Extension of Principal? 1

[ewh]

Can a circular pattern of features be used as a datum (see attached)?
I have my own ideas on this, but there is no example of this situation depicted in Y14.5 that I can find.
Thanks!


"Know the rules well, so you can break them effectively."
-Dalai Lama XIV

 

[Belanger]

Sure... See Figs. 4-26 and 4-29 and 4-39.

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

 

[ewh]

Thanks JP!
I knew I had learned it somewhere, but I guess I only checked Section 7.

"Know the rules well, so you can break them effectively."
-Dalai Lama XIV

 

[greenimi]

Should be the pattern callout at MMB or RMB?

See here for the discussion (one place is here, but there are plenty more)

http://www.eng-tips.com/viewthread.cfm?qid=305667

 

[ewh]

I was going to use MMB.
A related question - if the pattern were located with a composite positional tolerance, could it still be used as a datum? I don't see why not, but that doesn't mean I'm not overlooking something.

"Know the rules well, so you can break them effectively."
-Dalai Lama XIV

 

[axym]

ewh,

The pattern can still be used as a datum feature, even if it was located with a composite position tolerance. I don't know of any reason why it couldn't.

The composite position tolerance might come into play when calculating the size of the datum feature simulator, depending on how the datum feature is referenced (i.e. what the higher precedence datum features are).

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[ewh]

Thanks!

"Know the rules well, so you can break them effectively."
-Dalai Lama XIV

 

[tbuelna]

As shown, there is no specific definition of datum C. There are 4 holes in the pattern, and only 3 holes are required for establishing datum C. As for the angular positioning of the smaller holes, which of the larger holes do you use to establish the X-Y axis system?

 

[3DDave]

All four holes participate as Datum C. Ref ASME Y14.5M-1994, figure 4-22, page 69. The 2009 version adds the implausible explanation for the inspection of simultaneously expanding pins. I wonder how much those gages cost to build and what real-life assembly has that as an interface.