Composite Position vs. Two Single-Segment Position for a Clearance Hole Pattern
A cover plate has four Ø6.6-6.8 clearance holes for M6 bolts, with a pattern located by |⌖|Ø0.8(M)|A|B|C| and a second segment |⌖|Ø0.3(M)|A| applied as a composite feature control frame (single frame, two segments). Datum A is the plate face; B and C locate the pattern. The mating part has four M6 tapped holes with a pattern position of Ø0.2 at MMC. The plate's holes are produced at Ø6.75, and the pattern's hole-to-hole spacing varies such that the pattern as a whole is offset 0.25 mm from true position, while individual hole-to-hole locations deviate up to 0.28 mm from each other. A functional gage built to the composite requirement rejects the part on the lower segment.
1. Explain what the upper and lower segments of the composite position callout control, and evaluate whether the gage rejection is consistent with the callout.
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In a composite position frame:
- Upper segment (|⌖|Ø0.8(M)|A|B|C|) controls the location of the pattern as a group relative to the datum reference frame, plus orientation and hole-to-hole spacing. All four holes must lie within Ø0.8 zones located per A|B|C.
- Lower segment (|⌖|Ø0.3(M)|A|) controls hole-to-hole (pattern-internal) location and orientation only, relative to datum A alone. It does not control pattern location relative to B and C; the pattern may translate/rotate as a group. The Ø0.3 zones are oriented perpendicular to A and located only relative to each other.
Evaluation: pattern offset of 0.25 mm is within the upper Ø0.8 zone (and bonus from Ø6.75 holes adds 0.15, giving 0.95). Hole-to-hole deviation of 0.28 mm must be checked against the lower Ø0.3 zone plus bonus (0.3 + 0.15 = 0.45). A 0.28 mm internal deviation is within 0.45 mm, so the lower segment should pass on these numbers - the gage rejection on the lower segment is inconsistent unless the gage is built to the wrong MMB or the deviation data are incomplete (e.g., the 0.28 is in a direction that, combined with orientation, exceeds the zone).
2. The gage was built with fixed pins at Ø6.6 (MMB) for the upper segment and separate fixed pins for the lower segment. Explain why a single functional gage cannot simultaneously verify both segments, and what inspection approach is correct.
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A functional gage verifies the upper segment by simulating the datum reference frame A|B|C and the virtual condition boundary of each hole (MMC Ø6.6 minus the Ø0.8 position tolerance = Ø5.8 virtual condition pin, or equivalently pins at the virtual condition). This checks group location and spacing simultaneously.
The lower segment, however, references only datum A and controls pattern-internal location. A gage for it would need to allow the pattern to float relative to B and C - a different fixturing concept. A single gage cannot enforce both because the upper segment constrains the pattern to B and C while the lower segment deliberately frees it. Trying to do both with one set of fixed pins over-constrains the part.
Correct approach: either (a) use two separate functional gages - one per segment - or (b) verify with a CMM: check the upper segment by best-fitting/aligning to A|B|C and confirming each axis lies in its Ø0.95 zone; check the lower segment by aligning only to A and evaluating hole-to-hole relationships within Ø0.45 zones. The gage result alone is not sufficient evidence of compliance with both segments.