Composite Position Question

[[Er...]]

I have to put together a CMM program using different software. I'm trying to wrap my brain around the .25mm position requirement and it's struggling to compute. IS the B-C Datum meaning the symmetry of the 2 datums, and I would just need to hold it within .25mm position of said center point? And with B-C the secondary datum, would I still need to clock the holes to the datums, or do the holes get to float around as long as it's within?

[[Jo...]]

1. That is multiple single segment.
2. I would interpret that B-C is a "pattern" , and it CAN constrain 6 DOF with A. Effectively averaging the location.
3. I know it isn't Calypso, but the parallel cylinder option might work there.

The above are my opinion

wait, is B that side or the hole ??

[[To...]]

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In the top line, B is the origin and the X axis drives thru C. I guess you could interpret that on the lower line, the origin could be the midpoint of a 3d line between B and C or symmetry of B and C but I would guess B and C would also control rotation. And everyone thought that GD&T would eliminate ambiguity. 😃

[[Er...]]

Sorry, Datum B and Datum C are the holes. The only 2 holes related to this callout are the 2 middle holes as well. I don't know if this changes your interpretation or not. Thanks again in advance.

[[Er...]]

I Guess that's where i am at as well. Does B-C control rotation for this .25mm or not? My interpretation is that it does, and then the center point or symmetry becomes the origin. The holes would have to be within position, within .25mm from the origin. Unless someone else can tell me otherwise.

[[Jo...]]

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I would agree with that ,because:
1. multiple single segment does not release constraints, it follows the regular rules, treat it as a separate DRF
2. Unless there is a note "SEPT REQUIREMENT" or "INDIVIDUALLY", you have an implied simultaneous requirement by default for those two locations, which makes rotation a moot point. They must be evaluated in the same orientation.

[[Da...]]

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IMHO:

The A-B datum callout is suspect. Compound datums are used to treat multiple surface as a single one. Generally this is used for coaxial cylinders (journals on a shaft) or coplanar planes. I don't remember it being used for parallel holes.

I would argue that in this case, this would be equivalent to a pattern of holes datum. Which constrains up to 5 DOF. In this case, A controls 1T2R, and B-C controls 2T1R. Your DRF is fully constrained.

The lack of a diameter symbol means you get 2 parallel planes, basically located, oriented and separated by 0.25. Without more information it is hard to say which directions they should face though. This would give you a little more tolerance than same callout with the Dia symbol though, so I would ignore it unless you get fails.

[[Er...]]

My interpretation...I have seen many prints using position to control symmetry. Per 14.5 2009 this is legal. I looked up "multiple datum feature" in 14.5 and nothing seemed to cover this situation.

B-C implies create a symmetry between B and C to treat them as one feature per 14.5@4.12. Then the position would be to the symmetry only in the direction of the symmetry, say the X axis. The logic for this is that the symmetry is on that axis and there is no diameter symbol. Planer rotation would be controlled by B and C.

As an Alignment
Spatial -A-

Planer BC
Line from B and C

X B-C
Symmetry of B and C

Y B-C

Z -A-

As a Datum callout
-A-
BC
B-C