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A different kind of positional tolerance!

A different kind of positional tolerance!

A different kind of positional tolerance!

(OP)
Hello, just curious if anyone has run into this type of thing before.
I have a customer part with a 2D drawing which has no GD&T on it except in the drawing notes where you find the following:
"All holes to be position within .02 @ mmc to A from flat pattern periphery, and position within .005 @ mmc to A for hole to hole relationship."
This is followed by another note stating:
"Web face of flat pattern to be considered as datum A"
Some parts also add datum B to the callout and a note stating:
"Periphery of part to be considered datum B"
The question is how can you specify the entire periphery of a part as a datum?
I assume what they want is the holes best fit to the periphery but I'm sure this is not a legal callout!
Any thoughts?

Thanks

RE: A different kind of positional tolerance!

This sounds like a very rough attemt to get around the formal use of GD&T. This is very common practice but not well defined, as you realize. I imagine the person who did it might have known what he wanted. :)
Frank

RE: A different kind of positional tolerance!

I expect the person that did it didn't have a clue.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

RE: A different kind of positional tolerance!

Sounds more like half a$$ed attempt at hybrid MBD (Model Based Dimensioning) to me.

The datum B based on periphery sounds really odd - did -2009 version introduce irregular datums or similar that might even begin to cover this?

Posting guidelines FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: A different kind of positional tolerance!

2009 allows using “extruded shape” as a datum, but suggests the shape itself being controlled by profile all-around.

RE: A different kind of positional tolerance!

Just a reminder: Even in the 1994 version, any mathematically defined feature can be a datum. If a feature is in a CAD system, it is mathematically defined. Constraining the DOF could be tricky in the 1994 version though.

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