Bonus Tolerance at MMC: Rework, Ship, or Scrap?
A lot of 120 anodized aluminum housings is at final inspection. The drawing specifies a Ø10.0–10.1 through hole, position Ø0.2 Ⓜ to A|B|C, and a general note that position may be verified by functional gauging. The hole is a fastener clearance hole, not a sealing or locating feature. The CMM reports hole size Ø10.06 and a true-position deviation of Ø0.206. The source inspector on site wants a disposition in 30 minutes. Rework means reaming the bores and re-anodizing, at roughly $28 per part and three days; scrap is a $9,400 charge and a two-week slip. The functional gauge at MMC is on the bench, calibrated and in date.
1. Compute the acceptance limit for this hole and make the disposition. Show the arithmetic.
Read the worked answer
The part passes.
- Stated tolerance at MMC: Ø0.200
- MMC size of the hole = Ø10.000 (smallest permitted hole)
- Actual size = Ø10.060 → bonus tolerance = 10.060 − 10.000 = 0.060
- Total permitted position deviation = 0.200 + 0.060 = Ø0.260
- Reported deviation = Ø0.206 ≤ 0.260 → accept
Physical cross-check: the virtual condition boundary is 10.000 − 0.200 = Ø9.800. A Ø9.800 gauge pin that simultaneously engages all three datums must pass. Run it, record the result on the inspection sheet, and the disposition is defensible to the customer without a CMM argument.
Reasoning: the Ⓜ modifier is a statement that the hole does more good as a bigger hole; extra size is exchanged for extra location error. Ignoring the bonus is the classic way to scrap good parts.
2. The inspector says "0.206 is bigger than 0.2, so it fails - the modifier is just a drawing formality." How do you answer?
Read the worked answer
The modifier changes the acceptance boundary; it is not a formality.
Argue it three ways, in increasing strength:
- Standards basis: under ASME Y14.5, a position tolerance with Ⓜ defines a boundary (virtual condition) that the feature's axis must stay inside, and the zone grows as the feature departs from MMC. The drawing's own note authorizes functional gauging, which is the conformance test that implements that boundary.
- Physical basis: a fastener clearance hole exists to pass a fastener. A larger hole can be farther off-center and still pass the same fastener. The bonus is the drawing's way of saying that.
- Practical basis: the gauge is calibrated, in date, and built to the virtual condition. If the pin goes, the part conforms by the method the drawing specifies.
Caution before you celebrate: verify the CMM report is a diametrical deviation (2 × the radial distance), not a radius, and confirm the datum reference frame and the size measurement (a two-point micrometer on a three-lobe hole under-reads; use the CMM or a bore gauge). A sign or axis convention error here is worth 2×.
3. A second part measures Ø10.100 with a deviation of Ø0.305. What is the disposition and what are your options?
Read the worked answer
It fails, and reaming will not save it.
- Size is at LMC (Ø10.100), so bonus = 0.100 and the limit is 0.200 + 0.100 = Ø0.300.
- Deviation Ø0.305 > Ø0.300 → nonconforming by 0.005.
Why rework does not help: reaming a hole follows the existing axis, so the location error stays, and the size is already at LMC - there is no further bonus to earn.
Order of operations before you scrap anything:
- Re-measure. A 0.005 mm exceedance on a Ø0.300 zone is at or below typical CMM task uncertainty. Repeat the measurement, verify probe calibration and the datum setup, and consider a second method.
- If the fail is confirmed, ask whether the drawing is truly functional here (fastener clearance, M8 fastener with 1 mm radial clearance still assembles). That becomes a customer deviation/waiver request, not an internal decision.
- Ship the conforming parts now; quarantine the nonconforming ones and keep them intact until the customer answers - reworking early destroys your evidence.
Rule: when the margin is smaller than the measurement uncertainty, the correct action is measurement, not disposition.