Multi-segment FCF (refinement) 1

[SpaciouS]

Are the segments of a multi-segment FCF to be written from top down (& not bottom up)

And if so

Does the lower segment of a multi-segment FCF have to be a refinement of the preceding segments (like a composite FCF)?




Thanks,
Sean

 

[DeanD3W]

Sean,
I think the short answers to your two questions are "yes" and "yes"...

As a matter of practice, feature control frames are stacked with the one having a larger tolerance, and more constrained tolerance zone, on top, then as you go down in the stack, the tolerance values become tighter and the tolerance zones are less constrained, based upon the type of tolerance and/or the datum features referenced.

This means a location tolerance that references datum features that constrain all 6 degrees of freedom would be listed first, then maybe a location tolerance with fewer datum features referenced, then an orientation control, then a form control.

Having said this, I don't mean to imply that such a stack of feature controls frames is a common need though... Most common is that a size tolerance provides sufficient form control, and a location tolerance provides sufficient orientation control, or if using profile, all of the needed location, orientation, form, and possibly size, control may come from a single feature control frame (as I expect you already know - I add this just for the sake of attempting to answer this completely).

Even though stacking feature control frames with tighter values as you go downward in the stack, is normal practice, and to avoid meaningless controls as you go downward in the stack there are some dependencies to consider regarding datum features referenced (for instance, identical datum reference frames would not be specified for two stacked position tolerances), the requirement imposed by each is separate and independent from all others... Technically you wouldn't really have to stack the feature control frames following a normal convention, except section 3.4.5 and figure 3-26 of ASME Y14.5-2009 (or the same section and figure 3-22 in Y14.5M-1994) imply this convention as a "shall" type of requirement.

Dean

http://www.d3w-engineering.com

 

[ptruitt]

To your first question, there is no top or bottom, if I understand it correctly. "No" to your second question.

Peter Truitt
Minnesota

 

[SpaciouS]

Thanks, I agree.

Thanks,
Sean

 

[SpaciouS]

I've never seen a figure or any text, in any of the ASME standards, that supports stacking segments of a multi-segment FCF upward. I personally have always stacked one segment below the other, while refining tolerances & reducing datum references. I'm not saying "if an example doesn't exist in the standard, it can't be done in real life", because I think we all know that's not true, but I do tend to lean this way on stacking FCF's. But my question is does it have to be one way or the other. I don't want to falsely correct some bodies work if there is not a requirement on the matter.

Peter, do you disagree with both Dean and I?


Thanks,
Sean

 

[fsincox]

In the past it was not always done that way. I have seen drawings "in the field" and "in reference materials" before the 1994 version where they were reversed. It really did not matter until the committe had to deal seriously with the issue of the meaning of secondary or tertiary datum restatment in a composite feature control frame. Then standard got specific ASME Y14.5-1994, 5.4.1 states: "..but the lower segment is always a subset of the upper segment."
Frank

 

[ptruitt]

I had better do some homework. Others here are more proficient in this. I thought that composite call-outs were the only situations where the lower FCFs were refinements of the upper. I'll re-read the Standard in the next few days, but I should have avoided the question given my other obligations, today.

Peter Truitt
Minnesota

 

[SpaciouS]

ASME Y14.5M, 5.4.1 is talking about composite FCF when making that statement, however the idea is that the UPPER segment is for PLTZF and the LOWER segment is for FRTZF. Para. 5.4.1.3 "Two Single-Segment FCF" clarifies a bit on what the upper and lower segments are referring to in terms of PLTZF and FRTZF.

Even though, to me the standard is a little unclear, I feel segments should be stacked one below the other and that the lower segment is a refined tolerance to the upper.

Thanks Fank.

Thanks,
Sean

 

[fsincox]

ptruitt,
Do not fret, it really did not matter until troublemakers like myself tried to actually use this stuff. We wanted datum restatement to imply orientation only. The 1982 standard was not clear on this as they were only showing restatements of the primary datum which when used traditionally implied orientation only.
FYI, It is my understanding the ISO does not agree with ASME on this point, either. Here is one area I am firmly with ASME as a composite framework vs separate statements now gives us additional options, the ISO way I do not gain anything.
Frank

 

[fsincox]

I am sorry, you are right,
I am specifically refering to composite tolerances. Otherwise, as I have already stated, I do not believe it functionally matters.
Thanks, Spacious.
Frank

 

[SpaciouS]

What does not functionally matter?

Thanks,
Sean

 

[fsincox]

Spacious,
I do not believe the order of the tolerance statements was that critical until the composite issue I mentioned, above.
Frank

 

[Belanger]

Since the OP is talking about single-segment FCFs, then it is not mandatory that the bigger tolerance be on top. That is definitely the convention, but the real meaning to each FCF is determined by the datum references, not which one is on top.


John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems