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- 1. Chapter 6 Preparing and using lathes for turning operations © Pearson Education 2012 Printing and photocopying permitted Preparing and using lathes for turning operations
- 2. Chapter 6 Preparing and using lathes for turning operations © Pearson Education 2012 Printing and photocopying permitted The lathe
- 3. Chapter 6 Preparing and using lathes for turning operations © Pearson Education 2012 Printing and photocopying permitted Workholding devices: Four jaw chuck A versatile workholding device. Each jaw moves independently and can be reversed. 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 used on a machined surface. Which is the most accurate?
- 4. Chapter 6 Preparing and using lathes for turning operations © Pearson Education 2012 Printing and photocopying permitted 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.
- 5. Chapter 6 Preparing and using lathes for turning operations © Pearson Education 2012 Printing and photocopying permitted 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.
- 6. Chapter 6 Preparing and using lathes for turning operations © Pearson Education 2012 Printing and photocopying permitted 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.
- 7. Chapter 6 Preparing and using lathes for turning operations © Pearson Education 2012 Printing and photocopying permitted Setting and adjusting machine tool speeds Tool manufactures recommend cutting speeds that give the best finish and life 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
- 8. Chapter 6 Preparing and using lathes for turning operations © Pearson Education 2012 Printing and photocopying permitted 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.
- 9. Chapter 6 Preparing and using lathes for turning operations © Pearson Education 2012 Printing and photocopying permitted 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.
- 10. Chapter 6 Preparing and using lathes for turning operations © Pearson Education 2012 Printing and photocopying permitted 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.
- 11. Chapter 6 Preparing and using lathes for turning operations © Pearson Education 2012 Printing and photocopying permitted 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
- 12. Chapter 6 Preparing and using lathes for turning operations © Pearson Education 2012 Printing and photocopying permitted 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.
- 13. Chapter 6 Preparing and using lathes for turning operations © Pearson Education 2012 Printing and photocopying permitted 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.
- 14. Chapter 6 Preparing and using lathes for turning operations © Pearson Education 2012 Printing and photocopying permitted 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.
- 15. Chapter 6 Preparing and using lathes for turning operations © Pearson Education 2012 Printing and photocopying permitted 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.
- 16. Chapter 6 Preparing and using lathes for turning operations © Pearson Education 2012 Printing and photocopying permitted 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.
- 17. Chapter 6 Preparing and using lathes for turning operations © Pearson Education 2012 Printing and photocopying permitted 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.
- 18. Chapter 6 Preparing and using lathes for turning operations © Pearson Education 2012 Printing and photocopying permitted 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.
Hinweis der Redaktion
- Speaker notes The elements of the lathe are described below Headstock – This contains the gears or pulleys that allow the operator to change the speed of the spindle. Spindle – This is the rotating shaft that runs through the headstock and is often hollow to allow bar to be feed through the headstock. The workholding devices are held on to the spindle. Lathe bed and slideway – This is the backbone of the machine that the saddle runs on, it is parallel to the spindle axis. It can engage with the lead screw to cut threads and often has a power feed as well. Saddle – Supports the crosslide and allows the cutting tool to move along the spindle axis. Cross slide – This is mounted on the saddle and moves 90 to the spindle axis, it is used for facing off or for changing the depth of cut. This often has a power feed. Compound slide – Fitted to the top slide and supports the tool post. It can also be swivelled to different angles to cut tapers and chamfers. Toolpost – Holds the cutting tool at the correct height firmly to allow machining to take place. Tailstock – This runs on the lathe bed slideway and is inline with the spindle axis, it has a barrel that can be moved in and out by the handwheel. It can be used to hold drills, drill chucks, dead centres and live centres. These are held in a tapered hole bored down the centre of the barrel called a Morse taper. Leadscrew – This is an accurate screw thread that is geared to the spindle and allows threads to be cut with a single point tool.
- Speaker notes Before setting up work isolate the lathe from the power supply. The last method is the most accurate.
- Speaker notes Information about the cutting tools that can be used on lathes can be found on page 195 of the Candidate Handbook.
- Speaker notes Ask learners to investigate the different cutting speeds for different materials and tool types.
- Speaker notes Remind learners that every time a workpiece is removed from the workholding device, accuracy is lost, even if you painstakingly clock the workpiece up again using a dial test indicator. A skilled operator should be able to machine to an accuracy of 0.02 mm.
- Speaker notes Remind learners that as drills are harder to keep cool than external cutting tools, the cutting speeds are slightly slower.
- Speaker notes For efficient boring: Use slow feed rates (0.03 mm/rev). Use only light cuts (0.3 mm to 0.5 mm at a time). Remember to turn the cross slide handle anti-clockwise to remove metal. If the tools does chatter, try sticking a small weight on the tool near the tip. Use a Vernier height gauge or scribing block to transfer the height from the centre to the tool tip.
- Speaker notes Learners should make sure that they never use hand reamers under power.
- Speaker notes Remind learners that the tool has hardly any lateral strength and cuts along the axis of the machine should not be attempted.
- Speaker notes Remind learners form tools are not easy tools to use, as there is often a wide cutting edge in contact with the workpiece that can cause the tool to be overloaded. Low lathe speeds should be selected when using form tools to prevent heat build up and the tool snatching.
- Speaker notes Remind learners to make sure the knurling tool is dead square and on centre height and the workpiece securely held.
- Speaker notes Remind learners about checking work and carrying out quality checks – information about the tools that can be used for this can be found in the Chapter 4 Powerpoint.