Oversize Dowel Pins: Instrument Error or Real Rejects?
A trainee inspector in a press-tool shop is checking 50 hardened EN8 dowel pins, drawing size Ø12 h6 (0 / −0.011 mm), with a 0–25 mm outside micrometer. The micrometer was zeroed against its setting standard in the air-conditioned stores at 20 °C, then carried to the shop floor beside a heat-treatment furnace where the ambient is 34 °C. After ten minutes the readings cluster between 12.006 and 12.009 mm; six pins are logged as oversize and the supervisor wants them scrapped. A batch measured at the same bench in the morning passed. The inspector notices a faint bright ring on both anvils and that the friction thimble slips under light pressure. The calibration sticker is eight months old against a twelve-month interval. Nothing else has changed: same bar stock, same grinder, same operator. Take the coefficient of linear expansion of steel as 11.5 × 10⁻⁶ per °C. Production needs 40 good pins for an assembly due tomorrow.
1. Quantify the thermal contribution and decide whether temperature alone explains the rejections.
Read the worked answer
Compute the thermal effect. ΔL = α·L·ΔT = 11.5×10⁻⁶ × 12 mm × (34 − 20) = 0.0019 mm, i.e. about 2 µm. Because the micrometer frame and the setting standard also warm up, only the differential expansion of the pin against the standard matters in first order, so the figure stays near 2 µm; a worst case where the micrometer is re-zeroed on a cold standard at 34 °C adds roughly another 2 µm.
Compare with the tolerance. Total tolerance is 11 µm and the upper limit is 12.000 mm. The readings are 6–9 µm above that limit, so the observed error budget is 8–11 µm. Temperature can explain at most about 4 µm, leaving 4–7 µm unexplained.
Verdict. The rejections cannot be blamed on shop temperature. The residual is of the same order as the defects visible on the instrument (anvil ring marking, slipping thimble), so the measurement is not trustworthy and the batch must not be scrapped on these readings. The morning batch passing on the same bench is consistent with a gauge that has developed a zero error rather than with parts that suddenly changed.
2. List the checks you would carry out on the micrometer and on the parts before any disposition, and state the accept/reject criterion you would use for the gauge.
Read the worked answer
On the gauge, in this order:
- Clean anvils and re-zero on a gauge block of 10 mm soaked at shop temperature for 30 minutes, not on the 20 °C setting standard.
- Check anvil flatness and parallelism by wringing a gauge block and looking for a light gap or by checking the ring mark with an optical flat; a visible wear ring means the measuring faces are no longer flat and the instrument is out of service.
- Check thimble friction and spindle play; a slipping friction thimble gives variable measuring force and therefore scatter.
- Repeat-measure one pin ten times to get repeatability, and measure a known gauge block five times to get the zero error.
- Compare the six suspect pins with a second, in-calibration micrometer or with a comparator set on gauge blocks.
Criterion. If repeatability plus zero error exceeds roughly 20 % of the 11 µm tolerance (about 2 µm), the gauge is unfit for this feature under any rule-of-thumb gauge acceptance policy; the feature needs an instrument with resolution and uncertainty well inside one-tenth of tolerance.
On the parts: let them soak on a steel plate at shop temperature, measure at three positions and two directions to rule out ovality, and record temperature on the inspection sheet.
3. Recommend the disposition of the six pins and the corrective actions for the inspection system.
Read the worked answer
Disposition. Quarantine, do not scrap. If a second instrument confirms 6–9 µm oversize, the decision depends on the joint: a dowel pin used as a location pin in a hardened plate has little interference margin, and 9 µm extra interference can crack a plate or distort a bore. Re-grind to size if there is grinding stock (typically 0.05–0.10 mm on diameter); if there is none, downgrade to a non-critical use or hold as spares. Scrapping 6 pins on a suspect gauge is the one option that is certainly wrong, and the cost of re-inspection is minutes against the cost of 40 pins and a missed assembly date.
Corrective actions:
- Withdraw the micrometer from service, tag it, and send it for calibration and anvil lapping or replacement.
- Move inspection of h6 features off the shop floor next to the furnace; if it must stay, allow a 30-minute soak and record ambient temperature.
- Change the zeroing procedure to use a gauge block at shop temperature, and re-zero at the start of each shift.
- Shorten the calibration interval for this instrument or add a monthly in-house check against a reference block.
- Enter the event in the calibration record so the trend of zero drift can be seen.