Pp lathes

Chapter 6 Preparing and using lathes for turning operations

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Preparing and using lathes for turning operations



The lathe



Workholding devices: Four jaw chuck

A versatile workholding device. Each jaw moves independently and can bereversed. This allows it to hold castings, square material and work that needs to be set up accurately.

Methods of setting up a four jaw chuck:

• The tools are brought near the work and the jaws adjusted.

• A centre drilled hole is aligned with the tailstock centre.

• A scribing block or Vernier height gauge used on the cross slide to find a datum.

• A dial test indicator or clock gauge is usedon a machined surface.

Which is the most accurate?



Workholding devices: Face plate

• The workpiece can be aligned on the face plate in a similar way to the four jaw chuck.

• However everything must be bolted or screwed in place.

• A clamping set can be used to attach work to the face plate.

• This can only be run at very slow speeds.



Cutting tool materials

• High speed steel (HSS) – high carbon steel alloy of iron and carbon with small amounts of tungsten, chromium, vanadium and cobalt. They have high resistance to abrasion and remain effective at over 600°C. Available in round and square sections.

• Cemented carbides – made from mixing different metal carbide particles. Tungsten carbide is the most common. Cemented carbide tools can be run at faster speeds than HSS tools but are brittle and can shatter if used for intermediate cuts.

• Stellite – a cobalt chromium tungsten alloy, as hard as HSS but can withstand temperatures up to 700°C. It is slightly more brittle than HSS.

Use the Internet, catalogues and the workshop to find out more about the cutting tools that can be used for lathes.



Cutting tool angles

• Clearance angles – angle between the cutting edge and the surface of the material being cut. This prevents any other part of the tool touching the work other than the cutting edge.o There is front clearance, side

clearance and tool clearance.

• Rake – the angles of the face that the metal chip slides on while the tool is cutting. Materials that break into small chips have a small rake angle while soft tools have steep rake angles.



Setting and adjusting machine tool speeds

Tool manufactures recommend cutting speeds that give the best finish andlife for their tools. This is expressed in metres per minute.

The cutting speed is the speed the surface of the work passes the tool.

To calculate the correct spindle speed in revolutions per minute (rpm) use the following formula:

Rpm = 1000 x S

Π x d



Cutting fluids and compounds

These are used to:

• Keep the tool cool as heat can build up quickly.

• Prevent the work piece heating up from friction.

• Allow faster cutting speeds.

• Wash away chips and swarf.

• Give a good finish.

• Work as lubricant between the tool and the work piece making the tool last longer.



Getting ready to start machining

• Isolate the machine from the supply.

• All guards must be present and in good condition.

• Correct workholding device for the job must be fitted.

• No unsecured bar or material inside the headstock spindle.

• No automatic feeds have been left engaged and the lead screw has been disconnected unless needed.

• Correct levels of lubricant and coolant are in the machine, oil sight glasses and coolant tank checked.

• Coolant is not turned on and the nozzle is pointing down towards the machine.

• There is no excessive backlash on any of the machine slides.



Different operations to produce turned parts

• Facing – normally the first turning operation to produce the workplace datum. A right hand cutting tool can be used for this, the tool moving from the outside of the work to the centre. Special facing tools are available that have a larger radius on the cutting edge allowing the tool to take larger cuts.

• Turning – accurate turning of diameters is simple to carry out on a lathe. Try to arrange the smallest diameter on your right pointing towards the tailstock. This allows long cuts to be made using the right handed turning tool. Always turn the largest diameter down to size first if possible.



Drilling on the lathe

When drilling the cutting speed is calculated from the diameter of a drill rather than the diameter of the workpiece, so the bigger the drill the slower it goes.

Material to be cut Cutting speed in metres per minute

Mild steel 25

High carbon steel 10

Cast iron 15

Stainless steel 20

Aluminium 60

Brass 50

Bronze 20



Boring

• This is shaping a hole in the metal by cutting.

• Before boring a hole has to drilled to allow the tool to enter.

• Small boring tools lack strength and rigidity and may chatter – to avoid this take only very light cuts at slow feed rates.

• For a deep hole it is likely a tapered hole will be produced. To prevent this light cuts should be taken and two or more spring cuts made at the finished diameter.



Reamers

• This produces more accurate holes than twist drills with a good internal surface finish.

• They are not able to cut a hole on their own and must only be used to open out a hole to the correct size.

• A hole must be drilled with a twist drill first, 3% smaller than the original hole.

• Reamers are held in the tailstock, usually by a morse taper.

• They are run at a slow speed (80 rpm) and taping grease and straight oils work well with reamers.



Parting and grooving tools

To produce grooves and undercuts, a thin tool is plunged into the work using the cross slide to feed it in gently to the required groove.

• Use a slower speed than calculated (50% reduction will make a big difference)

• Lock the saddle to stop lateral movement.

• Use a slow automatic feed rate if possible, as when feeding by hand the movement is not so smooth and the tool will dig in.

• Check the tool is on centre height – any deviation will put extra strain on the tool.



Form tools

• A turned profile is needed made of short tapers, radiuses and complex forms.

• Form tools usually cut these shapes.

• These tools are ground to the required profile and then gently fed into the workpiece until the required depth or profile is reached.



Knurling

This is not a cutting process but an indenting process. A pattern is indented into the work to provide a grip to the surface. There are two types of knurls:

• Straight knurls put a series of longitudinal lines along the axis of the machine around the circumference of the workpiece.

• Diamond knurls form a diamond pattern on the work.

Set a slow spindle speed at 40 rpm and a fast feed rate of 0.5 mm/rev to 1 mm/rev.



Turning tapers

There are four ways or producing a taper on a centre lathe:

1. Using a form tool with a tapered surface to cut short.

2. Setting the compound slide at the required angle.

3. Using a taper turning attachment.

4. Setting the tailstock out of line with the machine axis.



Dealing with problems

The problems you might encounter include:

• Excessive spindle speeds causing the tool to wear or break.

• Incorrect feed rate giving the wrong surface finish and maybe damaging the tool.

• Tools blunt or incorrectly ground giving a poor finish.

• Damaged or un-calibrated measuring equipment.

• Damage from misused or incorrectly set work holding equipment.

• Lathe unsuitable for the task, worn or damaged.

• Incorrect coolant marking work.

• Failure to check work as it progresses.

• Excessive backlash causing errors.

• Poor or inadequate planning.

Pp lathes

Engineering

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