Position question

[[To...]]

Below is a part where Datum Feature A is a cylinder and Datum Feature B is a plane on the end. The drawing calls for Position of the cross-cut cylinder feature to A and B. We all know this means the rotation of the feature is not part of the position. However, the cylinder is rotated about the Datum A axis by 8° relative to other features on the part, which I left off the drawing for time sake. The locations of the cross-cut diameter are BASIC but the 8° angle is not.

Customer is currently creating an alignment using A for spatial, XY origin, B for Z origin and the cross-cut cylinder for planar. Then using this alignment in the position.

In theory, I think this might be ok but I'd like to hear how you guys/gals would go about creating the position.

Screenshot 2023-03-09 094621.jpg

[[Ma...]]

There must be something to stop rotation in A axis. Otherwise you can not measure angle 8° - unless crosscut cylinder will be used to stop rotation ( but then it would be as base C )

[[Ch...]]

you could have a position to -A- only (or A+B) without a tertiary, it would essentially be how "on center" the cutout is to -A-.

The 8° is questionable - as it is implied to what ? CL of -A-? nothing stops the roation on that, so could pretty much be anywhere you like.

By lining up to the cross cut as a tertiary - they are bascially doing a rotational best fit themselves, however this should not have an effect on how center its position is to -A-.; but to be accurate I would not use it. I would just A +B in a position letting rotate BF as needed.

[[To...]]

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As mentioned, there are other features on the part that are normal to coordinate system. I did not say the customer's drawing was a good one. 😃

[[To...]]

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There are basic dimensions from the end (B) and the centerline (A). The position is reporting the basic dimensions correctly. As is common, we can't assume the engineer understands GD&T.

The reason for my question is I am being asked to use the customer's customer's program to inspect some parts. The benefit is if the parts are good on my machine, then they should be very close to the same on the customer's customer's machine. (Sorry for the replicate replications...lol) I don't want to modify the program just because I don't like it but if something is wrong with the program, I want to share this info with everyone involved.

I also wanted confirmation that rotational BF would be applied using A and B.

[[Er...]]

It sounds like a really bad drawing.
In my opinion...
Datum B should be Datum C
The crosscut should be Datum B.

We have plenty of gages that don't use the drawings datums. No engineering was put into how the part could be checked. Or functionality.

[[To...]]

Here's a redacted copy.

Screenshot 2023-03-10 110855.jpg

[[Da...]]

The position call-out constrains five DOF with A|B, so for the remaining DOF (the clocking), you would usually need to find only one solution that gave you a good result.

However, since the position tolerance zone is diametrical and it uses only two coordinates, the remaining DOF has no influence on the result. The only thing the clocking does, is to provide a projection plane for the 2d tolerance zone. There will be a number of solutions that will work (even after a 180° turn), but if you use a best-fit rotation for the cut-out cylinder (you need good nominals), that should do it.

Don’t forget though, to check the angle 8° with your usual general tolerances.