 Continuous path operation, block transition with speed reduction. * G62 provides the

minimum deceleration zone at the end of each block of movement before continuing with the next block. Sometimes called round comer mode.

13 G70 Imperial input * Dimensions are interpreted as inches. 13 G71 Metric input * Dimensions are interpreted as millimetres

14 G50 G51

Cancel scale modification Scale modification * The size of a job may be increased or decreased. ego G51 P60 the job is reduced to 60% of the programmed size.

15 G90 Absolute dimensioning. * The co-ordinates are interpreted as distances from the absolute zero position.

17 G94 Feed rate, address F is in mm/min 17 G95 Feed rate, address F is in mm/rev 19 G80 Cancels G81 to G89 19 G81 Drilling and centreing 19 G82 Drilling and spot facing 19 G83 Deep hole drilling, pecking

19 G84 Threading (tapping with encoder) * see section on work cycles in this book for a full description of G81 to G84.

4.16.2 G codes for turning Okuma OSP5000L Control unit


This list is an edited list of the more common commands applicable to an OKUMA LB 15 two axis lathe.

G00 Rapid transverseG0l Linear interpolation G02 Circular interpolation C. W. G03 Circular interpolation C. C. W. G04 Dwell G3l Fixed thread cutting cycle G32 Fixed thread cutting cycle, End face (Transverse) G33 Fixed thread cutting cycle, Longitudinal G34 Variable lead thread cutting cycle (Increasing lead) G35 Variable lead thread cutting cycle (Decreasing lead) G40 Tool nose radius compensation: Cancel G4l Tool nose radius compensation: I. D. ordinary cutting (Left of programmed line) G42 Tool nose radius compensation: O. D. ordinary cutting (Right of programmed line) G50 Maximum spindle speed designation G50 Zero offset G7l Longitudinal thread cutting fixed cycle G72 Transverse thread cutting fixed cycle G73 Longitudinal grooving fixed cycle: OD/ID groove G74 Longitudinal grooving fixed cycle: Face groove/drilling G75 Automatic chamfering 45 0

G76 Rounding G77 set pitch error compensation: C axis G80 End of contour definition G8l Start of longitudinal contour definition G82 Start of transverse contour definition G84 Change of rough cut conditions for bar turning G90 Absolute programming G9l Incremental G94 Feed rate mode: mm/min. mode G95 Feed rate mode: mm/rev. mode G96 Constant speed cutting: On (meters per min) G97 Constant speed cutting’ OFF (R.P.M) 0180 MUltiple fixed cycle cancel: (LCM) Milling functions 0181 Drilling cycle : (LCM) Milling functions 0182 Boring cycle : (LCM) Milling functions G183 Deep hole drilling cycle : (LCM) Milling functions G184 Tapping cycle : (LCM) Milling functions G185 Longitudinal thread cutting cycle : (LCM) Milling functions

4.16.3 ‘X’ ‘Y’ ‘Z’ axis movements These addresses refer to the axis or group of axes that are required to move in the particular block. These can be programmed as incremental or absolute moves, dependent on the Modal G Code at that time. Modal codes will be dealt with in more detail shortly.


4.16.4 ‘T’, ‘J’ and ‘K’ arc centre offsets These words are used to further describe the motion required when travelling in a circular path. They do this by specifying the centre of the arc in relationship the start point of the curve. Arc centre offsets and their codes are dealt with in the section on programming for circular interpolation.

Some Machine Control Units also use the ‘I’, ‘J’ and ‘K’ values when defining cutter radius compensation.

4.16.5 ‘F’ feed rate designation The value associated with this address informs the MCU of the required vector speeds of the axes during the cutting motion. Exactly how the ‘F’ value will be interpreted depends on the modal ‘G’ command in force at any given time.

For example: If the modal ‘G’ command was G94 any ‘F’ value would be interpreted as mm/min.

If the modal ‘G’ command was G95 any ‘F’ value would be interpreted as mm/rev.

4.16.6 ‘T’ tool numbers This is usually a four digit designation where the first two digits refer to the actual tool number and the last two digits, calls up the appropriate tool offsets. Tool offsets are in fact the way we communicate to the machine the type and physical size of each individual tool being used. This topic will be discussed in detai11ater in this guide.

The command T0707 is interpreted as tool 7 and tool offset 7.

On some control units the tool offsets are called up using the letter address D or H. On the Siemens 820M control a tool offset is identified by the letter address D.

Thus the command T03D03 is interpreted as tool 3 and tool offset 3.

Note: Tool numbers and tool offsets are modal.

4.16.7 ‘S’ spindle speeds The address ‘S’ is followed by up to four digits that call up a particular spindle speed revolutions per minute (RPM). ego S2000. S codes are modal. In case of CNC lathes which have a constant cutting speed function, an S code is also used to set constant cutting speed. ego G96 S150 will maintain a cutting speed of 150 m/min regardless of changes in the diameter of the work. On a CNC lathe this feature will automatically increase or decrease spindle RPM in relation to the work piece diameter.


4.16.8 ‘M’ miscellaneous functions The ‘M’ code refers to any of the miscellaneous function the control should perform on command. Typical miscellaneous functions are: spindle start, stop, coolant on/off, rewind. As these codes listed below are based on ISO standards may not all be applicable to all CNC machines. Your machine tool manual should be used as the final guide.

Miscellaneous functions use an ‘M’ code up to two digits. Normally most controls will only read a maximum of two ‘M’ codes in anyone block of information.

For example: G01 X300. Y210, M03, M08

Sinmeric M codes for 820M Programmed stop, unconditional* The program will stop operation including spindle stop.

MOl Programmed stop, conditional * Same as MOO. Only active when the option stop switch in ON.

M02 Program end * Indicates the end of a main file. M03 Spindle start M04 Spindle start, counter clockwise. MOS Spindle stop, non oriented. M06 Automatically loads tool to the spindle. M08 Coolant on M09 Coolant off. M17 Subroutine end * Indicates the end of a subroutine (sub-program file) Ml9 Spindle stop with orient * Ready mirror image M20 Cancel mirror image M21 Mirror image in the X axis M22 Mirror image in the Y axis * The mirror image function reverses all X and Y coordinates to

produce a component of the opposite hand. M30 Program end, with rewind. * Indicates the end of a main program file and returns control to

the start of the program. M34 Selects high range for spindle speed. By selecting a spindle speed less than 960 rev's the

spindle would normally be in low range, and in this range the spindle inertia is such that the reversal of spindle direction, at tapping depth, the spindle takes a number of turns without Z axis feed tending to pull the tap from its holder (axial float holders must be used). Selecting high range reduces the spindle inertia and minimises the above effect.

Okuma CNC lathe This is an edited list of the more common miscellaneous function codes for the OKUMA LB15 lathe fitted with an OSP5000L MCU.

Program stopMOl Optional stop M02 End of program M03 spindle forward (C. W.) M04 Spindle reverse (C. C. W.)


M05 Spindle stop M08 Coolant ON M09 Coolant OFF MI2 Stops M-Tool rotation M13 Starts M-Tool rotation C. W. Ml4 Starts M-Tool rotation C. C. W. Ml5 Index ‘c’ -Axis in positive direction. Ml6 Index ‘c’ -Axis in negative direction. MI9 Spindle orientation M22 Cancel ofM23 (Chamfering) M23 Chamfering ON M24 Chuck barrier function/tool interference check OFF M25 Chuck barrier function/tool interference check ON M26 Thread lead along ‘Z’ axis M27 Thread lead along ‘X’ axis M30 End of tape M32 Straight infeed along thread face mode M33 Zig zag infeed in thread cutting M40 Spindle neutral M41 Spindle speed range selection M42 Spindle speed range selection M43 Spindle speed range selection M44 Spindle speed range selection M50 Spare air blower function OFF M51 Spare air blower function ON M55 Tailstock quill retract M56 Tailstock quill advance

4.17 Contour machining using circular interpolation Circles and curves can be produced on both the lather and on a Machining Centre by one of two methods, point to point programming or by circular interpolation.

4.17.1 Point to point contours This system of programming curves and contours is very much a thing ofthe past and would be used in the case where a machine was not fitted with a control capable of circular interpolation. When using the point to point method the curve is redefined as a series of straight lines within a calculated zone of tolerance. Refer to figure 4.23.


Figure 4.23 Curve high limit

4.17.2 Circular interpolation Most modem machines (CNC) have a circular interpolation function which will produce a curve or circle in one motion and to a much higher accuracy than the point to point system. With the exception of a fewer minor points, the calculations for circular interpolation for CNC Machining Centres are the same for CNC lathes.

Circular interpolation has three basic considerations.

• The direction of rotation • The end point of the curve and • The start point of the curve.

A typical command for circular interpolation

G03 X40 Y30 I -20 J 0

Figure 4.24


4.17.3 Direction of rotation On CNC lathes and machine centres the commands used to call up circular interpolation are the same.

G02 Calls up circular interpolation in a clockwise direction.

G03 Calls up circular interpolation in an anti-clockwise direction.

Figure 4.25 Directions of rotation

4.17.4 The end point The second of the three commands must inform the control exactly where the contour being programmed will end. This normally requires a calculation accuracy of two decimal places. Another important consideration is that the end point is an ‘X’, ‘Y’ or ‘Z’ axis co-ordinate defined as an absolute dimension.

Using the mill sample above you will see that the end point follows the circular interpolation command, ego G03 X40. Y30.

4.17.5 Arc centre offsets Put simply, an arc centre offset tells the control the point at which the centre ofthe arc is in relation to the start point of the curve.

This distance is always expressed as an incremental dimension accurate to two decimal places. To identify an arc centre offset, the dimensions are prefixed with a letter code; ‘1’ values ‘J’ values and ‘K’ values, these can be positive (+) or negative (-).

4.17.6 ‘I’ value statement An ‘1’ value statement is the incremental distance from the starting point to the centre ofthe arc measured parallel to the ‘X’ axis.



4.17.7 ‘J’ value statement A ‘J’ value statement is the incremental distance from the starting point to the centre of the arc measures parallel to the ‘Y’ axis.


4.17.8 ‘K’ value statementsIn the case of CNC lathes where the ‘X’ and ‘Z’ axes are the primary axes, a ‘K’ value statement is used to define the incremental distance from the starting point to the centre of the arc measures parallel to the ‘Z’ axis.



4.17.9 General rule for circular interpolation • CNC lathes operate in the X and Z axis only, therefore, the offsets

applicable are ‘I’ and ‘K’. • All programming for the machining centre in this course will be for

circular interpolation in the X and Y axis therefore the offsets applicable are ‘I’ and ‘J’.

• Offset values can be singular or double values ego J1 0 or J1 0 15. If an offset has a value of zero it need not be written into the programme. Example, 120. KO. can be simply written as 120.

• To determine the arc centre offset is to be positive or negative and a cartesian quadrant should be constructed at the start point.

• A clear plastic overlay of the cartesian quadrants as shown in below will be of great assistance in defining arc centre offsets. This overlay is of particular value in establishing if the offset is to be (+) or (-).

Figure 4.32 Lathe Figure 4.33 Mill


4.18 Programming examples Example 1: When programming a full circle the end point will be at the same point as the start of the circle. For this reason you need only to define the direction of travel and the applicable arc centre offset.

Figure 4.34

Program

G00 X125 Y60 G01 Z-5G03 I-42

Example 2: In this next example you will note that the end point is 84mm along the X axis therefore the value I will be + 42 mm.

Figure 4.35

For a semi-circle the programme could read:

G00 X0 Y0

G01 Z-5

G02 X84. Y0. I42

In the case where the arc only forms part of an arc it is often necessary to apply some basic mathematical principles in order to calculate all the coordinates required to define the machining of the arc or curve.


Figure 4.36

In the example above the Start Point and the arc centre offsets for the curve are lines AC and BC of the triangle ABC.

Referring to triangle ABC in figure 4.37

Figure 4.37

The direction of rotation is in an anti-clockwise direction.

The end point is X50 Z80

The arc centre offsets are 1-15 K-20 (measures from the start point)

The correct command: G03 X50 Z80 I-15 K-20

Exercise 6 — section 4 — circular interpolation programming

1. Write a tool path program for the job below.


Circular interpolation

Point

G X Y Z I J

12345

Exercise 7 — section 4 — circular interpolation programming2. Write a tool path program for the job below.


Point

G X Y Z I J

12345

4.19 Writing a program for a machining centre This instruction introduces a standard format for the writing of a program for the ZenJord Ziegler Machining Centre using the ‘word address’ system. Although there are other program formats which will work on this machine, students are advised that the standard format introduced in this instruction is the only format which will be accepted in this course of instruction.

4.19.1 The job The example to be used is the ‘slotted base’ shown below. The job is to be clamped as shown with the program ‘zero’ being placed in the bottom left hand comer and identified using the standard symbol.


Figure 4.38 Slotted base

4.19.2 The tool path The program is a point programme in which the centre of a 10 mm end mill will follow the path shown below.

Remember: Milling program paths are based on the path the centre of the cutting tool would take.


Figure 4.39 Slotted base showing the path of the tool

4.19.3 Operator set up sheets One of the first tasks in writing a part for any machine is to complete the ‘Operator's Set Up Sheet. See the following example:


Operators setting up sheet - example


The main functions of such a sheet is that it becomes a written copy of what tools offsets are to be used during the machining process. In addition the set up sheets show all of the physical clamping or work holding arrangements showing not only the program zero but just as importantly the position of any clamps or obstructions the programmer will have to consider when plotting the tool path.

4.19.4 Programming format Although we accept that there are a number of formats or approaches which can be used in writing part programs for a CNC Machining Centre, the format introduced in this guide is designed to provide a logical, simple and safe approach to programming.

4.20 CNC Machining centre program format You will note that the programming format it is divided into four basic groups, each group represents a particular phase or stage in the programming process.

Machine set up %MPFI (Program I. D.) G71 G90 G94 Gl7 G40 G00 Z0 D0 G54 X0 Y0 G55 X0 Y0

Tool set up Ml9 M00 T01 D01 (Tool I. D.) F320 S2000 M03

Machining cycle G00 Z10 X 10 Y45 Z-2 M08 G01 X130 …………. ................. …………..G00 Z40 M09

End of cycle G00 Z0 D0 Ml9 M00 (Remove last tool)


G53 G00 X0 Y0 M02

For the purpose of standardisation and ease of assessment, this programming format is to be strictly adhered to by students.

4.20.1 Group 1 — Machine set up

% MPFI: This is a program identification code which the Siemens 820M MCU requires to accept a part program. The number which follows % MPF identifies the program number.

Program ID: This is a man data which gives the operator prompts. Anything in brackets ( ) will be ignored by the MCU.

G90 G94 G71 G40 G17

Regardless of whether or note the machine is preset, it is good practice to confirm the modes in which you want the control to respond. The order in which you write these commands does not matter.

G90 : This command instructs the control that all tool movement will be ‘absolute ‘, that is to say that all dimensions will be taken from the one datum point. Should ‘incremental’ dimensions be required then the command 091 is used.

G94: The commands 094 and 095 are codes which relate to feed units, G94 will set the control in feed units of mm/min units while G95 sets the control in mm/rev.

G71: If a G71 command is specified all movements will be in mm. Should the inch mode be required then the command used will be G70.

G40: The subject of tool radius compensation will be covered at a later stage of this course however, at this point it is important that prior to any machining, all compensation from any previous programme be cancelled. This is done by including the command G40.

G17: This command sets any contour machining in the X and Y axis only.

G900 Z0 D0


G00: This is a rapid traverse command.

Z0: This directs the Z axis to return to its ‘home’ position.

D0: This cancels any tool offsets which may still be set as modal command.

G54 X0 Y0

G54: The G54 is one of four available positions preset commands which, when executed, will drive the table to what ever the X and Y values specify. These values are preset as ‘zero offsets’ on the MCU.

In this example no matter where the tool may be, the G54 X0 Y0 command will drive the table back to the machine home starting point is rapid traverse.

Figure 4.40

G55 X0 Y0

This command is the second of the four available position preset commands available and is used to drive to the point the programmer has used as the ‘program zero ‘.

Prior to entering a program into the MCU the machine setter must establish the exact distance from the ‘machine home’ to the ‘programme zero’, this is known as setting the zero offsets. The actual distance between the machine home and the programme zero is referred to the ‘zero offset’ and is expressed as an X and Y axis distance.

Setting zero offsets The methods used to establish these X and Y coordinates can vary from machine to machine. In the case of the Zenfold Machining Centre the zero offset is found by manually driving an edge finding device from the machine home to the selected program zero.


Figure 4.41

The X and Y coordinates, (in this example X-350 and Y-250), are registered in the MCV memory as a zero offset under G55. This remains in memory until deleted by the setter/programmer.

The G55 position preset has an additional function which is executed by the X and Y coordinates which complete the action statement.

In this example shown above:

G55: When this is read, the MCV searches its buffer storage and finds the coordinates registered as G55 -in this case X-350 Y-250. Once found the table is rapid traverse to these coordinates.

X0 Y0: Once the table has reached the target point which in this case is the program zero, the programmer has the option of giving this point any value he/she wants. Because we want this point to be recognised X0 Y0 we specified G55 X0 Y0. (If for some reason a programmer wanted the target point to be recognised by the MCV as X-25 Y-25 then the appropriate command would be G55 X-25 Y-25).

From here on, the MCU only recognises the program zero as X0 Y0 until such time as the G55 X0 Y0 command is cancelled with the modal command G53.

4.20.2 Group 2 —Tool set up M19: As part of the tool change routine M19 will both stop the spindle revs and jog the spindle into a tool change position.

M00: In most CNC controls M00 is positive stop only. On the Siemens 820M MCU, M00 is a program stop and tool load command. Once the tool has been loaded the MCU ‘cycle start’ must be pressed to resume the program.


T01 D01

T01: Each tool used is given an identification number prefixed by the letter T. T04 is tool 4 which for example may be and 80 mm four tooth carbide face cutter. Check with your class teacher for the correct tool identification codes.

D01: Because of the varying length of tools used on a machining centre, the Z axis distance from the point of the tool (in the home position) to the workpiece surface will vary for each tool. If the programmer had to compensate for the different length of tools when programming the job, it would become very complicated and the possibility of error more likely. To overcome this problem the programmer treats all tools as being the same length in his program. To compensate for the variations in tool length, tool offset codes are used to pre-set the MCU independent of the actual part program.

Setting tool offsets To set tool offset the setter loads the tool, say T01, into the machine and ensures the spindle is ‘home’ in the ‘z’ axis. The spindle is then manually driven down until the tool touches the top surface’ of the workpiece or a appropriated setting gauge. The distance the tool travelled from the home position to the top face of the work is the tool offset and, is a minus Z value, this distance is entered into the Tool Offset Register as TOOL 1 (T01) OFFSET 1 (D01). The same procedure is repeated for all other tools.

Once the offsets for each tool have been established and registered in the MCU, the programmer can assume that the top face of the workpiece will always have a Z value of Z=Zero. In addition, any Z value above the top face of the workpiece will always be a +Z value while any Z value below the top face reference plane will always be a -Z value.

Figure 4.42 Tool positions after the tool offsets have been established and registered


F 320 S2000 M03

F320: F codes always refer to feed rates, in this example, because the feed has been specified as G94 (mm/min) the 320 refers to 320 mm of feed per mm.

S2000: On the machining centre S values are always statements of R. P. M. In the example the spindle speed would be 2000 rpm.

M03: This command will cause the spindle to start revolving in a clockwise direction. M04 will cause anti-clockwise, MOS will stop all spindle revs until an M03 or M04 command is given.

4.10.3 Group 3 -The machining cycle G00Z10: This is often referred to as the Initial Level and is a rapid traverse to a safe distance above the Top Face of the workpiece in this case 10 mm above.

X-10 Y45: This position is to set the tool away from the workpiece ready to accept the first full depth cut. Note: This will be in rapid traverse because the previous G00 command remains modal.

Z-2 M08: Rapid traverse to Z-2 (a 2 mm depth of cut). M08 will tum the coolant ON.

G00Z40 M09: At the end of each cutting cycle the tool be raised to a safe stand off position ego Z40. With tool still below the splash guard tum to coolant OFF with the M09 command.

Group 4 -The end of the cycle G00Z0 D0: With the job completed drive the tool home in the Z axis first with a G00 Z0 D0 command. Note: The D0 is read first, cancelling the tool offset and so making Z0 again the machine home position.

Ml9: Stop spindle and orient for Tool Change.

M00: Stop program and tool change.

G53 G00 X0Y0

G53: This cancels the G54 and G55 position preset and action allows the MCU to once again recognise X0 and Y0 as the ‘Machine Home’.

G00 X0Y0: Rapid traverse back to the Machine Home position.

M02: End of program and reset to start o program.


4.20.4 Sample program

Task Using the co-ordinates previously calculated in exercise six and the standard program format from page 143, mill a groove 2 mm deep using a 4 mm HSS slot drill (tool number 3).

Result The following manuscript would represent the complete program ready for production.

%MPFI (SAMPLE PROGRAM) N05 G7l G90 G94 G17 G40 Nl0 G00 Z0 D0 N15 G54 X0 Y0 N20 G55 X0 Y0 (************) N25 M19 N30 M00 (L0AD 4 mm SL0T DRILL) N35 T03 D03 N40 F300. S3000 M03 (************) N45 G00 Zl0.0 N50 X195.0 Y62.32 M08 N55 G0I Z-2.0 N60 G02 X120.0 Y19.02 I-50.0 N65 G0I X55.98 Y55.98 N75 G0I X35.0 N80 G03 X15.0 Y80.0 I-20.0 N85 G0I X0 Yl05.98 N90 G00 Z40.0 M09 (**************) N95 G00 Z0 D0 Nl00 M19 Nl05 M00 (REM0TE LAST T00L) N110 G53 X0 Y0 N115 M02

Exercise 8 — section 4 — programming practiceStudent instructions


• Referring to NM09 -Exercise 8, and using coordinate geometry calculate the program points for the finishing operation. Remember that the accuracy of calculations is to be to two decimal places.

• Prepare a hand written manuscript of your program on the work sheet provided and include all appropriate commands written in the standard format introduced in these instructions.

• Check your program against the example format provided.


Worksheet

Exercise 8 -Hand written program

NC program Programmer's notes (as requested)


4.21 Writing a program for the 0kuma LB15 CNC lathe

4.21.1 Introduction

This instruction introduces a standard format for the writing of a part program for an Okuma LB15 lathe using the word address format. Although there are other program formats which will work on this machine, students are advised that the standard program format introduced in these instructions is the only format which will be accepted on this course of instruction.

4.21.2 The job For the purpose of these instructions, the job will be a tapered pin as shown below. The program zero will be the face of the chuck jaws. The work is held in the chuck on a pre¬machined step, 40 mm long and with a diameter of 55 mm +/-0.4 mm

Figure 4.43

4.21.3 The tool pathIn CNC lathe programming where tools tips are round, triangular or any other rectilinear shape, the programming point cam be generated by drawing a vertical line and horizontal line that touch the cutting edge of the insert.


Figure 4.44

The path the tool will be following in normal feed traverse is shown as a solid line. The path the tool will follow in rapid traverse is shown as a dotted line (see below)

Figure 4.45

4.21.4 Operators setting up sheet One of the first tasks in writing a part program for any machine is to complete an Operators Set Up Sheet. The main functions of such a sheet is that it becomes a written copy of what tools and tool offsets are to be used during the machining process.

Operators setting up sheet


Programming format

The program format shown on the following page is a standard format which is to be strictly adhered to. You will note that the programming has been devised into six basic steps.


4.22 OKUMA LBt5 CNC lathe program format

No action (Location pin) No action (57 x 160 m/s round) (Tool 1 = 80 degrees roughing) (Tool5 = 30 degrees finishing)

Action set up G90 G95G96 S180G40 G00 X300 Z300 T0101 M42G50 S2500 M42M03

First action G000 X55 Z120 F0.2 M08G01 X-1.6G00 Z121 X50G01 Z20 F0.3

Tool change — finishing G00 X300 Z3000 T0100 M09(Tool 5 = 30 degrees finishing)G00 X300 T050505G50 S2.500 M42G06 S200

Second action — finishing G00 X-1.6 Z124 M08G42 G02 Z120 F0.2X50

End of action G40 G00 X300 Z300 T0500 M09 M02

%

For the purpose of standardisation and ease of assessment, this programming format is to be strictly adhered to by students

4.22.1 Group 1 — No action ( ) As an ISO standard any information in brackets is man readable information to which the MCU will not react to:


4.22.2 Group 2 — Action set up G90 G95

G90: The G90 command instructs the MCU that all tool path coordinates have been expressed as absolute coordinate values.

G95: Given that convention has feed on a lathe expressed as mm/rev, G95 will ensure this mode of feed.

G96 S180

G96: This command maintains a constant cutting speed throughout the cutting operation. When G96 is evoked the MCU will automatically increase or decrease spindle RPM to main a constant cutting speed with each change in workpiece diameter.

S180: Although S values generally refer to spindle RPM, in this case when associated with G96, S 180 sets the constant cutting speed to 180 m/min.

G40 G00 X300 Z300 T0101 M42

G40: The G40 command when used at this point of a program is a precautionary move to ensure that any Tool Nose Radius is compensated is cancelled. More about Tool Nose Radius compensation shortly.

G00 X300 Z300: This command instructs the MCU to drive the tool slide to the Machine Home position G00 X300 Z300. This is required at the beginning of each part program as well as during all too change routines.

T0202: This command calls up TOOL No.1, (T0101) and applies the TOOL OFFSET No.1, (T0101). The OFFSETS for all tools to be used are established and registered in the MCU prior to machining.

G50 S2500 M42

G50: The G50 command is a ‘speed limiting’ command which prevents the machine chuck from exceeding a specified RPM. This is use particularly where large diameter work is required to be faced while a G96 command is in force. Without a speed limiting the G96 command would allow a workpiece being faced to reach maximum RPM as the tool reaches the centre of the job.

S2500: When used in conduction with the G50 command S2500 specified the maximum RPM which will be reached. In this example 2500 RPM.

M42 (M41): The RPM ranges on the OKUMA are controlled via a preselect gear box which is coupled to the electronic speed control unit. M42 sets the machine in high spindle range. M4l sets the low spindle range.


M03: This instructs the MCU to drive the spindle in a clockwise direction.

4.22.3 Group three — First action G00 X55 Z120 F0.2 M03

G00 X55 Z120:

This command will drive the tool to the ‘initial stand off’ position somewhere close to the starting point but still off the workpiece.

FO.2 : All ‘F’ words relate to feed rate, in this case a facing feed rate of 0.2 mm/rev. Because feed command are modal any new feed command will cancel any previous command, for example, the G01 Z20 F0.3 command will cancel the current F0.2.

M08: This command is a programmed ‘coolant ON’ command.

G01 X-1.6

At this point of the program the co-ordinate points of the tool path is entered. A widely accepted practice is to first machine the workpiece to the correct length by a series of facing cuts which, as shown in this example, extend 1.6 mm past the centre of the work in diameter terms as indicated by the minus (-) value of -1.6 mm.

4.22.4 Group four — Tool change routine G00 X300 Z300 T0100 M09

G00 X300 Z300: Drives the tool slide back to the machine home position.

T0100: Cancels the tool offset for Tool 1. T01 identifies the tool number while the 00 cancels the offset.

M09: Turns the coolant OFF.

G00 X300 Z300 T050505

G00 X300 Z300: Confirms the machine home prior to a tool change.

T050505: In this particular example a finishing tool is being called up. Because the finish cut will involve a taper or a contour, Tool Radius Compensation must be applied. The order of instructions are:

T05 — Call up Tool 5

05 — Call up length offsets 5

05 — Call up value Nose Radius for Tool 5


G50 S2500 M42

G50: The G50 command is a ‘speed limiting’ command which prevents the machine chuck from exceeding a specified RPM (S2500).

G96 S200: Resets the constant cutting speed to 200 m/min.

4.22.5 Group 5 —Second action G00 X-1.6 Z124 M08

Rapid traverse to initial stand off position and tum the coolant ON.

G42 G01 Z120 F0.2

G42: In the next section of this guide the topic of Tool Nose Radius Compensation will be discussed in some detail. At this point of time it is sufficient to state that Automatic Tool Nose Radius Compensation must be applied during a motion command at using either a G42 or G4l command.

G01 Z120: This is a motion command during which the Radius compensation is applied.

F0.2: This feed rate of 0.2 mm/rev over rides all previous feed rate commands.

4.22.6 Group six —End action G40 G00 X300 Z300 T0500 M09

G40: At the end of each action you are required to cancel all Tool Nose Radius Compensation with a G40 command.

G00 X300 Z300: At the end of the cutting routine drive the tool slide to the machine home position.

T0500: Cancel the Tool No.5 and Tool Offset No.5 with the command T0500.

M02: End of program —go to starting block.

% End of tape marker


4.23 Points on CNC lathe programming

4.23.1 Facing As a general rule the overall length of the workpiece is faced to length as the first machining operation. In this case the direction of feed is normally towards the centre of the work with a width of cut of no more than 1 mm. Refer below. When facing cuts are taken, the first cut is a light reference cut as the end of the bar stock rarely has a perfectly square face. The facing out is driven past the centre line of the work to a point equal to twice the nose radius of the insert. For example, if the insert has a 0.8 mm nose radius then the facing cut will be G01XC-1.6

Figure 4.46

Note: The machine has no way of knowing if the workpiece blank is too long. Consequently, if the job was 5 mm longer than what you have programmed as a Z axis value, the lathe will attempt a 5 mm facing cut and so creating a very dangerous situation.

When facing to a shoulder as shown below, the direction of the cut is normally away from the centre of the work with a depth of cut of no more than 0.1 mm. The reason for this is that most finishing tools have a 3 to 5 degree negative approach angle which is used to finish up a shoulder.


Figure 4.47

Roughing out a profile Tool number 1 is the roughing tool used for facing and turning. This tool can take a 3 mm depth of cut using a feed rate of approximately 0.2 to 0.35 mm/rev. When roughing remember to leave the work 1 to 2 mm oversize for the finishing cut as carbide tools do not perform satisfactory when taking light cuts below 0.5 mm depth.

When roughing machining large diameter stock remember to include a G50 command line to limit the RPM to a safe value which will not throw the work out of the chuck jaws. Remember that this command requires a speed limit value. For example G96 S200 = constant cutting speed of200 m/min, G50 S2000 = a maximum RPM of 2000.

Also remember when machining black bar it is never exactly the size stated, it is always larger. For example, Ø 100 black bar is probably Ø 101 or larger. It is best to go and measure your material to check this out, then allow for this in your depths ofuts. The material may also be out of round.

Note: It is strongly recommended that the tool path co-ordinates for roughing are establishes by drawing the component on graph paper using a 2: 1 scale and then plotting each roughing cut on the graph paper.

4.23.3 Roughing out tapers When roughing tapers, it is important that the finishing tool has a reasonably even depth of cut. ‘Steps’ in tapers should be avoided.


Figure 4.48

Method one Using ‘method one’ the steps are removed by programming the tool movement in such a way that, at the end of each successive cut, the tool is fed from the end point of one cut to the end point of the previous cut along a line approximately the same as the required taper.

Figure 4.49

Method two When the last depth of cut is programmed, feed up the taper leaving a suitable finishing allowance.


Figure 4.50

4.23.4 Roughing out radii and fillets A method similar to that of roughing out tapers is also used to rough out radii and fillets. Again the important factor to remember is that steps should not be left for the finishing tool to clean up. These steps must be eliminated during the roughing process.

Figure 4.51

Method 1 Perhaps the simplest method of roughing out a radius as shown is to first draw the profile on a sheet of graph paper to a scale of 2: 1. Having done this, the procedure now only requires a series of point to point coordinates from the end of point of one cut to the end point of the previous cut. This generates an approximate radius without the need for the calculations associated with true form circular interpolation.


Figure 4.52

Method two This uses a preliminary roughing out cycle in which the bulk of the material is removed in a manner which will leave undesirable steps. The final roughing cut, is programmed to remove these steps by cutting a true form radius smaller than the design size using the circular interpolation function.

Figure 4.53

4.23.5 Roughing out chamfers

Method one Because chamfers generally require only a small amount of material to be removed, the simplest way of cutting chamfers is to program each chamfer, small fillet or radius as part of the finishing cut as shown.


Figure 4.54

An alternative method is shown. This method which is slower, provides for a greater dimensional accuracy in that any backlash which might be present is eliminated even though in theory there should be no backlash present. The method illustrated here should only be considered if the dimensional accuracy of the diameters of the shaft were critical.

Figure 4.55

4.23.6 Programming the finishing cut When programming the finishing cut for a component the programmer has to consider four basic things:

1 The finishing size 2 The finishing tool 3 The finishing feeds and speeds 4 Tool nose radius compensation.


Finishing size When programming the diameter of a finished size, the size programmed should always be the mid point between the high limit and the low limit of size.

Figure 4.56

Finishing tool The practice of only using one tool for all roughing and finishing operations is one which is not generally recommended because the one tool would wear quickly and as a result the dimensional accuracy of the tool will be effected. In addition surface finish would also be effected. Usually one tool is used for roughing, leaving a small amount of material to be removed by the finishing tool. With this method the finishing tool maintains a working tolerance of size and finish over a greater number of components.

Finishing speeds When using finishing tools, speeds should be increased. For example, when finished machining mild steel and coated carbide tools, use a cutting speed of200-300 m/min as a starting point. Feed rates for finishing vary, depending on the tip radius and surface finish required. As a rule of thumb finishing feeds should not exceed one quarter of the Tool Nose Radius -the feed rate for a tool with a 0.4 nose radius would therefore not exceed 0.1 mm/rev.


4.24 Tool nose radius compensation

Point one If a lathe tool was ground so that angles formed a sharp point then tool nose radius compensation would not have to be considered as this point of the tool would always locate at the programmed point. Referring to the figure below, you will see that if you programmed a tool movement of say X50, Z100 then the point of the tool would be located at this program point.

Figure 4.57

To have a tool without a nose radius will produce an often undesirable surface finish, with the added problem of the tool point frequently burning away. To overcome these problems a tool nose radius o/between 0.4 and 0.8 is normally applied.

Point two When considering a tool with nose radius as shown below, it is important to note that during the setting of the tool offsets, the tool contact point ‘a’ located in the datum in the Z plane while the contact point ‘b’ located the datum in the X plane.

The intersection point of a tangent line drawn through points ‘a’ and ‘b’ forms the tool program point. This means that when a tool position of say, X5, Z100 is ordered, the tool program point would locate at these coordinates thus putting the two contact points (a and b) in their correct X and Z axis positions.


Figure 4.58

Point three Considering a tool with a nose radius. If the tool movements were all X and Z values at 90° to each other then no special considerations regarding nose radius is required. However, where a taper or an arc is to be cut, tool nose radius compensations must be applied otherwise an oversize or undersized taper or arc will be cut. The example illustrated below shows how, even though the tool program point follows the finished size geometry of the workpiece, the taper cut will be oversize.

Figure 4.59


4.24.1 When do I use it? Because tool nose radii only affects tapers and arcs, radius compensation need only be applied on the finishing cuts of programs in which arcs and tapers are required.

4.24.2 How do I apply it? The application of tool nose radius can be delivered by a number of ways including the use of reference charts or by trigonometry calculations. At this level of training the method which will be used is a programming command to which a Machine Control Unit (MCU) equipped with the appropriate ‘G’ code function will respond by calculating and automatically applying tool nose radius compensation until cancelled by another ‘G’ function command.

4.24.2 Tool nose radius compensation commands G41 —The command used to apply tool nose radius compensation to the left of the line of travel.

G42— The command used to apply tool nose radius compensation to the right of the line of travel.

G40 — Cancel all nose radius compensation.

4.24.3 A method of remembering whether to use G41 or G42 Think of yourself as being the centre of the tip or the centre of the cutter, standing in line with the edge of the job, facing the direction of the feed.

Figure 4.60

Ask yourself: ‘Which way must I move so that the cutting edge of the tool will move along the edge of the job? ‘


In the example shown below, the direction of motion required was to the right of the direction of feed and so a G42 command will be required.

Figure 4.61

In the example below, the direction of motion was to the left of the direction of feed and so a G41 command will be required.

Figure 4.62

4.25 How to call up and use tool nose radius compensation The program block in which the mode changes to G41, G42 or G40 is called the start up block. During this block, the MCU will respond to the selected command by calculating and then physically adjusting the position of the centre of the tool nose radius and the distances required in both the X and Z axis. This adjustment will position the radius portion of the tool's cutting edge at a tangent to the


required curve or taper as shown below. Once applied, the tool noise radius compensation selected will remain in force until cancelled by the G40 command.

Figure 4.63

When tool nose radius compensation is called up, the command, whether it is G41 or G42 must be called up during a motion in either linear feed rate or rapid traverse. The preferred method is to call up tool nose radius compensation on a feed rate motion line. Referring to the figure below, the tool is driven in rapid traverse from the point XI00, Z100 to the point X80 Z100.

As this motion is simply in the X axis no compensation is required. As this job involves the machining of a taper the finishing cut must incorporate tool nose radius compensation. This is done by including the G42 command on the feed rate traverse to the point X70 Z90, as shown in the diagram. It is during this motion that the required compensation is set.

Figure 4.64

The program movements for this job would be written as follows:


G00 X80 Z100G42 G01 X70 Z90

4.25.1 The rules All non motion ‘G’ commands must be written before any ‘G’ command which refers motion control. This means that a G41 nose radius compensation command must always proceed any G01 X. Z. command. Eg. G41G01X20Z50.

Once radius compensation has been set, do not interrupt the read ahead function with non motion commands as this may cause the MCU to miss applying the required compensation for one or more blocks.

For example:

G41 G01 X50 Z100 Z50 G00 X60 Z100 G96 S200 FO.2 M09 G01 X44 Z60 In this example the block G96 S2000 F0.2 M09 contains non motion commands to which the MCU will try and apply nose radius compensation. Because no radius compensation is required for such a block no slide position adjustment calculation is made for this and the next block which is a taper. The result will be an incorrect taper.

Exercise 9 — section 4 — lathe programming

Programming practice

Students instructions 1 Referring to NM09 -Exercise 9 draw a centre line half view of the locating

pin on an appropriate sheet of graph paper to a scale of 2: 1. 2 Using the scaled drawing determine the program points for the roughing

operation. Remember that the first operation will be to face the length. 3 Using coordinate geometry calculate the program points for the finishing

operation. Remember that the accuracy of calculations is to be to two decimal places.

4 Prepare a hand written manuscript of your program on the work sheet provided and include all appropriate commands written in the standard format introduced in these instructions.


5 Check your program against the example format provided.

Exercise 9 — CNC lathe programming


Worksheet Exercise 9 — Hand written program NC lathe program Programmer’s notes (as requested)

4.26 Quality control There may be many ways in which a program can be checked for errors, but a program can only be proved 100% by running the machine and producing a part.

Error checking can be performed in a variety of ways:

Verification: Read through the print-out (NOT the handwritten manuscript) carefully— sometimes mistakes can be seen easily.

Trialing: This involves the execution of the program without actually cutting the part of may be carried out in several ways depending on the philosophy of the person in charge. Adhere to the latter unless you can put up good reasons for alteration.

Trialing usually consists of running the machine with the single block switch active —that is, each block will only be executed by pressing cycle start -in conjunction with the program being displayed on the screen.

Quite often the dry run mode is switched -in to hasten proceedings. Dry Run' results in all machine motion being executed at a preset rate, usually in the region of 50% to 80% of the rapid traverse capability of the machine. The actual axis velocity can be overridden from 0% to 100%. The disadvantage of dry running a program is that feed rates will be masked, and attention must be paid to determining the actual programmed feed rate for each block. This may be displayed on the screen.

Every movement the machine makes during program trialing should be expected and accountable to the programmer, if not, those motions should be checked for viability, and if necessary a more thorough understanding of the machine operation should be sought.

Editing: Wherever errors are found, they should be corrected and rechecked, be it on the machine or at the programming station. Whenever a program is edited on the machine, a note should be made on the print-out so the matter or original program can also be corrected. A better method is to punch out a program from the control after successfully producing a component.


4.27 Canned cycles

Canned cycles may be defined as a set of preprogrammed instructions stored away in the computers memory. The word ‘canned’ has probably been borrowed from canned goods which one usually stores away for later use.

Because the instructions represented a set of routine like repetitive patterns, the word cycle was found to best express what was taking place.

These canned cycles are filed away under a ‘G’ code address. To a large extent the G¬codes are standardised.

As an example (see sketch below), a G84 code, usually representing right-hand tapping in a machining centre and will consist of the following steps:

• Clockwise rotating of the tap at the correct rpm • Rapid advancing of the tap to a set clearance from a pre-drilled hole

(R-level) • Feeding the rotating tap to a set depth at a rate of one thread pitch

per revolution • Reversing both feed and spindle rotation until the tap reaches the R-

level. • Returning spindle rotation to its original clockwise direction.

Figure 4.65

CNC mills, drills and machining centres are all equipped with cycles to perform drilling, reaming, counter boring, boring and tapping operations. Some others


have pocketing cycles, slot cutting cycles, hole pattern cycles etc, all of which are designed to save programming time and effort.

CNC lathes usually have cycles to cover drilling, grooving/parting, screw cutting, repetatives cut (automatic roughing) operations and others. Each cycle has its own G code to control the sequence of motions and an accompanying set of words to define the parameters of those motions. These words have addresses such as: R,P,Q,D,E,I,K,H,B etc.

Review questions — section 41. List four techniques used to prepare a NC program. 2. Why would a program checked using the computer graphics cause an

error when checked by the MCU? 3. Which type of tape material is best suited to long production runs on NC

machines without too much expense. 4. How is one character represented on a punch tape? 5. What is meant by a block of program? 6. What are the two tape formats commonly in use and what parity are

they? 7. How can you determine if a tape is odd or even parity? 8. What does CAD/CAM stand for? 9. In a block of program, which word must appear first? 10. What are I, J and K words used for? 11. Give two examples where the letter S is used in a program and what the

meaning is . 12. What are M codes used for? 13. How do you determine which G code to use to produce an arc?

Answers at the end of the topic CNC Machining


Section 5: Transfer programs to CNC machines

This section covers methods of transferring programs to CNC machines and loading programs into the CNC machine memory.


list methods of transferring a program into the memory of a CNC machine load a program into a CNC machine memory identify which method of program transfer is suitable for given situations maintain workplace safety standards at all times.



5.1 Setting up a CNC Milling Machine Note: Operation of the machine tool is only to be done under the teacher's supervision.

The setting up of the CNC milling machine involves the following procedures:

• Entering the program in the machine control • Checking and editing the program • Preparing the work holding devices • Mounting tools in the tool changer • Setting the tool offsets


• Selecting and setting a tool change position • Establishing the tool and work piece datums • Dry running the program.

5.2 Program entering and checking Programs may be entered into the machine controller by several methods:

• By direct entry via the machine control keyboard • By tape prepared on a punch tape machine from the editor • By disc from the editor computer to the on board computer on the

machine or a computer set up next to the machine and connected to the machine

• By direct transfer from the editor computer to the machine controller.

Once the program is entered into the machine control, editing is carried out via the controls keyboard.

The procedures for transfer via disc or direct line vary from one machine to another but the basic principle is to prepare the machine to receive the information and then to prepare the computer to send the information and finally to actually send.

A trap that can be fallen into when transferring information via computer is the fact that the baud rate on both the receiver and the sending machines must be the same.

The correct procedures for transferring data to and from the CNC machine controller will be demonstrated to you by your teacher.

5.3 Safety Safety should be to Occupational Health & Safety (OH & S) standards before working on CNC machines, in particular attention to:

• Personal safety • Workplace safety • Machine safety.

Briefly and generally the following checklist should be covered as a minimum requirement.

• Personal attire (safety glasses, clothing, shoes etc). • Personal attitude (awareness, concentration, distraction, behaviour). • Machine condition (operation of guards, contamination level of

coolant, security of holding devices and tooling, interlocks operational, hydraulic pressures checked).


• Machine operation (correct and careful trialing, gripping force at high speed, materials handling/lifting, use of settable barriers, effect of out of balance work).

• Workplace (oily floors, cluttered work area, lighting ventilation).

Refer to OH & S documents for fully detailed requirements on all aspects of work safety.

Exercise 10 — section 5 — program transferNote. This exercise should only be carried out under the supervision of your teacher.

Program transfer Use available options to transfer the completed program from Exercise 9 into the CNC machine.


Section 6: CNC machine preparation

This section covers the preparation of a CNC machine for operation including work holding, tool installation, pre start checks and machine adjustments.


• mount work holding devices • install work material • install tooling as required • set/adjust machine to meet the required specifications • maintain work place safety standards at all times • explain the concepts of compensating for tool radius • identify and carry out workpiece checks before and after machining • identify and carry out machine checks before and after machining.



Special rules for this section • take care when handling sharp cutters and tooling • do not work on the machine unless your teacher has cleared you to

do so • ensure all tools will clear other machine parts when mounted in

magazine • do not make any adjustments to the machine unless your teacher is

present.


6.1 Work holding devices

Machine vice A machine vice can be used in conjunction with parallel strips, although it is preferable to machine soft jaws in order to achieve workpiece support with no loose pieces involved in the set up. For greatest accuracy, soft jaws should be machined in position on the machine.

Fixtures Similarly, fixtures are most accurate if their locating surfaces are machined in position with the fixture accurately positioned on a riser table or some form of adaptor.

If any fixture can only be located in one certain position on the machine, then the machine position that locates the program origin can be stamped onto the fixture to eliminate setting and checking each time the fixture is used.

Fixtures should at least be aligned accurately in one plane via tee slots.

Simple fixtures designed to hold a part for a once-only run of a single part or a few parts may be simply bolted or clamped.

6.2 Mounting tools in the tool changer Where possible, the tools should be placed in the tool changer adjacent to each other and in the sequence desired for the machining operations. There may be some limitation to this ideal if the tools are heavy or have a large diameter.

A few heavy tools placed together can unbalance or subject the tool changer to undue stress and a large tool may overlap the adjacent tool pockets, forcing them to be left empty. Also, if jobbing work is frequently performed rather than long run production work, it may be desirable to mount a series of 'standard' tools in certain positions in the tool changer magazine, to reduce the set up time on job change overs.

The actual placing of the tools in the magazine is usually done by hand, but some machines may have to transfer the tool from the spindle to the magazine using the auto changer.

Your teacher can demonstrate the correct method to you.


6.3 Offsets The tools used on CNC milling machines usually require a tool length offset and some tools also require a radius offset. The values for these various offsets must be determined and entered into the respective offset register in the control unit.

There are a few variations to the way tool length offsets can be used, such as:

1. Each tool has an offset equal to the distance the Z-axis must move from home position to bring that tool to the work surface (which is usually taken as the Z program origin).

2. A reference tool (usually the longest tool) is used to set the Z program origin by entering the tool length offset value in the workpiece zero offset register instead of the tool length offset register. This results in the Z-axis moving towards the work by the offset amount each time the workpiece zero offset is commanded. Any additional movement towards the work to make up for shorter tools is accomplished by initiating the tool length offset for that tool, the offset amount being equal to remaining motion required.

3. By using the program commands available on most controls to reset a machine position to represent a workpiece position. The codes used between controls may be different and the terminology used to describe the technique may be different but the results are similar.

4. Using the gauge line on the machine spindle in conjunction with a known length setting bar.

Your teacher can detail the technique used at your college.

6.4 Tool change position Tool changing should take place at a position that removes the risk of any tool colliding with any part of the machine or fixturing during the tool change sequence.

On some machines, the position may vary, on others the position may be fixed. Your teacher can supply this information for the machine at your college.

6.5 Workpiece datum As with tool offsets, there are a few variations to the method of locating the three dimensional workpiece zero (program origin) within the total motions of the machine, the differences mainly depend on the control executive program and are basically:


1. By entering the XY machine positions where the program origin is desired into an offset register known as a zero offset or a Workpiece Coordinate System register, or similar.

2. By using the program commands to reset a machine position to represent a workpiece position usually used with a G50 or G92 code, depending on the control.

3. By utilising the tool offset register on those controls that will accept X and Y offsets as well as Z offsets.

Again, your teacher can assist you in this regard.

6.6 Workpiece and machine checks before and after machining Before and after machining, a range of checks should be carried out on both the material and the machine to ensure that all is in readiness for the machining operation to be performed safely and in the most efficient manner with regards to the quality of the part being produced and care of the machine tool itself.

These checks will include the following points:

6.6.1 Workpiece checks before machining 1 Correct material specification 2 Correct size allowing for initial roughing cuts 3 Pre-machining of datums carried out if required 4 Castings and forgings checked for clean and removal of excess flash 5 Cored holes in castings and forgings clear and in correct position

6.6.2 Workpiece checks after machining 1 Sizes correct to gauge or manual measurements taken 2 Surface finish specification 3 Burrs and sharp edges removed where necessary.

6.6.3 Machine checks before machining 1 Machine lubrication system levels correct 2 Tooling correctly loaded in tool changer with position numbers matching

program 3 Machine referenced (machine home) 4 Workpiece datum set 5 Tool offsets set (length and radius) 6 Safety guards in position 7 Coolant levels checked.


6.6.4 Machine checks after machining 1 Machine returns to safe position for loading next component 2 Program returns to start (production sequence)

6.7 Dry running program Dry running a CNC program is a procedure that allows the program to be run with or without tool movements in order to check that the program will actually run on the machine.

The initial dry runs are usually done without tool or axis movement so as to verify that the program can be understood by the machine.

After the initial dry run a trial work piece may be machined using machinable wax, foam or timbers specifically developed for machine trialling.

During a dry run certain alarms may come up because the machine cannot carry out the commands given to it by the program. These alarms must be cleared and the program edited to remove the cause of the alarm.

After the program is proved to run correctly on the machine, a dry run may be made with the Z axis set a safe distance above the Z zero so that the tools will not come in contact with the workpiece and all tools and axis movements used. Your teacher will assist you to carry out the dry run.

Practical exercise 11 — section 6Note: This exercise should only be carried out under the supervision of your teacher.

Instructions

1 Take the program tape (or computer disc) to the machine along with a print out and a part drawing ofNM09 -Exercise 8 as well as the operators setup sheet

2 Read the tape into the machine (or send the program direct from the computer).

3 Perform a verification check on the program through the graphic simulation function, if fitted.

4 Prepare the work holding device according to the operators setup sheet 5 Mount the tools in the tool changer and verify that they match the tools

and tool numbers in the program 6 Set the tool offsets and register in the appropriate part of the memory 7 Select a suitable tool change position and enter this into the program


8 Establish the work piece datum and enter this in the appropriate part of the machine memory

9 Dry run the program and edit errors that may occur.

Complete the ‘Operators Set up Sheet’ for job NM09 -exercise 8

Figure 6.1

Exercise 11 — Operators setting up sheet (NM09 exercise 11)



Section 7: CNC machine operation

This section covers the operation of a CNC machine in manual, jog and automatic mode to machine a work piece to drawing, including verification and editing of programs on machine.


operate the CNC machine using manual data input (MDI) mode safely verify safe operation of program edit a program using the CNC control panel produce a pre determined component using automatic mode compare completed component to required specifications and complete a

report on the comparison maintain work place safety standards at all times.



Special rules for this section • take care when handling sharp cutters and tooling • do not work on the machine unless your teacher has cleared you to

do so • ensure all tools will clear other machine parts when mounted in

magazine • do not make any adjustments to the machine unless your teacher is

present.

Do not work on the machine unless your teacher has cleared you to do so. Dry run program first with tool clear of work piece by a minimum 50 mm. Do not run the


program unless your teacher has given approval to do so. Ensure all guards and safety devices are in position before operating the machine.

7.1 Quality Assurance

7.1.1 Surface finish There are two types of surface finish obtained from milling operations:

1 A circular pattern, produced by the cutter teeth when the surface is at 90 0 to the spindle axis, such as flat faces that have been face milled.

2 A linear pattern, produced by the teeth during peripheral milling, such as the sides of a slot that have been end milled.

Peripheral surfaces generally are easier to obtain a good surface finish on because of the sliding effect as the tooth engages or disengages the act at the thin end of the chip (depending on whether up cut or down cut milling is used.)

Axial surfaces are produced by teeth with sharp comers (HSS endmills, etc), or radiused comers (inserts), or by milling inserts with parallel lands -in which case the finish may be stepped because of uneven insert height.

In this case, one wiper insert can be fitted to the cutter and set to be the lowest insert, thereby acting as a finishing tool built-in. The length of the flat (wiper) is usually about 10 mm for most insert shapes, enough to cover the feed distance for one revolution of most cutters except perhaps large diameter fine pitch face mills.

Surface finish is also dependent on:

• machine condition • cutter design • cutter geometry • cutting data (speed, feed, depth) • workpiece shape • workpiece clamping.

7.1.2 Size The sizes produced by milling are a result of the correct positioning of the cutter, which in tum is under the direct influence of machine offsets.

The height of faces is controlled by tool length offsets (Z-axis), increasing the offset value results in a lower machined surface.

The size of peripherally milled surfaced is controlled by cutter radius offset in conjunction with cutter radius compensation. Increasing the radius offset will


increase the external size of the part for a given tool and decrease the internal size of the part for the same tool.

You will be required to measure the first part from your program and report on the measurement obtained. You will then make any necessary adjustments to offsets to compensate for inaccuracies found.

7.2 Machine operations Note: Operation of the machine tool is only to be done under the teacher's supervision.

7.3 Control Panel Most controls offer similar functions and uses, even though the panels differ in appearance.

For you to operate the machine correctly and confidently, it is important that you understand the functions of, and uses for all the buttons and switches on the control panel.

The most frequently used controls are:

• The mode selector switch. This selects o auto operation o jog (or manual) operation o MDI o program edit facility o zero set o tool offsets o etc.

• The cycle start button. This initiates program operation, either complete by one press or a block at a time depending on the setting of the single block switch.

• The feed hold button. A red or an orange coloured button immediately adjacent to the cycle start button (usually green). Pressing the feed hold button will halt slide motion only, pressing cycle start will resume the operation of the program.

• Spindle and feed override switches. Multi-position switches used to vary the programmed rates. The feed override usually overrides rapid traverse motion, but some machines have a separate switch for this.

• Emergency stop. A large red button placed conspicuously on the control panel, it may also be duplicated elsewhere on the machine. Pressing this cuts power to the machine (and on some machines to the control as well), thereby effecting a halt to all machine motions.


This button should only be pressed in an emergency situation where personal injury or machine or tool damage is likely, and not for clearing alarms or other minor problems.

• Other switches or buttons. Dry run, block delete, optional stop, Z-axis inhibit, axis jog, axis zero, homing (zero return), keyboard, handwheel and 'soft' key which are buttons placed under the VDU (screen) and have a variety of functions as indicated on the bottom of the screen above them. They are so named because their function is controlled by software, they are not 'wired-in' for just one function.

7.4 Machining Simple shapes resulting from motion of one axis at a time can be machined by manual control of the machine through the handwheel and axis selector switch. Actual positions can be displayed relative to the start position to easily allow accurate distances to be traversed.

More consistent motion may be obtained through the MDI function which then places the machine under programmed conditions. Some machines will allow several blocks of MDI to be entered resulting in a 'one shot' program with most functions available but usually excluding canned cycles.

Other machines will only accept one MDI block at a time, still without canned cycles and the like.

Another disadvantage of MDI is that each command must be correct, no trialing is possible and pressing the feed hold will defeat the MDI block (or program).

The third method used for machining is the execution of a properly prepared program, carefully trialed and checked.

7.5 Proving Program proving (or trialing) is the careful and concentrated execution of the program by various means in order to eliminate all errors that could cause machine or tool damage or spoiling the workpiece before actually machining the part under automatic cycle conditions.

The technique used may differ from one machine to another even from one person to another, but the result of trialing must be a knowledge that the program is as close to 100% operational as can be ascertained.

Use such methods as:


• Graphic display on the machine. This is very useful, but won't show everything, ie. if the spindle is running backwards, coolant is not on, tool offsets are incorrect, etc.

• Dry run motions with Z-axis inhibited. Similar results are obtained to the previous method on machines without graphic displays, except spindle rotation and coolant can be observed. Feed rates and Z-axis motions are not exhibited.

• Run the program without cutting the part. This is the slowest but probably the surest that can be used for trialing, and may be used after the above ifthe program is fairly long and/or complicated.

Finally, and only when it appears the program is 100% correct on all counts, cut the part and check sizes to determine if all offsets are set to allow tolerances to be achieved.

7.6 Editing If the program results in many alarms during the graphic display, it is probably wise to move off the machine and carefully check the print out, correcting faults at the program preparation area, rather than perform involved editing at the machine.

Any editing, be it simple or involved will alter the execution of the program and therefore program re-trialing may be necessary before safe and sure execution of the program is proved.

7.7 Machining There occasionally comes a time during machining when something goes wrong; perhaps a cutter chips or breaks, an insert comes loose, the part moves during heavy machining the coolant falters to a dribble, there is a general failure etc.

In any situation like these, it is advisable to stop the machining process temporarily, if not totally and immediately.

In other situations, such as cutter dulling, the machining process can be nursed to completion by using the override controls. It should be noted, that if the spindle is slowed with a G94 command active (feed/minute) the effective chip load on the cutter increases, but if a G95 command (feed/revolution) is active, the feed slows proportionally. In either case, feed can be overridden independently of the spindle.

Some controls have only one override control for all axis motions, and will usually be effective in all modes that is, cycle, single block, dry run, jog (or manual), feed and rapid traverse.


Other controls will override rapid motions only if the single block switch is on, while others again have separate override controls for feed and rapid motions and may also have a jog rate switch marked in feed rate per minute values, which can be advantageous if machining in the jog mode.

The hand wheel (manual pulse generator, or MPG) works independently of any override controls, is effective on only one axis at a time and can be switched to different sensitivities -usually 0.001 mm, 0.1 mm to allow precise positioning.

The effect of pressing the emergency stop button may be different between one control and another.

The emergency stop on some controls switches everything off, including the control itself, but most will cause all motions and miscellaneous functions to cease. Some machines may require re-homing after an emergency stop, but most won't.

After an emergency stop, power must be connected again to the machine via the reset button before the machine can be moved, and if the reason for the emergency stop was tool breakage, the motion directions must be chosen with a view to minimising any further damage to the tool or work piece.

The cause of the damage should be determined before the program is restarted, in case the cause was program related.

After reset and damage repair, the block search function can be used to restart the program at any point providing that block has a sequence number.

The use and effect of block search and program restart will be different on different controls; some process the program while searching, which slows up the procedure on long programs, but results in all modal conditions being set and active at the block searched to.

Other controls ‘jump’ straight to the block required with only the default modal conditions active. In this case all necessary modal conditions must be entered through the MDI function before restarting the program.

Possibly the most successful way to restart a program is to block search to a tool change command -from there all necessary modal conditions will be commanded in the program for that tool. Another advantage is that the wrong tool cannot be in the spindle for the sequence required, although any machining already completed will be re-executed, and that may cause a problem on parts having a fine tolerance, or where tapped holes will be re-tapped.

Some controls allow subsequent block searching (once the tool is in position and running) to bypass machined sections without a loss of modal conditions.

A program can be written with easy and accurate restarting in mind by judicious use of sequence number and perhaps skip block commands.


Practical exercise 12 — section 7Note: This exercise must be done under the supervision of your teacher.

Refer to the drawing NM09 -Exercise 12 and prepare the material for machining.

• Using MDI and jog modes machine the two reference edges and the bottom face ofthe material.

• Set the material in the work holding device and reset work datums. • Set the machine to 'Auto' and machine the component. • Perform a program restart after your teacher has simulated a tool

failure • Communicate back to your teacher all the instructions necessary for a

machine operator to take over the machine operation to complete another 20 components.

• Take measurements of the first off component and edit tool offsets to compensate for any errors found

First off part measurements Referring to the drawing of the part that you have machined, measure at least five features specified by your teacher and fill in the chart below.

Under supervision of your teacher modify the tool offsets necessary to correct any errors in the part dimensions.

Tool No. Drawing size Measured size Offset correction radius

Offset correction length

1

2

3

4

5

Exercise 12



Answers to questions

Review questions — section 11. ‘alpha’ stands for letters and ‘numeric’ stands for numbers. Alphanumeric

characters are any number (0 to 9) or letter (A to Z). 2. Program of instruction. Control unit or MCU. Machine tool. 3. MCU stands for Machine Control Unit. 4. CNC stands for Computer Numerical Control. 5. To read and store program information. To interpret the information in a

logical command sequence. To control the motion of the machines mechanical members. To monitor the status of the machine.

6. Lathe. Mill. EDM 7. Pick and place robot. Spot and seam welding machines and robots 8. Carpet dying and weaving. Wood machining. Warehouse control and

material handling 9. By the capability to program the NC machine and to save that program for

subsequent use in the future. 10. Reduced non production time. Reduced fixture costs. Reduced

manufacturing lead time 11. High investment costs. Higher maintenance costs. Finding and/or

training NC personnel 12. The machine tool. The control unit. The machine positioning system 13. Thermal stability means that the heat generated by the machine, by the

process of metal cutting or through external sources have little or no affect on the accuracy of the machine tool.

14. After the Z axis is established, the X axis is the next largest major axis of machine tool movement.

15. Advantage: Lower friction than hydrostatic slides and therefore quicker. Disadvantage: Reduced load carrying capacity.

16. Provides an interface between the operator and the NC machine tool 17. Photo-electric type 18. Read Only Memory 19. Tool ring results when the tool is allowed to dwell in the same spot during

cutting. 20. The tape is read only once. Program editing can be done at the control.

Programs can be converted to metric or imperial. Buffer storage. Control can read ahead.

21. Direct Numerical Control 22. A, B and C


23. Point your thumb in the positive direction of the linear axis and the curl in your fingers will be pointing in the positive direction of the rotary axis.

24. X and Z

Review questions — section 21. A transducer. 2. Hydraulic motors. 3. Hydraulic ram. Pack and pinion. Leadscrew and nut 4. To eliminate backlash. 5. 90% 6. A transducer to send a position signal back to the control. 7. An analog transducer produces a variable electric voltage whereas a

digital transducer produces discrete electrical voltages. 8. Resolvers and tachometers. 9. An axis limit position stored within the control. 10. To permit smooth continuous motion from block to block. 11. Machine vice. Fixtures. Dummy tables 12. Computer Integrated Manufacturing Systems 13. Annual production rates from 1500 to 15000. 14. Flexible Manufacturing System.


1. Process planning. Part programming. Tape preparation. Tape verification. Direct Link. Transfer to machine memory. Production

2. Sketch the part. Select work holding. Select datum. Plan operation sequence. Record data. Operators instructions

3. The machine tool the program was written for. What tooling is being used. Work fixtures used . Sequence of operations. Inspection requirements. Program notes

4. To achieve the correct size and shape of the part. 5. Manual compensation by calculating and programming different co-

ordinates. Using the automatic compensation available in the control. 6. Program the tool to machine past the centre of the job a distance equal to

twice the nose radius of the insert. 7. Suitable job plan similar to exercise shown in 3.3 8. Raw material preparation. process selection. process sequencing.

machining parameter selection. tool path planning. machine selection. tool selection. fixture or work holding method.



Question 1



Question 2



Question 3



Question 4




1. Manually. Direct input into CNC control. Computer aided programming. Voice numerical control

2. The graphical tolerance acceptable by the computer is often greater than what the MCU will permit.

3. Laminated paper tape with reinforce mylar plastic. 4. Each character is represented by a row of holes across the punched tape. 5. This is a combination of words that represent one complete sequence of

commands. 6. EIA standard is odd parity. ASCII / ISO standard is even parity 7. Count the number of holes punched across the tape for any character.

Odd number of holes is EIA and even number of holes is ASCII / ISO. 8. Computer Aided Drafting / Computer Aided Manufacturing 9. The sequence number (N100, N110 etc) 10. I, J and K words are used to define arc centre offsets. On some controls

they are also used when defining cutter radius compensation. 11. G97 S2000 Set spindle speed to 2000 RPM. G96 S180 Set Constant

Surface Speed (CSS) to 180 Metres / minute 12. M codes are used for miscellaneous functions such as turning the coolant

on and off. 13. G02 for clockwise arcs and G03 for anti-clockwise arcs.


Answers to exercises — section 4 — exercise 1














Section 4 — Exercise 8 — slotted base plate

Sample program millingShown below are all of the dimensions required to complete the part program as a functional example of a Machining Centre Program

%MPF 1 N05 (PR0GRAM PRACT1CE) N10 G71G90 G17 G40 G94 N15 G00 Z0 D0N20 G54 X0 Y0 N25 G55 X0 Y0 N30 (////) N35 M19 N40 M00 N45 (L0AD 10mm END M1LL) N50 T01 D01N55 F600 S3000 N60 (///) N65 G00 X-10 Y45 N70 G00 Z10 M03 M08N75 Z-2 N80 G01 X30 N85 X-10 N90 G00 Y65 N95 G01 X35 N100 X-10N105 G00 Y100 N110 X30 N115 G01 X60 Y70 N120 Y20 N125 X90 Y-10 N130 G00 X110 N135 G01 Y25 N140 X90 N145 Y33 N150 X115 N155 X-10 N160 G00 Z40 M05 M09 N165 (////) N170 G00 Z0 D0 N175 M19 N180 M00 N185 (REM0VE LAST T00L) N190 G53 X0 Y0 N195 M02


Section 4 — Exercise 8 — slotted base plate

Sample program millingShown below are all of the dimensions required to complete the part program as a functional example of a Machining Centre Program

%MPF 1 N05 (PR0GRAM PRACT1CE) N10 G71G90 G17 G40 G94 N15 G00 Z0 D0N20 G54 X0 Y0 N25 G55 X0 Y0 N30 (////) N35 M19 N40 M00 N45 (L0AD 10mm END M1LL) N50 T01 D01N55 F600 S3000 N60 (///) N65 G00 X-10 Y45 N70 G00 Z10 M03 M08N75 Z-2 N80 G01 X30 N85 X-10 N90 G00 Y65 N95 G01 X35 N100 X-10N105 G00 Y100 N110 X30 N115 G01 X60 Y70 N120 Y20 N125 X90 Y-10 N130 G00 X110 N135 G01 Y25 N140 X90 N145 Y33 N150 X115 N155 X-10 N160 G00 Z40 M05 M09 N165 (////) N170 G00 Z0 D0 N175 M19 N180 M00 N185 (REM0VE LAST T00L) N190 G53 X0 Y0 N195 M02


Section 4 — Exercise 9Practice program turning

Locating pin

N10 (MATERIAL 57 DIA X 160 LONG M.S.)N15 (TOOL T01 – 80 DEGREE ROUGHING)N20 (T00L T05 -30 DEGREE F1N1SH1NG) N25 G90 G95 N30 G96 S180 N35 G40 G00 X300 Z300 T0101 N40 (T00L T01 R0UGH1NG) N45 G50 S2500 M42 N50 G00 Z108 M03 M08 N55 G01 X-1.6 F0.15 N60 G00 Zl09 N65 X5l N70 G01 Z5.1 F0.3 N75 X57 N80 G00 Z109 N85 X45 N90 G01 Z18.1 N95 X51 N100 G00 Z109 N105 X39 N110 G01 Z79.7 N115 X41 Z78 N120 Z18.1 N125 X45 N130 G00 Z109 N135 X33 N140 G01 Z84.2 N145 X39 Z79.7 N150 G00 Z109 N155 X27 N160 G01 Z88.7 N165 X33 Z84 N170 G00 Z109 N175 X21 N180 G01 Z93 N185 X27 Z88.7 N190 G00 Z109 N195 X19


N200 G01 X25 Z106 N205 G00 X300 Z300 T01N240 X20 Z107 N245 Z93 N250 X40.01 Z78 N255 Z18 N260 X50N265 Z5N270 X60 N275 G40 G00 X300 Z300 T0500 N280 M05 M09 N285 M02 %


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