Combining Multi Segment and Composite Position Tolerance 1

[soloeng21]

Simple yes or no question (hopefully).
Is it valid to combine the use of multi-segment and composite position tolerances?
See attached file. Let me know if seeing the actual part would be of any help.

 

[3DDave]

Yes - there is no rule or interpretation that prevents it. Not sure why one would do so, but it's allowed to pile them as high as required.

It isn't required that each segment contain at least one datum reference. Where did that idea come from?

 

[soloeng21]

There are no examples within the 2009 standard showing multi-segment position tolerances with the lowest segment containing no datum references. Are you saying it is valid to have the lowest segment contain no datum references?

From my understanding, the only cases where it is valid to not include datum references for a position tolerance are:
1.) To control coaxial features to each other (section 7.6.2.3 of 2009 standard)
2.) In the lowest segment of a composite position tolerance (section 7.5.1.8 of 2009 standard)

If there are additional cases where datum references don't need to be included I'd like to know (per the 2009 standard).

 

[3DDave]

I understand that you believe that only every single case that the standard explicitly shows an example of is allowable.

In that case no, you cannot do this. But everyone else can.

 

[soloeng21]

Haha. If I read between the lines here it sounds like you're implying that both FCF's would mean the same thing and the multi-segment callout I attached is in fact valid. Therefore, a datumless position tolerance essentially acts like a FRTZF.

 

[chez311]

I agree with 3DDave, we can't possibly expect the standard to cover every conceivable application. We have to take the basic concepts contained in the standard and apply them to our use cases. Theres even cases where we stretch the concepts a bit into "extensions of concept" but that tends to be a bit of a gray area, which this is not.

Therefore, a datumless position tolerance essentially acts like a FRTZF.

Well this is true insofar as the main difference between composite and multiple single segment tolerances is the way DOF are constrained wrt your DRF (lower segments/FRTZF of a composite tolerance only constrain rotational DOF wrt your DRF). If you remove datum feature references, then sure a datumless segment pretty much behaves the same in either case.

 

[chez311]

From my understanding, the only cases where it is valid to not include datum references for a position tolerance are:
1.) To control coaxial features to each other

Just to also add to my above comment, it might help you to think about how certain use cases are actually subsets of others even if they seem different. Coaxial features are just features which have a zero basic distance between their axes. While features with a non-zero basic distance might look different, the position tolerance does not care whether this distance is zero or non-zero, the same rules apply.

 

[axym]

Hi All,

I would agree that the single-segment callout and the composite callout are both valid. There is no reason why the lower segment in a multiple single segment callout cannot be datumless, even though Y14.5 only shows examples of this for composite FCF's (Fig. 7-49 and 7-51 in 2009). These could have been multiple single-segment, with no difference in the coaxiality control being applied in the lower segment.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca