How to control features separately 3

[bxbzq]

Take fig 7-54 in '09 std as an example, the four holes are treated as two separate patterns. If I want each hole to be controlled separately, how do I specify on the drawing?

 

[pmarc]

Dimension size of each hole separately and attach position FCF with SEP REQT note to each of these dimensions.

Of course one may ask, what if there are 20 or 40 holes and not 4? Should 20 or 40 separate FCF be shown then? In such a case I would go with some kind of tabulated tolerances and additional note clarifying that each tolerance in the table shall be verified as a separate requirement.

 

[CheckerHater]

May I ask stupid questions such as "why" and "how"?

Why - what exactly are the benefits of creating a pattern that is not a pattern? Are the benefits functional, manufacturing cost, or any other kind?

How - how process of creating "independent" holes will be different from creating a pattern? Will you take part out of the machine after creating of each hole and then put it back in to drill another hole?

 

[Belanger]

CH...

As to why: Suppose there are two separate things plugging into that part. One plug might have two small pins that insert into the two small holes, and then a totally different free-floating plug that inserts into the larger holes.
With that type of assembly, it would be too restrictive to have all four holes be toleranced simultaneously.

As to how: You are assuming that this part is machined. Why? Couldn't there be another process that could have different results?
Plus, the standard says more weight is to be given to the function, so we should use SEP REQ if the function allows it, regardless of the mfg process.

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

 

[Tunalover]

I suppose you have to have a copy of the 2009 standard to participate in this thread? There are lots of people who haven't adopted that yet, instead relying on the 1994 standard.

Tunalover

 

[Belanger]

Tunalover -- it's the same as Fig. 5-18 in the 1994 edition.


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

 

[CheckerHater]

@ Belanger:

I am not surprised you missed the fact that OP asked how to make "2 small holes" independent from each other.
That is 2 holes both controlled wrt A|B|C to the same tolerance, but allowed to translate from each other within larger tolerance.
Imagine FRTZF tolerance being larger than PLTZF tolerance. Can you?

Pmarc suggested that you can attach FCF with "SEP REQ" note to every single hole and thus create pattern that is not a pattern.
I see it as a physical and mathematical impossibility, so let me repeat same question" why and how?

 

[pmarc]

CH,
Are you sure bxbzq asked: "to make "2 small holes" independent from each other. That is 2 holes both controlled wrt A|B|C to the same tolerance, but allowed to translate from each other within larger tolerance."?

The way I understood his question was: how to make each of 4 holes separate of each other.

 

[CheckerHater]

If all 4 holes are separate from each other, doesn't it mean that every 2 of them are also separate from each other, including 2 small holes?
Plus, it was Belanger who mentioned "2 small holes" being separate from 2 big holes, so I had to correct him saying that 2 small holes are to be separate from each other as well.

 

[pmarc]

Yes, if all 4 holes are independent from each other, it means that every 2 of them are also independent from each other. But that still does not mean that OP asked how to >>make "2 small holes" independent from each other<<. He asked how to make each hole (not just "2 small holes") controlled separately. So if our understanding of OP's question is identical, I do not really get the part of your comment about PLTZF and FRTZF. I also do not understand why you think it is "a physical and mathematical impossibility" to control each of the holes separately.

As to J-P's answer on your "why" question, I believe he just tried to explain why someone might want to use SEP REQT for part shown in figure 7-54. So yes, he did not exactly answer to your question, but I think it is not that difficult to imagine that the very same logic can be used when 4 separate things are plugged into this part.

As to your "how" question, I would answer that the process of creating these holes as a pattern does not have to be different from creating them as independent features. This can be the very same process (regardless if it is machining, molding or something else). It is just that SEP REQT defines looser relationship between holes than SIM REQT, so gives more possibilities to manufacturing to produce features that meet trully functional requirements.

 

[CheckerHater]

@ pmarc:

How "SEP REQT defines looser relationship between holes" is different from "FRTZF tolerance being larger than PLTZF tolerance"?

Isn't FRTZF tolerance control relationship between holes?

 

[pmarc]

Sorry CH, but since these four holes are not going to be treated as a pattern (because SEP REQT note was applied), I do not understand why you are using terms PLTZF and FRTZF in this particular case.

 

[Belanger]

CH -- yes, I guess I did miss the part about "each hole" meaning each one of the four holes.

That said, you needn't be so snotty about it. My challenges to your post were at least polite!

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

 

[CheckerHater]

JP, where exactly I was impolite?

Looking back I can only see mention of you missing OP's point. All I wanted to say was that request was quite unusual, so it was easy to make mistake.

If I also offended you with something else, I apologize. Didn't mean it.

I have rough day today and probably won't post anything else. I would like to come back with more detailed explanation but don't know if will get a chance.

 

[Belanger]

Come on. You said "I'm not surprised..."

What, do I have a track record of not being able to read?

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

 

[CheckerHater]

Should I repeat? All I wanted to say was that request was quite unusual, so it was easy to make mistake.

Truly being offended is America's favorite past time.

 

[bxbzq]

Thanks guys. The comments and answers from pmarc and Belanger are clear to me.

 

[CheckerHater]

OK, let me make myself clear and explain (in form of a comic book) why you cannot turn pattern on and off like a light bulb, that pattern may exist regardless of you wishes and not everything you put on your drawing will make a difference.

 

[pmarc]

CH,
Cool graphics. Unfortunately it does not take into account key aspect of the problem - your both top drawings are not showing datum features B and C referenced at MMB (and they can't, because you used planar datum features B and C, which is totally diffetent from what is shown in fig. 7-54). In other words you simply got rid of datum feature shift effect (I don't know why), whilst this is absolutely essential to understand the difference between having and not having SEP REQT in fig. 7-54 and in the scenario with 4 "separate" holes.

Side notes (not related to the subject of discussion):
1. Since none of positional callouts on your picture is specified at MMC, per the standard you should not be using term "VIRTUAL CONDITION represented by each pin".
2. It looks like on your both bottom pictures there is a loose between gage pins and corresponding holes. This can't happen, because both pins have to expand until maximum contact with their holes is achieved - all because of the position tolerance value specified RFS, not at MMC.
3. Believe it or not, but there are some GD&T authorities claiming that specifying SEP REQT for both pins on the top right drawing makes these two drawings different in geometrical sense. Imagine that as-produced datum feature A is convex (within its form tolerance), so that the part can wobble when brought into contact with datum feature simulator A. This means there is more than one candidate datum A and in consequence more than one possible datum reference frame A, B, C. When SIM REQT is invoked by default, position of both holes shall be verified simultaneously relative to the very same datum reference frame. When SEP REQT is defined both holes can be verified relative to two different datum reference frames A, B, C, because the part can be laid down on datum feature simulator A (oriented to this datum feature simulator) in more than one acceptable way.

 

[CheckerHater]

pmarc,
I can read as well as JP and I have a copy of 2009
When you mentioned "20 or 40 holes", did you mean Fig.7-54, because I don't remember it having 20 or 40 holes. This makes me think you were talking about applying SEP REQT in general, and in general the idea still flawed.
1. Yes, I missed MMC on my position; I do not always have time to proofread when posting from work.
2. Same as 1. You didn't have to make it 2 paragraphs.
3. Borderline demagoguery. "What if" is not an argument. What if datum A is represented by 3 target points so the same part is always restrained exactly the same way?
The idea that simply applying SEP REQT to several holes will automatically dissolve the pattern is still questionable to me.