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.

 

[powerhound]

I don't see any issue with scenario one as you have presented it.

Scenarios 2 and 3 are incorrect. The controlled face has to be oriented per the callout to the primary datum.

What would be your reason for wanting the orientation of the controlled feature to be controlled to the secondary and not the primary?

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

 

[Sem_D220]

powerhound,

Here are examples (can't upload sketches in the next 2 days so I hope the literal descriptions are clear enough).

Scheme #2: a part containing a tab (a rectangular feature of size). The face against which the part is clamped (by screw joints, for example) in the assembly is a bottom face designed perpendicular to the faces of the tab. I call it datum feature A. The part is supported against and oriented by a planar side face which is nominally parallel to the tab. I call it datum feature B. It is desirable to make sure that when the part is assembled, the center plane of the RAME of the tab is parallel to the plane against which the side face is supported, and the part is oriented. The DRF- primary datum is A, secondary datum is B for a parallelism control on the tab.

Scheme #3: A part (a kind of connector) in real application assembly is rigidly clamped by fasteners against a bottom face - datum feature A, to the base of the assembly. The part includes a top face designed at 150° to A. That 150° face connects a component of rectangular cross section in the assembly and it is the main mounting base for that component. I call that 150° face datum feature B. A tab on the extension of the part is designed at 90° to B. It is designed to function as a secondary support for the component. (Edit- since A constrains the most degrees of freedom of the part in the real assembly), when contolling the contact face on the last feature, it may be desired to control perpendicularity, with A as the primary datum of the DRF, and B as a secondary, in correspondance with the functional interface of the part.

 

[CheckerHater]

There is something I am missing here.
Let say, you have rectangular room with the door in the wall
You want to describe door as half-open (45 deg. for example) so you declare angularity between door and a wall.
Door is also always perpendicular to the floor.
Why would you want to make floor your primary datum?
You'll have to try and produce some sketches

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

 

[Belanger]

And I'm curious as to the "various reasons" that you don't want to use profile for these situations. If GD&T is about clarity, simply use the clearest solution.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[powerhound]

I guess I'm going to have to wait those 2 days. I don't follow your descriptions enough to see why you want to do things they way you're doing them.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

 

[Sem_D220]

CheckerHater, I don't consider the floor as a primary datum a bad idea. After all, if the door is installed tilted enough relative to the floor, it might be impossible to use

powerhound, I will definitely make some sketches when I can. In the meanwhile, I have question - you said you don't see issues with the first scheme (angularity), but the other two are incorrect. As far as I can tell, the only difference between angularity, parallelism and perpendicularity is that for angularity the basic angle between a controlled feature and the datum reference can be of some other value than 0° or 90°. In contrast, for parallelism and perpendicularity, it is only allowed to be 0° or 90° (implied). For me, there is no principal difference. So why is angularity allowed to be specified on a controlled feature oriented by basic angle to the secondary datum, unlike the other 2 controls?

Belanger, I guess my best "excuse" will be that something tells me to utilize orientation controls, where pure orientation of planar surfaces needs to be specified. As for profile, I think it is more appropriate in other cases. To quote 14.5: "Profile tolerances are used to
define a tolerance zone to control form or combinations of size, form, orientation, and location of a feature(s) relative to a true profile." For me to use it where only orientation control is needed is "pulling out the big guns" where you can do without them. I suppose it is possible to remove orientation controls from the standard alltogether and use profile whenever a surface needs to be controlled, but it's not done...

 

[powerhound]

So why is angularity allowed to be specified on a controlled feature oriented by basic angle to the secondary datum, unlike the other 2 controls?

Because what angularity means will change depending on the orientation of the feature. If you call out angularity to a surface that is perpendicular to a datum then it is considered perpendicularity. Same goes for parallelism. With angularity--used in this context--you aren't specifying an angle.
With perpendicularity or parallelism, you are specifically stating 0 or 90 degrees so the feature being controlled has to be at that basic angle to the primary.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

 

[Belanger]

To quote 14.5: "Profile tolerances are used to define a tolerance zone to control form or combinations of size, form, orientation, and location of a feature(s) relative to a true profile." For me to use it where only orientation control is needed is "pulling out the big guns" where you can do without them.

I think you're forgetting that the orientation symbols inherently control form.
So to use profile of a surface would do exactly what you want. It's not a bigger gun -- it's an equal caliber gun as long as there is no location being invoked, and profile is much more compliant with Y14.5 for your situation.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[Sem_D220]

powerhound, Belanger, those are very good points that you brought up.

But before I get convinced, I'd like to show a sketch. This is what my scheme #2 describes (It's somewhat different and simpler than the example I brought earlier). I'd like to get your opinions - if this is not legitimate, what is the reason? Is it illegality (per ASME Y14.5), lack of clarity, redundancy, or a perhaps combination of such reasons, and why?

Thank you very much.

 

[greenimi]

Why you do not think in the tolerance zones and degrees of freedom terms? What is and what can do - translate, rotate- the TZ of the controlled feature?

TZ is two perfectly parallel planes perfectly ....what ???? to A primary...
Parallel to A ? Perpendicular to A?


Then if the above answer is perpendicular to A then the FCF should reflect that.

Also what DOFs primary A is stopping? Then you determine what DOFs are left for the secondary B. And if B is determined that cannot stop any DOFs that have been already stopped by A (primary) then someone shouldn' have it there as a secondary in the FCF. If it does stop soome of remaining DOFs then is correct to be added as secondary.

 

[Sem_D220]

greenimi,
A stops 2 rotations and 1 translation.
B stops 1 translation and 1 rotation.
TZ should be parallel to B and perpendicular to A. The part should make contact on 3 high points at datum feature A and 2 high points at datum feature B. With that description do you consider the scheme shown in the sketch correct or not?
Side note that might be relevant - for many cases of this type I encountered, parallelism to B is actually more important than perpendicularity to A. However, I do not consider it wise to break it down to two separate orientation segments (parallelism and perpendicularity) with different values, each referencing a single datum. That is because such scheme would not reflect the real application interface of the part. It will create 2 separate tolerance zones within which the face should be contained, and none of those TZs will be required to be oriented as I described above. It is important to orient the part for inspection as it functions in the assembly.

 

[Belanger]

Check out the definition of parallelism in paragraph 6.3.2 and see if your sketch complies with that definition. The last part of paragraph 6.4 may be helpful too.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[3DDave]

semiond - you should send a change request to the Committee to change the definition of parallelism from:

"equidistant at all points from a datum plane" to

"equidistant at all points from the primary datum plane in the FCF"

so that ambiguous examples like the ones you come across in so many cases, such as your diagram, can be immediately rejected.

By extension of principles, there are multiple dozens of similar wording changes required. It's imperative to submit them before the next version can be issued.

 

[Sem_D220]

Belanger, thank you for directing to the relevant references:

"6.3.2 Parallelism
Parallelism is the condition of a surface or feature’s center plane, equidistant at all points from a datum plane;
or a feature’s axis..."

Well, "a datum plane" in my sketch which is relevant for this definition is datum plane B. Full compliance so far.

The end of para 6.4 states:
"If the primary datum feature alone does not constrain sufficient degrees of freedom, additional datum features
may be specified."

That reminds of the fact that sometimes we can do without the secondary datum at all for orientation tolerances, so the secondary datum can be considered, perhaps - "less fundamental" so to say, and therefore can't take the "main role" of constituting the parallel ingridient of the DRF for parallelism. Not very convincing. I get the hint, but I see it more as a basis to a convention than an obligating rule.

Edit: 3DDave, just saw your post. You list the diagram among the "ambiguous examples", would you mind to detail about the ambiguity? Is it the tolerance zone that is ambiguous?

 

[3DDave]

semiond, it is obviously ambiguous - you could be referring to any datum plane, not just one in the current FCF. The current definition doesn't provide any limits at all as to what datum plane might be involved.

Perhaps you could elaborate on where you come across all the many cases you come across. It suggests that those cases must number in the hundreds.

 

[Sem_D220]

3DDave, The tolerance zone is not ambiguous, just as it wouldn't be ambiguous if the symbol was replaced by angularity, for instance (which is not different in this case from any application of parallelism or perpendicularity). The datum reference frame is defined, 3 rotational degrees of freedom are constrained and the tolerance zone is oriented to the two datums referenced.
Fig. 6-4 clearly shows that there is no requirement from both datums in a FCF to have the basic angular relationship associated with the symbol to the controlled feature. It shows that one of the datums in a perpendicularity FCF may not be designed perpendicular to the controlled feature. The reasoning behind what should be the primary and what should be the secondary datum must first and foremost be based on functional considerations, rather than on the angle you associate with the symbol.
If the committee members want to forbid schemes such as shown in my sketch, they are free to change the wording as you suggested. But if there is a reason to prevent the use of a functional (per the application of the part) feature control frame for orientational controls and instead force a datum precedence order according to some symbolic aesthetics, that reason is definitely not ambiguity of the shown scheme.

I'd like to clarify that I am not trying to advocate the use of orientation controls the way I described. If I was sure it is OK, I wouldn't be asking the question on the forum. I just want to understand thoroughly why it is considered illigitimate/illegal.

 

[3DDave]

semiond - your example is obviously ambiguous as to which datum the tolerance zone is to be parallel. It requires the user to conduct a search through the list of datum references an compare them to the geometry on the drawing to see which is applicable instead of settling on the first one. A typical use of orientation tolerances is that the primary datum is closely aligned to the datum feature surface and the feature surface is therefore controlled to that.

In this case there is not any sign that there is close alignment beyond two points. It would be no different than if datum feature [B} was at 45 degrees to datum feature [A]. One would still establish just the same. And that would be just as legitimate. You can sketch it and see. See figure 4-7 for inspiration.

In Fig. 6-4 the perpendicularity tolerance zone is perpendicular to the primary datum to which the surface is nominally perpendicular.

I can't help but notice the way "perpendicularity" is repeated in the preceding sentence.

In your example the parallelism tolerance zone is perpendicular to the primary datum to which the surface is nominally perpendicular.

One of those orientation words is not like the other.

You already know that there is nothing in the standard that lists the infinite possibilities of "not-allowed." You are certain to have carefully combed the standard for something that disallows your example and, not finding any, posted here.

There is no functional reason to use a parallelism control when the primary relation is perpendicularity, but you claim to have seen it as a solution in "many cases." Unless someone can show a rule against it, knowing there is no rule against it. I only suggested you submit the change so that you can be 100% sure, which seemed to be your goal.

Your proposal is neither illegitimate or illegal, just ill advised as it is likely to provoke distracting discussions like the ones you have here. You are free to proceed with it.

Perhaps you could elaborate on where you come across all the many cases you come across.

 

[Sem_D220]

3DDave,
Searching the parallel one among the 2 datums in the FCF is a task that even the laziest of the drawing users are not expected to complain about (by the way, ever encountered a 3 datums FCF for orientation? Something about the way you presented the "search through the list of datum references" implies that you're used to orientation FCFs with more than 2 datums) But since I already understand that it's controversial, I no longer consider using parallelism, or perpendicularity the way described.

Do you agree with powerhound that angularity according to scheme #1 is legitimate?

Perhaps you could elaborate on where you come across all the many cases you come across

This is the second time in a row you ask this question. And the second time in a row I'm not going to answer on it.

 

[3DDave]

semiond, do you agree that the secondary datum feature is not required to be nominally parallel to the secondary datum and therefore no datum feature referenced in the FCF would need to be parallel to the indicated feature? That lacking that obvious parallelism makes it more difficult to understand the reason for using that as a reference? And that even a nominally parallel surface may be far from parallel in the realized product, perhaps far in excess of the parallelism tolerance?

I don't have to agree with anyone in particular on angularity - I have proposed before that it replace both parallelism and perpendicularity in their entirety.

However, using angularity will not force the zone to be closely parallel to the secondary datum feature.