Lathe and Basic Machining Operations
Questions cover lathe parts and their functions, cutting parameters, taper calculation and thread cutting practice.
Main parts
Bed, headstock with spindle and chuck, tailstock for centres and drilling, carriage made up of saddle, cross slide and compound rest, tool post, apron with half-nut lever and feed selection, lead screw for threads and feed rod for turning, plus steady and follower rests for slender work. Specification terms include swing over bed, swing over cross slide, distance between centres and spindle bore.
Operations and parameters
Turning reduces diameter with longitudinal feed; facing squares the end using cross feed; also chamfering, grooving, parting off, knurling, drilling from the tailstock, boring, taper turning and thread cutting. Cutting speed V = pi x D x N / 1000. Depth of cut = (D - d)/2. Parting off uses low speed, coolant and a tool set exactly on centre.
Taper turning
tan(alpha) = (D - d) divided by 2L, where alpha is the half taper angle set on the compound rest; the included angle is twice this. Methods: compound rest swivel, tailstock offset for small angles, taper turning attachment, and form tool.
Thread cutting
Select the gear ratio between spindle and lead screw, engage the half nut on the same thread dial mark for every pass, feed with the compound rest set at about 29-30 degrees for metric threads, and take several light passes.
Worked example
A taper from 40 mm to 30 mm over 100 mm: tan(alpha) = (40 - 30)/(2 x 100) = 0.05, so alpha is about 2.86 degrees, roughly 2 degrees 52 minutes on the compound rest, and the included angle is 5.72 degrees. Reasoning: the tool travels along one side, so the formula uses the radius difference, not the diameter difference.
Traps that cost marks
- Setting the compound rest to the included angle instead of the half angle.
- Using (D - d)/L and getting double the correct angle.
- Measuring while the job rotates.
- Leaving the chuck key in place or turning slender work without a steady.