Composite callout

[aniiben]

Does the addition of B secondary and C tertiary in the lower segment of the composite (FRTZF) brings any value to the callout? If yes, what is the value added?

What would be the differences between only A primary shown in the lower segment (FRTZF) on the composite versus what is shown on the embedded picture and circled in red?

 

[chez311]

aniiben,

The only thing that additional datums in the FRTZF (lower FCF) of a composite tolerance can do is constrain additional rotational DOF. The addition of B and C in the FRTZF (lower FCF) constrains rotation of the pattern's tolerance zone governed by the FRTZF and only allows it to translate relative to the DRF inside the PLTZF. Note that my conventions assume the z-axis is perpendicular to the datum A.

If only [A] were called out, it constrains the 2 rotational DOF about A (u,v) (ie: pattern axes are held perpendicular to A) but it would allow both translation (x,y,z) and rotation (w) of the FRTZF. (see Fig 7-38 and 7-41))

If only [A|B(M)] were called out, since datum B does not constrain any additional rotational DOF that is not already constrained by datum A, I believe it would still allow both translation (x,y,z) and rotation (w) of the FRTZF. There is no example for this in the standard, however it seems to me to be a logical conclusion.

If all [A|B(M)|C(M)] were called out, it would constrain the final rotational DOF (w) and only allow translation of the FRTZF (x,y,z) (see Fig 7-39 and 7-42).

Note that it is my understanding that even if adding an additional datum in the FRTZF would not constrain any additional DOF (for example, if in 7-39 the DRF was [A|B|C] in both the PLTZF and FRTZF) it could still be added for gauging/measurement reasons - especially if one wanted to add a simultaneous requirements note to the FRTZF/lower FCF.

 

[greenimi]

chez311,
You had me untill "If all [A|B(M)|C(M)] were called out, it would constrain the final rotational DOF (w) and only allow translation of the FRTZF (x,y,z) (see Fig 7-39 and 7-42)."

If all [A|B(M)|C(M)] were called out, it would constrain the final rotational DOF (w) and only allow translation of the FRTZF (x,y,z) (see Fig 7-39 and 7-42).

My follow-up question is: How?
How "C" is able/capable of constraining the final rotational DOF (w)?

 

[chez311]

greenimi,

Because secondary datum B constrains translation (x,y - which doesn't matter in this case because we are talking about the lower FCF/FRTZF of a composite tolerance and only rotation can be constrained in that case) and then tertiary datum C constrains rotation (w) about secondary datum B.

Am I missing something?

 

[greenimi]

Because secondary datum B constrains translation

Is "B" (as a datum feature in the FRTZF) allowed to constrain translation?

 

[chez311]

no - which is why i said "datum B constrains translation (x,y - which doesn't matter in this case because we are talking about the lower FCF/FRTZF of a composite tolerance and only rotation can be constrained in that case)", however I do believe the combination of B and C called out together as shown [A|B(M)|C(M)] could constrain rotation (w) of the FRTZF in this case.

edit: formatting

 

[chez311]

Sorry - I realize how I phrased that could be confusing, it made sense in my mind. Please ignore what I said about datum B constraining translation in x/y, I should not have mentioned it at all in this context.

The point is that I believe the combination of B and C constrains the final rotational DOF (w). Is there any reason to believe otherwise?

 

[greenimi]

The point is that I believe the combination of B and C constrains the final rotational DOF (w).

Exactly, ...that is my main question too.... what means "combination" in the context that "B" one should take precedence? Or what means "combination" when B is not allowed to stop any translation?

 

[chez311]

greenimi,

I would treat it like you would any other datum callout/DRF as in any other FCF in regards to datum precedence and constraint of DOF, with the exception that any translational constraints are ignored due to the rules governing the behavior of the FRTZF.

I know its not a perfect analog, but taking a look at the figure 7-42 that I noted, the combination of B and C constrains the final rotational DOF. Now I know that C could accomplish this on its own as it is a planar datum, but I still think it is still treated as a tertiary datum and follows standard datum precedence, per the note on the figure saying "FRTZF is constrained in rotation to datum plane A, datum axis B, and datum center plane C". I think you would have the same result if your tertiary datum C was an axis formed by a round FOS.

 

[TheTick]

Why would you have a composite tolerance where both lines have the same datums? All you do is duplicate and nullify the larger tolerance.

 

[greenimi]

Why would you have a composite tolerance where both lines have the same datums? All you do is duplicate and nullify the larger tolerance.

It is not true.
It is a composute tolerance and not multiple single segments.

 

[greenimi]

...but taking a look at the figure 7-42 that I noted, the combination of B and C constrains the final rotational DOF.

It is NOT.
In fig 7-42 "C" is capable to constrain the remainign degree of freedom (rotational one as that's only that is allowed to arrest) due to the part configuration.

In the OP case, "C" is not able/capable to arrest / constrain the remaining rotation DOF

 

[chez311]

greenimi,

I don't want to put words in your mouth, but it sounds like you think that datums called out in the FRTZF ignore all previously established rules of datum precedence and formation of a DRF (ie: primary planar datum, secondary axis datum, and tertiary axis datum constrains all rotational DOF) and are treated individually. I don't think you will get me to agree with you there, I would consider it more of a special case where all previously established rules apply, except that any translational DOF constraints are ignored - this seems to follow more logically for me, but I would say it is not explicitly stated in the text unfortunately.

Perhaps one of the more seasoned experts could weigh in? I could well be wrong in this matter.

 

[greenimi]

My concern is how come “B” individually as secondary in the FCF does nothing to arrest the remaining rotation DOF
and
also “C” ” individually in the FCF does nothing to arrest the remaining rotation DOF,

but together, as a combination “B” secondary and “C” tertiary (not B-C) are able to do something useful (constrain remaining rotation DOF)?

How come the combination have “additional “ power to do it?

 

[3DDave]

greenimi - you opened the can. Now the worms are loose.

 

[chez311]

My concern is how come “B” individually as secondary in the FCF does nothing to arrest the remaining rotation DOF

Because thats how it operates in every other case? B cannot constrain any other rotational DOF that A has not already constrained - this would be the case if it were in the FRTZF, PLTZF, or in its own single control frame standard position control.

also “C” ” individually in the FCF does nothing to arrest the remaining rotation DOF

There is no example here of only datum C in the FRTZF because it must follow the same structure/precedence as the PLTZF.

but together, as a combination “B” secondary and “C” tertiary (not B-C) are able to do something useful (constrain remaining rotation DOF)?

How come the combination have “additional “ power to do it?

Because, once again, thats how it operates in every other case? My position is that constructing a DRF in the FRTZF of a composite callout would be considered a special case (instead of following a totally different set of rules) of constructing any other DRF in any other FCF - just with the exception that translational DOF are not constrained.

I don't really think this is much of a stretch.Actually I don't think its a stretch at all - I think its wholly consistent with the rest of the text in regards to datums and setting up a DRF. Allow me to pose a question to you - why do you think that in every other case the DRF as shown [A|B(M)|C(M)] constrains all rotational DOF (u,v,w) but in this case it doesn't?

 

[aniiben]

Because, as I said before, C is capable of doing its job but in the OP case it is not.

 

[chez311]

Aniiben,
I'm not sure what you mean by "as you said before" - i only see your OP, do you have another response in this thread I'm missing?

Aniiben/greenimi,
Is there any example from the standard, either in verbiage or figure, that says that the datums in a FRTZF should be treated individually? ie: that all interactions between datums in the FRTZF should be ignored, in this case that a pair of round FOS datum features that would constrain the final rotational DOF in any other case, does not here?

 

[greenimi]

Agreed with OP. I learned,long time ago granted, about can, may must rule from training class held by Multimetrics.
If a datum feature can and it may then it must arrest DOF.

 

[greenimi]

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