fig 4-16 -Y14.5-2009 and datum feature C -as tertiary

[gabimo]

If we remove datum feature C from the positional callout of Ø7.0 OD, does the meaning of the drawing or 4.11.6.1 (a, b and c)subsequent explanations change?
If yes, how?
If no, why?

 

[greenimi]

Not an expert. I would say : No (does not change the meaning for 4.11.6.1 )
I can provide a picture for quick reference.

If I am wrong, I will stand corrected.

 

[mkcski]

Yes. The boss will not be located on the center-plane that passes through datum axis B and the datum center-plane established from/by the keyway - Datum C. Without Datum C the bosses can be located anywhere "around" datum axis B.

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[JNieman]

Datum C is not directly referenced at all in that section, so it doesn't appear to. It referenced Datum D, which is located using C as a tertiary, but I don't see how removing that would have any real impact on how to interpret the conditions explained in 4.11.6.1

 

[mkcski]

Sorry. I missed the intent of you question. Removing C will NOT change the determination of MMB as discussed in 4.11.6.1 because C is not part of the calculation. It does, as I mention in my previous post, change the motion controlled for locating Datum feature D inself

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[greenimi]

And the drawing will still be fully defined (well only for the features shown anyway) due to the simultaneous requirement implied A primary and B(MMB ) secondary.

Therefore, C does nothing?
Can we conclude just that?
Or that is a stretch/ unfortunate misleading?

 

[mkcski]

greenimi:

I agree with your statement for the simultaneous reqmnt. But I always waffle at not adding the "extra" datum (C). Although redundant, datum C adds clarity for those who are not cognizant with simultaneous reqmnt conventions.

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[AndrewTT]

For my clarity, are we agreed that the material boundary at D does not change if there is no Datum C call out on the keyway and Datum C is not called out on the ∅7 post? But the part could now look like the attached image.

In reality you would only not have a datum C if there were no keyway, just a simple hole for Datum B, correct? But then the Datum D would become your clocking feature, that is needed, to use a profile geometric tolerance to describe the outer periphery of the part. Still correct?

 

[greenimi]

"Re:"But the part could now look like the attached image."
No.
Zero basic implied.
Sim Req. in effect.

 

[greenimi]

 

[3DDave]

Does simultaneous requirements apply to sub-sets of the FCF? That would indicate that A|B and A|D would not allow repositioning relative to A. I'm not sure there is anything that supports something besides the entire Datum Reference Frame.

The language in the standard is vague on this. Perhaps:

When a Datum Reference Frame is referred to in multiple feature control frames, including datum feature modifiers, then the part shall not be repositioned when checking individual features that have tolerances based on that Datum Reference Frame.

 

[greenimi]

3DDave,
Well nobody said it does apply to sub-sets. If you are reading a little more careful the thread the OP talked about removing C and then DRF to become A and B, same as the slot[pre][/pre]

 

[3DDave]

greenimi - Sure, when you write it that way. I do dislike this figure, as discussed previously.

I should have fallen back to an investigative method I suggested before, which is to envision the gauge required for each case and see if there would be any difference between them. This avoids trying to figure out which part variations are compliant in favor of looking at the compliance criteria. Since the simulator for C{M} is identical to the one that verifies the slot location, there is no difference, so I agree.

If [-D-] is changed to be controlled by [A|B(M)|C], in which case the gauges would not be the same for the removal of C.