Orientation controls referencing 2 datums 2

[Sem_D220]

What are the opinions on the following schemes?

1. Angularity control referencing 2 datums in the FCF, when the basic angle is specified between the controlled face and the secondary datum. The primary and the secondary datums are perpendicular to each other.

2. Parallelism control referencing 2 datums in the FCF, when the controlled face is nominally parallel to the secondary datum and perpendicular to the primary datum.

3. Perpendicularity control referencing 2 datums in the FCF, when the controlled feature is nominally perpendicular to the secondary datum and at some other angle to the primary datum.

I haven't seen any of these brought as an example in the Y14.5 standard (unless I'm missing one), or in any other sources I was exposed to, but I also don't see how the contents of chapter 6 may reject those schemes. Schemes #1 and #2 are ones I wanted to implement for real cases, but hesitated (eventually I did ). As for #3, I haven't encountered a case requiring this, but I can imagine one. I think I once heard a GD&T professional say that there should always be a basic implied 90° angle (for perpendicularity), or a basic angle of some other value (for angularity) between the controlled feature and the primary datum feature whereas the secondary datum may only constrain DOF / orient the tolerance zone. But, if the DRF should first and foremost reflect the functional interface, there certainly may be cases where a vice-versa scheme is justified. The problem is - there are no figures to point to if such position needs to be supported. In Y14.5, looking for orientation controls that reference more than one datum, I find only figures 6-4, 6-8, and even 6-17, all show an implied right angle relationship or basic angle between the controlled feature and the primary datum reference, never to the secondary.

Has anyone else dealt with this dilemma? Maybe it's only my lack of knowledge / experience, and such schemes are either commonly practiced or clearly not supported? Whatever the case is your input will be very much appreciated.

Edit: I'd like to add that I realize that one solution could be to use profile of a surface for orientation, but from various reasons I prefer to utilize orientation controls and reserve this solution only as a last resort, if needed.

 

[pylfrm]

It depends on what the designer wanted to say. If it's "the face should be parallel to datum B, while datum B simulator contacts the datum feature at two points, and constrains the part in 1 rotation", then it does represent the problem.

The tolerances in Fig. 6-4 require that the surface fall within a zone bounded by two planes which are parallel to datum B in the |A|B| datum reference frame. I don't know if that's what the imaginary designer wanted to say, but it matches what you are saying.

Do you consider the perpendicularity callout shown in that figure a clear way to say what the designer means to say in that case? Only in terms of design intent clarity, that is.

Yes I do. I consider the angularity alternative even more clear. I would consider parallelism completely unclear and incorrect.

Or perhaps you consider the design intent I described is irrelevant to any real possible case in the first place?

I thought you wanted to avoid discussing specific real-world applications, so if you want my opinion on that you're going to have to ask for it.


pylfrm

 

[greenimi]

it is taken out of it's context when separated from everything else in that post. To make it complete as a stand alone statement it should be added:"parallelism to B - in |A|B| DRF, is actually more important than perpendicularity to A in |A|B| DRF"

So what are your objections against what I proposed: surface located (profile) to A primary and B secondary in one FCF then a refinement of the same surface such as parallelism to B primary? That I was asking and you haven't answer.

 

[Sem_D220]

pylfrm, I agree with everything you say. Thank you for the input.

greenimi, I think an orientation refinement should be in the same DRF as the profile for location. It represents the functional requirement better, and as another advantage - it wouldn't require to fixture the part differently for each control.

 

[chez311]

pylfrm,

In regards to Figure 6-4, semiond has already stated that if for some reason the important control per the functional/assembly requirements is of parallelism to datum B at 2-points, he is willing to sacrifice the clarity of the shown design scheme and instead specify parallelism to [A|B] even though, going by his interpretation and assuming for a second its at least can be interpreted in this manner, it would be geometrically equivalent to perpendicularity or angularity to [A|B]. I would agree with you that this is at best confusing and at worst outright wrong, however I do not believe there is anything that you or I can say at this point that will convince him otherwise.

Now why anything with secondary 2-point contact could be the most important control (instead of requiring 3-points and referencing parallelism to only instead of to [A|B] ) that would be worth these kind of sacrifices for clarity is beyond me, but that is what he has stated several times. Saying something is of such primary importance per the design intent that it needs to be communicated in this manner but can be controlled at 2-points to a secondary datum seems to be inherently contradictory.

 

[Sem_D220]

chez311,
they key to understanding why controlling an angular orientation to a secondary datum might be functionally important, is to realize that controlling an angular relationship to a datum plane is like controlling an angular relationship to a mating face in assembly. The part can be supported on 3 or 2 points on that mating face, but that doesn't suggest if you care more, or care less, about a particular orientation to that mating face. I encountered situations where an orientation of a feature to a mating face at assembly that contacts the part on 2 high points, is more important than the orientation to a mating face that contacts the part on 3 high points. Therefore, there is no contradiction here.

 

[greenimi]

semiond,

I think an orientation refinement should be in the same DRF as the profile for location

Why ? Please answer this question: why the refinament, (because we already that should be a refinament) should be the same DRF?

It represents the functional requirement better

And no, this is not good enough answer for the above question. You can do better than that.
The follow up question is how? (how it represents the functional requirement better). AGAIN, we are talking about the refinament only. Not about the location. We already located (profile) the feature.

What means better? Only a refinament of the parallelism to B is needed for a functional requirement (again, I repet it again, for orientation, refinament only). What means better in the context of the refinament. How you define better in this context?

Conveniecnce for measurement? (by not requiring the fixture)? Sorry this argument is not good enough as you are not defining your print by a convenience for measurement. The driving force is the function not the measurement/ quality. (if you do that means the tail wagging the dog and not the dog wag the tail, correct)

Other reasons are expected. Sorry, I don't want to put you in the corner, but you are not far away from that either.

 

[chez311]

The part can be supported on 3 or 2 points on that mating face, but that doesn't suggest if you care more, or care less, about a particular orientation to that mating face.

It has nothing to do with how much you or I or anyone else "cares" about it. An orientation control requiring 3 points (primary datum) will be a tighter and more direct control than one requiring 2 points (secondary datum) - thats just a fact. You are welcome to say, imply, or notate that the 2 point reference is more important, I'm just saying logically it doesn't make sense. I know the crux of your entire point is that its about intent - and my continual rebuttal has been that your communication of intent changes nothing geometrically and does nothing but create confusion.

greenimi,
Your point about refinement is a good one - it pairs well with pmarc's concept of local and global DRF's. If the feature is located to the global DRF, if necessary to ensure proper orientation of a particular feature a subsequent refinement to a local DRF or to another feature other than the primary datum in the global DRF does not necessarily violate the functional/assembly requirements somehow.

 

[chez311]

Semiond,

I realize after re-reading it that you were referring to the assembly condition. My point really doesn't change though. If we look at Figure 6-4 again, if you change the shown callout to parallelism you can say as many times as you want that your "intent" is to say that parallelism to B is more important - that will not change the fact that perpendicularity will be FIRST and MORE TIGHTLY controlled to A (through 3 points) and parallelism will be SECOND and LESS TIGHTLY controlled to B (through 2 points). If that doesn't seem like contradiction to you, then I don't know what else to say but to repeat that logically it doesn't make sense. And if that is acceptable for the design, then so be it - you have successfully created the potential to confuse or mislead anyone who reads the drawing, leaving perpendicularity or angularity as shown in 6-4 would accomplish the same thing without said confusion. I would think however that chances are that if the parallelism to B is truly more important, in 99% of cases one would have to add a refinement of parallelism referencing B as a primary datum.

 

[Sem_D220]

greenimi, chez311,
I'm sure that you know that simulating datums of a datum reference frame by datum feature simulators, if the DRF and datum sequence were specified correctly in the drawing, is ideally equivalent to creating real application assembly conditions during fixturing for inspection. Assuming we agree upon this concept, I can now address the issue of location+orientation requirement versus the orientation refinement. You seem to support the idea that if the part (any part) is mounted in assembly in a way that suggests |A|B|C| datum sequence, then the profile tolerance that should locate a controlled surface should in most cases reference those datums in that same order. If you add an orientation refinement, naturally the orientation tolerance should be tighter than the profile tolerance. That means that orientation should be controlled tighter than location for the part to be good enough, in other words it is more critical. If you applied the above mentioned functional DRF to the looser tolerance of location, you should probably specify |A|B| for the orientation refinement. Since you probably still disagree let me ask you this - why would you want a different DRF datum sequence and consequently - different simulated datums, which represent a different relationship between the part and the mating surfaces, to be called out at the tighter, more critical tolerance zone of orientation? And when I say different datums, I mean fundamentaly different ones. A single datum B which stabilizes the part on 3 points, is not the same datum B from |A|B| DRF, and not the same datum B from |B|A| DRF (this was a mistake). Don't be confused by the fact that it is named by the same letter. Those datum planes will not coincident with each other for a given imperfect as-produced part. They are separate planes located and oriented differently in space relative to a given as-produced part. Edit: when you have different datums established, consequently you create different tolerance zones basically related to those datums as well, also located and oriented differently in space. One tolerance zone may reject parts that are acceptable by the other I wish I had the time to model something like that and present an image here. If you can visualize this picture in your head, you should be able to at least relate to the reasoning, even if you don't agree.

 

[Sem_D220]

chez311,
Forget already about parallelism to |B| in |A|B| DRF for fig. 6-4. I was merely asking for opinions on similar schemes in the beginning of the post thread and explaining the logic behind it later. I have also said on numerous occasions that I would not recommend something like this to anyone. I was also asking for some reference about its illegality but haven't found or been pointed to something that clearly rejects it. Besides, I also said more than once in this thread that angularity or profile are apparently the preferred substitutes to similar schemes. I would also appreciate it if you don't make statements in my name, directed to other people, especially provided they don't represent my point of view and contradict things that I said.

 

[greenimi]

Why would you want a different DRF datum sequence and consequently - different simulated datums

Because you said: " parallelism to B is actually more important than ......" My "....." can replace anything else.

when you have different datums established, consiquently you create different tolerance zones basically related to those datums as well, also located and oriented differently in space. One tolerance zone may reject parts that are acceptable by the other

Still not sure what is the problem. The feature must fit in both tolerance zones. No difference versus any other refinament controls.

I see what are you saying and I agree with most of your points.

But not sure I do understand your problem with my proposal. I do not see your arguments listed anywhere. You are just giving some general statements.

 

[greenimi]

OHHHH, I understood. This thread is about beating the number of replies..... So far we got 130 and counting.

 

[Sem_D220]

Still not sure what is the problem. The feature must fit in both tolerance zones. No difference versus any other refinament controls.

I was talking about 2 alternatives for refinement, it's either that tolerance zone or the other tolerance zone. The feature is not required to fit both because you specify only one refinement. You're not paying attention. Doesn't matter, It seems we both waste time here.

 

[greenimi]

Well, the feature/surface must fit between the location TZ and also between orientation TZ (refinement). That I meant by both.

I did not compare anything about you A/B proposal for refinement because I do not consider it valid for the intended purpose of "parallelism to B is actually more important than....."
For me your solution is no alternative for refinament. But again it is just me.....

 

[Sem_D220]

"parallelism to B is actually more important than....." still stands, and once again, it was meant as parallelism to B in conditions that simulate assembly, which means |A|B| DRF, not |B| DRF. This is the last time I mention this and I think you was already provided a detailed explanation about the difference between the two.

 

[chez311]

semiond,

So I think here lies my main issue with your thought process - you've said that orientation to a secondary datum (secondary dictated by assembly/functional requirements) is of primary concern, however not enough to alter the "global" DRF and call it out as a primary datum in a "local" DRF. This means that actually what is being controlled first, most tightly, and I would say most importantly is orientation to the primary datum. So actually what you are saying when you state that you are concerned about orientation to a secondary datum with 2 points of contact it is only after orientation to the primary datum with 3 points of contact has been established (which I am saying is "most" important as it is controlled first, and more tightly). I believe we have been sort of talking past each other this whole time just with different paradigms or points of view which really don't differ that much, at least I don't think. At this point I think you and I both are just debating over semantics (you and I both, I'm not just saying that you are) and our preferred way of thinking about the subject, while still not resulting in a huge (if any) difference in the actual result.

I guess I missed your post on 26 Jul 18 12:48 saying that you agree with what pylfrm said about that figure and the related scheme, you were still discussing it up to that point and I did not realize it had been settled. I apologize if I referenced your name in a way that disagreed with you or summarized your point of view incorrectly/inappropriately, I did not mean to speak for you.

 

[greenimi]

"parallelism to B is actually more important than....." still stands, and once again, it was meant as parallelism to B in conditions that simulate assembly, which means |A|B| DRF, not |B| DRF.

"

Semiond,

I think you are missing some fundamentals of the language. The functional requirements (for example requirement of a feature to be parallel to a datum plane created from another feature, refinament in our case) will come before the DRF is deveolped or chosen or decided. Not the other way around. The designer or the drawing creator cannot change the law of physics. Those requirements (parallelism) belong to the design. The drawing creator is only recognizing them but cannot change them.

 

[Sem_D220]

chez311, I was trying explain that difference in pereceptions all along. Perhaps you also missed my post from 25 Jul 13:53. I will spare you the scrolling and copy paste it:

I describe a case where control with datum feature simulator A contacting at 2 high points rather than 3 and constraining 1 rotation rather than 2 a more accurate control, not a sloppier control. This is because datum feature simulator A that will be made to behave this way will represent the actual mating surface in real application better than a primary datum feature simulator A. This is the most important point I wanted to convey.

 

[Sem_D220]

greenimi,
Nothing that you said contradicts my approach.

It all comes down to the fact that I say that the functional requirement which decides the DRF is an "x" and you insist it's a "y", ignoring explanations about what the functional requiremnt is based on. I think this is a dead-end.

 

[Sem_D220]

I stumbled upon this interesting article published in Alex Krulikowski's blog on behalf of Rick Hughes about misconceptions related to GD&T. "Silly thing #10" might be relevant here, as it reflects some of the points I tried to convey. I'd like to quote this excerpt:

"SIlly Thing #10: The datum to which a hole is perpendicular must be PRIMARY (or; a feature must always be perpendicular to the PRIMARY datum
DATUMS are chosen based on part function; some part features will be perpendicular to the primary datum, some will not. Required orientation depends on function and, therefore, varies from part to part and from feature to feature within a part."