Multiple Datums for Same Feature

[jmarkus]

Does ASME Y14.5M allow or restrict the application of multiple datums on the same feature?

For example, if I have a part which is going through multiple manufacturing steps, can I do the following:

- Side 1 of the part is both datum B & datum X, and side 2 of the part is both datum C & datum Y (and primary datum A is common)
- various FCF call outs to A|B|C and/or to A|Y|X

If it is allowed, do I simply show that the feature is both datums by putting the datum boxes next to each other, sharing the same datum leader? Or do I need to place them on the drawing completely separate?

Thanks,
J

 

[3DDave]

The datum feature symbol identifies a datum feature. Why give a single feature multiple names?

 

[CheckerHater]

The question is "why?"
Is your B feature before machining and X after?
If it is absolutely the same feature, there is no need to assign different symbols to it.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[Kedu]

- various FCF call outs to A|B|C and/or to A|Y|X

Regarding the above statement only:

If you have Y14.8 Casting standard see fig 4-9
position 0.5(M) |Z|Y|X|
position 0.2 |A|B|
which is datum feature's C definition

 

[CheckerHater]

@Kedu:

On Fig. 4-9 X, Y, Z and A, B are ALL completely different features.

OP is asking if exactly the same feature ("side 1") can be both B and X at the same time.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[jmarkus]

All,

The request for additional datum features has come from a supplier. I'm not entirely sure that they understand what they are requesting as I am receiving the request 3rd hand from our supplier contact who is likewise unable to explain it to me, but seems to be content to "just give them what they want". It doesn't make much sense to me either, but I wanted to understand if there was any situation where it might make sense. The only sense I can make of the request is that the supplier wants to check some features to one datum scheme (with secondary datum as Y and tertiary datum as X), and our internal quality wants to check them to another datum scheme (with secondary datum as B and tertiary datum as C). If I keep the same name for the datums, but then call out a tolerance from one feature to A|B|C and a different tolerance for the same feature to A|C|B then it would appear to someone else as either a mistake on the drawing or simply a conflicting requirement (which I think it is anyways).

I'm going to push back again and see if this request can be withdrawn.
Thanks.

 

[Belanger]

Are the two datums sampling different portions of the feature? There is a provision called "partial surface as a datum feature" which would then permit the two different datums.

 

[jmarkus]

According to what they are asking me to put on the drawing, it is not different portions.

I managed to get the request dropped, so there will be no "same but different" datums.

Thanks all for letting me hash it out with you.
J