Composite/Multi Single Segment FCF @ MMC 2

[JakG]

Hi everyone,

I have a question regarding Composite/Multi Single Segment FCF and application of MMC. Are there any rules to this? How would it work if you had bottom frame at MMC but the top frame at RFS. Since top is locating it, and bottom refines the location. Thank you for any insight!

 

[pmarc]

To clarify, are you asking about the modifiers after the tolerance value (material condition modifiers) or after the datum letter (material boundary modifiers).

Your description implies the former, but the thread title implies the latter.

 

[JakG]

To clarify, are you asking about the modifiers after the tolerance value (material condition modifiers) or after the datum letter (material boundary modifiers).

Your description implies the former, but the thread title implies the latter.

Youre correct, I was just reviewing a drawing with MMB and had that in my mind. Apologies hah. Fixed the tittle.

My question is directed to MMC application in second frame, but nothing on top frame.

 

[3DDave]

Single segments are entirely independent and what is in one doesn't affect any other.

Composite demands that whatever is in the top is in every lower level except for the removal of datum feature references from right to left and decreasing the tolerance value top to bottom.

 

[JakG]

Single segments are entirely independent and what is in one doesn't affect any other.

Composite demands that whatever is in the top is in every lower level except for the removal of datum feature references from right to left and decreasing the tolerance value top to bottom.

Understood, so if MMC is applied on a Composite, both frames must posses it. Now how would a gauge to check a multi single segment work if you had the top frame at RFS and bottom at MMC. Do we just do the math as normal but use the bottom frame tolerance and make that the pin/hole size?

 

[pmarc]

Material condition modifiers may be different in different segments, regardless of whether composite or multiple single segment.

As a matter of fact RFS in the upper segment and MMC in the lower segment of composite FCF may be a very reasonable choice in certain cases.

 

[JakG]

Material condition modifiers may be different in different segments, regardless of whether composite or multiple single segment.

As a matter of fact RFS in the upper segment and MMC in the lower segment of composite FCF may be a very reasonable choice in certain cases.

I can see the logic behind that... so upper segment located roughly, and bottom at MMC refines the located and allows us to create a hard gauge, that is located off upper segment.

 

[3DDave]

RFS on the upper means a hard gauge cannot be used on the upper and can be used on the MMC lower, to simulate those cases of simultaneously having an interference fit and a clearance fit with the mating part.

I'll have to be shown an example of how that works.

 

[pmarc]

It does not have to be about interference fit at all.

 

[3DDave]

No, it doesn't. Give an example.

 

[pmarc]

A square part with a square pattern of holes in the center for which bonus tolerance resulting from the presence of the MMC modifier in the upper segment of the composite position FCF might/would be detrimental to the "symmetry" of the location error of the pattern.

Examples may even get more exotic and still make sense from functional point of view. For example, LMC in the upper segment to protect wall thickness between the holes and the outer contour of the part; MMC in the lower segment to ensure assembly.

 

[3DDave]

The first example appears to be a complicated way to enforce a smaller MMC requirement on the upper segment without making obvious what the real limitation is.

The second example doesn't appear to offer any advantage over two single segment tolerances.

They may make sense per the rules, but they don't make sense as an engineering requirement.

 

[pmarc]

They don't make sense to you, but they make sense to me.

 

[3DDave]

Is there a product you have shipped where this was analyzed and peer reviewed?

 

[pmarc]

Yes, there is.

 

[3DDave]

Glad to hear it.

 

[Burunduk]

The first example appears to be a complicated way to enforce a smaller MMC requirement on the upper segment without making obvious what the real limitation is.

The second example doesn't appear to offer any advantage over two single segment tolerances.

They may make sense per the rules, but they don't make sense as an engineering requirement.

Enforce a smaller MMC requirement on the upper segment?
Do you mean it would somehow effect the minimum allowed size of the holes (I guess not) or would it somehow enforce a smaller positional tolerance at MMC for the upper segment even though it is specified at RFS. Why and how? Would you mind to clarify?

For the second example two single segments would tighten location to the DRF by the secod segment, while a composite tolerance may only refine orientation and within-pattern distances by the second segment.

Anyhow the OP asked about the rules ("Are there any rules to this?") so, "Composite demands that whatever is in the top is in every lower level except for the removal of datum feature references from right to left and decreasing the tolerance value top to bottom" was not a true answer.