Micrometer Readings That Do Not Agree
A batch of 25 turned pins is specified as 12.000 ± 0.010 mm. A machinist measures five pins with a 0–25 mm outside micrometer and records 11.996, 11.998, 11.994, 11.997 and 11.995 mm - all within tolerance. A second inspector uses a different micrometer and gets readings about 0.012 mm larger on the same pins, putting two of them out of tolerance. Both micrometers were zeroed that morning. The workshop is warm and the pins came straight from the lathe. The first micrometer was checked against a 10 mm setting rod and read 10.003 mm. The second micrometer's anvil was wiped but its spindle was not.
1. Identify the most probable sources of the disagreement and state which instrument is more trustworthy.
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Three factors explain most of the 0.012 mm gap:
- Zero error. The first micrometer reads 0.003 mm high on a 10 mm rod, so every reading from it is about 0.003 mm large; its true values are nearer 11.993–11.995 mm. A zero check on a rod only verifies one point, but it is evidence of a small systematic bias.
- Cleanliness. The second micrometer's spindle was not wiped. A film of oil or a fine chip on the measuring faces adds directly to the reading, easily 0.005–0.010 mm, and it is one-sided - it only makes parts measure larger.
- Temperature. Pins straight from the lathe are warm. Steel expands about 0.0000115 per °C, so a 12 mm pin at 10 °C above the micrometer reads roughly 0.0014 mm large - small, but it adds in the same direction.
The first micrometer is more trustworthy because its bias is known and can be subtracted, and its faces were clean. The second inspector's readings are inflated by an unknown, one-sided contamination error and cannot be corrected reliably.
2. Describe the verification steps and the decision on the batch.
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Steps:
- Wipe both measuring faces of each micrometer with a clean lint-free cloth and check for burrs or nicks on the anvils.
- Close each micrometer on its own zero and record the error; repeat with the 10 mm rod to confirm the error is consistent across the range, not just at zero.
- Let the pins cool to room temperature on a surface plate, then re-measure. Never measure hot work for acceptance.
- Measure each pin at three positions 120° apart and at two axial locations to catch ovality and taper, and record the maximum and minimum.
- Use a third instrument or a comparator setting to arbitrate any remaining disagreement.
Decision: the batch cannot be accepted or rejected on the disputed readings. Once the faces are clean and the pins are at room temperature, re-measure with the corrected first micrometer. If the corrected values sit inside 11.990–12.010 mm with margin, accept; if any pin falls outside, quarantine it. The 0.012 mm discrepancy is larger than the tolerance band's practical margin, so the batch must be held until the clean, temperature-corrected readings are recorded.