Positional tolerance clarification 2

[rwild]

Hello all,

We have a GD&T postional tolerance call out that we need help with interpreting. The feature control frame uses 2 datums, and 1 of the 2 datums,(datum A), is the surface that the hole is on. My question is where do we get the second deviation from for the true position formula? Please see the attached drawing. Thanks in advance for your help.

Randy

 

[CheckerHater]

Please rename your picture without using symbol "&" and re-post

 

[rwild]

Thanks CheckerHater,

Please see attached drawing.

 

[CheckerHater]

Creators of the drawing wanted to use the round hole as datum to control profile of the formed part.

The idea by itself is legal, but something is missing from the drawing.
According to standard, the feature being controlled should be tied to datum features using basic dimensions.

I see following possibilities:
They forgot to put dimensions on the drawing
Somebody erased dimensions before posting to Eng-tips.
The complete drawing has a note saying something like "CAD data is master. Dimensions derived from CAD are basic"

So my question is: are there any more dimensions somewhere?

 

[rwild]

Yes, the master definition is the model. We also have a note stating that the internal features have a postional tolerance of .060 with respect to datums A B and C.

 

[pmarc]

Coming back to the question about position tolerance, there seems to be a diameter symbol missing in the FCF in front of .028 value. If this is corrected, then I think nothing else will be needed for this positional callout - the only basic linear dimension that could be given for this kind of datum features configuration has been already given - it is basic 3.120 from datum plane B.

 

[CheckerHater]

Question to pmarc:

What is the point of having diameter symbol if position is only defined in one direction wrt B?

 

[rwild]

That was my bad. The feature control frame should have the diameter symbol. But I am not sure I understand how nothing else is needed. Shouldn't there be 2 basic dimensions to calculate the true position.

 

[Belanger]

Yes, add a dia symbol. Otherwise the tolerance zone is two parallel planes, and the drawing isn't clear as to where those planes would be.

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

 

[pmarc]

Answer to CH:
The point is that without this symbol the axis of the hole could tilt freely in a direction parallel to datum plane B.

Answer to rwild:
No, the second basic dimension is not needed. The only locational aspect that positional tolerance on datum feature C controls is location relative to datum plane B. Or in other words, the positional tolerance for datum feature C is constrained in location to datum plane B (at basic distance 3.120 from that datum), but it can freely translate along the line parallel to datum plane B.

That brings up another point. If presented precedence of datum features is trully functional (so that the datum feature C is really tertiary and not secondary), and if I were designer of this part, I would at least consider changing round hole to a slotted hole in order to avoid overconstraining. Some useful info can be found under this link:

http://www.tec-ease.com/gdt-tips-view.php?q=167

Since the tertiary hole is chosen to constrain last remaining translational degree of freedom (along horizontal direction), the slotted hole could be vertical and only the width of the hole (between two flats) would be assigned as tertiary datum feature). Just a thought.

 

[CheckerHater]

Sounds meaningless to me.

Position specified wrt A already controls perpendicularity

Round tolerance zone being able to translate in one direction creates flat tolerance zone

Makes more sense to add diameter symbol and remove reference to B from position callout. You can replace position with perpendicularity as well

Then you control edge B wrt A and hole C using profile. Then control the rest of the part to A, B, and C using profile as original drawing suggests.

 

[pmarc]

It is not meaningless, CH.

It is true that cylindrical tolerance zone being able to translate in one direction creates "resultant" tolerance zone that is a space between two parallel planes, but without the diameter symbol the axis of the hole could tilt without any limitation in the unconstrained direction. The diameter symbol makes this impossible to happen.

In order to say what dimensioning scheme makes sense and what does not, we would probably need to know how this part is mounted against its counterpart(s) in an assembly, don't you think?

 

[axym]

Hi All,

I agree with pmarc (as usual). If the diameter symbol is not specified, then the feature's tilt is not controlled. A parallel-plane zone does not apply the same control as a cylindrical zone that translates. This is often overlooked.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca