Controlling Coplanarity of n surfaces in lower segment of Composite Profile Tolerance

[Nescius]

Forgive me if this is a simple matter, but I've been unable to find any examples relating to my question.

Of course, we can control the coplanarity of n surfaces by noting "nX" before a profile FCF; no datums need be referenced. As Y14.5 says, "Each surface must lie between two common parallel planes...". Emphasis mine.

Now, consider a composite profile FCF, preceded by "nX", where the lowermost segment references no datums. As above, we're refining the form of the surfaces, but must each of the n surfaces fall within two common parallel planes?

I say yes (all surfaces must conform "together"), but can't back it up with hard fact. My only evidence comes from this thought experiment: Consider a rectangular part outline controlled by an all-around composite profile tolerance, the lowermost segment referencing no datums. The rectangular profile, as a whole, is allowed to move in all 6 DOF. It does not simply boil down to 4 independent flatness controls.

So, does the coplanarity of n surfaces follow this same logic?

 

[pmarc]

Yes, it does. In case of two nominally coplanar surfaces controlled with two-tier composite profile FCF with no datum features referenced in the lower segment, the lower segment controls form of each individual face as well as mutual orientational and locational relationship between them, commonly called coplanarity. If this is not required, use flatness callout in addition to a single segment profile FCF, or place 'INDIVIDUALLY' after the lower segment of the composite FCF.

 

[Nescius]

Thank you very much, pmarc. I cannot pinpoint where my doubt came from in this case, probably just because it's something specific that I've never applied before.

It does make sense that the "nX" notation before the composite FCF is simply another way to group the surfaces together...much like an all-over symbol, all-around symbol, or the double-ended "between points" arrow. Even multiple leaders originating at the FCF appears to have this same effect (Fig. 8-16 in the 2009 standard).

You mention placing 'INDIVIDUALLY' after the lower segment in question of the composite FCF if we wish to negate the mutual location and orientation requirements that would normally be required by the segment. This makes perfect sense. How would you feel about noting 'INDIVIDUALLY' after such a segment where the profile tolerance is an all-around variety? Allowed, but strange? Let's assume a number of non-planar surfaces, so using flatness isn't an option.

 

[pmarc]

You mean more or less something like fig. 8-23 in Y14.5-2009?

 

[Nescius]

Not exactly, I think, but I may not be knowledgeable enough about composite profile patterns to really grasp the implications of Fig. 8-23, relative to my question.

You clarified for me that a lower segment in a composite profile tolerance does control the mutual orientation and location the governed surfaces. When you indicated that adding the "INDIVIDUALLY" callout after the lower segment would break that segment's mutual requirement, I wondered where else this principle could be used. See my attached drawing.

 

[pmarc]

A picture is worth a thousand words, as they say. This indeed is different situation from fig. 8-23.

First off, regarding the use of 'INIDIVIDUALLY' after the lower segment of composite profile callout (cases 6 through 9), I thought about it a little bit more and I am leaning towards saying that better option would be to go with two single segment profile callouts instead of composite (the second FCF would still have INDIVIDUALLY added). Not saying that composite callout would necessarily be bad or illegal, but two single segments seem to me more direct and better supported by the standard.

And secondly, your pictures reminded me about one more thing, and actually I would like to see what others think about it. It starts in case 2, where two leader lines are used but no '2X' precedes the profile FCF. I know that fig. 8-16 in the standard shows it exactly this way, meaning that basic distance between two tolerance zones does apply, but knowing what pattern definition (1.3.42) says, should the locational relationship between the zones really apply here? The definition does not mention that the use of 2 leader lines is enough to define a pattern of features. So shouldn't the profile FCF in fig. 8-16 be preceded by '2X' to make the surfaces a pattern, like in figs. 8-14 or 8-15 for instance?

Going back to cases in question, shouldn't 'nX' prefix be attached to profile FCFs in cases 2, 3, 4 and 7?

 

[patdh1028]

I agree with pmarc about pattern representation - you want the nX or some other 1.3.42 notation to indicate a pattern. If I was interpreting the drawing I would interpret multiple leaders sans nX as the same as multiple individual callouts.

As to cases 6+, it seems like the notation "efficiency" of case 6 doesn't translate well when used in cases 7-9. I would ask what you gain, other than avoiding putting a second note on the drawing, by using a composite note in the first place. A composite note is meant to control + refine a group of surfaces; if you put "INDIVIDUALLY" to indicate a form control to each surface i.e. breaking up the group, what is the purpose of using a composite note to begin with? Just use two notes: one with the datums to locate, and one with no datums and the "individually" note to control form if indicating a group as in an "all around" application.