Primary Datum Parallel to Hole Axis 1

[mhorbacz]

I have a part (see image below), where a hole is located on a small face. As such, it doesn't make sense to make that face the primary Datum. For my part, this forces the secondary or tertiary datum to be the face the hole is on. My question is, how do I locate the hole? I can locate the hole to datum A, and orient to datum C, but then how do I locate it to datum B? My intuition is that I can't dimension it to datum C, since C is the primary Datum and only serves to orient the hole.

 

[3DDave]

The current location of B is not required for this hole.

A different way to look at it. If you were to build a fixture for a drill press, what surfaces would you need to push the part against to drill the hole in the end of this part that would affect where the hole got drilled?

If B is used elsewhere, that's fine; but only two of the sides are required to locate and orient this part and that would be C and A.

 

[mhorbacz]

The current location of B is not required for this hole.

A different way to look at it. If you were to build a fixture for a drill press, what surfaces would you need to push the part against to drill the hole in the end of this part that would affect where the hole got drilled?

If B is used elsewhere, that's fine; but only two of the sides are required to locate and orient this part and that would be C and A.

So am I allowed to have a basic dimension to datum C, even though it's the primary Datum and technically only orienting (and not locating) the hole?

 

[3DDave]

Datum features should, mainly, represent how the part is oriented and located. Basic dimensions should originate from datum features.

Since you cannot locate the hole from B in this part, you need some features to dimension from.

 

[mhorbacz]

Datum features should, mainly, represent how the part is oriented and located. Basic dimensions should originate from datum features.

Since you cannot locate the hole from B in this part, you need some features to dimension from.

Ok, so despite the primary Datum being used to orient the hole, it seems like I can still tie a basic dimension back to it. Thanks for the help!

 

[pmarc]

mhorbacz,
In this case the primary datum is not only used to orient the hole but also to locate it from.

Despite that most figures in the standard (Y14.5) shows primary datum established from a surface that is nominally perpendicular to the hole(s), there is no rule that would say the primary datum in a position tolerance is for orientation only.

 

[greenimi]

pmarc,

May I ask one related question:

In this case the primary datum is not only used to orient the hole but also to locate it from.

Despite that most figures in the standard (Y14.5) shows primary datum established from a surface that is nominally perpendicular to the hole(s), there is no rule that would say the primary datum in a position tolerance is for orientation only.

In 2009 or 2018 standards, can we legally apply MMB on datum feature C ? Or it is not allowed because C it is primary?
(I know C is a planar surface)

 

[pmarc]

greenimi,
I don't think C at MMB would make sense in this case. In my opinion, if it was secondary or tertiary (with A and/or B being the higher order datums), it wouldn't make sense either.

 

[greenimi]

greenimi,
I don't think C at MMB would make sense in this case. In my opinion, if it was secondary or tertiary (with A and/or B being the higher order datums), it wouldn't make sense either.

Pmarc,

Just for my own edification (and I won't bother you again, ON THIS SUBJECT)

So, you don’t think C(MMB) primary make sense, or A|B|C(MMB)| or A|C(MMB)| won’t be valid either because there is NO location relationship between C (planar surface) and its higher precedence datum?

Or what is the reason?

Are you looking for the location relationship between the datum features OR you are looking for the location relationship between the concerned/ dimensioned feature and datum features? (because if it is later we have a location relationship between the hole (concerned feature) and datum feature C).

I guess it is the first part (“there is NO location relationship between C (planar surface) and its higher precedence datum?”)

Am I correct on this?

Sorry, for some reason when I was asking my original question (addressed directly to you) I was thinking about the later (“the location relationship between the concerned/ dimensioned feature and datum features”)

You see ….I had a brain fart in this area…..maybe my lack of fundamental(s) knowledge.

 

[pmarc]

Yes, referencing flat surface at MMB/LMB makes sense if there is a location relationship between that datum feature and the higher order of precedence datum feature(s). At least that's how I like to think about this.