A different kind of positional tolerance!

[thecmmguy]

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

 

[fsincox]

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

 

[dgallup]

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.

 

[fcsuper]

Sounds like someone trying to state GD&T without the symbols to get around a rule given to them to not use GD&T.

Matt Lorono, CSWP
Product Definition Specialist, DS SolidWorks Corp
Personal sites:
Lorono's SolidWorks Resources & SolidWorks Legion

 

[KENAT]

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

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[CheckerHater]

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

 

[CheckerHater]

So, the note could be possibly interpreted like this:

 

[Tarator]

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.