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?

 

[greenimi]

CH and pmarc,

I would say if the MMC is present on the feature and on the datum (datum shift available) then the SEP REQT idea is NOT flawed.
SEP REQT: You are checking one hole (the GO pin has to go in the part and in the gage) and then you can REMOVE the first pin (A) from the assembly and check the second hole positional requirement (the second GO pin (B) has to go in the part and in the gage base)

When SEP REQT is not present (SIM REQT is implied) them you are not allowed to remove the first pin (A) and both pins (A and B) must go simultaneously to meet the print requirement.
If a part meet SIM REQT for sure meet SEP REQT which is not true vice-versa. SIM REQT make the requirements more stringent and I am sure you know that.

I don’t know if I agree with your last sentence in the posted picture: “ ….so the part passing inspection in the right fixture will pass inspection in the left one and vice-versa”. I have a small problem with this “vice-versa” added on the end of the sentence. But I might be wrong. Probably in your depicted scenario (RFS scenario/case) wouldn’t make any difference, but I am sure in the MMC case WILL make a difference.

 

[CheckerHater]

@greenimi:
In my example datums are definitely not at MMB. "vice-versa" will take place when datum shift is not possible, that's for sure.
The datum shift is treated differently by "GD&T authorities", Everybody agrees that it exists, but not everybody agrees that is should be used as bonus tolerance, so some authors even suggest to ignore it.
So I still behind my idea that SEP REQT will automatically dissolve any pattern, and some other, less exotic control should be used.

 

[3DDave]

"What if datum A is represented by 3 target points so the same part is always restrained exactly the same way?"

What if parts are not made perfectly? Then the datum B feature isn't always straight and the part can rotate even on the three target points defining A. SEP REQ is to identify when the condition of imperfect repeatability and lack of definite relationship between features is acceptable so that parts might be less expensive to make and might be simpler to inspect.

Vocabulary builder:

A demagogue /ˈdɛməɡɒɡ/ or rabble-rouser is a political leader in a democracy who appeals to the emotions, fears, prejudices, and ignorance of the lower classes in order to gain power and promote political motives. Demagogues usually oppose deliberation and advocate immediate, violent action to address a national crisis; they accuse moderate and thoughtful opponents of weakness. Demagogues have appeared in democracies since ancient Athens. They exploit a fundamental weakness in democracy: because ultimate power is held by the people, nothing stops the people from giving that power to someone who appeals to the lowest common denominator of a large segment of the population.

Source:

http://en.wikipedia.org/wiki/Demagogue

 

[CheckerHater]

@3DDave:

Speaking about vocabulary, demagoguery is an appeal to people that plays on their emotions and prejudices rather than on their rational side.

You have searched for the wrong word.

 

[3DDave]

Demagoguery is practiced by demagogues. Others might want to know precisely what the accusation meant.

 

[axym]

Hi All,

I'll offer a couple of comments. There are several things going on in Figure 7-54.

The 2X multipliers are the "grouping mechanisms" that define each of the patterns in Figure 7-54. The SEP REQTS annotation overrides the simultaneous requirements default and results in two separate patterns.

There is another mechanism that Y14.5 defines for breaking the simultaneous requirement within a pattern. The annotation INDIVIDUALLY is placed next to the FCF, and this overrides the grouping effect of the nX multiplier. There's an example in the Profile section, in Figure 8-23. This annotation would be necessary in order to achieve separate control of each of the 4 holes in Figure 7-54.

I agree that the separate requirement only makes a practical difference when there is datum feature shift on the cylindrical datum features or rocking/instability on the planar datum features.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[greenimi]

Evan,
Does it *HAVE* to be cylindrical datum feature? Or could be a regular feature of size (width/slot, length)?
Just curious.

 

[CheckerHater]

@greenimi:

Everything that rattles in physical world may create datum shift or bonus tolerance in the world of GD&T (providing MMB / MMC is specified)

 

[axym]

greenimi,

No, the datum feature(s) don't have to be cylindrical. The same kind of shifts can occur on other datum features of size (parallel-plane widths, spheres, irregular features of size referenced MMB or LMB).

CH,

Yes, the MMB modifier results in what is usually called datum feature shift. There are those (myself included) who argue that similar instability can also occur even on planar datum features or when the RMB modifier is used (e.g. a cylindrical primary datum feature that is produced with a tapered condition). But there are definitely differing opinions - some feel that the RMB modifier implies a stabilized datum.

I would agree that the use of datum targets can greatly reduce or eliminate the instability and shift.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[pmarc]

CH,
Allow me to answer to your last post addressed to me:

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.

Of course, it wasn't specifically about fig. 7-54. It was about a case where there are so many features to be controlled separately that associating size dimension, positional feature control frame and SEP REQT note with each and every hole can be really painful. That is why I suggested to go with tabulated tolerances. And it is not my job to judge whether you can read (with comprehension) or not, so I am choosing to stay as far away as possible from this part of your comment.

This makes me think you were talking about applying SEP REQT in general, and in general the idea still flawed.

So far you have not proved that the idea is flawed (at least your graphics have not done it), so let me show you that the idea does make sense, i.e. there is a geometrical difference between considering features simultaneously and separately. In the attachment you will find a modified version of fig. 7-54. For simplicity I just considered 2 holes, not 4. I also added geometrical tolerances to datum features B and C to make the proof possible. To keep pictures clear, I used axis interpretation of positional tolerance at MMC.

http://files.engineering.com/getfil...48-b7df-f3fb9812c267&file=sim_vs_sep_reqt.pdf

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.

You are right, I didn't have to make it 2 paragraphs. My apologies.

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?

Like Evan said, using datum targets significantly reduces instability of a part, but this is not what my comment was about. I clearly said that in case of your both drawings (where datum targets have not been used) the difference between the prints is possible. And I provided a brief description of when this can happen. If you think it is not true, constructively prove it. But please use entire surface as primary datum feature, not just some portions of it.

 

[CheckerHater]

please use entire surface as primary datum feature, not just some portions of it

May I ask why?

 

[pmarc]

You are joking, right?

 

[CheckerHater]

I am not joking.
When you were writing about 20 or 40 holes you did not add disclaimer that your advice only works on your terms: when datum is wobbling, when datum shift is present, etc., etc. You just made general statement.
I stated that it will not work on my terms and I clearly showed my terms on my sketch (sorry for missing MMC requirement).
Even on your terms it’s questionable: Who said that every time you make a check, new different candidate datum should be selected? Standard says you have to make “best effort”. Why it should be different “best” datum every time? Just follow my description: put part into the fixture, insert pin A, take it out, insert pin B, take it out, separate req’t checked. Insert both pins, simultaneous req’t checked. No need to re-fixture between the checks.
Same with datum shift. It will work with functional gage, but not all the CMM. The shift is sometimes ignored and datum is considered RFS anyway, just because it’s easier to have the datum that doesn’t move.
I think the burden of proving that your suggestion will work under all circumstances is on you.
And if you agree that the method may have its limitations, maybe we better turn to discussing when and how it may be useful in civilized manner. (I promise not to use big words)

 

[pmarc]

I give up, CH.
I am afraid our understanding of GD&T concepts mentioned in this discussion is so different that we will never find an agreement.

P.S. For those who have already opened the attachment with 23 page pdf file in my reply from 8 May 14 9:25, I asked forum admin to replace it, so now a 3-page document should be available. My apologies for that mistake.

 

[3DDave]

"Just follow my description: put part into the fixture, insert pin A, take it out, insert pin B, take it out, separate req’t checked. Insert both pins, simultaneous req’t checked. No need to re-fixture between the checks."

If the part fails inspection on one of the holes, the separate requirements allowance lets the inspector adjust the part to a new orientation and possibly accept the part - the one that would otherwise be thrown out.

 

[CheckerHater]

3D, I was talking about datum that wobbles, not datum specified at MMB.
My argument was that you will not wobble the datum indefinitely, once you found your "best" candidate datum there is no need to re-fixture the part. On my drawing there was no "shifting" datums and no way to "adjust" the part.
I suggest we put argument for the sake of argument to rest, and make sure it doesn't wobble anymore.

 

[pmarc]

My argument was that you will not wobble the datum indefinitely, once you found your "best" candidate datum there is no need to re-fixture the part.

This actually proves that your understanding of the whole "instability-when-SEP-REQT-is-specified" concept is wrong. There is indeed no need to re-fixture the part, BUT if you are not able to find your best candidate datum (so that axes of both holes fall into their positional tolerance zones simultaneously), you are allowed to re-fixture it in order to fit the holes into their positional tolerance zones separately. If SIM REQT was specified, the re-fixturing would not be allowed. And the very same logic applies to situation when datum features are specified at MMB and actual geometry of a part allows for datum feature(s) shift.

 

[CheckerHater]

Maybe I don't understand imaginary world, maybe you don't understand reality.

In your example with 20 or 40 holes nobody will build 20 or 40 fixtures or re-fixture parts 40 times; they will build 1 fixture with removable pin or pins and clamp the part down just once as soon as suitable position is found. Not to mention that all 40 holes will be definitely machined at once, from one single fixturing.

This is what I started with my very first post: what are the actual benefits from functional, machining or quality control point of view? What is the point of using technique that even experts will disagree about? Nobody answered.

And by the way, do you agree that practical application of separate requirement is not universal, but only useful when datum shift is present? (Counting datum wobble as logical subset of datum shift)

 

[pmarc]

In your example with 20 or 40 holes nobody will build 20 or 40 fixtures or re-fixture parts 40 times; they will build 1 fixture with removable pin or pins and clamp the part down just once as soon as suitable position is found. Not to mention that all 40 holes will be definitely machined at once, from one single fixturing.

Nobody has to build 20 or 40 fixtures.
And the part will be clamped down just once in two situations:
1. If removable gage pin or pins fit into each of 20 or 40 holes in that single position of the part relative to the gage.
2. If a gagemaker does not understand what SIM REQT note on the drawing means and what benefit it can give to the part.
I just do not understand why you are so sure that all gagemakers are not aware of the meaning of SEP REQT.

Not to mention that all 40 holes will be definitely machined at once, from one single fixturing.

I already answered to that argument, so here I will simply paste what I wrote on 5 May 14 7:35:
"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."

This is what I started with my very first post: what are the actual benefits from functional, machining or quality control point of view? What is the point of using technique that even experts will disagree about? Nobody answered.

Isn't defining a looser relationship between holes a benefit from functional and especially from manufacturing point of view? Isn't the additional possibility to verify each hole separately a benefit from QC point of view? Especially in cases where simultaneous verification does not give positive results? This was already said (not only by me), so again I have no idea why you are saying that nobody answered to your question.

And by the way, do you agree that practical application of separate requirement is not universal, but only useful when datum shift is present? (Counting datum wobble as logical subset of datum shift)

No, I disagree with that. SEP REQT can be applied even when datum feature shift is not present. Fig. 4-41 in Y14.5-2009 shows how this is possible. Generally, all depends on geometry of a part and configuration of its datum features. In case of fig. 7-54, for example, datum feature shift will be not present only when datum features B and C are produced at their MMBs at the same time. This is quite unlikely to happen in reality, don't you think? And in case if you still believe that the application of SEP REQT is limited to very special cases, think how similar MMC modifier after geometric tolerance value works. Bonus tolerance will not be present only when a feature of size is produced at its MMC size. When the size departs from MMC, we can take advantage of that. Why not to take the very same advantage of presence of datum feature shift?

 

[CheckerHater]

I just do not understand why you are so sure that all gagemakers are not aware of the meaning of SEP REQT.

They are. It's just nobody will build more complicated setup to produce less accurate part. As many of holes as possible machined all together at once, as many of holes as possible checked all together at once, out of the door.

It is just that SEP REQT defines looser relationship between holes than SIM REQT

Could we use easier way to specify looser requirement?

Isn't the additional possibility to verify each hole separately a benefit from QC point of view?

Not really because it takes more time.

No, I disagree with that. SEP REQT can be applied even when datum feature shift is not present. Fig. 4-41 in Y14.5-2009 shows how this is possible.

We were talking about hole pattern, I don't see any hole pattern. We were talking about fully constrained parts, the cylindrical part on the figure doesn't have to be fully constrained, in fact it only has 1 datum. It looks like you are avoiding direct answer again.

Are you saying, that because separate requirement works on Fig 4-41 it will work always, all the time?