Machining and Machine Tools: Lathe and Cutting Calculations
Machining and machine tools tests the parts and operations of the lathe, drilling machine, shaper and milling machine, and simple cutting calculations. Cutting speed is surface speed, so rpm must be calculated, not guessed.
Lathe
Bed, headstock with chuck and spindle, tailstock for support or drilling, carriage with saddle, cross slide, compound rest and tool post, apron with feed and half-nut levers, lead screw for threads and feed rod for ordinary feeds. Operations: turning, facing, chamfering, parting off, knurling, boring, taper turning (compound rest, tailstock set-over, taper turning attachment) and thread cutting with the half-nut engaged on the lead screw.
Drilling machine
Drill geometry: point angle 118° for general steel, helix angle for chip removal, lip clearance. Operations: drilling, reaming, counterboring, countersinking, spot facing, tapping. Hold work in a vice or clamp; never hold small work by hand.
Cutting speed formula
V = pi × D × N / 1000 m/min, so N = 1000V / (pi × D) rpm.
Worked example
Turn a 50 mm mild-steel shaft with a high-speed steel tool at 30 m/min. N = 1000 × 30 / (3.14 × 50) ≈ 191 rpm. At 600 rpm the surface speed would be about 94 m/min, far too high for HSS, and the edge would burn. Bigger diameter means lower rpm for the same surface speed. Feed is in mm per revolution; depth of cut is half the diameter reduction.
Traps
Treating rpm as cutting speed. Using the lead screw for ordinary turning feeds. Forgetting to disengage the half-nut before reversing. Knurling is a forming operation, not cutting. Reaming a rough hole without checking alignment.