using a datum and within datum frame using same datum MMC 1

[Harol]

I am looking at a print that has a datum C call out on a hole. The hole dimension has a frame with .004 MMC, A, B MMC and C MMC. Can a frame contain the datum that it is controlling? If so how does this funcion?

 

[Belanger]

It doesn't add any value, so it really shouldn't be there. But it's not creating any harm, either, other than the confusion factor.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[Harol]

John-Paul,
If I understand C mmc could/should be removed from frame? Eliminate confusion?

 

[axym]

Harol,

A feature control frame can reference the considered feature, if it is part of a multiple datum feature.

Does the feature control frame look like this:

POS|Dia .004(M)| A | B(M) | C(M) | (B is secondary and C is tertiary)

or like this:

POS|Dia .004(M)| A | B(M)-C(M) | (B and C comprise a multiple datum feature)

If the FCF has the first option with the tertiary reference to C, then the C reference is "self-referencing" and should be removed.

If the FCF has the second option with the multiple datum feature, then removing the C reference will change the meaning.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[Harol]

John-Paul,

Thank you. Feature is called out as your first option defines.

 

[pmarc]

Evan,
You said: "A feature control frame can reference the considered feature, if it is part of a multiple datum feature."

Is this, in your opinion, always true?

 

[Belanger]

Proper credit to Evan

I guess the standard reply in these cases is to ask for you to post a sketch, but I think the gist of the question has been answered: They shouldn't have included C. Yank it.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[axym]

pmarc,

You tend to ask these questions that seem simple but really are not. Keep it up!

I wouldn't say that it's always true that a FCF can always reference the considered feature as part of a multiple datum feature. In fact, I really don't like the practice but I mentioned it because the standard allows it and it seems to be commonly used.

I've mentioned this in many other threads that discussed runout FCF's for A and B both referencing A-B. To me this is conceptually flawed - the FCF's ideally should not reference any datum features and have the "datum axis" be arbitrary (as is done for Position tolerances that control coaxiality).

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[pmarc]

Evan,
I would just like to have clear understanding of first part of your reply, therefore allow me to ask differently.
I took your "A feature control frame can reference the considered feature, if it is part of a multiple datum feature" more or less as "When a FCF applied to a feature references the considered feature in a non-multiple-datum-feature way, this is always invalid callout". Did I read the message correctly?

 

[Tunalover]

I think the correct term is not "multiple datum" but rather "compound datum". Regardless, I don't think it is proper to use datum C at all in this case.



Tunalover

 

[3DDave]

It seems to me the initial description is insufficient, but self-referential tolerance zones don't seem like a good idea.

A picture would clarify the situation.