Sixty pins in one shift: speeds, stock and a drifting diameter
A multi-skill assistant in a small workshop must turn 60 mild-steel pins from 25 mm bar to Ø20 mm over a machined length of 120 mm, with a 2 x 45° chamfer at one end. The lathe offers spindle speeds of 180, 280, 380, 560, 800 and 1,200 rpm and has a 2.2 kW motor. The foreman specifies a cutting speed of 30 m/min for the available HSS tool, a feed of 0.2 mm/rev and a maximum depth of cut of 2.5 mm per pass. Bar stock comes in 3 m lengths, and each pin needs 125 mm of stock to cover facing and chucking allowance. The shop works one 8-hour shift. Setup and first-piece inspection take 25 minutes; loading, unloading and measuring each pin takes about 0.5 minute; the assistant is also expected to spend 45 minutes on daily machine cleaning and lubrication. At the twentieth pin the micrometer reads 20.06 mm against a drawing limit of 20 ± 0.05 mm, and twelve pins have already been accepted.
1. Select the spindle speed, decide the number of passes, and calculate the machining time per pin. Show why the next higher speed is not acceptable.
Read the worked answer
Speed N = 1000v / (πD). Roughing at Ø25: N = 1000 x 30 / (3.142 x 25) = 382 rpm, so select the available step 380 rpm. The next step, 560 rpm, gives v = π x 25 x 560 / 1000 = 44 m/min, about 47% above the HSS limit - the tool would lose its edge in a few pieces and the finish would suffer.
Passes Radial stock = (25 - 20) / 2 = 2.5 mm, exactly the maximum per pass. Taking it in one pass is technically allowed but leaves no finishing cut and risks chatter and taper on a small lathe, so split it: 2.0 mm rough to Ø21 and 0.5 mm finish. For the finish pass at Ø21, N = 455 rpm, so 380 rpm is still the correct step.
Time Time per pass = L / (f x N) = 120 / (0.2 x 380) = 1.58 min. Two passes = 3.16 min; add 0.5 min handling/measuring = 3.66 min per pin (the chamfer is done inside the handling allowance by using the same setting). Total = 60 x 3.66 = about 220 min of floor time at the machine.
2. Can the 60 pins be completed in the single 8-hour shift? Include the bar stock requirement and state the assumptions you rely on.
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Time available 480 min shift - 25 min setup and first-piece inspection - 45 min cleaning and lubrication = 410 productive minutes. Machining with handling needs about 220 min, so there is roughly 190 min (46%) of slack. The batch fits comfortably, provided cleaning is done at the end of the shift rather than in the middle, and provided no tool breakage or rework occurs. A first-piece check after setup and an in-process check every tenth piece should be scheduled inside the 220 min.
Bar stock
- Total length = 60 x 125 = 7500 mm.
- Pieces per 3 m bar = 3000 / 125 = 24 (with no remnant usable for another pin).
- Bars needed = 60 / 24 = 2.5, so 3 bars (capacity 72 pieces, about 1.5 m left over as offcuts).
Assumptions One operator on one lathe, single-shift working, no power interruption, HSS tool holding its edge for the whole run at 30 m/min, and the 0.5 min allowance genuinely covering load, unload, measure and deburr. If any of these fails - and the tool life is the weakest one - the slack shrinks quickly, which is why the drift in the next question matters.
3. At the twentieth pin the micrometer reads 20.06 mm and twelve pins have already been accepted. What is happening, what do you do with the batch, and what do you correct before restarting?
Read the worked answer
What 20.06 means The limits are 20.00 ± 0.05, i.e. 19.95 to 20.05 mm. 20.06 is 0.01 mm over the top limit - the part is out of tolerance, not 'close enough'.
Diagnose before touching anything Measure the last 5-10 pins at both ends. If the whole diameter has grown progressively, the cause is tool wear or thermal growth of the work - typical on a 60-piece HSS run. If one end is 20.06 and the other near 20.00, the cause is taper from tailstock alignment or bed wear, and changing the tool will not fix it. Write the readings down; they decide the corrective action.
The batch Stop the machine. Segregate the drift pieces plus the last accepted ones (say pins 10-20) as suspect, tag them, and tell the foreman. Do not polish them down with emery cloth and pass them - that destroys size control and roundness. Re-measure the suspect pins properly and let the supervisor decide rework or scrap; never adjust the record to match the drawing.
Before restarting Change or regrind the tool, take a trial cut, set the diameter to mid-tolerance (about 20.00, not 20.05), confirm with two measurements at 90°, check taper again, and only then continue - with a check every tenth piece instead of only the first.