Is it acceptable if I create a datum feature and do not refer it anywhere in the drawing? 1

[ninjaz]

For example, suppose I mark a plane surface of a component a datum feature A and now I do not refer it anywhere on the drawing for marking any tolerance feature. Is that acceptable ?

 

[CheckerHater]

May I ask... why?
Datum / datum feature by itself doesn't have any magical property / meaning.
By definition, datum is a reference for measuring something.
If I create a dimension with only one extension line, half of dimension line and no value, would you find it acceptable?

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[mkcski]

I totally agree with CheckerHater. Adding "useless" symbols to a drawing only adds confusion to interpretation process, which is NOT productive time (lower profits). Someone will be looking for the other "end" of the controlled feature-to-datum relationship and never find it.

Certified Sr. GD&T Professional

 

[ninjaz]

Thanks for your comment CheckerHater,mkcski.

This is my case.
In inspection point of view, In-order to arrest all six degrees of freedom , I have defined three datum feature.
Datum Feature A - Planar surface
Datum feature B- Planar surface vertical to the one defined above.
Datum feature C- A mounting hole of the component.

Now I define another datum feature D for another hole in the component . And I am referring this datum feature to define the concentric tolerance to be maintained with another profile.

Do you feel, Defining datum feature A,B, and C are not necessary?. Then how could we locate the component in the inspection jig correctly?

Thank you..

 

[mkcski]

Can ya provide sketch?

Certified Sr. GD&T Professional

 

[mkcski]

To be sure: The name fo the person posting is CheckerHater not Clarke. Arthur C. Clarke is the author of the quote he is using in the closing.

Certified Sr. GD&T Professional

 

[chez311]

In inspection point of view, In-order to arrest all six degrees of freedom , I have defined three datum feature.

Now I define another datum feature D for another hole in the component . And I am referring this datum feature to define the concentric tolerance to be maintained with another profile.

Do you feel, Defining datum feature A,B, and C are not necessary?. Then how could we locate the component in the inspection jig correctly?

As CH stated - simply defining a surface/feature as a datum does not create any inherent meaning or special properties. It is only when those datums are then referenced in the other feature control frames applying tolerances and controls to other features on the part that those datums actually mean something. See below for an example of this. Hopefully its not too basic - I'm just trying to get the point across that datums by themselves don't have any inherent meaning.

I agree though, a sketch/drawing in this case showing how your datums are defined and utilized would go a long way in answering your questions. Without that its not really possible to answer with any certainty, especially since your original question asks about defining a datum without referencing it and its not clear which datums you have mentioned this applies to. Additionally you mentioned concentric tolerance - I'm pretty sure you mean something other than the "concentricity" symbol as it probably doesn't accomplish what you want it to.

 

[CheckerHater]

Thank you mkcski, I didn't notice.
I also really want to see some sketch.
The only other possibility I can see is combining the datum(s) with text notes like
"UNLESS OTHERWISE SPECIFIED, ALL SURFACES ARE PROFILE 0.25 WRT [A|B|C]"

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[Belanger]

In inspection point of view, In-order to arrest all six degrees of freedom, I have defined three datum feature.

This is the key statement...but why do you feel that it's necessary to constraint all DOF for the general part? Instead, you should use datums to provide constraint for individual tolerance callouts. And concentricity -- or a similar control for coaxiality -- won't need to constraint all six DOF.

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

 

[dgallup]

If the datums are not called out in a tolerance, inspection has no reason to use them during measurement.

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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.

 

[ninjaz]

Its clear now. Thank you chez311 for the great explanation.

@CheckHater, Due to confidential reason, I couldnot share the sketch, Sorry.
BUt the sketch shared by chez311 almost supports my query and clarified it.

Thanks..