CHAP 1 Introduction to CNC Technology MODIFIED

8/3/2019 CHAP 1 Introduction to CNC Technology MODIFIED

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BMFS 3373

CNC TECHNOLOGYDR. AHMAD KAMELY BIN MOHAMAD

ROOM: BLOCK FTMK GROUND FLOORTEL. EXT. NO: 063316488


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INTRODUCTION TO

CNC AND METALCUTTING


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HISTORY

US Air Force commissioned MIT to develop thefirst "numerically controlled" machine in 1949. Itwas demonstrated in 1952.

At 1970-1972 first Computer Numeric Controlmachines were developed.

Today, computer numerical control (CNC)machines are found almost everywhere, fromsmall job shops in rural communities tocompanies in large urban areas.


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DEFINITION

In CNC (Computer Numerical Control), theinstructions are stored as a program in amicro-computer attached to the machine.

The computer will also handle much of thecontrol logic of the machine, making itmore adaptable than earlier hard-wired

controllers.


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CNC APPLICATIONS

Machining

2.5D / 3D

Turning ~ Lathes, Turning Centre

Milling ~ Machining Centres Forming

2D

Plasma and Laser CuttingBlanking, nibbling and punching

3D

Rapid Prototyping


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EXAMPLE OFCNC MACHINES


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CNC TURNING/LATHE


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CNC MILLING


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CNC LASER CUTTING


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CNC PLASMA CUTTING


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CNC PRESS


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CNC RAPID PROTOTYPING


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INDUSTRIES THAT USE CNCMACHINES

Aerospace

Machinery

Electrical Fabrication

Automotive

InstrumentationMold making


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SAMPLE PRODUCTS

OFCNC MANUFACTURING


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AUTOMOTIVE INDUSTRY

Engine Block


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AUTOMOTIVE INDUSTRY(Contd)

Various Products


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AEROSPACE INDUSTRY

Aircraft Turbine Blade Machined by5-Axis CNC Milling Machine


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CNC MOLD MAKING

Front car bumper mold


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ELECTRONIC INDUSTRY

Fabrication of mold for various plastic casings


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RAPID PROTOTYPINGPRODUCTS


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ADVANTAGES AND

DISADVANTAGES OF CNCMACHINES

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ADVANTAGES OF CNC


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Utilization of computers in

manufacturing applications hasproved to be one of the most

significant advantages &developments over the last coupleof decades in helping to improve

the productivity and efficiency ofmanufacturing systems.


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1. CNC machines can be used continuously 24 hours a day,and only need to be switched off for occasionalmaintenance.

2. CNC machines are programmed with a design which canthen be manufactured hundreds or even thousands of

times. Able to produce same quality products. Productsare more repeatable and less waste due to scrap.

3. Reduced setup time permits smaller economic batchquantities. Lower lead time allows lower stock levels.

4. Less skilled/trained people can operate CNCs unlikemanual lathes / milling machines etc.. which need skilledengineers.

ADVANTAGES


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ADVANTAGES (CONTD)

5. Training in the use of CNCs is available through the use of virtual

software. This software allows the operator to practice using CNC

machine by the use of a computer. The software is similar to acomputer game.

6. CNC machines can be programmed by advanced design softwaresuch as CARTIA, Pro/DESKTOP, and etc., enabling themanufacture of complex products that cannot be made by manualmachines, even by skilled designers / engineers.

7. Saves time and money. Modern design software allows the designerto simulate the manufacture of his/her idea. There is no need tomake a prototype or a model.


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8. One person can supervise many CNC machines asonce they are programmed they can usually be left towork by themselves. Sometimes only the cutting toolsneed replacing occasionally.

9. Automation Increase productivity. Reduced setup timewill increase utilization. Therefore will increase profit.

ADVANTAGES (CONTD)


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Transferred

skillResults

muscle power engine drivenmachine tools

First industrial

revolution

manipulating

skill

mechanization hard automation

vision skill use of positiontransducers,

cameras

increase of

accuracy, part

recognition

brain power cnc machines, industrialrobots, soft

automation,

computer control of

manufacturing

systems

second industrial

revolution


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DISADVANTAGES

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1. High initial capital cost. CNC machines are moreexpensive than manually operated machines, althoughcosts are slowly coming down.

2. High maintenance requirements and cost. Maintenance

personnel must have both mechanical and electronicsexpertise.

3. Not cost-effective for low-level production on simpleparts.

4. Less workers are required to operate CNC machinescompared to manually operated machines. Investment inCNC machines can lead to unemployment.


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5. Many countries no longer teach pupils / studentshow to use manually operated lathes / millingmachines etc... Pupils / students no longer developthe detailed skills required by engineers of the past.

These include mathematical and engineering skills.

6. The CNC machine operator only needs basictraining and skills, enough to supervise severalmachines. In years gone by, engineers needed

years of training to operate centre lathes, millingmachines and other manually operated machines.This means many of the old skills are been lost.

DISADVANTAGES (CONTD)


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FUNDAMENTAL OFMETAL CUTTING


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The metal cutting operations (also

called machining) is one of themost important manufacturing

processes in industry today.


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MACHINING IS THE REMOVAL

OF MATERIALS IN FORMS OF

CHIPS FROM THE WORKPIECE

BY SHEARING WITH A SHARP

TOOL.


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The main function of a machinetool is to control the workpiece-

cutting tool positional relationship in

such a way as to achieve a desiredgeometric shape of the workpiece

with sufficient dimensionalaccuracy.


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Machine tool provides:

work holdingtool holdingrelative motion between tooland workpiece

primary motionsecondary motion


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Primary motion

Relative motionbetween tool and

workpieceSecondary motion

Cutting motion

Cuttingspeed

Feed motion

Feed rate


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CLASSIFICATION OF THE CHIP REMOVINGMETHODS ACCORDING TO THE RELATIVE MOTION


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CLASSIFICATION OF MACHINE TOOLS

THOSE USING

SINGLE

POINT

TOOLS

THOSE USING

MULTIPOINT

TOOLS

THOSE USING

ABRASIVE

TOOLS

lathes

shapers

Planers

Drilling

boring m/csetc.

milling m/cs

broaching m/cs

hobbing m/cs

etc.

grinding m/cs

honing m/cs

etc.


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BASIC COMPONENTSOF CNC SYSTEMS


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ISO MACHINE TOOL AXIS DEFINITION

ISO MACHINE TOOL AXES DEFINITIONS


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AXIS MACHINE TOOL WITH SPINDLE MACHINE TOOL WITHNO SPINDLE

Z axis of spindle,(+Z) as tool goes away from the work piece

perpendicular to work

holding surface, (+Z) as

tool goes away from theworkpiece

MACHINE

TOOL WITH

ROTATING

WORKPIECE

MACHINE TOOL WITH

ROTATING TOOL

HORIZONT

AL AXIS

VERTICAL

AXIS

X radial andparallel to

cross slide,

(+X) when

tool goes away

from the axis

of spindle

horizontal

and parallel

to work

holding

surface,

(+X) to the

right whenviewed

from

spindle

towards

work piece

horizontal

and parallel

to the work

holding

surface,

(+X) to the

right whenviewed

from

spindle

towards

column

parallel to and positive in

the principal direction of

cutting (primary motion)

Y apply right hand rules


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RIGHT HAND RULEVertical Machine Horizontal Machine


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STANDARD LATHECOORDINATE SYSTEM

S G C


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STANDARD MILLING MACHINECOORDINATE SYSTEM


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NUMERICALLY CONTROLLED MACHINE

TOOLS:An NC machine tool is functionally the same

as a conventional machine tool. The NCmachine tools technological capabilities interms of machining are no different from

those of conventional ones. The differenceis in the way in which the various machine

functions and slide movements arecontrolled.


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The functions and motions such as;

turning the spindle on and offsetting cutting speeds

setting feed rate

turning coolant on and offmoving tool with respect to workpiece

are performed by Machine Control Unit(MCU) in NC machine tools.


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MACHINE TOOLAUTOMATION


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CNC SYSTEM ELEMENTS

A typical CNC system consists of thefollowing six elements:

1. Part program

2. Program input device3. Machine control unit

4. Drive system

5. Machine tool6. Feedback system


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NC SYSTEM ELEMENTS

OPERATIONAL FEATURES f


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OPERATIONAL FEATURES ofCNC MACHINES


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PART PROGRAM

A part program is a series of coded instructions required

to produce a part. It controls the movement of themachine tool and the on/off control of auxiliary functionssuch as spindle rotation and coolant. The codedinstructions are composed of letters, numbers andsymbols and are arranged in a format of functional

blocks as in the following exampleN10 G01 X5.0 Y2.5 F15.0| | | | || | | | Feed rate (15 in/min)

| | | Y-coordinate (2.5")| | X-coordinate (5.0")| Linear interpolation mode

Sequence number


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PROGRAM INPUT DEVICE

The program input device is themechanism for part programs to beentered into the CNC control. The most

commonly used program input devices arekeyboards, punched tape reader, diskettedrivers, pen drive, through RS 232 serial

ports and networks.

MACHINE CONTROL UNIT


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MACHINE CONTROL UNITThe machine control unit (MCU) is the heart of a CNC

system. It is used to perform the following functions:

Read coded instructions

Decode coded instructions

Implement interpolations (linear, circular, and helical) togenerate axis motion commands

Feed axis motion commands to the amplifier circuits fordriving the axis mechanisms

Receive the feedback signals of position and speed foreach drive axis

Implement auxiliary control functions such as coolant orspindle on/off, and tool change


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TYPES of CNC CONTROLSYSTEMS

Open-loop controlClosed-loop control

OPEN LOOP CONTROL


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OPEN-LOOP CONTROLSYSTEM

In open-loop control system step motors areused

Step motors are driven by electric pulses

Every pulse rotates the motor spindle through acertain amount

By counting the pulses, the amount of motioncan be controlled

No feedback signal for error correction Lower positioning accuracy


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OPEN-LOOP CONTROL SYSTEM

CLOSED LOOP CONTROL


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CLOSED-LOOP CONTROLSYSTEMS

In closed-loop control systems DC or ACmotors are used

Position transducers are used to generate

position feedback signals for errorcorrection

Better accuracy can be achieved

More expensive Suitable for large size machine tools


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CLOSE-LOOP CONTROL SYSTEM


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CONTROL

Desired path (p, v, a)

3-axis position control (encoder feedback)

Velocity control (tachometer feedback) Torque control (current feedback)

Path generator

Linear interpolation Circular interpolation

Complex path interpolation (contouring)


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DRIVE SYSTEM

A drive system consists of amplifiercircuits, stepping motors or servomotorsand ball lead-screws. The MCU feeds

control signals (position and speed) ofeach axis to the amplifier circuits. Thecontrol signals are augmented to actuatestepping motors which in turn rotate the

ball lead-screws to position the machinetable.


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STEPPING MOTORS

A stepping motor provides open-loop, digitalcontrol of the position of a workpiece in anumerical control machine. The drive unitreceives a direction input (cw or ccw) and pulse

inputs. For each pulse it receives, the drive unitmanipulates the motor voltage and current,causing the motor shaft to rotate by a fixed angle(one step). The lead screw converts the rotary

motion of the motor shaft into linear motion ofthe workpiece .


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STEPPING MOTORS

RECIRCULATING BALL


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RECIRCULATING BALLSCREWS

Transform rotational motion of the motorinto translational motion of the nut attached to themachine table.

RECIRCULATING BALL


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RECIRCULATING BALLSCREWS

Accuracy of CNCmachines depends ontheir rigid

construction, care inmanufacturing, andthe use of ball screwsto almost eliminate

slop in the screwsused to move portionsof the machine.

COMPONENTS OF


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COMPONENTS OFRECIRCULATING BALL SCREWS

Ball screw

Ball nut (anti-backlash)

Ways Linear bearings


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POSITIONING

The positioning resolution of a ball screw drivemechanism is directly proportional to thesmallest angle that the motor can turn.

The smallest angle is controlled by the motorstep size.

Microsteps can be used to decrease the motorstep size.

CNC machines typically have resolutions of0.0025 mm or better.


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MACHINE TOOL

CNC controls are used to control varioustypes of machine tools. Regardless ofwhich type of machine tool is controlled, it

always has a slide table and a spindle tocontrol of position and speed. Themachine table is controlled in the X and Y

axes, while the spindle runs along the Zaxis.


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FEEDBACK SYSTEM

The feedback system is also referred to asthe measuring system. It uses position andspeed transducers to continuously monitor

the position at which the cutting tool islocated at any particular time. The MCUuses the difference between referencesignals and feedback signals to generate

the control signals for correcting positionand speed errors.


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CNC MACHINES FEEDBACK

DEVICES

ENCODERS


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ENCODERS

A device used to convert linear or

rotational position information intoan electrical output signal.


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ENCODERS

INDUSTRIAL APPLICATIONS of


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INDUSTRIAL APPLICATIONS ofENCODERS


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RESOLVERS

A resolver is a rotarytransformer that producesan output signal that is a

function of the rotorposition.

SERVOMOTOR with


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S O O O tRESOLVER


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DRIVE MOTORS

DC servo motors

AC servo motors

Stepper motors

Hydraulic motors

OS O C


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POSITION FEEDBACK

Incremental encoder

Quadrature

Absolute encoder

Resolver Tachometer

No feedback (open

loop)

CNC P i


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CNC Programming

Manual

Write code directly

Computer-assisted

Draw cutter path

CAD/CAM

Draw the part

Cutter path is generated

VELOCITY FEEDBACK


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VELOCITY FEEDBACK

Tachometers:Electrical output is proportional to rate ofangular rotation.

Encoders, Resolvers, Potentiometers:Number of pulses per time is proportionalto rate change of position.

POTENTIOMETERS


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POTENTIOMETERS

POTENTIOMETERS


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POTENTIOMETERS


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CNC MACHINES

CUTTING TOOLS (CUTTERS)

CNC CUTTERS


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CNC CUTTERS

Turning center cutters

Machining center cutters

TURNING CENTER CUTTERS


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TURNING CENTER CUTTERS

Types of cutters used on CNC turning centers:

Diamond

CBN

Carbides (and other hard materials) insert turning andboring tools

Ceramics

High Speed Steel (HSS) drills and taps

STANDART INSERT SHAPES


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STANDART INSERT SHAPES

V used for profiling, weakestinsert, 2 edges per side.

D somewhat stronger, used forprofiling when the angle allows it,2 edges per side.

T commonly used for turning

because it has 3 edges per side. C popular insert because the

same holder can be used forturning and facing. 2 edges perside.

W newest shape. Can turn andface like the C, but 3 edges per

side. S Very strong, but mostly used

for chamfering because it wontcut a square shoulder. 4 edgesper side.

R strongest insert but leastcommonly used.

TYPICAL TURNING


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TYPICAL TURNING,THREADING and PARTING TOOLS

MACHINING CENTER CUTTING


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MACHINING CENTER CUTTINGTOOLS

Most machining centersuse some form of HSS orcarbide insert endmill asthe basic cutting tool.

endmills insert cut manytimes faster than HSS,but the

HSS endmills leave a

better surface finish whenside cutting.



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C G C CU GTOOLS (contd)

Facemills flatten largesurfaces quickly andwith an excellentfinishing. Notice theengine block beingfinished in one passwith a large cutter.



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TOOLS (contd)

Ball endmills (bothHSS and insert) areused for a variety ofprofiling operations

such as the moldshown in the picture.

Slitting and sidecutters are used when

deep, narrow slotsmust be cut.



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TOOLS (contd)

Drills, Taps, andReamers

Common HSS tools suchas drills, taps, andreamers are commonlyused on CNC machiningcenters. Note that a spotdrill is used instead of acenterdrill. Also, spiral

point or gun taps areused for through holesand spiral flute for blindholes. Rarely are handtaps used on a machining

center.

TOOL HOLDERS


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TOOL HOLDERS

All cutting tools must be held in a holderthat fits in the spindle. These include endmill holders (shown), collet holders, facemill adapters, etc. Most machines in theUSA use a CAT taper which is a modifiedNST 30, 40, or 50 taper that uses a pull

stud and a groove in the flange. Themachine pulls on the pull stud to hold theholder in the spindle, and the groove inthe flange gives the automatic toolchanger something to hold onto. HSK toolholders were designed a number of years

ago as an improvement to CAT tapers,but they are gaining acceptance slowly.

Documents

CHAP 1 Introduction to CNC Technology MODIFIED