Micrometer zero error on a batch of locating pins
During a batch of 60 turned locating pins, nominal Ø12.00 mm with a tolerance of ±0.04 mm, a learner uses a 0–25 mm outside micrometer. With the jaws closed, the micrometer reads 0.03 mm. The learner records readings from 11.98 mm to 12.08 mm and asks whether the batch can be passed. The pins are at 28 °C and the inspection room is at 20 °C. The micrometer was dropped once, and the thimble now moves stiffly. The learner measured each pin once, over the same spot, and used the thimble to force the spindle against the work. A go/no-go gauge is available but has not been checked. The customer requires 100% inspection and a record of actual sizes.
1. Correct the readings for zero error and decide which pins conform. Show your calculation clearly.
Read the worked answer
With the jaws closed the micrometer reads +0.03 mm. That means it over-registers by 0.03 mm, so every reading is 0.03 mm too large and the true size is the reading minus 0.03 mm. The limits are 12.00 ± 0.04 mm, i.e. 11.96 mm to 12.04 mm. A recorded reading of 11.98 mm gives 11.95 mm, which is below 11.96 mm and must be rejected. A reading of 11.99 mm gives 11.96 mm, which is on the lower limit and acceptable. At the top end, 12.07 mm gives 12.04 mm and is acceptable, while 12.08 mm gives 12.05 mm and is oversize. Therefore the batch cannot be passed as it stands: the recorded spread produces true sizes from 11.95 mm to 12.05 mm, so some pins are outside both limits. The pins should be sorted, not accepted on the raw readings.
2. Explain the sources of error in this inspection and what you would check before accepting any result.
Read the worked answer
The first error is the zero error, which must be corrected or the micrometer reset. The dropped micrometer and stiff thimble make the instrument suspect: it may be bent, have a damaged thread or give different readings at different openings, so it should be checked at several sizes with gauge blocks or a setting ring and sent for calibration if necessary. The learner used the thimble to force the spindle, which can compress the work and the instrument; the ratchet or friction stop should be used. One reading at one position does not detect taper or ovality, so measure at three positions around and along each pin. Temperature has a small effect: 12 mm steel at 28 °C compared with 20 °C expands by about 0.001 mm, which is negligible here, but the pins should be allowed to stabilise. The unchecked go/no-go gauge could itself be worn, so it must be proved against a known setting ring before use.
3. Recommend a practical inspection method and records for the remaining pins.
Read the worked answer
Quarantine the batch and inspect 100% as the customer requires. First prove the micrometer against a setting ring or gauge blocks and record the zero check. Measure each pin with the ratchet stop at three positions, record the actual corrected size to 0.01 mm, and mark accept, rework or scrap. Use the go/no-go gauge only after proving it on a known setting ring; a go/no-go gauge is fast for production but does not give an actual size, so it supplements rather than replaces the recorded micrometer readings. Record the instrument identification, zero error, temperature, inspector and date. Any pin outside 11.96–12.04 mm should be segregated. If the process is stable, ask the customer whether sampling could replace 100% inspection, but do not change the agreed method without approval.