How to control features separately
How to control features separately
(OP)
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?





RE: How to control features separately
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.
RE: How to control features separately
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?
RE: How to control features separately
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
RE: How to control features separately
Tunalover
RE: How to control features separately
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: How to control features separately
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?
RE: How to control features separately
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.
RE: How to control features separately
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.
RE: How to control features 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.
RE: How to control features separately
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?
RE: How to control features separately
RE: How to control features separately
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
RE: How to control features separately
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.
RE: How to control features separately
What, do I have a track record of not being able to read?
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: How to control features separately
Truly being offended is America's favorite past time.
RE: How to control features separately
RE: How to control features separately
RE: How to control features separately
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.
RE: How to control features separately
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.
RE: How to control features separately
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.
RE: How to control features separately
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.
RE: How to control features separately
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
RE: How to control features separately
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.
RE: How to control features separately
RE: How to control features separately
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.
www.axymetrix.ca
RE: How to control features separately
Evan,
Does it *HAVE* to be cylindrical datum feature? Or could be a regular feature of size (width/slot, length)?
Just curious.
RE: How to control features separately
Everything that rattles in physical world may create datum shift or bonus tolerance in the world of GD&T (providing MMB / MMC is specified)
RE: How to control features separately
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.
www.axymetrix.ca
RE: How to control features separately
Allow me to answer to your last post addressed to me:
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.
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.
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You are right, I didn't have to make it 2 paragraphs. My apologies.
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.
RE: How to control features separately
May I ask why?
RE: How to control features separately
RE: How to control features separately
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)
RE: How to control features separately
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.
RE: How to control features separately
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.
RE: How to control features separately
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.
RE: How to control features separately
RE: How to control features separately
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)
RE: How to control features separately
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.
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."
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.
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?
RE: How to control features separately
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?
RE: How to control features separately
RE: How to control features separately
For what it's worth, I agree with pmarc on all counts. His explanation of separate requirements, and the additional freedom it allows, is correct and accurate. This guy knows what he's talking about, and I hope that people appreciate the high level of knowledge being demonstrated here.
The objections that are being raised seem to relate more to how one might choose to utilize the additional freedom in manufacturing and inspection (or choose not to utilize it). But hypothesizing that manufacturing or inspection might choose to ignore part of the specification does not make it worthless. At this point I'm not sure if the objections are fully genuine or are just arguments for argument's sake. The dismissive and sarcastic language doesn't help.
The specification of separate requirements allows each characteristic to be inspected in different candidate datum reference frames. In other words, inspected in different setups. Candidate DRF's occur when the degrees of freedom between the datum features (i.e. the part) and the datum feature simulators (i.e. the inspection fixture) are not fully constrained. Examples include the following:
The distinction between simultaneous requirements and separate requirements only becomes significant when the degrees of freedom are not fully constrained, so to keep directing the discussion back to fully constrained cases is pointless.
Also, the concept of candidate DRF's is general. It doesn't depend on the geometry of the considered feature. So it doesn't really matter if we have a hole pattern or not, it's just that the examples illustrating separate requirements usually have hole patterns in them. But we can have the same thing for other features with profile controls as well.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: How to control features separately
I agree wholeheartedly.
RE: How to control features separately
A lot of people here promised him lunches, booze etc. for the ABSOLUTELY FREE help they got from pmarc.
THANK YOU PMARC
RE: How to control features separately
Did pmarc agree with that statement? Because that's all I wanted from him.
RE: How to control features separately
IOW, if Fig. 7-54 didn't use any MMB modifiers, everyone would agree that all DOF are constrained. But with the MMB modifiers, is it "fully constrained"?
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: How to control features separately
To look from the other side: can we call it a "datum shift" if datum is not referenced at all, so we can "shift" indefinitely?
RE: How to control features separately
Probably you already know.
AGI: Datum shift explained: page 33 and up.
Found this:
http://www.conceptmachine.com/files/zeiss/GD&T...
Same thing pmarc explained in his sketches.
RE: How to control features separately
John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: How to control features separately
Your questions bring up a subtle issue, that is unfortunately a major digression in a thread like this.
The issue could be described as "partial constraint". Y14.5 does not directly address this - the standard describes degrees of freedom as being either constrained or unconstrained. But when there is shift or instability between the datum feature and its datum feature simulator, the constraint is somewhere in between.
With MMB modifiers, there is a high probability of datum feature shift between the datum feature and its fixed-size simulator. I think we can safely say that the DOF's are not fully constrained.
With planar datum features, there is a small (but significant) probability of rocking and instability. It would be most prevalent on primary datum features and possible on secondary. I would have to say that the DOF's are not fully constrained.
With RMB modifiers, there is a small possibility of datum feature shift even with a snug-fitting simulator. This issue could conceivably occur on primary cylindrical or width datum features, when a tapered condition exists. I would say that the DOF's are not fully constrained, but many others would disagree with this one. Almost fully constrained, but not quite ;^).
CH,
If a datum is not referenced at all, meaning that one or more degrees of freedom are completely unconstrained, then I would say yes - we can shift indefinitely. It's generally not described that way, but the effect is the same as datum feature shift on an MMB datum feature reference (only with no limits).
powerhound,
I'm not sure if the above comments address your questions or not - what do you think?
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: How to control features separately
John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: How to control features separately
Would that be safe to say that the benefits of separate requirement are more significant when the datum(s) are allowed to shift / translate (use your favorite terminology) and less obvious when part can be “fully immobilized” (thanks powerhound)?
RE: How to control features separately
Thank you very much for these kind words. I am really flattered.