Machine tools and machining operations
Machine tools and machining operations questions cover the lathe, drilling and milling machines, cutting data, workholding, and the logical sequence of operations for a simple part.
Cutting data
Spindle speed follows n = 1000v/(π·d), with v in m/min and d the diameter in mm. Feed is mm/rev on the lathe and mm/min on a milling machine; for milling, table feed vf = fz × z × n (feed per tooth × number of teeth × rpm). Carbide runs three to five times faster than HSS: C45 steel with carbide about 150–200 m/min, aluminium 300–500, cast iron 100–150. Depth of cut is typically 2–4 mm for roughing and 0.2–0.5 mm for finishing.
Lathe work
Face the end first to establish a reference, then centre-drill before a long drill. A three-jaw chuck suits round or hexagon stock; a four-jaw chuck with a dial indicator is used for eccentric or square work. Support long shafts with the tailstock centre. For threads, set the pitch on the gearbox, grind a 60° tool and take spring passes for the final size.
Milling and drilling
Conventional (up) milling is safe on machines with backlash; climb (down) milling gives a better finish but needs a backlash eliminator. Standard drill point angle is 118°; use coolant, reduce feed as the drill breaks through, and always clamp the work - never hold it by hand. For an H7 hole, drill 0.2–0.3 mm under size and ream at about one third of the drilling speed with double the feed.
Worked example
Turn a Ø60 mm C45 shaft with carbide at v = 180 m/min. n = 1000 × 180/(π × 60) = 955 rpm, so set 950 rpm with a feed of 0.25 mm/rev. From a Ø65 blank to Ø60, allowance is 2.5 mm radially: one roughing pass of 2 mm and a finishing pass of 0.5 mm. Note that d is the diameter, not the radius - using 30 mm would double the speed and burn the tool.
Traps
Using radius for diameter in the speed formula, running HSS speeds with carbide, leaving a long shaft unsupported, drilling without a centre hole, and measuring while the spindle is turning.