DRAFT - Pearson Education... · 2019-06-20 · 134 BTE EVEL 2 TEHNICAL PLOMA ENGEERING Tools for milling Face mill – a cutter with multiple cutting tips –, often called inserts

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Machining Techniques5Have you ever thought about how products are made and the types of processes that are involved? Throughout industry, machine tools are used extensively to change the shape of workpieces into finished components, using the engineering machining techniques of drilling, turning and milling.

In this unit you will learn about the selection and use of a range of tools and work-holding devices so that you can carry out these machining operations. This will include how to set up the machines, monitor machining processes and finally inspect workpieces for accuracy and compliance with the required design. You will also learn about the importance of safe machine operation.

This will prepare you for a mechanical or manufacturing engineering apprenticeship, and for technician-level roles, for example as a machine setter and setter-operator.

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U N I T 5 M A C H I N I N G T E C H N I Q U E S

How will I be assessed?

For this unit, you will be assessed through internal assessment. This will allow you to apply technical knowledge and demonstrate an understanding of the required practical and technical skills through realistic tasks and activities.

Internal assessment is through completion of three assignments, set and marked by your tutor. So that you can demonstrate what you have learned, you will need to ensure that you have clear evidence of what you have achieved and how you did it. Your tutor will help guide you on the most suitable forms of evidence – some are listed in the Ready for assessment feature on page xx.

To complete the assessment activities for this unit you will need to know how to:

• Select, set up and safely use the tooling and work-holding devices associated with three machining techniques to manufacture workpieces.

• Set, monitor and adjust machining parameters to ensure required accuracy, compliance and surface finish, demonstrating a good understanding of safe working practices and general safety awareness.

• Complete a quality control inspection of some other workpieces to ensure their fitness for purpose.

Assessment criteria

Pass Merit Distinction

Learning aim A: Prepare for machining operations

A.P1 Select tools to safely drill, turn and mill simple and complex features to a given brief.A.P2 Select appropriate work-holding devices to safely drill, turn and mill simple features to a given brief.

A.M1 Select the most appropriate tools and work-holding devices to safely drill, turn and mill simple and complex features to a given brief.

A.D1 Justify the selection of the most appropriate tools and work-holding devices to safely drill, turn and mill simple and complex features to a given brief.

Learning aim B: Make workpieces using drilling, turning and milling techniques safely

B.P3 Set up work-holding devices, tooling and initial positional and dynamic parameters for a given brief.B.P4 Manufacture functional workpieces using drilling, turning and milling, showing some skill.B.P5 Demonstrate compliance with safe working practices when setting up and using drilling, turning and milling machining techniques.

B.M2 Monitor and adjust positional and dynamic parameters when safely drilling, turning and milling workpieces, showing good skill.

B.D2 Monitor and adjust positional and dynamic parameters effectively when safely drilling, turning and milling workpieces, showing a high level of skill.

Learning aim C: Undertake a quality control check for the compliance and accuracy of a workpiece

C.P6 Carry out quality control checks for compliance and accuracy on machined workpieces, maintaining a record of the inspection.

C.M3 Carry out quality control checks for compliance and accuracy on machined workpieces, maintaining accurate records of the inspection.

C.D3 Carry out a comprehensive assessment of workpieces, maintaining detailed records of the inspection and making recommendations for improvement.

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A Prepare for machining operations

A1 Tools

In order to machine a workpiece the material being used to generate or form the workpiece must be held firmly in the machine. A range of different tools are then used to shape the material and produce the required features of the workpiece. A WORKPIECE is the piece of material that is machined to produce a finished component.

There is a wide range of machining techniques, but in this unit you will be setting up machine tools safely and using standard drilling, turning and milling techniques (see Figure 5.1) to produce and inspect several workpieces.

• Drilling – the workpiece is held stationary on a drilling machine while the tool rotates and is pressed into the workpiece forcing the cutting edge against the workpiece, cutting off chips (swarf) from the hole as it is drilled.

• Turning – the workpiece is rotated on a centre lathe while, typically, a single-point cutting tool is fixed in a given position, moves in one direction (axially) and is forced into the workpiece. Turning can be done on the external surface of the workpiece as well as internally (boring).

• Milling – a milling machine cutter is used to remove material from the surface of a workpiece. The milling cutter is a rotary cutting tool, often with multiple cutting points. As opposed to drilling, where the tool is advanced along its rotation axis, the cutter in milling is usually moved perpendicular to its axis so that cutting occurs on the circumference of the cutter. Material is removed by performing many separate, small cuts.

Figure 5.1 Machining techniques

To find out more about the features of drilling machines, centre lathes and milling machines, go to Unit 2: Processes and Materials.

Link it up

Practise

Make a visit to an engineering workshop, perhaps one in your centre. Make a list of the drilling machines, lathes and milling machines that you find there.

1 See if you can find and then record the serial number of each one.

2 Talk to your workshop supervisor and find out the capacity of each machine. Again, record this information.

MachiningtechniquesTurning Milling

Drilling

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There is a range of different tools used for the three different machining techniques; however, some that are used for one technique can also be used for the others.

Tools for drilling

Centre drill – the tool often used to machine the first indent in the workpiece, as a guide for machining operations using a drill bit.

Drill bit – the tool (sometimes called a twist drill) used to produce drilled holes; often follows the use of a centre drill to ensure the tool is accurately aligned to the workpiece.

Flat-bottomed drill – a drill bit without a point that is used to produce blind holes (holes that do not go all the way through the material but need a flat bottom).

Counterboring tool – a flat-bottomed drill with some of its FLUTES (lines cut in a spiral in the surface of the drill bit) removed that is used to enlarge an existing hole, allowing a fastener to sit below the surface of the workpiece. Spotfacing is a similar but shallower version of a counterbore and is used to ensure that a small flat area is created for a seal or washer to fit into.

Countersinking tool – similar to a drill bit but smaller in length; used to produce an angled start to a hole for a screw or bolt of similar shape and form to fit into.

Reamer – a tool used after drilling to produce an accurate hole with a very smooth finish.

Tap – a tool used after drilling to produce a thread in the hole.

Some of these tools can be seen in Figure 5.7 on page xx.

Tools for turning

Turning tool – used to remove material from the workpiece along its circumference.

Facing tool – used to remove material from the end of a workpiece to produce a flat surface. The tool is moved at right angles to the workpiece.

Form tool – produces a very specific shape or profile, such as a radius or stepped feature.

Parting off tool – produces deep grooves, which will cut off the workpiece at a specific length.

Single-point threading – used when a thread form needs to be cut into the outside diameter of the workpiece.

Boring bar – used on a lathe to carry out boring operations along the axis of the workpiece, such as increasing the size of a drilled hole.

Recessing tool – often used after boring operations to produce internal features, such as a recessed groove.

Die – similar to a tap but cuts a thread on an external diameter.

Knurling tool – produces a diamond pattern, known as a knurl, on the outside of the workpiece.

Centre drill, twist drill/drill bit, reamer and tap are the same as for drilling.


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B T E C L E V E L 2 T E C H N I C A L D I P L O M A E N G I N E E R I N G

Tools for milling

Face mill – a cutter with multiple cutting tips –, often called inserts –, designed to move across the face of the workpiece.

End mill – the most common tool used in milling, which has cutting teeth at one end and along the sides.

Slot drill – a type of end mill designed to plunge into the workpiece like a drill bit, then be moved across to create a groove or an enclosed slot.

Slotting cutter – a cutter with multiple tips designed to move through the workpiece to create a channel or slot.

Slitting saw – a disc with saw teeth around the perimeter, designed for cutting deep thin slits into the workpiece.

Profile cutter – designed to produce a specific shape or profile, such as gears or corner radii.

Twist drill/drill bit, reamer and boring bar are the same as for drilling and turning.


Tooling materials

The choice of tooling materials depends on the material of the workpiece and the conditions that the tool is being used in. Some examples are shown in Table 5.1.

Table 5.1 Examples of tooling materials

Material Application

High-speed steel (HSS) Used for a wide range of applications, it can withstand high temperatures without losing HARDNESS (the ability to resist wear and abrasion or to resist indentation by marking or scratching). Maintaining hardness at high temperatures is known as ‘red hardness’. It is called high-speed because it can cut material much faster than high-carbon steels.

Cobalt steel Similar to HSS but with additional cobalt. The advantage of drill bits made from this material is that they remain very hard even at high temperatures, so they are used to drill hard materials such as stainless steel.

Tungsten carbide Tungsten carbide is a very hard material that can cut faster than HSS and may produce a better surface finish. However, it is very expensive, so only the cutting edge of the tool is made from tungsten carbide, and this is attached by brazing to a steel body or by inserting the tool into a cutter body. Often, to help keep the costs down, the inserts have more than one cutting edge and can be turned around and relocated when a new edge is needed.

Diamond A cutting tool that has diamond particles bonded to the cutting surface. Because diamond is much harder than steel or tungsten carbide, diamond tools last longer and can be used on very hard materials to a very high degree of accuracy. Diamond is often used in machining operations where high speeds are required, even though the material being machined may be easy to machine, such as aluminium pistons for the automotive industry.

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Go to Unit 7: Computer Numerical Control to see how some of these tools are used when machining components using a CNC machine. This could also be important when you come to complete the synoptic tasks in Unit 9: Delivering Engineering Solutions.

Link it up

Practise

Find one image of the tools mentioned for each of the drilling and turning techniques; then share and discuss these with others in your group.

For milling, look up an image of a slot drill and an end mill. Can you see the difference between the two, particularly the number of flutes? Why is this?

During your visit to the engineering workshop, ask if you may take some photographs of the tooling used for drilling, turning and milling; make a note of the photo reference number and find out/record the material the tool is made from.

A2 Work-holding devices

It is important to think about how workpieces can be safely held in position to allow them to be machined accurately.

There is a range of different devices that can be used for the three different machining techniques.

Work-holding devices for drilling

Machine vice – this is the most commonly used method to secure the workpiece (see Figure 5.2); the workpiece is placed between two jaws which are then tightened to hold the workpiece firmly in position so that machining can take place.

The machine operator in a workshop has become so used to selecting the correct tools for carrying out machining techniques that she does it without thinking.

What would happen if she:

1 Selected an incorrect tool?

2 Used a tool that was not hard enough, e.g. used a high-speed steel tool when a tungsten carbide one was needed?

What if...?

Figure 5.2 Machine vice

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Figure 5.3 Clamping plate

Angle plate – this allows the workpiece to be positioned at any given angle, with slots or holes allowing the workpiece to be bolted onto the angle plate (item 1 in Figure 5.4).Figure 5.4 Slotted angle plate

Vee block and clamps – a vee-shaped block of metal for round workpieces. A clamp fits over the top to secure the workpiece into the vee block so that it cannot move (see Figure 5.5).Figure 5.5 Vee block with clamp

Work-holding devices for turning

Chuck – the workpiece can be secured in a lathe by placing it in the jaws of a chuck, which are then tightened with a large key until the jaws hold the workpiece in place. Chucks normally have three jaws but there are also 4-jaw chucks. The jaws grip the workpiece quickly and accurately. Hard jaws are often serrated to increase the grip, but they can damage the surface of soft materials when tightened, so in these cases soft jaws should be used.

Centre – a lathe centre is an accurately machined cone-shaped tool that is clamped in a lathe’s chuck or TAILSTOCK (the adjustable part of a lathe which contains the dead centre holding the workpiece) to position a workpiece precisely and securely. The workpiece has a hole drilled centrally in its end, and the cone of the centre work-holding device is located in this hole. This ensures that the workpiece remains central when machining takes place. Two different types can be used:

• A dead centre does not rotate with the workpiece, so must be lubricated regularly to prevent friction that would cause heat to build up, start to melt the material and cause the centre to bond or friction weld to the workpiece.

• A live centre has a bearing incorporated, allowing it to rotate, and so does not require LUBRICATION (using a lubricating substance to minimise friction and wear).

Clamps – these clamp directly to the machine table using special T-shaped fixings (bolts and nuts) that slide into grooves on the table or base of the machine. Figure 5.3 shows such an arrangement where the bolt and nut (item 1) is holding the clamping plate (item 2) in place, securely holding the workpiece in position.

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1

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Faceplate – a circular metal plate with slots machined into it. It is used instead of a chuck when an awkward workpiece has to be secured. The workpiece is secured to the plate by nuts and bolts. T-nuts are often used as they can be locked into the slots. The faceplate fits onto the lathe spindle where it is securely fixed by being screwed into place or held by a LOCKNUT (a nut that resists loosening) onto a taper, or by using cam locks.

Fixed steady and travelling steady – used to support long workpieces, held in a chuck or between centres. A fixed steady is clamped to the bed of the lathe. It normally consists of three jaws with plastic or bronze rollers that can be adjusted to support the workpiece. A travelling steady is attached to a saddle, which allows it to be moved along as machining takes place.

Work-holding devices for milling

Machine vice, clamping direct to the machine table, angle plate, and vee block and clamps are the same as for drilling.

Indexing head/device – made up of two parts: one similar to a faceplate or chuck on a lathe, and one similar to a centre. The device is clamped to the table of the milling machine and the workpiece is positioned between the two work-holding parts. The purpose of an indexing head is to allow movement through 90 degrees.

Rotary table – this is clamped to the table of the milling machine and the workpiece is clamped to the rotary table. This allows the operator to machine at regular indexed intervals around the workpiece.

Practise

Consolidate some of the points about tooling and work-holding devices for drilling, turning and milling by carrying out a search on the internet for a sketch of each machine.

1 Highlight or annotate the sketch to show the features of each machine. Add a list of the tooling and work-holding devices that could be used on each.

2 Now search the internet for images of work-holding devices from your list and share these with other members in your group.

A3 Selecting appropriate tooling and work-holding devices

Having established which tooling and work-holding devices are available to engineers when carrying out machining operations on drilling, turning and milling machines, it is important that you understand which should be selected to produce different features on different workpieces. Table 5.2 shows examples of the selection of tools and work-holding devices for some workpiece features.

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Practise

Figure 5.6 shows the machining tolerances for a vee block.

1 Make a list of the tooling and work-holding devices that could be used when making this vee block.

2 Write a paragraph that describes how the features of the vee block, such as the 40 mm diameter holes, should be produced.

Table 5.2 Selecting appropriate tooling and work-holding devices

Machining technique

Operation Tooling Work-holding device

Drilling Drilling a through hole in a round bar

Centre drillDrill bit

Vee blockClamps

Drilling a blind hole in a square bar

Centre drillDrill bitFlat-bottomed drill

Machine vice

Turning Turning a flat face on the end of a round bar

Facing tool 3-jaw chuck

Knurling diameters on a round bar

Knurling tool 3-jaw chuckLive (rotating) centre

Producing an M8 thread in the end of a square bar

Centre drillDrill bitTap

4-jaw chuck

Milling Producing flat faces on a square bar

End mill Machine viceClamping direct to machine table

Producing an enclosed slot in a round bar

Slot drill Vee blockClamps

Producing a gear profile on a circular blank

Profile cutter Indexing head/device

Figure 5.6 Autocad drawing of typical vee block with machining tolerances

These and other workpiece features are covered in more detail in the next Learning aim in this unit, so you may wish to come back to this section later.

To enable you to become familiar with and knowledgeable about selecting tooling and work-holding devices, you will need to visit an engineering workshop on several occasions and physically see these devices being used on machines.

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Skills and knowledge check

Name at least two tools that can be used in all three machining techniques.

Name at least two work-holding devices for each of the three machining techniques.

In drilling, what tooling and work-holding devices would be used for a through hole in a square bar?

I know how to select different tooling and work-holding devices for all three machining techniques.

I can state what the hardest tooling material is.

I understand the advantages of using tungsten carbide tooling.

B Make workpieces using drilling, turning and milling techniques safely

B1 Techniques for machining features of the workpiece

The selection of appropriate tooling and work-holding devices will depend on which specific machining activities are required to be carried out on a workpiece. For this, you will need to know which features are required.

These features can normally be found by referring to the specification for the part, component or product that you are working on. This will be in the form of an engineering drawing, which specifies sizes, TOLERANCES and features, allowing you to select the correct tools, work-holding devices and machining techniques. (Tolerance is the allowable variation in sizes/dimensions of the workpiece and its features.)

Complex workpieces have different features. Sometimes a workpiece may have a feature produced on a lathe; it may then have holes produced using a drill; and, finally, extra features may be added using a milling machine.

The following information will help you appreciate the requirements when machining features of a workpiece. It is anticipated that your workshop supervisor will be spending a lot of time with you, demonstrating and practising the skills you will need.

Features produced by drilling

Drilling is normally used for producing or altering holes:

• through hole – a hole that passes all the way through a workpiece such as a square bar

• blind hole – a hole that does not pass all the way through a workpiece • flat-bottomed hole – a blind hole that has a flat base

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Figure 5.7 Examples of features produced by drilling

Features produced by turning

Turning is normally used to produce different features and profiles on round components:

• flat face – a square end of a round workpiece • parallel diameter – a circular profile that is produced using turning tools • stepped diameter – a circular profile of a different diameter produced

on one workpiece • tapered diameter – a sloping circular profile in a conical shape • drilled or reamed hole or internal thread – these have similar features to

those mentioned under drilling • bored hole – a drilled hole that has been enlarged using a special tool

providing dimensional accuracy.

Figure 5.8 Example of lathe tooling

• counterbored/countersunk hole – an additional element machined at the entrance of a drilled hole to allow a part or bolt to sit below the surface of the workpiece

• reamed hole – a drilled hole that has been enlarged so that it is dimensionally accurate and/or has a very smooth finish

• tapped hole (internal thread) – a drilled hole that has a thread cut into it.

• profile form – a special feature on a workpiece that has complex contours

• thread – formed either on the outside or inside of a circular component to allow a nut or other threaded component to be secured

• chamfer – an angled form, forming a bevel or small taper where sections or sizes change

• knurl – a diamond pattern allowing the workpiece to be gripped • groove/undercut – a ridge on the outside or inside of a workpiece;

when an undercut is made all the way through the workpiece, this becomes a parting-off feature.

Partingtool

Radiustool

Threadingtool

Chamfertool

Turningtool

Right-hand

turningtool

Left-hand

turningtool

Drilling Reaming Boring

Counter-boring

Counter-sinking

Tapping

Drilling Reaming Boring

Counter-boring

Counter-sinking

Tapping

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Features produced by milling

Milling is used to produce a variety of features, normally on non-circular workpieces:

• flat/square/parallel face – this allows surfaces to be accurate and can be used as a DATUM (a fixed starting point of a scale or operation) for other operations

• angular/rotated/indexed feature – a feature specified by angular dimensions or a number on a pitch circle diameter and produced using special indexing devices

• step/shoulder – produced by milling part of a surface away to leave a regular profile

• slot/recess – a cutaway part of a solid workpiece; can be an open-ended, enclosed or a tee slot

• drilled or bored hole or profile form – these have similar features to those mentioned under turning

• serration – a regular pattern on the surface of a workpiece.

Figure 5.9 shows some examples of standard milling operations.

Figure 5.9 Examples of standard milling operations

The features produced by the three machining techniques can be summarised as simple or complex, as presented in Table 5.3.

Table 5.3 Simple and complex features produced by the three machining techniques

Drilling Turning Milling

Simple features Through holeBlind hole

Flat faceParallel diameter

Flat faceSquare face

More complex features

Flat-bottomed holeCounterbored holeCountersinkingReamingTapping

Stepped diameterTapered diameterDrilled holeBored holeReamed holeProfile formInternal threadExternal threadParting offChamferKnurlGrooveUndercut

Parallel faceAngular faceStepShoulderOpen-ended slotEnclosed slotRecessTee slotDrilled holeBored holeProfile formSerrationIndexed formRotated form

Convex millingcutter

Workpiece Workpiece Workpiece

Concave millingcutter

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Practise

By now you should be realising that machining a workpiece using any of the three machining techniques is a complex process. To consolidate some of the points covered, ask your workshop supervisor for a range of machined workpieces, and discuss with another learner the machined features of each workpiece.

1 Make a list of the features of each workpiece.

2 Make a note of how each feature can be machined.

Now ask your supervisor for an engineering drawing of each workpiece and add your observations to the drawing to show a non-engineer what the features are.

B2 Machining parameters

During the rest of this unit you will need to show that you can successfully do the following.

1. Set up work-holding devices, tooling and initial positional and dynamic parameters for a given brief.

2. Manufacture functional workpieces using drilling, turning and milling.3. Demonstrate compliance with safe working practices when setting up

and using drilling, turning and milling machining techniques.4. Monitor and adjust positional and dynamic parameters when

machining workpieces.

To determine which parameters need to be used when machining a workpiece you will normally refer to the controls on the machine and/or the associated operating manual for that machine. Your tutor or workshop supervisor is likely to demonstrate the use of the machines and then let you practise, gradually building up your skill level until you are able to use the machine safely. Please remember that the use of these types of machines can be dangerous, so extreme care must be taken at all times.

There are two sides to understanding parameters when using machining techniques. First, you have to set the machine so that the workpiece and tool are correctly positioned relative to each other. This is the case for all three techniques. Likewise, the consideration of swarf clearance and removal is something that has to be done for each machining technique; each technique has different requirements, as summarised in Table 5.4.

Parameters for drilling, turning and milling techniques

Practise

Make a visit to the engineering workshop in your centre and using the list of the drilling machines, lathes and milling machines you made earlier:

1 Find out how the cutting speed is changed or set.

2 Find out if the machines have a facility to set a FEED RATE; if they do, make a note of how this is achieved. (Feed rate is the velocity at which the tool is moving during machining.)

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Swarf clearance during: How achieved?

Drilling • Using drill bits with correct helix angle forming the flutes on the drill bit.

• The use of COOLANT (a substance used to reduce heat and friction) also helps to control the swarf flow.

Turning • Having the correct cutting angles and faces on the cutting tools. The swarf normally flows across the top of the tool face as the tool is generating a wedge-shaped action.

• Often chip breakers are used to control the flow of the swarf.

• Also, a change in cutting speed or feed rate will alter the characteristics of the swarf.

• Again, the use of coolant helps to control the swarf flow.

Milling • Similar methods to those used for drilling and turning – a sort of combination of these according to the milling operation being performed.

• Again, the use of coolant helps control the swarf flow.

Table 5.4 Swarf clearance considerations for the three machining techniques

Dynamic parameters for drilling

The two most important parameters for drilling are:

• tooling revolutions per minute (speed) • linear feed rate (feed).

In determining both the speed and the feed, several aspects of the process need to be considered. You will need to consider the finish required, the amount of metal to be removed, the type of tool being used, the properties of the tool and workpiece materials, and the power and capability of the machine. In drilling, the spindle speed N can be calculated using a formula, but reference must be made to an engineer’s handbook to find some of the values used in the formula, such as the overall cutting speed, which is given in metres per minute (m min−1) and is represented by S in the following formula:

N =1000S

πD

where:

N = spindle speed of the work-holding device/chuck (rpm)

S = cutting speed (m min−1)

D = diameter of the drill (mm)

Table 5.5 shows the recommended values for S in metres per minute for some common workpiece materials when using either high-speed steel (HSS) or tungsten carbide tooling. Don’t expect to see these exact figures in a table representing cutting speeds. You will find variation across textbooks, the internet and other sources of engineering information.

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Table 5.5 Recommended cutting speeds for drilling

Workpiece material HSS cutting speed (m min−1)

Tungsten carbide cutting speed (m min−1)

Aluminium 60–90 Above 350

Brass 45–70 350–500

Cast iron 25–50 90–120

Bronze 40–70 150–250

Mild steel 25–35 100–190

Stainless steel 10–15 50–100

High-carbon steel 15–20 60–120

Example:

Calculate the required revolutions per minute setting on a drilling machine when a 12 mm HSS drill bit is to be used when machining cast iron.

N =1000S

πD

=1000 × 30

π × 12

A value of 30 has been taken for S, which is within the range 25–50 given in the table for cast iron. Therefore:

N = 796 rpm

To be on the safe side, the nearest rpm setting below this on the machine would be selected.

Practise

Calculate the required revolutions per minute setting on a drilling machine when an 18 mm tungsten carbide drill bit is to be used when machining mild steel. Don’t be surprised if you get a different answer to other learners in your group.

Why might this be the case?

Often in drilling, the feed rate is manually applied and controlled by the operator. However, on some machines a linear feed rate is applied by a motor that has been set by the operator. Generally, the feed rate is quoted in mm per revolution (mm rev−1) and again varies according to conditions. A typical value for a 12 mm drill bit would be 0.2 mm rev−1 for the ideal conditions of a sharp tool, use of cutting fluid, secure work-holding and a rigid machine.

Practise

Find and compare several sources of information about feed rates for drilling, including those shown in a range of engineering handbooks. Discuss with other learners.

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Dynamic parameters for turning

The most important parameters for turning are similar to those for drilling. However, in the case of turning the workpiece rotates at a predetermined speed (rpm) and not the tool. So the main parameters are:

• workpiece revolutions per minute (speed) • linear feed rate (feed)

plus there is a need to consider:

• the depth of cut for ROUGHING (where machining quickly removes excess material from a workpiece) and FINISHING (machining operations to obtain the final surface finish of the required features).

In drilling, one cut produces a hole. In turning, several cuts may be necessary to produce the final required outside diameter of a workpiece.

For turning operations you can use the same formula to work out the workpiece speed of rotation N, where D is the diameter of the workpiece:

N =1000S

πD

The selection of an appropriate feed rate is usually down either to the experience of the operator or to trial and error, or to a combination of both. The range of feed rates is often displayed on an instruction plate on the HEADSTOCK of the lathe (the headstock is the part of the machine that supports the revolving spindle). A high feed rate will limit the depth of cut and produce a poor finish on most materials, while a low feed rate will enable a greater depth of cut. A good starting point would be to use 0.2 mm rev−1 for roughing cuts and 0.02 mm rev−1 for finishing and accuracy cuts. Unfortunately, there are no hard and fast rules – you will have to see where your experience takes you. Even so, whenever you are selecting the feed rate or speed for a lathe, always have your selection checked out by your workshop supervisor before you commence machining.

Dynamic parameters for milling

Just like drilling, the important parameters are:

• tooling revolutions per minute (speed) • linear/table feed rate (feed), but this time you have to consider the

number of teeth on the cutting tool

plus again the need to consider:

• the depth of cut for roughing and finishing.

In milling, the tool rotates and not the workpiece. So, by using the same rpm formula as for drilling and turning you can calculate the rotational speed of the cutter N, where D is the diameter of the cutter.

The selection of an appropriate feed rate is usually achieved by referring to a feed rate chart for the particular tool in question and using the data to calculate what table feed rate to set the machine to.

Table 5.6 presents the recommended feed rates for the ideal conditions of a sharp HSS tool, use of cutting fluid, secure work-holding and a rigid machine.

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Table 5.6 Recommended feed rates for milling

Cutter diameter (mm) Feed per tooth mm for slot drills

Feed per tooth mm for end mills

2 0.0075–0.013 0.013

3 0.013–0.025 0.018

6 0.025–0.037 0.025

12 0.037–0.050 0.050

25 0.050–0.064 0.090

50 0.064–0.075 0.130

The linear feed rate is represented by f in the following formula:

feed per min (f) = rev per min (N) × no. of teeth on the cutter × recommended feed per tooth

Example:

Calculate the feed per minute setting on a milling machine when a 50 mm HSS end mill with 6 teeth is to be used to machine a metal component. Assume a cutting speed of 25 m min−1.

N =1000S

πD

=1000 × 25

π × 50

= 159.2 rpm

feed per min (f) = rev per min (N) × no. of teeth on the cutter × recommended feed per tooth

= 159.2 × 6 × 0.130

= 124 mm min−1

A machine operator is using a range of workshop machining techniques for the first time.

What would happen if he:

1 Set the workpiece in the wrong position?

2 Positioned the tool in the wrong place relative to the workpiece?

What if...?

B3 Safe working practices

Safety in the workplace should not be overlooked or treated lightly. The Health and Safety Executive (HSE) reports that in 2015/16:

• 144 workers were killed as a result of a workplace accident under the Reporting of Injuries, Diseases and Dangerous Occurrences Regulations 2013 (RIDDOR). Falls from height (26%), being struck by a moving vehicle (19%) or being struck by a moving object (10%) were the main types of accident, accounting for just over half of all fatalities.

• An estimated 621,000 workers sustained a non-fatal injury at work according to self-reports from the Labour Force Survey (LFS). Of these injuries, 200,000 led to absences of more than 3 days from work, of which 152,000 led to absences of more than 7 days. Being injured handling, lifting or carrying (20%), slipping or tripping (19%) and being hit by a moving object (10%) were the main types of accident, accounting for around half of all non-fatal injuries.

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General safety awareness

Machining operations usually take place in an engineering workshop – a potentially dangerous working environment where safety awareness is critical. An evaluation of identified potential risks to health and safety should therefore be carried out before you undertake any machining operations. It may be the case that your tutor or supervisor has already carried out a RISK ASSESSMENT for the work areas and workshop activities you will be using. The purpose of the risk assessment is to ensure that all risks have been identified, all associated hazards have been considered and appropriate measures and ways to control the risks/hazards have been put in place. It is essential that you familiarise yourself with the risk assessment, in addition to seeking guidance on the use of drilling, turning and milling techniques. Things you need to be aware of include:

• Moving parts – be alert to which parts are moving; do not touch or approach the moving parts of a machine.

• Ensuring machine guards are in place – guards are designed to protect you from getting trapped in a machine or being hit by parts coming off the machine or workpiece. You should know exactly which guards are required and how they work. Never try to reach over or around a machine guard.

• Use of the EMERGENCY STOP – a pushbutton designed to stop the machine quickly. Before operating a machine, make sure you know the location of all emergency stops and check that they work. Remember the machine guard is often INTERLOCKED; this means that the machine will be stopped if the guard is moved or opened during a machining operation.

• Machine isolation – a switch that ensures that the machine is properly switched off (isolated) and cannot be accidentally activated. Make sure you know where all power switches and buttons are located and check that they work.

• Wearing appropriate personal protective equipment (PPE) – you should not enter a workshop unless you are wearing the correct protective equipment. Overalls, safety boots and safety glasses are normally the minimum requirement, with other equipment such as safety goggles and gloves being used for specific machining operations.

• Keeping a clean and tidy work area – all tools should be put away after use, swarf should be safely removed and all spills should be quickly cleaned up to prevent accidents. Never rest tools or workpieces on machines or machine tables – vibration can cause them to fall off, leading to injury.

• Removing burrs and sharp edges – always use a tool to do this as they can be very sharp and can cut fingers easily.

Go to Unit 2: Processes and Materials to find out more about risk assessments, health and safety signs and symbols and the uses of PPE. In addition, learn about preparing and closing down a work area in Unit 4: Workshop Skills.

Link it up

Safe working practices on drilling machines

How to handle drilling tools safely

Be very careful of sharp edges when handling drilling tools. For example, twist drills should never be held by the cutting surface or flutes – hold the smooth shank of the drill bit instead.

What to do when a tool breaks

Make sure that there are guards in place to prevent broken cutting tools or parts coming off the machine. The use of eye protection is mandatory when using a drilling machine.

How to remove and dispose of swarf

Swarf should be carefully removed using a brush and/or hook tool, taking care to use an appropriate container to remove and recycle the material.

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Keep the brush and/or hook well away from any moving components. Do not handle the swarf with bare hands as the edges are very sharp.

How to safely handle cutting fluids

Cutting fluid must be applied carefully, ensuring that it cannot splash onto hands, into eyes or come into contact with any other parts of the body or overalls/clothing. Cutting fluids can be a skin irritant, and barrier cream should be applied on the hands prior to use. After a period of time, cutting fluid deteriorates and must be changed as recommended by its manufacturer. The applicable COSHH (Control of Substances Hazardous to Health) regulations data sheet should always be consulted when handling substances such as cutting fluids.

Safe working practices on lathes

How to handle tools safely

Like drilling, be very careful of sharp edges when handling lathe tools. They should never be held by the cutting surface – hold them only by the tool’s shank.

The considerations you need to make regarding tool breaks, handling swarf and using cutting fluids require exactly the same care and attention as for drilling operations.

Safe working practices on milling machines

How to handle tools safely

Once again, be extremely careful of sharp edges when handling milling tools. Never hold them by the cutting surface – always hold the tool by its shank. It is even more important to be careful with milling cutters because of the extra number of sharp teeth on each cutter.

The considerations you need to make regarding tool breaks, handling swarf and using cutting fluids need exactly the same care and attention as required for drilling.

A further consideration when milling, which potentially makes it the most dangerous of the three machining techniques, is being aware of BACKLASH in machine slides. While this is a feature of lathes, it is more so for milling machines. Backlash is caused by a poor fit between a drive screw and nut, resulting in backward/forward movements. Contaminants between the drive screw and nut can increase the problem, causing the machine to lose accuracy, and issues such as vibration and tool/workpiece movement can become a danger. With horizontal milling machines, it is imperative that the correct process is used to minimise the problem of backlash: use up-cut milling on these machines, making sure that any backlash eliminators are working correctly.

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Practise

For the drilling machines, lathes and milling machines in your engineering workshop, find out about and make a written record of the following details:

1 The safety guards on each machine.

2 The machines’ emergency stop buttons. In addition, locate the emergency stop for the whole workshop.

3 How the machine tools are stored.

4 What information exists for cutting fluids.

5 The PPE required for each machine technique.

With the information you have recorded, prepare a PowerPoint® presentation and present it to the other members in your group.


Name a feature that can be machined by all three techniques – drilling, turning and milling.

Describe how the revolutions per minute setting on a drilling machine is worked out when a HSS drill bit is to be used to machine a mild steel workpiece.

What are recognised as being the ideal conditions when setting up a milling machine?

Explain why compliance with safe working practices when setting up and using machining techniques is so important.

I know how turning tools are stored safely.

I understand the dangers of working on a milling machine.

I can manufacture functional workpieces using drilling, turning and milling.

I can monitor and adjust positional and dynamic parameters when safely drilling, turning and milling workpieces.

C Undertake a quality control check for compliance and accuracy of a workpiece

C1 Quality control checks for compliance and accuracy

You will also need to know how to carry out both visual and specific quality control checks on finished workpieces.

Visual checks are required to ensure that:

• a workpiece is free from false tool cuts • all burrs and sharp edges have been removed.

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It is also important that the machined workpiece is FIT FOR PURPOSE (meaning that the workpiece is good enough to do the job it was designed to do) and is manufactured to the accuracy required. This can be done by carrying out QUALITY CONTROL checks – the inspection and/or measurement of the workpiece for compliance and accuracy in accordance with both the required sizes and tolerances specified in the engineering drawing and/or specification of the workpiece and the applicable British Standards. A tolerance is given where a controlled accuracy is needed and specifies the limits of what the size of the feature of the workpiece can be, allowing the workpiece to still function correctly when in use.

For example, if you refer to Figure 5.6 on page xx, machining tolerances are shown on a drawing to manufacture a typical vee block. There are two holes to be drilled with a specified tolerance: O40 ± 0.05 mm means the maximum size is 40.05 mm and the minimum size is 39.95 mm. Incidentally, this cannot simply be measured using a rule, so accurate measuring instruments have to be used.

Measuring instruments

It is important that you are aware of the available types of measuring equipment and have an appreciation of how these instruments should be used correctly to achieve accuracy during inspection.

Typical instruments used for checking measurement accuracy include the following:

• Engineer’s rule – made from hardened and tempered steel with graduations that correspond to measurements at millimetre intervals scribed accurately into the surface.

• Vernier callipers – like the engineer’s rule, it features a linear calibrated scale that has both a fixed and a moveable jaw attached (see Figure 5.10). The moveable jaw is adjusted to fit across a workpiece and the measurement is read off the scale.

Figure 5.10 Vernier callipers

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• Vernier height gauge – uses the same principle as the Vernier callipers; however, it is fixed to a stand (see Figure 5.11) and used to measure the height of a workpiece.

Figure 5.11 Vernier height gauge. Electronic height gauges with digital readouts are also available.

• Micrometer – normally consists of a C-shaped frame; a calibrated screw rotates inside a sleeve passing through one end of the frame and traps the workpiece being measured against the anvil that is attached to the opposite end of the frame (see Figure 5.12). Markings scribed on the outside of the measuring screw align with markings on the sleeve, allowing accurate readings to be taken.

Figure 5.12 Micrometer

• Dial test indicator (DTI) – records the displacement of a plunger that is very sensitive to movement. This movement is amplified by a series of gears before a rack and pinion turns the linear movement into rotation of a large pointer (see Figure 5.13).

Figure 5.13 Dial test indicator (DTI)

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Figure 5.14 Engineer’s square

• Surface table and surface plate – a surface table is a large, heavy, flat table manufactured from self-lubricating cast iron. Being perfectly flat means that workpieces placed on it are also flat, and levels or heights etc. can be accurately measured. A surface plate is a smaller version of a surface table and is designed to be used on a workbench.

• Plug gauge – a gauge that is pushed into the machine hole on the workpiece. For the hole to be considered the correct size, one end of the gauge should go in and the other should not; these are classified as ‘go’ and ‘no-go’ plug gauges.

• Surface texture gauges – a set of comparison specimens that have different roughness finishes so that they can be compared with the finished surface on the workpiece.

Go to Unit 4: Workshop Skills to find out about the use of tools and equipment for measuring and Unit 7: Computer Numerical Control for more information on checking the conformity of a machined component. This could be important when you come to complete the synoptic tasks in Unit 9: Delivering Engineer Solutions.

Link it up

Practise

In the engineering workshop, see what measuring equipment you can find.

1 Take a photograph of each piece of equipment.

2 Make a note of what it is called.

3 Now search the internet for information about the equipment you have photographed.

4 Make a note of how and where this equipment could be used.

With the information collected, prepare a presentation and discuss your findings with other members in your group.

What if...?

An engineering quality control inspector usually carries out the final inspection of machined components. He uses a range of measuring techniques and measuring instruments.

What would happen if he:

1 Used the wrong plug gauges?

2 Did not check to see if a DTI was reading accurately?

3 Used Vernier callipers when he should have used a micrometer?

• Engineer’s square – an engineer’s square is used to ensure objects/features are at right angles (90°). It is constructed by securing a steel blade into a more substantial steel handle (see Figure 5.14). Both parts are manufactured to a high standard to ensure accuracy.

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Specific checks for drilling

Drilled holes will be as per the size of the drill bit, whereas reamed holes will have a tolerance specified which should be used to check if the size of the hole is within the tolerance required. These tolerances will be determined using the British Standards (BS) set by the British Standards Institution (BSI), in this case BS EN 22768-1:1993 or BS 4500:2009 for size and BS 1134:2010 for surface texture.

H8 on an engineering drawing, when shown in reference to a hole, indicates the position of the tolerance relative to the standard size quoted, e.g. 25 mm, and the difference between the biggest and smallest dimension that the hole is allowed to be. The difference will depend on the standard size of the hole – the bigger the standard size of the hole, the wider the tolerance.

Surface texture is due to small local deviations of the surface from what would be a perfectly flat, ideal, true plane, and the requirements are stated as a surface roughness value in µm, e.g.1.6 µm.

Screw threads can be classified as fine, medium or coarse. Again, the best way to assess their accuracy is by the use of gauges. Thread gauges are available for the different classifications and standard thread sizes. Thread gauges comprise a thin steel strip with serrations that match a particular thread size. They usually come in sets that are rotated out of a casing.

Figure 5.15 Imperial thread pitch gauge

Specific checks for turning

All specific checks mentioned for drilling are also appropriate for turning. Additionally, any tapers or angles that are turned will have a tolerance that is quoted as plus (+) or minus (–) a certain number of degrees, e.g. ±1.0°.

Specific checks for milling

All specific checks mentioned for turning are also appropriate for milling. In addition, flatness and squareness may also have to be checked. FLATNESS is defined as how much a surface on a real workpiece may vary from the ideal flat plane. SQUARENESS is the degree to which the orientation of a right-angled workpiece feature may vary. In both of these cases the accuracy would be stated as mm per mm. This may sound confusing, but as an example this could typically be 0.125 mm per 25 mm.

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Practise

Search the internet to find out which British Standard is used to check or specify:

1 Hole sizes

2 Sizes of shafts

3 Surface texture

STEP BY STEP CHECKING A REAMED HOLE

STEP 2STEP 1 STEP 3


What does a plug gauge check?

What unit is used to quote surface texture?

Describe what a tolerance is.

I know how to carry out visual checks on machined workpieces.

I know how to use a range of measuring equipment.

I know how to measure a range of external features produced by turning techniques.

Checklist

PPE Tools and equipment Source information

Workshop overalls Reamed workpiece Appropriate drawing of workpiece

Protective footwear Plug gauge BS EN 22768-1 or BS 4500

Safety glasses Quality control checklist

Wear appropriate PPE and obtain the tools, equipment and engineering information, including the drawing of the workpiece

Use the ‘go’ and ‘no-go’ plug gauge to check whether the hole size conforms to the requirements on the drawing

Record whether or not the holes are within tolerance

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You will be assessed for this unit by completing three assignments. For each of these assignments, you will need to show you have understood the practical and technical requirements of the necessary Learning aims (see Assessment criteria on page xx) by undertaking a practice activity and/or completing a subject-related checklist.

Practical activities

The following are practical examples of the type of activity you may be asked to carry out.

A Researching material removal processes of drilling, turning and machining. When manufacturing specific workpieces you should identify which types of tooling and work-holding devices should be selected and explain why you chose them. This could include a discussion of the alternatives and why these would be considered less suitable than the ones you finally decided on.

B Manufacturing workpieces by carrying out drilling, turning and milling. The workpieces will have a required number of features, and you must show how you monitored and adjusted positional and dynamic parameters effectively and systematically. Demonstrate a clear understanding of these parameters of accuracy and surface finish of the finished workpieces.

C Inspecting finished workpieces. Carry out a quality control inspection of the finished workpieces, identifying the appropriate measurement instruments, and record the results of the process. Describe why it is essential to undertake these inspections, and what could happen if they are not performed.

Providing assessment evidence

This is required to show what you have learned and that you fully understand the content of this unit. The types of evidence used could be any of the following:

• tutor or work placement supervisor observation records

• written documents, notes, sketches or annotated photographs

• completed inspection records and any checklists

• written justifications and evaluations

• workshop/training logbook.

Also include videos, along with any narration and notes you have added.

If this evidence is gathered in the workplace, don’t forget that it will need to be countersigned and dated by your workplace supervisor.

Pull all your evidence together and present it in your assessment portfolio.

Ready for assessment

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Here are some important skills and competencies to practise and develop when studying this unit, which could help you gain employment in an engineering workplace.

1 Show you can respect work colleagues and other professionals

• Talk to your tutor or workshop supervisor and find out as much detail as you can about each machine in the workshop.

• Discuss with them, adding your observations, the features of a workpiece that are shown on an engineering drawing, and work out the best way to carry out the machining activities.

2 Use appropriate verbal communication

• Before you start machining, ask your supervisor if you have set up the machine correctly.

3 Problem solving

• Think about ways to improve your knowledge and experience when selecting appropriate machining techniques, tooling and work-holding devices to manufacture a range of features.

4 Management of information

• Carry out quality control inspections of completed workpieces, record visual measurements and interpret the results to confirm that the workpiece complies with the drawing and is fit for purpose.

WORK FOCUS

HANDS N

Ready for work?ake this short quiz to see if you can use machining techniques appropriately in a workshop. Answer the following questions; tick the answer you think is correct.

When you are preparing a lathe to carry out machining, which of the following is not important regarding your safety?

A Types of tools to use

B The workpiece material

C The customer

D The speed to set the spindle

Why is it important to produce workpieces accurately when carrying out machining techniques in an engineering company?

A To impress the boss

B To ensure they are fit for purpose

C To make them last longer

D To ensure they are made on time

When working in a quality control department and checking workpieces, why

is it important to make sure measuring equipment is maintained correctly?

A It looks better

B To ensure it works correctly

C So customers think you have good equipment

D You would lose your job otherwise

1

2

3

T

Answers: 1C, 2B, 3B

Your score:

3 out of 3: Well done! You’re getting the idea of what an employer is looking for.

2 out of 3: Understand why the answer to the question you got wrong is the right one.

1 or less: Look again at this unit and/or discuss the answers with your tutor.

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Documents

DRAFT - Pearson Education... · 2019-06-20 · 134 BTE EVEL 2 TEHNICAL PLOMA ENGEERING Tools for milling Face mill – a cutter with multiple cutting tips –, often called inserts