Compound datums for hole intentionally not perpendicular to surface into which it is drilled

[Stephen Robertson]

Is this use of compound datums correct (see uploaded image)? The holes are features drilled into a drafted surface, but for easier machining set up the holes need not be perpendicular to the drafted surfaces (a very slight ellipse will be fine for the cable gland). This leaves no surface to use as a projection datum. My thinking is, for example in the case of the 22 mm hole, that the projection must be established with a combination of A-C, then position established from A then C.

 

[3DDave]

These multiple datum feature references do not make sense and do not seem necessary. Perhaps there is clarification in the 2018 version of the standard that would explain precisely how to evaluate arbitrary combinations, but before then there isn't much as to how they are evaluated, except for a few special cases that look nothing like your example.

 

[cr7]

Stephen,

first of all, is datum A some sharp edge? Those "A-B"s and "A-C"s, yeah what Dave said. I know you don't like quality department, but please take your time and think/re-think how will one measure this?
I see it, on the right side, this could easily be [box][⌖][/box][box][⌀]2[Ⓜ][/box][box]A[/box][box]C[/box]
and on the left side, [box][⌖][/box][box][⌀]2[Ⓜ][/box][box]A[/box][box]B[/box] Or do you need to control those holes on the left as a pattern?

 

[Burunduk]

Stephen Robertson,
Where a multiple datum reference is specified such as A-C it means that both datum features constrain the applicable degrees of freedom together without datum precedence priority and are used together to establish the datum(s).
Datum features A and C in your drawing constrain n total 5 DOF, but it is not likely that they can be made to do so "together" without one taking priority over the other, and referencing A and then C separately as secondary and tertiary after A-C as primary is questionable at best. The intent isn't clear and the meaning is vague.

 

[jassco]

Hi, Stephen:

"A-B|A|B" does not make sense. "A-B" controls 5 degrees of freedom. So, you need to restraint just one more degree of freedom which is translation parallel to both A and B. However, neither A and B has ability to restraint this translation.

Academically, "A-B" is Ok, but it does not make sense here as Burunduk indicated. Multiple Datum Features are used to be called Compound Datum Features. What that means there is no precedence priority. A good example is a shaft with two bearings.

Datum feature "B" is for an axis rather than a center plane.

Happy Thanksgiving!

Alex

 

[aniiben]

Is this ISO or ASME drawing?

Because in ISO "A-B" (where "A" is a planar surface and "B" is a cylindrical surface) DOES make sense and it is valid.

However "A" SECONDARY and "B" TERTIARY are pushing the line of absurdity.