Datum Definition and Datum Usage

[weisusu]

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?

 

[KENAT]

weisusu, I'm not sure what you mean by 'defined as Datum B with RFS'. Is datum B placed on an FCF in which RFS is stated?

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 recently, or taken a look at posting policies:

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

 

[weisusu]

Yes RFS is defined in Datum B FCF. The drawing define a feature with its FCF relates to Datum B, but the modifier for Datum B calls out MMC. Just curious if that is a correct call out.

 

[KENAT]

OK, now I'm more confused, can you put a sketch of what you mean.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

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

 

[powerhound]

The datum is what it is. 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.

Powerhound, GDTP T-0419
Production Supervisor
Inventor 2008
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[ewh]

powerhound is correct. The RFS locating datum B is for just that, locating datum B. Datum B will still exhibit a feature size, and can be used as a secondary datum at LMC, RFS, or MMC.

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. - [small]Hunter S. Thompson[/small]

 

[drawoh]

weisusu,

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

 

[ringster]

Just for the heck of it, suppose we stated that 'the inside diameter of the part is defined as DATUM FEATURE B.' Would not this be more technically correct and to a degree clarify the situation?

Agreed the FCF could specify the simulated datum at LMC RFS or MMC condition.

 

[powerhound]

There are certain methods shown in the standard for defining datum features and this is not one of those methods. The idea of GD&T is to reduce the notes on a print, not add notes. Having a box with a datum letter inside of it with an extension and a "sucker" on the end of it attached to the exact datum feature referred to, should be clear to even a rookie that it is a datum feature.

Powerhound, GDTP T-0419
Production Supervisor
Inventor 2008
Mastercam X2
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[ctopher]

weisusu,
A picture would help.

Chris
SolidWorks/PDMWorks 08 3.1
AutoCAD 08
ctopher's home (updated Aug 5, 2008)
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[axym]

I agree that some sort of picture would help.

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.

http://www.axymetrix.ca

 

[pmarc]

As I can imagine weisusu's case looks like on attached picture (weisusu, could you confirm?). But this is only my assumption.

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".

 

[ringster]

Let me see if this gets action. I believe that datum feature B, a diameter, could be used as a primary, secondary or tertiary datum feature as required. And additionally could be specified at MMC, RFS, or LMC,as dictated by the design.

Yea or nay?

 

[KENAT]

pmarc, that isn't defining an ID as B at RFS. In fact that isn't defining datum B at all.

weisusu if you want an answer post a sketch showing what you mean.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

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

 

[powerhound]

ringman,

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

 

[axym]

Labeling a cylindrical feature as Datum Feature B does just that - labels it. The labeling does not specify anything to do with precedence or material condition. These are only specified when the datum feature is referenced in a feature control frame. Further, modifiers on a datum feature don't really specify material condition - they specify the behavior of the gage element that will contact or "simulate" the datum feature.

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.

http://www.axymetrix.ca

 

[ewh]

Labeling a cylindrical feature as Datum Feature B does just that - labels it. The labeling does not specify anything to do with precedence or material condition.

I missed the tree for the forest!

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. - [small]Hunter S. Thompson[/small]