Bonus Tolerance Interaction Between Position at MMC and a Datum Feature at MMC
A bracket has a Ø10.0-10.2 hole controlled by |⌖|Ø0.3(M)|A|B(M)|, where B is a Ø30.0-30.2 boss that serves as the secondary datum feature. The produced hole measures Ø10.18 and the produced boss measures Ø30.06. The inspector computes allowable position tolerance as 0.3 + (10.18 − 10.0) = 0.48 mm and accepts the part. A second inspector recomputes and gets a different number, claiming the boss size also contributes bonus tolerance. The measured hole axis deviates 0.44 mm from true position relative to the datum reference frame established with the boss at its actual size. The part is a safety-critical bracket and the drawing is silent on any note about datum feature shift.
1. Determine the correct total allowable position tolerance, accounting for both the feature-of-size bonus and any datum feature shift.
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Two separate allowances apply:
- Bonus tolerance from the toleranced feature (hole): MMC of the hole is Ø10.0, actual is Ø10.18, so bonus = 0.18 mm. Base tolerance 0.3 → 0.48 mm.
- Datum feature shift from B(M): MMC of the boss is Ø30.0, actual is Ø30.06, so datum shift = 0.06 mm. This permits the part to shift relative to the datum axis by up to 0.06 mm (radially, in the direction that helps).
These are additive in effect but distinct in nature. The available position tolerance is 0.48 mm, and additionally the part may shift up to 0.06 mm relative to the true datum axis B. The measured deviation of 0.44 mm is within the 0.48 mm zone, so the part passes even before considering shift; with shift available, the margin is larger still.
2. The second inspector claims the answer should be 0.54 mm as a single number. Is that a valid way to combine the two allowances? Explain.
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It is not strictly correct to add them into one number. Bonus tolerance enlarges the position tolerance zone (a Ø0.48 cylinder about true position). Datum feature shift does not enlarge the zone; it allows the entire part (and thus the datum reference frame) to translate relative to the true datum axis by up to the shift amount. The two effects are geometrically different: bonus is a zone-size increase, shift is a frame translation.
The practical consequence: you cannot simply say the axis may deviate 0.54 mm from true position, because shift is directional and its benefit depends on the direction of the deviation. The rigorous evaluation is to check whether the measured axis lies within a Ø0.48 zone when the DRF is permitted to translate up to 0.06 mm. In many cases the combined effect permits up to 0.48 + 0.06 = 0.54 mm of apparent deviation in the favorable direction, which is why the shortcut often gives the same accept/reject answer - but the reasoning must be correct, and in unfavorable directions shift does not help.
3. If the drawing instead used B (no MMB modifier) as the secondary datum, how would the analysis change?
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Without the (M) modifier, datum B is established at its actual mating envelope (AME) regardless of size, and no datum feature shift is allowed. The part must be oriented/located to the AME of the boss exactly. Only the 0.18 mm bonus from the hole applies, giving a 0.48 mm zone. The 0.06 mm boss-size allowance disappears.
This matters for inspection fixturing: with B(M), a functional gage with a fixed pin at Ø30.0 (MMB) is acceptable and the part may shift on the pin; with B (RFS), the gage must simulate the actual mating envelope, typically requiring a variable/expanding mandrel or CMM evaluation. The choice of modifier is a design decision reflecting whether looseness at the datum interface is functionally tolerable.