difference between composite and multi segment pos tol in asme y14.5 2009 5

[Gopinath K]

Hi All
Can any one explain exact difference between composite and multi-segment pos tolerance, is Multi segment used for refinement of uppersegment ?
in both lower segment controls orientation and feature relation then what is the difference and how to choose which one to use with same datums.
refer images from asme y 2009 and list the difference pls
image from fig 7.39a and 7.46 c

 

[3DDave]

A star for Burunduk for his last two replies as they, in my opinion, summarize the two topics - 1) the need for B in the composite FCF and 2) the rotation constraint by C - very well.

No one has argued that B is not needed in the composite FCF.

It should not be required as decoration in the lower segment of the FCF.

 

[pmarc]

3DDave,
You know exactly what I meant.

In the discussed case, B in the lower segment of the FCF is not a decoration. That's what I and, if I understand correctly, Evan and Burunduk have been trying to explain.

You have a right to believe it is otherwise.

I think it's now time to say that we agree to disagree.

 

[3DDave]

Evan, it cannot be a refinement of a requirement if the basis for that refinement is allowed to change.

Are you allowing for the adjustment of the part between inspection steps to generate different datum simulators/true geometric counterparts for the same datum feature references on a composite tolerance?

That each level is allowed different orientations of the part relative to the identified datum simulators/true geometric counterparts?

How much tolerance is allowed for those shifts in orientation?

How does that affect tolerance stacks?

Then comes this question: If the part orientation in a composite tolerance is allowed to change while using same references/same order why then the requirement for the order being the same?

My position is rational - fix the location and orientation of the part relative to the datum simulators/true geometric counterparts established by the first segment and use those datum simulators/true geometric counterparts as references to establish orientation as mentioned in the lower segments. If a reference does not establish orientation or is not desired to control orientation, it can be omitted.

What is irrational - requirement to include references that cannot control orientation in a segment with the limited purpose of controlling orientation.

 

[axym]

3DDave,

You asserted "for the lower segment only: since datum axis B is nominally perpendicular to datum plane A, then it is redundant and controls nothing". Now you're saying that no one has argued that B is not needed in the composite FCF. Most of us would take the phrase "it is redundant and controls nothing" to mean that you think that it is not needed. If you meant something else by that, we're not going to get it. If you're still thinking that B would be referenced only for decoration, to satisfy Y14.5's arbitrary rule, then you disagree with pmarc and I. We're saying that B is needed in the lower segment because referencing it affects the orientation of the DRF. If you disagree, that's fine.

I'll try to at least briefly answer your other questions from the last post:

I know that Y14.5 makes some statements about the lower segment being a refinement of the upper segment, but I don't think that this kind of description should be given very much weight. To me, what really matters are the rules governing the tolerance zones and datum feature simulators. So if you want to argue that the lower segment is not really a refinement of the upper segment if the DRF's are different, then I suppose that's probably right.

If the datum feature references in the upper and lower segments are exactly the same, then the simulators would be exactly the same. In this case, I would say that the actual part must be adjusted in the same way for the inspection of each segment. If the datum feature references in the lower segment are different than in the upper segment, then the simulators for the lower segment would be a subset of the simulators for the upper segment. In that case, I would say that the actual part can be adjusted differently for each segment. Regarding how much tolerance is allowed for those shifts in orientation or how that affects tolerance stacks, there's no simple answer to that.

I'm not really sure what the reasons were for Y14.5's requirement for the order to be the same. I believe it was mainly the idea that the lower segment is a refinement of the upper segment. I believed they used to use the term "liberation within given limits" or something like that. I wasn't around when composite FCF's were first introduced, but I have heard from friends that there was a lot of debate over how the lower segment should work. I would agree that if the datum features in the lower segment are different than in the upper segment, it's not really liberation within the same limits. They could have just stated that the datum features in the lower segment must be exactly the same as in the upper segment, and not allowed the subset cases. Then the orientation of the DRF would be the same for both. But they didn't do that.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[3DDave]

Evan,

"We're saying that B is needed in the lower segment because referencing it affects the orientation of the DRF. If you disagree, that's fine."

You say that because you accept that the part can be repositioned for the lower segment and therefore the tolerance zone for the lower segment can be allowed to exit the tolerance zone for the higher segment. It is your statements that support it isn't a refinement. You misread the conclusions your statements support for my understanding.

I say there is only one DRF in a composite tolerance. Every level has access to exactly what is established in the top level. They cannot be redefined.

If they can be redefined then there is no need to say they have to be repeated, although they aren't repeated in their entirety or at all - then any references should be accepted. Why not, in the example, allow [A|C(M)] if the user wants that to be repositioned basis?

That the committee screwed this up is clear to me. That the result is not clear to so many users brings me to think maybe the same logic used to eliminate symmetry and concentricity for being misunderstood needs to be used to eliminate composite tolerancing as well. If, as you suggest, the tolerance stack caused by repositioning cannot be calculated then that is a huge problem.

 

[Burunduk]

3D: "The FRTZF has no requirement for any datum references, so this is a flawed statement. If you had parroted what I wrote you would have been correct."

If the FRTZF has datum references, then it is constrained in rotation to a DRF. The actual part has also constraints applied to it by the datum simulators (fixture).  If it has no datum references, it only refines the form.

3D: "So the DRFs can be entirely different? That doesn't seem in the spirit of the composite rules."

No, if there are DRFs for both, they can't be entirely different. They can only differ in terms of establishment and in terms of the constraints being applied as much as A, B or A is different from A, B, C. If the last segment doesn't reference datum features at all then I guess the question is irrelevant to that case, and whatever you consider the "spirit of the composite rules" is also irrelevant to it.

3D: "But if the datum reference frame is already established then there is no need for that."

Already established when? You keep insisting on some made up theory in which the datum references in the lower segments do not establish regular datum reference frames, but that is not supported by anything.

 

[Burunduk]

Evan, it cannot be a refinement of a requirement if the basis for that refinement is allowed to change

That is obviously not true, unless you believe that a best-fit floating tolerance zone of a flatness control can't be a refinement of a rotation constrained orientation tolerance or a rotation and translation constrained profile tolerance.

At this point I think the best solution for you would be to forget everything you think you know about this entire subject and attend a good training course, or carefully read some professional literature to learn it.

 

[3DDave]

"If it has no datum references, it only refines the form."

Also not correct. It refines feature-to-feature location and orientation in composite tolerance FCFs.

"No, if there are DRFs for both, they can't be entirely different. "

If they are different they are different. There are no partial differences. There is one frame. Singular. The lower segments may refer to parts of that one frame but that does not redefine the common elements.

It is supported by reason rather than rote. If you are uncomfortable with working the reasoning you can just stop.

As Mark Twain wrote - it is impossible to reason a person out of a position they did not reason themselves into. You take the book at face value and apply no reason to it's rules. So I end replying on your rote responses.
---
"Evan, it cannot be a refinement of a requirement if the basis for that refinement is allowed to change"

"That is obviously not true, unless you believe that a best-fit floating tolerance zone of a flatness control can't be a refinement of a rotation constrained orientation tolerance or a rotation and translation constrained profile tolerance."

That is so far from a composite tolerance construction as to need a thread on Quora.

 

[Burunduk]

You seem not able or refusing to see the difference between "different DRFs" and "completely different DRFs". You also have proven not to understand what DRFs that are different but not entirely different have in common, which makes them work together in a composite tolerance to achieve refinements.

"It is supported by reason rather rote" - No, what you say is only supported by shutting your eyes and ears to whatever anyone else says and repeating statements that make no sense.

Also not correct. It refines feature-to-feature location and orientation in composite tolerance FCFs.

That's true, you got me on this one. From some reason I was thinking about profile tolerances rather than composite tolerances applied to patterns. I got tired from trying to explain stuff to you, I guess. Maybe it's a sign to stop.

 

[3DDave]

When datum simulators/true geometric counterparts are fixed relative to the part for a composite feature control frame at the top level they should remain fixed for all lower levels and not be redefined at any level.

Does that make sense?

Yes or no.

If "no" then explain how it is useful and accountable for the lower level controls of the feature relating frame to escape from the limits of those above them.

 

[Burunduk]

No, it does not make sense.
Consider a simple rectangular block that has 3 nominally perpendicular surfaces designated as datum features A, B and C, and two through holes perpendicular to datum A.
You can use a composite position tolerance for the holes referencing A, B, C in the top segment and A, B in the following segment. After the top segment requirement is verified, there is no more need to keep the part fixtured to the simulator of datum C. For the second segment, the part can only be supported by the datum A and B simulators. The holes would be evaluated for perpendicular orientation to datum A, lack of rotation in the direction constrained by B, and mutual location. You don't have to keep the datum C simulator or the part static in the directions that C arrested for the first segment for that. If the bottom segment does not reference any datums, then you don't need any simulators or constraints to verify it. Only the mutual location and orientation of the holes are then considered.

Unlike you may conclude from just reading the descriptions in the standard without giving any thought to the practical application of the concepts, You don't need to actually verify for each lower segment that the "feature relating" tolerance zones do not exit the tolerance zones of the segment above it. In practice, each segment's tolerance zones are respected as long as the features are found to fall within them: Once it's found that the features are within the PLTZF tolerance zones constrained by A, B, C, you no longer need to worry about that DRF when verifying the following segments.

 

[3DDave]

So - you say it makes no sense to leave an inspection element in place that isn't being used - which is exactly what I said was the reason to remove an element that wasn't being used.

Your agreement with me is astonishing and welcome aboard.

"You don't need to actually verify for each lower segment that the "feature relating" tolerance zones do not exit the tolerance zones of the segment above it."

That isn't what I asked. The portions that do exit cannot be accepted and removing a means to find out allows accepting out-of-tolerance parts.

 

[axym]

Hi All,

I would say that comparing the tolerance zones from one segment to the tolerance zones from the other segment doesn't add much (if any) value. The part features need to conform to the requirements from both segments, and each requirement can be verified on its own. We don't have to be concerned with the lower segment zones exiting the upper segment zones, or as Y14.5 puts it "portions of the FRTZF may lie outside the PLTZF but are not usable". They're still usable, for verification of the lower segment. The features can conform to one of the requirements but not the other - this doesn't mean that out-of-tolerance parts are accepted.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[3DDave]

The problem is when the part is repositioned in a way that would have been rejected the upper upper requirement so it passes on the lower one.

If the standard shows that the lower segment DRF is no longer oriented to match the upper DRF, that the typical datum simulator for A is different between the two, and this motion is acceptable, that would support the contention that lower level DRFs are entirely independent.

Is that what the standard has ever shown or described?

 

[Burunduk]

The problem is when the part is repositioned in a way that would have been rejected the upper upper requirement so it passes on the lower one.

This is actually prevented by keeping the datum precedence order in the lower segments. Like not skipping the B of A,B,C in the A,C manner for a lower segment even when B does not constrain rotations.

 

[3DDave]

Burunduk,

It is (perhaps) prevented by a rule, maybe. But there is zero reason to not make a rational rule that says they are not repositioned or restablished in special cases such as composite FCFs or under simultaneous requirements.

You should discuss with Evan as his belief they should be repositioned/repositionable regardless for composite tolerances.

 

[3DDave]

Per my suggestion one could limit the lower segments to any datum derived in the top segment; in the example, rotation could be unlocked for A, skip the useless B reference, and retain orientation of the feature relating frame to only C.

Perhaps, in that case, a new notation for all the lower segments.

Examples:

[<A,C>] or [<C>] indicating translation is allowed.