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.

 

[Sem_D220]

This is incorrect. The parallelism will be to datum B since it is secondary. Datum feature B can be as out of parallel as much as it wants.

You are absolutely right. Parallelism to datum plane is what I meant. This was only a momentary confusion. I am usually very strict with this differentiation. You can notice it in my other posts, and can also see that parallelism to datum B sits well with the functional description I provided. I hope the functional requirement is detailed enough, and I don't consider that a specifying what the part does will add any value to the discussion. In other words it is the translation of a given functional requirement to GD&T language that is of interest, not questioning that requirement.

Regarding the other issue, there is a good reason why I refused to answer his question, espcially provided the way it was formed and previous discussions. Let's just leave it at that.

 

[pmarc]

Considering the modified fig. 6-8,
Which is the more problematic scenario?

The designer having to specify parallelism wrt primary datum C and secondary A, when what he really cares about and intends to say in the drawing is that perpendicularity to secondary datum A is needed.

Or, the confusing perpendicularity call out wrt |C|A|, which will establish the same tolerance zone as the previous option, translating the intent of designer from English to GD&T language more directly, probably not illegal, but doesn't make a familiar use of the orientation control?

You have to be joking. There is no other explanation that I can find to understand the reason you asked this question. It seems like almost everything that was said up to that point was nothing but a waste of time.

------

John,
I am not sure what you exactly want me to sketch. Could you clarify?

 

[powerhound]

I am not sure what you exactly want me to sketch. Could you clarify?

No, I guess not. I thought maybe you had figured out his point and could help me along.

Sem,
Please explain how the scenario in the attached print is functional (just make something up if you have to. It just needs to make sense). Also help me understand why it is a better callout than perpendicularity to A. If you refuse or can't think of anything then I'll just have to concur with pmarc that this has been an enormous waste of time.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

 

[powerhound]

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1532544271/tips/parallelism_km8ses.pdf[/url]

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

 

[greenimi]

pmarc and John,

What did I say on July 23rd?

At this point I'm not sure if your objections are fully genuine or are just arguments for argument's sake.

 

[greenimi]

But to give the OP some credit, I would say that the entire thread forced me to go to the standard to lookup all those referenced figures from y14.5, understand their intent and learn more about the theory itself. Therefore a good academic excercise.

 

[Sem_D220]

pmarc,
The purpose of that post was explained in the following one.
Perhaps you have all the answers - but you were not the one asking the questions in the first place.
Even if this discussion is considered completely exhausted by you - it doesn't mean that it is so for everybody else. The proof for that is the following posts, with people still showing interest and will to engage in the discussion.

powerhound, the best I can say to answer your question about functionality is to repeat that the part assembled on the controlled face needs to be oriented parallel to the face that supports datum feature B (in other words, to what is simulated as datum plane B as you correctly noted). The other possible callout to produce the same tolerance zone would be perpendicularity to A in an |A|B| DRF. Geometrically - no difference. In such situation, the reason why there is still a dilemma is described in the post which pmarc critisized for it's existance and you quoted from.

 

[Sem_D220]

greenimi,
Yes, the feature is first located and then oriented as a refinement. Orientation needs to be controlled in conditions that imitate real assembly, therefore the chosen DRF.

 

[greenimi]

semiond,
Okay. Understood.
The feature is already located based on (your words): in conditions that imitate real assembly, threfore the chosen DRF.

Orientation needs to be controlled in conditions that imitate real assembly

I would just correct your above statement a bit: Orientation needs to be controlled in conditions that imitate real FUNCTION. And the function you described as "parallelism to B is actually more important than perpendicularity to A", hence my proposal of refinament.

 

[powerhound]

the part assembled on the controlled face needs to be oriented parallel to the face that supports datum feature B

Then call out that surface parallel to B primary. If the mystery part mounts against the controlled surface then there is zero relationship to A as primary.

The other possible callout to produce the same tolerance zone would be perpendicularity to A in an |A|B| DRF.

Yes, of course. This is how this should be done. There would be absolutely no confusion about it and this thread wouldn't have gone past 5 or 6 posts.

Geometrically - no difference.

Geometrically, maybe not. Functionally, the difference is profound and the reluctance of you to see that is baffling.



John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

 

[Sem_D220]

greenimi,
Assembly is function, and the assembly suggests an |A|B| DRF. When I talked about parallelism to B I meant that B for that control should be the secondary datum. I just didn't consider it was needed to repeat that again in the sentence you refer to since it was stated earlier in the same post, and probably in every other post I submitted here.
You don't think an orientation refinement should also reference the functional datums?

 

[Sem_D220]

Then call out that surface parallel to B primary. If the mystery part mounts against the controlled surface then there is zero relationship to A as primary.

The part in the sketch (call it part no. 1) is clamped against datum feature A to part no.2 in a way that constrains 2 rotations, and the mating surface to datum feature B at part no.2 constrains 1 rotation. Part no.3 is mounted on the controlled face and needs to be oriented parallel to the face at part no.2 that supports datum feature B.
Do you understand now the relationship to A as primary? If not, what does a DRF or datum precedence order mean to you anyway?

Yes, of course. This is how this should be done. There would be absolutely no confusion about it and this thread wouldn't have gone past 5 or 6 posts.

Sorry, but I will not explain again the problem I see in this scheme.

Geometrically, maybe not. Functionally, the difference is profound and the reluctance of you to see that is baffling.

If there is no geometrical difference, there is no functional difference. The only difference that might be has to do with clarity/ legality. This was already discussed to death here.

At this point this is getting too much even for me. pmarc, I think you were right after all.

 

[powerhound]

Yeah. We're done.






John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

 

[greenimi]

So why "parallelism to B is actually more important than perpendicularity to A" is not function?

Assembly is function. I agree.

"You don't think an orientation refinement should also reference the functional datums?"

Why B as a standalone datum feature is only for assembly and not for function?
B is function because you added the parallelism requirement above for a reason ( hopefully good read functional one), otherwise why this requirement is added by you?

 

[Sem_D220]

greenimi,
The sentence you quote - 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"
Edit:That refers to the specific case discussed, not as a general statement. And I hope you know the differences that might occur for any given control when you change the datum precedence order or reference 1 datum versus 2 datums.

This is for both assembly/function, or function of the assembly, or function as an assembly, or assembly for function, or whatever combination of the 2 terms you find appropriate.

 

[Sem_D220]

powerhound,
Thank you for the input.

 

[pylfrm]

If that is good enough, you can address my sketch from 21 Jul 07:47, and consider that the main mounting face against which the part is clamped firmly is datum feature A, the mating surface at datum feature B locates the part at the direction perpendicular to it and constrains another rotation. The controlled face is a clamping surface for another rectangulsr part that once mounted should be parallel to the face that supports datum feature B. Therefore parallelism to secondary datum feature B is applied, and the datum precedence order is chosen in a way that represents real assembly conditions. I hope that this clarifies the intent, and thank you for trying to help.

Your problem as described in this post and the referenced sketch is exactly the same problem that ASME Y14.5-2009 Fig. 6-4 shows you how to solve. The fact that angularity is listed as an acceptable alternate practice but parallelism isn't tells me that parallelism is wrong.

Again, I'd recommend using angularity to avoid the issue entirely. Do you have some specific objection to that?


pylfrm

 

[Sem_D220]

pylfrm,
No, no objection to it at all. This is my current conclusion too. If the main clamping face for the part in fig. 6-4 was datum feature B and perpendicularity to datum feature A in |B|A| datum reference frame was the main concern, I would apply angularity rather than perpenducularity or parallelism.

 

[pylfrm]

If the main clamping face for the part in fig. 6-4 was datum feature B and perpendicularity to datum feature A in |B|A| datum reference frame was the main concern, I would apply angularity rather than perpenducularity or parallelism.

Do you mean "datum A" instead of "datum feature A"?

Anyway, why are you changing the problem now? Fig. 6-4 was an exact match for the functional problem you described in your 25 Jul 18 15:54 post, with datum feature A as the main clamping surface and datum feature B as the secondary locating and orienting surface.


pylfrm

 

[Sem_D220]

pylfrm, the word "feature" was a mistake (it's the second time I do it here ), I corrected it by editing immediately after the post. Look again.

I don't know if fig. 6-4 is an exact representation of my described issue.
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. 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. Or perhaps you consider the design intent I described is irrelevant to any real possible case in the first place? Because this is what the comments by other members imply.