huangyhg

datum shift on a restrained gd&t part
datum shift on a restrained part - can you legally use 鈥淐heck in a restrained condition鈥?note with a profile or position callout with mmc (shift)? can you have shift on a restrained part? sounds like an oxymoron or contradiction in terms at first look. is there a way to restrain a part and shift at the same time with fixtures (like clamping on a movable plate) or in ccm?
all parts are checked or confirmed in a "free state" unless otherwise noted as per asme y14.5m-94 .
yes, you legally can check a profile tolerance in a restrained condition but one should note the restrained force on the drawing and where the part is to be restrained which will most likely be on the datum targets.
but
if you have positional with both the tolerance and the datums in mmc, do not use a restrained state since the part should be allowed to float.this condition simulates assembly.
if positional tolerance are in rfs, then restrained state could be used.
dave d.
i don't like my original answer in one area.
i keep thinking about a thin part with positional tolerances at mmc including the datums. the part could have an "oil canning" situation where the centre is not quite flat.
once the part is placed in a checking fixture the person confirming the positional toleraces would push the part flat and the insert the positional checking pins simulating an assembled condition. the company in this situation stated on the drawing that the positional tolerances are to be checked in an "assembled condition" but make sure the customer agrees with this. it this note was not stated, the part is confirmed in a free state.
i feel better about this answer.
hope it helps.
dave d.
we inspect composite parts in the restrained condition, and note such on the part drawing. if we didn't, the part springback would cause many parts to fail, even though they are good parts when assembled.
can you have datum shift on a restrained part? short answer - yes.
if a non-rigid part needs to be clamped into a fixture (i.e. restrained), there can still be clearance on one or more datum features (e.g. the part's holes on the fixture pins). this means that the part could be clamped into the fixture in more than one position. if the optimal position is not found the first time, the operator can unclamp the part, shift it, and reclamp. this accomplishes the datum shift - the part does not have to "freely float" on the fixture.
evan janeshewski
axymetrix quality engineering inc.

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