Machine Tools and Machining Operations
Machine Tools and Machining Operations
The exam tests lathe and drilling operations, cutting-speed calculations, milling types and cutting-tool materials.
Lathe
Parts: bed, headstock, tailstock, carriage (saddle, cross-slide, compound rest), tool post, lead screw, chuck. Operations: turning, facing, chamfering, knurling, thread cutting (lead screw with half-nut), drilling and reaming from the tailstock, boring, and taper turning by swivelling the compound rest, set-over of the tailstock, or a taper-turning attachment. Cutting speed V = πDN/1000 (m/min, D in mm), so N = 1000V/(πD). Feed is given in mm/rev, depth of cut in mm. Milling: in up (conventional) milling the cutter rotates against the feed, which suits rough castings; in down (climb) milling it rotates with the feed, giving a better finish but requiring a backlash eliminator. Shaping reciprocates the tool while the work feeds; grinding is an abrasive finishing process. Tool materials: high-carbon steel, HSS (usable to about 600 °C), carbide, ceramic, diamond.
Worked example
Turn a 60 mm diameter mild-steel bar at V = 30 m/min. N = 1000 × 30 / (π × 60) = 30 000 / 188.5 = 159 rpm. With a feed of 0.2 mm/rev the tool advances 159 × 0.2 = 31.8 mm/min along the bar.
Traps
- Omitting the 1000 in N = 1000V/(πD), or entering D in metres.
- Quoting feed as mm/min when it is mm/rev.
- Swapping climb and conventional milling.
- Calling knurling a cutting operation - it is a forming operation.
- Using HSS at carbide speeds; carbide is harder but more brittle.