Rules for composite profile, datums in lower segment

[Nescius]

I have read that the datums in the the lower segment of a composite profile tolerance must appear in the same order as they appear in the upper segment. While this makes sense, I have also read that the lower segment datums must repeat those in the upper segment without skipping any, meaning that if the upper segment includes "A B C", the lower segment could include just "A", or "A B", but not "A C".

This makes sense in all the convenient textbook examples, but what if your datum B is a hole perpendicular to datum A? See my attached example.

To me, it seems silly to include a third, "clocking" datum in the lower segment of a composite profile tolerance when the second datum listed should define this orientation. Am I not thinking about this in the right way, or is the information I read not correct? I am happy to link said information if the rules of the forum allow it; I'm new here and not sure about those types of policies.

 

[3DDave]

You don't have to make the control frame composite if you don't want it to be limited by the rules that apply to composite control frames.

In this case, I suspect there is no real-world application that allows float for the location of the rectangle, but requires it to be precisely oriented to the width without using an explicit orientation tolerance. This kind of unrealistic requirement often results in difficult to create datum references.

If required, I'd use profile to A|B|C and refined to A and add a parallelism tolerance to the feature relative to C to be explicit in what the orientation control required.

Alternatively, in the 2009 version of '14.5, you can add the datum control extensions to negate the location contribution to refinement of datum C when it's repeated in a separate FCF.

Or be satisfied with the composite tolerance definition that, since B does not affect orientation, but is required per the Standard, you just have to repeat it even if it doesn't contribute an orientation refinement in this special case.

 

[Nescius]

Thank you for your help and insight. My doubt arose because I could find only one source that truly explicitly stated that the lower segment datums must not "skip" any, for lack of a better word. Instead of doubting that source, I suppose I should have been praising it for being the clearest. Really, I didn't interpret the word "repeat" in a strict enough sense.

I do see that in 2009, 7.5.1(b)(3) (dealing with composite positional tolerancing of patterns) says "In some instances the repeated datum feature references may not constrain any degrees of freedom; however, they are necessary to maintain the identical datum reference frame..." It then points to Fig. 7.42, in which the secondary datum is a hole perpendicular to the primary datum, a surface.

Also, in 2009, 8.6.1.3(c) (dealing with composite profile tolerancing of multiple features) says "If different datums, different datum modifiers, or the same datums in a different order of precedence are specified, this constitutes a different datum reference frame and design requirements. This is not to be specified using the composite profile tolerancing method." I should have been more strict in my interpretation of the phrase "order of precedence". For example, the numbers 1, 2, 3, 5 are "in order", but 5 appears in the fourth position, a violation as I now understand it.

Regarding the function of the part, the all-around profile and orientation of the square boss must truly be controlled more tightly than the location of the boss. In a nutshell, this is the case because other components in the assembly have actual adjustment mechanisms that can account for this translation. There is no way to accommodate the boss being much out of orientation, though. Don't worry, the adjustment mechanisms exist for another (justified) reason, not just to allow slop in the part in question. The fact that they exist, though, does allow me a little leeway in the location of the boss. My exploration into using a composite profile tolerance for this feature was an attempt to take advantage of this leeway.

Thanks again!