Composite tolerance

[3DDave]

Ref ASME Y14.5-2018

Compare:

Figure 10-51 Positional Tolerancing for Coaxial Holes of Same Size, Partial (Parallelism) Refinement of Feature of
Feature-Relating Axis

to

Figure 11-35 Composite Profile With Dynamic Profile to Control Form

If Figure 10-51 was altered to be a single hole in a block the following observation comes to mind:

If it's a profile control a composite tolerance can be applied to a single feature, but if it's a position conntrol, a composite tolerance is not allowed for a single feature even though it may be just as useful to refine the orientation of that single feature to one or more of the upper segment datums.

 

[Burunduk]

"but if it's a position conntrol, a composite tolerance is not allowed for a single feature"

Why not?

 

[Burunduk]

It's not disallowed. But may not be "just as useful". You can always use orientation tolerances to refine the orientation of a single axis or center plane. Not so for non-planar surfaces or complex features.

 

[pmarc]

I don't think that a composite position applied to a truly single feature makes sense.

In altered fig. 10-51, an orientation tolerance could easily replace the lower segment of the composite callout.

However, if in that altered figure there was a second hole controlled with a similar composite position tolerance and the design intent was to use the lower segments of both composite FCFs to control spacing between the holes (in addition to the orientation control wrt A and B), then addition of SIM REQT next to both lower segments would make both composite callouts meaningful, even though technically both would be applied to individual features.

 

[3DDave]

Why not?

Because for position the lower segments are feature relating tolerance controls. Cannot relate just one feature.

Unless the Book says so.

 

[Burunduk]

I would still say an extension of a principle is possible, but again, not very useful.

 

[3DDave]

It should have been clearly defined independently of the geometric control instead of being declared as Feature-Relating Tolerance Zone for position tolerance and just excepted for profile. Instead the application is specifically omitted for position tolerances in spite of spending hundreds of words and dozens of examples.

Specifically, under 10.5.1 Composite Positional Tolerancing:

(b) FRTZF. Each lower segment is a feature-relating control that governs the smaller positional tolerance for each feature of size within the pattern (feature-tofeature relationship).

With only one feature there is no pattern for position, but the profile subgroup carved out an exception for a non-pattern use which could be applied to a nominally circular feature in both cases.

This is a frequent flaw in the construction of the standard - the use of by-example rather than by-rule to define the way the symbols are interpreted means that only the given examples and explanations of those examples are acceptable, leaving "extension" largely an uninterpretable mess of arguments that cannot be traced with a by-rule certainty. They did the same for number of levels in a composite tolerance - spent 50 years limiting to two levels. Now it's three, but with customized datum references it could be six, one level for each degree of freedom.

 

[Burunduk]

The definitions for a pattern of features profiled by a composite control are the same as for position, including PLTZF and FRTZF.
The Profile section also includes a separate description for a single feature controlled by a composite FCF, which uses different terms. The first segment is a "profile-locating control" and the following segments are "profile-feature controls" (and I'm not sure why they came up with such a name. It seems "profile-orienting control" could be more appropriate). I suppose they didn't make a similar distinction between patterns and single features in the Position tolerances section because it is much less useful from the reasons already mentioned.