Multiple Bore Features as a Single Datum 1

[resjsu]

Hello,
So as topic states, 1 of our customer drawings has 4 holes called out as a single datum. Nothing I have read in the ASMEY14.5 standard (I could be missing it) really touches on this issue.

I have basic questions; is it allowed? How would you take dimensions using a datum composed of multiple axis etc.

If anyone has a link or some supporting information on this issue itd be appreciated.

I can post a pic if it helps but its pretty straight forward.

Thank you

 

[pylfrm]

resjsu,

I generally think of datum feature references as requirements for the relationship between the actual part geometry and the theoretical geometry. If the hole pattern is referenced as primary datum feature at MMB, then that relationship must be such that the actual part does not violate the boundary of 10X cylinders of diameter 15.9 located at the exact theoretical positions defined by the basic dimensions.

It should be relatively easy to implement a conservative inspection method that only accepts good parts. For instance, if the holes meet the position tolerance that references | A | B | C |, then you could just re-use that datum reference frame and forget about D. Your proposed method of establishing a datum reference frame from the pattern using two pairs of holes will also work, provided you ensure all 10X holes meet a tolerance of | position | diameter 1.5 (M) | D |. Unfortunately these conservative approaches will lead to some degree of error in the determination of actual values for the position tolerances, so a lot of good parts could be rejected.

Depending on the capabilities of the software available, implementing a "correct" inspection method may be substantially more difficult.


pylfrm