Composite Position Tolerance

[[Mi...]]

ASME 1994 Standard.

Struggling to understand the right way to setup this composite tolerance for a pattern of 3 bore holes. The bore holes are interrupted cylinders, machines through 2 levels of material hence 6 in the drawing note.

A is a Plane
B is a Circle/Cylinder with an axis perpendicular to A (and parallel to the pattern of holes)
C is a Point that controls the rotation of the pattern

Top line, easy. Setup the BFBP, allow no rotation or translation.
The lower two I'm struggling to grasp for this particular situation (With a circular/cylindrical pattern and secondary datum). I understand that for lower segments only orientation is controlled.

So does that mean the Position to AB essentially controls the parallelism of the Pattern's axis to B's axis? Meaning rotation and translation both enabled in the characteristic, A and B in the DRF.

And then the position to A is essentially the perpendicularity to A, again meaning rotation and translation enabled? only A in the DRF.

[[To...]]

Good video to watch but around the 51 minute marker goes into the area I think you might be looking for.

https://www.youtube.com/watch?v=iSdMpgVxeDg

[[Mi...]]

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Thanks Tom, that explains it the same way I read it everywhere else. I have found 0 examples of a composite callout with a cylindrical secondary datum, must be pretty uncommon.

Which means, I think I'm correct in saying that for this particular callout with a cylindrical Datum B, the second line is mostly controlling the parallelism of the axis of the holes to Datum B. In the video he says translation only, no rotation, but I *think* in this case since the secondary datum is a central cylindrical feature rotation would be left on as the pattern needs to be free to rotate around it.

Does that sound right?

[[Ch...]]

Michael,

I would say second row is controlling how centered the pattern is to B (&A). So in Calypso would check rotation but no translation as there is no modifier on -B- to allow any datum 'shift'.

The 3rd row is essentially perpendicularity refinement of the holes to -A-.

Good luck !

[[Mi...]]

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That's how we would treat it if it was a multiple single segmented position tolerance, this is exactly how we have been treating this so far, it's not a new part.

However, with composite as I understand it any subsequent lines after the first are released from the basic dimensions and control orientation only.

I'm just struggling to visualize, and therefore explain it in regards to a cylindrical secondary datum.

Short version is we have an engineer who's almost always right with GD&T arguments saying translation should be allowed, I'm leaning towards him being right - but I need to be positive.

[[Ro...]]

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He's correct. Composite Position allows for translation.

I realized your secondary datum is a cylinder. So, rotation is allowed around B.

So top row is as you stated.
Second tier is a BF to A-B of the pattern allowing rotation around B.
Third tier is BF to A only allowing translation and rotation.

[[Mi...]]

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Just making sure we don't have a miscommunication.

Engineer (and now myself) think that the second tier allows for both rotation and translation. Translation because it's the second tier of a composite callout, rotation because it's a Cylindrical secondary datum.

[[Ro...]]

That is correct. Composite position allows for all rotations and translations that aren't constrained by a datum.

Clearly you won't get any translation out of B, as it's an origin.

[[He...]]

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That is correct.
First row: No rotation or translation is allowed.
Second row. Rotation around Plane A normal allowed because no third datum. Translation parallel to A allowed because it is the lower segment in a composite position tolerance.
Third row: Rotation around Plane A normal allowed because no second or third datum. Translation parallel to A allowed because it is the lower segment in a composite position tolerance or that there is no second or third datum. Basically the same as second row but do it twice with 3 holes each.

The reason you never see any examples with composite tolerances with a plane and cylinder as datums is that it doesn't make any sense. B in this case doesn't do anything.

However since B is there in the second row it tells me that the designer doesn't really understand this part of the GD&T and if I were you I would try to talk to the designer to understand design intent. The last thing you want is to check the parts as conforming and then they are not working in assembly. Better to discuss the thing with the customer.

[[Ri...]]

I've met only a handful of companies that actually design and tolerance their drawings correctly per the standard they use.

GD&T standards are just a suggestion to the majority of design engineers.

I've had design engineers throw a straightness callout on an arc....