Taper, Chatter and Rising Rejects on a Centre Lathe
An operator is turning 200 plain-carbon shaft blanks (0.35 % C) from Ø45 mm to Ø40 mm over a 300 mm length between centres on a centre lathe, using a coated carbide insert, 600 rpm, 0.2 mm/rev feed and 2.5 mm depth of cut per pass. The insert was indexed 18 pieces ago; supplier data suggests about 40 pieces per edge at these conditions. After 12 pieces the operator reports chatter marks concentrated at mid-length, a dull screech and surface finish that has fallen from Ra 1.6 µm to about Ra 4.0 µm. A micrometer check at three points shows the diameter is 0.05 mm larger at the tailstock end than at the headstock end, over the full 300 mm. The coolant is a milky 4 % emulsion with a rancid smell and a visible tramp-oil layer. Three of the last 12 shafts are rejected on size and finish. The foreman blames the machine; the operator blames the insert. A delivery of 150 shafts is due in four days.
1. Calculate the cutting speed and the machining time per pass, and judge whether the programmed values are a credible cause of the trouble.
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Cutting speed. v = πDN/1000. At the start of the pass, D = 45 mm, N = 600 rpm: v = π × 45 × 600 / 1000 = 84.8 m/min. At the finishing diameter of 40 mm it falls to 75.4 m/min.
Machining time. T = L / (f × N), with L = 300 mm plus about 5 mm approach = 305 mm: T = 305 / (0.2 × 600) = 2.54 min per pass.
Judgement. For coated carbide in 0.35 % C steel, 80–110 m/min is a normal to conservative window; the speed is not excessive and cannot by itself produce chatter. A feed of 0.2 mm/rev is also normal for roughing. The depth of cut of 2.5 mm is the aggressive element - it is a roughing value, and using it on the finishing pass would explain both the finish and the accelerated insert wear (18 pieces against an expected 40). The evidence therefore does not support a simple 'machine is bad' or 'insert is bad' verdict for the speed and feed: the parameters are sane for roughing and wrong for finishing.
2. Diagnose the taper and the mid-length chatter, and state the corrective adjustment with numbers.
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Taper. The diameter is larger at the tailstock end, meaning the tool is further from the work axis there. With work held between centres the usual cause is tailstock set-over, not bed wear: bed wear near the headstock would make the diameter larger at the headstock end, the opposite of what is measured. Correction: shift the tailstock towards the operator by half the measured taper, 0.05 / 2 = 0.025 mm, then re-cut one test piece and measure at both ends. A dial indicator on a test bar in the spindle taper, traversed along the tailstock centre, confirms the direction before cutting metal.
Chatter. The marks are worst at mid-length, which is the point of maximum workpiece deflection - a 300 mm length at Ø40 gives an L/D near 7.5, and 2.5 mm depth of cut is enough to excite that. Also verify the tailstock centre: a dry or brinelled centre hole, or a worn live centre, produces exactly this pattern.
Actions: reduce depth of cut to 1.0–1.5 mm for the finishing pass, fit a travelling steady or reduce the unsupported length, re-centre the work, check the centre hole for scoring and lubricate it, and change the insert before it reaches the wear land that generates the screech.
3. The coolant is rancid at 4 % concentration. What is the maintenance significance, and what would you change about tool life management?
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A 4 % emulsion is below the usual 6–9 % working range for turning, so cooling and lubricity are both poor, which accelerates flank wear and worsens finish - part of the 18 versus 40 piece tool life gap. The rancid smell and the tramp-oil layer indicate bacterial contamination and a sump that is not being maintained; this is a skin-sensitisation and odour hazard as well as a performance problem, so it belongs in the safety/housekeeping scope, not only in the coolant scope.
Immediate action: dump the sump, clean it mechanically, charge fresh emulsion at 6–8 % with good-quality water, fit or restore a tramp-oil skimmer and an aeration or circulation arrangement, and start weekly checks of concentration with a refractometer and pH with strips.
Tool life management: keep a simple per-edge log of pieces, finish and power, and index on a fixed interval (for example 30 pieces) rather than on noise. If the finishing pass ran at 2.5 mm depth, the insert was doing roughing duty and the 40-piece figure never applied to it; separating roughing and finishing inserts and recording them separately would have shown that before three shafts were rejected.