Multiple datum reference

[lowerp]

A plastic injection molded automotive panel is designed with 6 holes intended for assembly to a chassis with fasteners. Of the 6 total holes, there are 3 separate sets (2X 6.0 dia, 2X 5.5 dia, 2X 5.0 dia). All of these hole features have equal importance and there is no specific order of fastener assembly.
Datum feature A is the large surface of the panel that butts up to the chassis. Datum feature B is intended to be the mounting holes. I'm a little familiar with multiple datum references per ASME Y14.5-2009 and I believe that concept pertains to this part. However, I'm not sure how to specify it. What would the dimension of each hole and FCF look like to control such a scenario? If I had to guess, I think each hole set dimension is 2X x.x dia, and FCF with perpendicularity to A, and datum feature symbol B attached to the FCF? Meaning there would be 3 datum B feature symbols shown? Thanks

 

[pmarc]

There are different options to do that. One that I would consider in the first place would be to specify 2X dia. 6.0 A, 2X dia. 5.5 A, 2X dia. 5.0 A as separate size callouts, then specify a single position (not perpendicularity) FCF to A with additional 6X prefix in front of it and a note HOLES INDICATED A beneath it and then associate datum feature B symbol with that FCF.

So in that solution only one datum feature B symbol would be needed.

 

[Burunduk]

I would indicate each set by a letter, attached by a leader to each of the holes (each leader directed letter shown twice). Then I would dimension each set, each dimension shown once with a '2X' notation and the letter specification, such as: '2X Ø5±X.X INDICATED K', with a Position FCF with ref. to A under each diameter dimension. The 3 position tolerances would apply in a Simultaneous Requirement because of referencing the same primary and only datum, thus controlled for location relative to each other (and within each set).
Separately of any FCF, I would show a datum feature symbol attached by the triangle directly to the outline of any of the holes, with the notation such as '6X (the datum feature symbol) INDICATED K,L,M'.

 

[greenimi]

Okay. Understood both options.
Now, the follow up question is: what is the difference (if any) between Pmarc's solution and Burunduk's one?
It is just drafting or is more subtle than that?

 

[Burunduk]

greenimi,
Using 3 indication letters would also make it easier to determine the required diameter dimension and tolerance for each hole. Especially given that the visible difference between Ø5, Ø5.5, and Ø6 is not great, and we don't know how the holes are arranged in the pattern.

 

[pmarc]

greenimi,
I don't see a difference between the two proposals regarding geometric requirements they define.

So in such cases usually personal preferences come into play. One could, for example, say that in Burunduk's proposal the tricky part (especially to less knowledgeable folks) would be to identify that all 3 position FCFs relative to A work as a simultaneous requirement.

Also, if all 6 holes would be of a different size, I think it is quite likely that someone might say that specifying 6 different letters next to each size callout and then doing the same thing next to the datum feature symbol is an overkill.

I am pretty sure that some folks might have some issues with my proposal as well.

 

[Burunduk]

Also, if all 6 holes would be of a different size, I think it is quite likely that someone might say that specifying 6 different letters next to each size callout and then doing the same thing next to the datum feature symbol is an overkill.

Yeah in the case of 6 different hole sizes it might look a bit cumbersome. In such case I would probably do something similar to what you suggested, with the minor difference that I would place a notation near each hole (without leader) and designate them K1 to K6 similarly to how it's done in tabular dimensioning. The position FCF referencing datum feature A would be preceded by '6X' and followed by 'INDICATED K', and datum feature symbol B would be placed on the FCF, which is similar to your suggestion, I think.

 

[lowerp]

Thanks for the responses. Am I correct in saying to dimension basic between all the holes but in any configuration I want?

 

[pmarc]

lowerp,
Yes, the relationship between all the holes in the pattern should be basic. As long as it is, the configuration of dimensions is irrelevant.