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.

 

[pmarc]

As stated before, I'm aware it's a problematic concept, but I don't want to compromise on functional DRF just to make the symbol specified as everyone is used to.

You do not have to make compromises on functional DRF. If functional DRF in the modified version of fig. 6-8 is |C|A|, then instead of applying perpendicularity symbol, use parallelism and that is all you have to do. Not to mention that in |C|A| DRF referencing to A secondary adds no geometrical value to the callout.

I hope that using angularity the way it is used in fig. 4-7 (when the main goal is to control the 60° angle to the secondary datum and not perpendicularity to the primary) is acceptable by most. By your last post I conclude you are not against such use.

Yes, I am not against such use, and I have a feeling that most of people will have nothing against it too. But I would not say the main goal of the angularity callout in fig. 4-7 is "to control the 60° angle to the secondary datum and not perpendicularity to the primary". The physics of the process of setting up the part against |A|B| DRF is that the main job of the angularity callout is the control of perpendicularity of the face C relative to datum plane A. The 60°-angle control is merely a secondary function of the callout.

So you have to pick either side:
If you "believe that in case of perpendicularity and parallelism callout the symbol used in the FCF should match the relationship between primary datum and the toleranced feature", you have to extend your belief towards angularity as well.
Or 4-7 approach should equally apply to all cases of orientation.

As long as any rule of the standard(s) is not violated, I do not have to pick anything if my main concern is clarity of the callouts. Use of angularity in fig. 4-7 does not violate any mathematical rule of the standards (both Y14.5 and Y14.5.1M), but offers one big advantage over perpendicularity callout - clarity. So I choose clarity over the general rule that it is always the relationship between toleranced feature and the primary datum that determines the selection of symbol for orientation control.

 

[CheckerHater]

@pmarc:
Would you agree with the following interpretation of 4-7:
"Orientation tolerance specifies that the toleranced surface must lie in a zone bounded by two parallel planes separated by the specified tolerance and basically oriented to the primary datum [A] (implied 90 deg. basic angle) and to the secondary datum (60 deg. basic angle)?
Nevertheless I will be glad to see you on my new thread.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[pmarc]

CH,
I would just change the very first word in your statement. Instead of "Orientation" I would use "Angularity".

 

[Sem_D220]

You do not have to make compromises on functional DRF. If functional DRF in the modified version of fig. 6-8 is |C|A|, then instead of applying perpendicularity symbol, use parallelism and that is all you have to do. Not to mention that in |C|A| DRF referencing to A secondary adds no geometrical value to the callout.

I generally agree, but what if my purpose as a designer for that particular case is to make sure that the hole is perpendicular to the face that contacts datum feature A at the assembly? I care less about parallelism to C, but the part happens to be oriented in assembly in a way that implies that C is primary. On the other hand If I ignore C and reference A only, or reference A as primary and C as secondary, the way the part will be fixed for inspection will not represent the exact way it is assembled when used. That means less qualitative control.
Again, I know that geometrically both alternatives are the same (edit: I mean in terms of tolerance zones - parallelism wrt |C|A| versus perpendicularity wrt |C|A|), and your suggestion is less controversial. But you said you care about clarity. GD&T is a language. Do you consider parallelism to C a clear way to say that perpendicularity to A is needed?

 

[CheckerHater]

@pmarc:
I did it for clarity. Because replacing angularity with perpendicularity in 4-7 wouldn't change anything. In fact, it would even make things better.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[pmarc]

semiond,
If the design intent is to make sure that the hole is perpendicular to the face that contacts datum feature A at the assembly, then the orientation callout should be perpendicularity to A primary, even though the component is oriented in assembly in C|A sequence.

Do I consider parallelism to C a clear way to say that perpendicularity to A is needed? Yes, I do, because I think you do not really say that perpendicularity to A is needed in the first place. By referencing C primary you are saying that your main concern is parallelism to C.

Side note: I made a mistake in my previous reply to you where I said: "Not to mention that in |C|A| DRF referencing to A secondary adds no geometrical value to the callout".

 

[chez311]

semiond,

If I understand the question correctly - definitely not. No datum precedence should be overriden. This is the most important thing. The whole point here is that I am trying to preserve the datum precedence order and the constrained degrees of freedom, as derived from the real application assembly. This is why I avoided from swapping the precedence order between datums A and B in the parallelism sketch embedded

I can relate to the reasoning, and the assumption that since the secondary datum is not mandatory, the nominally parallel datum should be the primary. I don't see how this is stated as an obligating rule, it's rather a common conclusion.

These two statements contradict each other. Your initial statements regarding a parallelism control where the feature is not parallel to the primary datum (as referenced by your sketch from 21 Jul 18 07:47 with parallelism to [A|B]) or a perpendicularity control where the feature is not perpendicular to the primary datum would be attempting to, or at least conflicting with, the datum precedence suggested by the DRF. Saying that this is because you want to preserve the datum structure from an assembly standpoint does not justify this to me, and if you MUST preserve the datum structure on this feature the callout should match this - in the case of your sketch it should be perpendicularity to [A|B]. If you want to do something else with a custom note that is up to you to determine what that would look like, but as you initially presented it I would say it is not compliant with Y14.5

I hope that using angularity the way it is used in fig. 4-7 (when the main goal is to control the 60° angle to the secondary datum and not perpendicularity to the primary) is acceptable by most.

As far as I know that is NOT the main goal? The angularity in any orientation symbol, as I and others here have said, is to the PRIMARY DATUM - in this case even though the surface is inclined at a 60deg angle, because datum A is primary in the DRF the surface is controlled first perpendicular to A! I'm not sure why you would think its the other way around? The DRF would have to be [B|A] for that to be the case. It seems pmarc agrees (see below).

But I would not say the main goal of the angularity callout in fig. 4-7 is "to control the 60° angle to the secondary datum and not perpendicularity to the primary". The physics of the process of setting up the part against |A|B| DRF is that the main job of the angularity callout is the control of perpendicularity of the face C relative to datum plane A. The 60°-angle control is merely a secondary function of the callout.

 

[chez311]

It is not about violating the standard or not. I don't think that the kind of control I use should determine the datum precedence order.

But it absolutely does in the case of orientation! Consider the general case of angularity - say you have a feature that is at a compound angle to all three datum planes. If you have an angularity callout with a DRF having reference to 2 or 3 datums how else is one supposed to determine where the primary/direct angularity control is? Guessing? It will ALWAYS be to the primary datum, any further datums will control additional DOF only and will only indirectly control angularity.

 

[Sem_D220]

If the design intent is to make sure that the hole is perpendicular to the face that contacts datum feature A at the assembly, then the orientation callout should be perpendicularity to A primary, even though the component is oriented in assembly in C|A sequence .

On thread1103-441690 you said:

The drawing should exactly represent function of the part and gage design should reflect that as close as possible.

If datum features, their precedence and material boundaries at which they are referenced on the drawing do not reflect how the part is oriented and located in real assembly, then no matter how precise the gage is and how well it simulates specified datums, it will not prevent from accepting non-functional workpieces.

I recognize contradiction between the two statements, and I relate much more to the content of the second quote.

 

[CheckerHater]

@chez311:
Could we talk about general case of angularity a bit more: thread1103-442354?

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[Sem_D220]

As far as I know that is NOT the main goal? The angularity in any orientation symbol, as I and others here have said, is to the PRIMARY DATUM - in this case even though the surface is inclined at a 60deg angle, because datum A is primary in the DRF the surface is controlled first perpendicular to A!

I know that it might not be the main goal for whoever made that figure. However I was talking about cases where it does happen to be the main goal for the designer. What would be your advice to a designer who wants to control the 60° angle from fig. 4-7? I suppose it is to reference |B|A| in the angularity FCF. Provided that the main mounting surface is datum feature A, can you relate to the idea that |B|A| callout would override the functional assembly sequence, and mean a different orientation of the part at inspection than the one in assembly? If you inspect orientation of a feature when the orientation of the part is not the same as in real use, don't you think there might be a "slight" problem there?

 

[Sem_D220]

But it absolutely does in the case of orientation! Consider the general case of angularity - say you have a feature that is at a compound angle to all three datum planes. If you have an angularity callout with a DRF having reference to 2 or 3 datums how else is one supposed to determine where the primary/direct angularity control is? Guessing? It will ALWAYS be to the primary datum, any further datums will control additional DOF only and will only indirectly control angularity.

Where there is more than one datum in the DRF, the tolerance zone will be determined as the part is oriented according to the datum precedence order, and with consideration of the basic angle specified. If the basic angle is specified to the secondary datum and not the primary, there is still no problem, as the secondary datum plane must be basically oriented to the primary datum plane. The tolerance zone will not be ambiguous, and there will be no need to guess anything.

Edited for typos correction

 

[CheckerHater]

If the basic angle is specified to the secondary datum and not the primary, there is still no problem, as the secondary datum plane must be basically oriented to the primary datum plane.

Genious! Since all the setup is done relative to basic geometry, there will be no difference.

Now take a look at the enclosed picture and tell if setup for B|A would be any different?

And finally take a leap of faith and answer this: is datum order relevant at all?

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[chez311]

CH,
Absolutely. I was actually going to reference your figure in my post but I wasn't going to get too deep into it, I just utilized the example because I think it supported my point well.

semiond,
When I said "main goal" I didn't mean just the intent of the designer who made the figure, I meant the main goal of the angularity control per the Y14.5 standard.

Yes my advice would be to utilize [B|A] in that case if the 60 degree angle is the most important dimension you want to directly control, there is no other way around it. Essentially your suggestion to use [A|B] but somehow notate B as most important you are doing the same thing, but not changing the DRF to support it which I believe leads to ambiguity and confusion - if you think you can accomplish that with a custom note feel free, just know that it cannot be inspected per Y14.5

If that feature truly requires to be directly controlled to another feature/datum other than the primary datum dictated by assembly/functional requirements then no - I see no issue. Otherwise follow the datum structure dictated by assembly/functional requirements and accept an indirect control. Or use profile. EDIT: on second thought, I'm not sure profile would be any less of an indirect control to a secondary datum. Please disregard that - it is tangential to my point anyway.

If the basic angle is specified to the secondary datum and not the primary, there is still no problem, as the secondary datum plane must be basically oriented to the primary datum plane. The tolerance zone will not be ambiguous, and there will be no need to guess anything.

I know this interpretation was brought up before, I do not agree with this. An implied basic angle is still a basic angle, whether not shown (0 or 90deg) or determined from another basic angle. The term specified adds no additional special meaning, at least to me, and does not provide the ability to select anything other than the primary datum in the DRF to which the angularity control is directly held to. Regardless, in my example all of the angles to primary/secondary/tertiary would have "specified" basic angles - it would be ambiguous to say anything other than the direct angularity control is to the primary datum.

 

[Sem_D220]

CH,
Datum precedence order matters.
The way the part will be stabilized on the as produced surfaces, and thus oriented in the fixture, is part of the process of establishing the theoretical datum planes, (which are basically oriented to each other, as stated in my previous post). Once the datum planes established, the tolerance zone can be established as well according to FCf specification and basic angle.

 

[pmarc]

semiond,
There is no contradiction between the two statements. The fact that the part is oriented and located in assembly by "global" datum features specified in a certain order of precedence does not mean that all other features of the part must be toleranced with respect to same datum features in the same order of precedence. This does not have to be a violation of functional datum features selection paradigm because very often from functional standpoint it is beneficial to control features relative to some "local" DRFs rather than "global" DRF.

 

[chez311]

pmarc,

Thank you for that explanation of "local" vs. "global" DRF's - I fully agree but I didn't have the exact terms to describe it until now. I will certainly be utilizing those in the future.

 

[Sem_D220]

This does not have to be a violation of functional datum features selection paradigm because very often from functional standpoint it is beneficial to control features relative to some "local" DRFs rather than "global" DRF.

pmarc, you said "often". You didn't say "always", and I appreciate that. What about those special cases where even orientation controls are preferred to be specified to the "global" DRF?

For example, when from functional reasons some feature must be controlled for orientation to a simulated mating face, and that same mating face happens to be not the one which constrains the largest quantity of degrees of freedom in assembly, and therefore not represented by the primary datum plane/datum feature simulator.

Keeping in mind the "functional datum selection paradigm" I further analyze this case:
If the required orientation control happens to be angularity, then according to your previous statements you would consider it legitimate to control a basic angle wrt non-primary datum (emphasizing that the "main" control applied by angularity in this case, whatever "main" might mean in this context, will still be to the primary datum. But, that will not work against the cause, as far as I'm concerned). On the other hand, if the considered control is perpendicularity - you would recommend to change the DRF from a functional one to a non-functional one, or specify a control other than perpendicularity, which fits the basic sorry - nominal relationship of the controlled feature to the primary datum better, and thus "disguise" the described above design intent, all in the name of clarity.

 

[CheckerHater]

What about those special cases where even orientation controls are preferred to be specified to the "global" DRF?

You can use Profile wrt A|B|C instead of orientation controls.

Then you will be rock-solid related to your DRF

Edited: your Profile will be orientation control if you only specify angles.


"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[chez311]

CH,

I was thinking about that earlier - if the relation (orientation or location) to something other than the primary datum of the global DRF is desired, wouldn't it still be best to reference a local DRF with that non-primary datum? Ie: your global DRF is [A|B|C] but the orientation/location of the feature being controlled is really critical in relation to datum feature B more so than A then wouldn't something like [B|A|C] or any variation with B as primary be what you want to utilize?

Yes profile orients/locates basically to your datums but since datum features are not perfect (as opposed to datums/datum simulators) its still not a "direct" control to the secondary/tertiary datums. Or am I nit-picking too much since profile can be such a tight control as it is?