The 18 Bushes That Were Never Oversize
Your workshop supplies 60 spacer bushes to a pump-assembly unit. The drawing calls for outside diameter 40.00 ±0.05 mm and the lot is inspected 100 per cent. The inspector, working fast before the weekend, rejects 18 bushes and records the readings in groups: 6 at 40.06 mm, 5 at 40.07 mm, 3 at 40.08 mm, 2 at 40.09 mm, 1 at 40.10 mm and 1 at 40.11 mm. Before you sign the rejection note you notice that the micrometer used was checked against a slip gauge only at the start of the shift: with the anvils closed it reads 0.03 mm instead of 0.00 mm, and nobody has re-zeroed it since. The calibration sticker on the instrument expired four months ago. The production supervisor wants all 18 bushes scrapped and the operator's incentive cut. The customer needs 60 good bushes by Friday; you already have 44 accepted bushes and bar stock for 20 more. The operator insists the machine has held size all week.
1. Explain what a reading of 0.03 mm with the anvils closed means, correct the recorded readings, and determine how many of the 18 rejected bushes are actually outside the limits 39.95 to 40.05 mm.
Read the worked answer
The 0.03 mm is a zero error of the instrument, not a size of the part. With the anvils closed on each other the micrometer should read 0.00 mm; it reads +0.03 mm, so every reading is 0.03 mm too large. True size = reading − 0.03 mm.
| Recorded (mm) | Qty | Corrected (mm) | Against 39.95–40.05 | Status |
|---|---|---|---|---|
| 40.06 | 6 | 40.03 | inside | conforming |
| 40.07 | 5 | 40.04 | inside | conforming |
| 40.08 | 3 | 40.05 | exactly on upper limit | conforming |
| 40.09 | 2 | 40.06 | 0.01 over | non-conforming |
| 40.10 | 1 | 40.07 | 0.02 over | non-conforming |
| 40.11 | 1 | 40.08 | 0.03 over | non-conforming |
14 of the 18 rejected bushes are within tolerance and only 4 are oversize. A size sitting exactly on a limit (40.05 mm) is acceptable, because the limits are inclusive. The rejection note is therefore wrong, and the arithmetic also shows the real defect lies in the measuring system rather than in the machining.
2. State the disposition of the 14 wrongly rejected bushes and of the 4 that are genuinely oversize, and what must happen before anything is released to the customer.
Read the worked answer
- The 14 bushes: do not release them on my corrected arithmetic alone. Re-measure all 18 with a calibrated micrometer or on a comparator set with slip gauges, write the actual value of each bush on the inspection sheet, have a second person countersign, and attach a note explaining the zero-error correction. Only then release the 14 to stores with a corrected inspection record.
- The 4 oversize bushes: true sizes 40.06, 40.07 and 40.08 mm, that is 0.01 to 0.03 mm above the upper limit. Because the error is on the plus side and only the outside diameter is controlled, the practical route is to ask the customer or designer whether the OD may be reworked. If the wall thickness and length still meet the drawing after skimming 0.03 to 0.05 mm off the OD to a target near 40.00 mm, rework, re-inspect and accept. If rework is not permitted, mark them scrap or diverted and record the loss. Do not ship them hoping the customer will not measure.
- The balance of the order: do not top up the remaining 16 with freshly turned bushes until the gauge problem is closed, otherwise the same error repeats in the next lot.
- Traceability: record which gauge measured which bushes, so the records can be corrected later if the gauge proves worse than 0.03 mm.
3. Identify the root cause, propose corrective actions, and respond to the supervisor's demand to cut the operator's incentive.
Read the worked answer
Root cause is the measuring system, not the operator: calibration expired four months ago, the zero was checked only once at the start of the shift, and there was no second check or independent verification before a rejection was raised.
Corrective actions:
- Withdraw the micrometer from service, tag it, and send it for calibration; do not use it until the sticker is renewed.
- Introduce a written daily zero check against a slip gauge, done before and after the shift and after any knock or drop, with a signature.
- Review the inspection records made with this instrument since the calibration expiry to decide whether any lot already sent out needs re-verification.
- Require a second inspector to confirm any 100 per cent rejection before the note is signed.
On the incentive: the parts were in size; the gauge was out. Cutting the operator's incentive would punish the wrong person and would also teach the shop to hide gauge problems. Say this in writing, and route the incentive question to the training or HR in-charge with the corrected data attached, rather than settling it on the floor.