Datum Definition and Datum Usage
Datum Definition and Datum Usage
(OP)
If an ID is defined as Datum B with RFS on a drawing, can the Datum B be used as a secondary datum for a different feature on the same drawing with MMC call out for Datum B?





RE: Datum Definition and Datum Usage
Certainly once you have datum B defined then other FCF which relate to it may or may not use MMC as required.
KENAT,
Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?
RE: Datum Definition and Datum Usage
RE: Datum Definition and Datum Usage
KENAT,
Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?
RE: Datum Definition and Datum Usage
Powerhound, GDTP T-0419
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Inventor 2008
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RE: Datum Definition and Datum Usage
The Edge... there is no honest way to explain it because the only people who really know where it is are the ones who have gone over. - Hunter S. Thompson
RE: Datum Definition and Datum Usage
When you identify your feature as a datum, there is no capability in the standard to call it RFS, MMC or LMC. When you attach a feature control frame calling up your datum, then you can call it RFS, MMC or LMC.
Your datum should be defined by a square with a letter in it. There should be no additional boxes.
Like everyone else here, I am trying to visualize what you are looking at.
JHG
RE: Datum Definition and Datum Usage
Agreed the FCF could specify the simulated datum at LMC RFS or MMC condition.
RE: Datum Definition and Datum Usage
Powerhound, GDTP T-0419
Production Supervisor
Inventor 2008
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RE: Datum Definition and Datum Usage
A picture would help.
Chris
SolidWorks/PDMWorks 08 3.1
AutoCAD 08
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RE: Datum Definition and Datum Usage
Purely speculation, but I can think of one possibility for what weisusu might be referring to. I've seen something in a very popular GD&T book called an "implied self-datum". It's put forth as an alternative for controlling mutual coaxiality. For example, a pattern of coaxial holes will have a Position FCF that references Datum A, and also has a datum feature label A attached to the FCF. Something like this:
3X dia 1.000 +/- .005
|POS|dia .010(M)|A(M)|
-A-
I'm not a fan of this technique at all, but I have seen it used.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: Datum Definition and Datum Usage
I totally agree with powerhound - "If you call out a feature relative to datum B with datum B at RFS in one place on a drawing and another feature relative to datum B with datum B at MMC somewhere else on the same drawing, that is legal but it is a separate requirement and need not meet it's tolerance simultaneously with the first feature".
RE: Datum Definition and Datum Usage
Yea or nay?
RE: Datum Definition and Datum Usage
weisusu if you want an answer post a sketch showing what you mean.
KENAT,
Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?
RE: Datum Definition and Datum Usage
In response to your broadly posed hypothetical; what you said could be true if specified properly. It could also be false if specified improperly. Of course the designer could also improperly specify proper specifications for an improper design OR he could properly specify what could have been improper, had the requirements not been what they could be. That being said, it is true that while your query could be true, what it could be and what it couldn't be, could be neither...or both.
Powerhound, GDTP T-0419
Production Supervisor
Inventor 2008
Mastercam X2
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: Datum Definition and Datum Usage
B or B(S) means "Datum Feature B simulated regardless of its size"
B(M) means "Datum Feature B simulated at its virtual maximum material condition"
B(L) means "Datum Feature B simulated at its virtual least material condition"
It is legal to reference a datum feature in several different feature control frames, at different precedence, with different modifiers.
Ringster,
Your question may be a broadly based hypothetical, but I understand what you're asking. I say yea.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: Datum Definition and Datum Usage
The Edge... there is no honest way to explain it because the only people who really know where it is are the ones who have gone over. - Hunter S. Thompson