DRF for a pattern of holes 1

[Rwelch9]

Hi guys,

Looking back to a previous thread of mine.

I had 3 holes equally spaced on a PCD. This consisted of 2x Ø4 and 1x Ø3 dowel holes.

The DRF that fitted best after some discussion on this forum was the plane perpendicular to the dowel hole was Datum A, 2x Ø4 was Datum B and 1x Ø3 was Datum C.

Datum feature B was position to A and Datum feature C was position to AB. The CRITICAL part was the rest of the features on the component was position to A B-C. B-C being a common datum


However the drawing got sent away to what gets described to me as a drawing checker. Who has changed the rest of the features to be positionally to A B C.
I didn't like this as much as the idea for me was that none of the dowel holes with the B-C call out had any preference over another and this matched the functionality of the part.

Can anyone shed some light as to why a DRF consisting of A B C makes more sense ?


Thanks

R

 

[chez311]

Can anyone shed some light as to why a DRF consisting of A B C makes more sense ?

Functionally it doesn't, based on your description. In fact, if referenced at RMB in |A|B|C| datum feature C does nothing and constrains no further DOF. If at MMB then |A|B(M)|C(M)| would be equivalent to |A|B(M)-C(M)|.

 

[Rwelch9]

Chez311,

If this is the route the customer went down regrading the print, does datum C have no place in a feature control frame along side Datum A and B ?

A being a plane ( 3 degrees of freedom )

B being 2x hole pattern ( 3 degrees of freedom )


Thanks


R

 

[chez311]

If referenced at RMB |A|B|C| then yes, C serves no purpose and should be removed as B already constrains (x,y) and (w). As referenced in the previous thread |A|B-C| would be perfectly acceptable as B and C work together to constrain these remaining 3 DOF and would seem to better reflect function. If these holes are a slip fit you may want to consider use of MMB, in which case as I said either notation would be acceptable.

If i had to guess I would say your checker either wasn't familiar with the Multiple Datum Feature designation, thought it was too complex, or needs a review on DOF constraint. Likely some combination.

 

[Rwelch9]

Chez311


Thanks for the help much appreciated.

R

 

[drawoh]

Rwelch9,

Could you please show us a picture?

If your secondary datum feature is a pattern of holes, a tertiary datum feature could show clocking. It depends on how many features are on your part.

--
JHG

 

[Rwelch9]

drawoh

There are a lot of features on the part which will be position to the 3 dowel holes,

This part is a shaft, so i have 2x Ø25 external diameters which are position now to Datum A B C rather than A B-C .

There are also two angled mechanical stops on this part and thread holes on the opposite end of the component, all now to be in position to A B C.

This Feature control frame of A B C, I thought A ( plane ) and B ( 2x Ø4 dowel holes ) these alone would constrain 6 degrees of freedom.

I know however the 3 dowel holes work simultaneously as a functional part. So I though B-C common Datum worked better ?


Thanks

R