Spinoff: Using position to only control perpendicularity 1

[Belanger]

I know this has been discussed several times as an offshoot of other threads, but I can't recall if it's had its own thread. The dilemma is whether a position tolerance can be applied to a traditional single hole through a plate, but the only datum referenced is the face of the plate. The objective is to simply let it control perpendicularity. I say, nay nay

The ASME standard tells us that position is "the location of one or more features of size..." (paragraph 7.2, emphasis added). And each of the subparagraphs of 7.2 indicate location as the main objective. So by definition, a position tolerance must control location.

We all agree that in the classic hole-through-plate example a position tolerance will have perpendicularity come along for the ride. But that's just it -- it comes along for the ride! If you want perpendicularity to be the vehicle, then you should use the perp symbol (or even angularity or profile!).

An analogy: if I say to you, "Please stop by the grocery store. We really need some bread for tonight. And if you want to get some cookies, that's OK too." Suppose you come home with only cookies. The main objective hasn't been met.

Sometimes folks will toss out the profile symbols as examples where different aspects can be opted in or out. But at its base, a profile tolerance is a form control. That's it. If you add datums, then you get orientation and maybe location and size (all around can do that too). But notice that profile is "upwardly mobile." That's fine.

But to use position to achieve only perpendicularity is "downwardly mobile," and the symbol fails to do its inherent job, which is location. It's akin to using parallelism to control ONLY flatness -- a higher-order symbol can't be used to achieve a lower-order symbol while cutting out its own meaning.
Thoughts?

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

http://www.gdtseminars.com

 

[dingy2]

J-P:

I agree with you that positional on one hole (as an example) relative to the primary datum is not quite correct. We should use perpendicularity with a diametrical tolerance zone of some value if the hole is round relative to the primary datum.

Dave D.

http://www.qmsi.ca

 

[pmarc]

J-P,
I am totally with you on this, though am afraid you won't stop seeing position instead of perpendicularity as long as any of future editions of Y14.5 clearly states that such practice is definitely prohibited.

For you, and probably for many of us here, paragraph 7.2 does the job, but it seems not enough to solve the issue once and for all.

 

[axym]

J-P,

I was initially going to say that using Position in this case would be inadvisable but legal, but it looks like you're correct that the standard does in fact disallow it. 7.2 states that Position is "the location of one or more features of size relative to each other or to one or more datums". We don't have location relative to each other, because there is only one feature. We also don't have location relative to a datum, because the feature is nominally perpendicular to the datum. So the standard says nay nay.

Despite this outcome, I will continue to argue ;^). We also wouldn't have location relative to the datum if the hole was at a 45 degree basic angle, or at any basic angle other than parallel. So we need to have a parallel relationship to have location. For the hole to be located to the datum, it needs to be parallel to the datum.

Imagine that we have a block-shaped part with datum features A,B,C on three mutually orthogonal planar surfaces. There is a hole in the part at some skewed angle, that isn't parallel to any of the datum features. We specify a Position tolerance on the hole, and reference datum features A, B, C. Does this locate the hole? Of course it does, but does it also violate the requirement of 2.7? Strictly speaking, the feature is not located relative to any of the three datums.

Or do we need to bring in a different definition of "location" ?

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[fsincox]

I agree with you that, in the simple cases as shown in the standard and as described here, perpendicularity is the best choice. I think blind rules are just that, though, "blind". I do advocate for options, and while many parts fit these basic molds, some do not. We must be able to tailor the GD&T to fit them, too.
Say the primary datum was a spherical feature and now you want to locate a through hole in the sphere, an orientation only tolerance may not be sufficient to achieve what is desired, but, I could see someone saying the second feature "must be orientation", if we are not careful. People who are using the 2009 standard can now override what a datum controls, but for those of us tied to the older versions we have to reference a sphere feature before other features that could also exert a control in the same direction if we want the sphere to represent “0”.
I have seen GD&T people advocate no tolerance statement at all, just make it a datum. The round piloted bearing retainer, The mounting face is the primary datum and the pilot diameter is secondary. There are people who advocate not bothering to put an orientation tolerance on the pilot at all as this secondary datum is not "sufficient" to inspect for perpendicularity (it is sufficient enough to do the function that it is intended to do in the real world, by the way) but we find it hard to measure so why bother. I find that to be a more grievous offence than a position for perpendicularity, IMHO.
Frank

 

[Belanger]

Evan, certainly a hole at an angle is still controlled for location from the edges. In addition to a basic angle, we would have a basic dimension to locate either the entry or exit of the hole.

Frank, I'd have to think more about the sphere example... a hole through the sphere is still located to the center point of the sphere, although orientation may be irrelevant. My head still kinda hurts from all that other stuff about spheres
I do agree about the business with perpendicularity on a secondary datum back to the primary plane. Even on a stubby pilot diameter where perpendicularity might be negligible in a numeric sense, perpendicularity does maintain the relationship between the two datums, which is important especially for gaging.

I know that sometimes I seem too legalistic about the standard, but because position is a location tolerance, I don't see this topic as a "nuanced" thing but rather the very heart of what position means. But opposing views are welcome...



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

http://www.gdtseminars.com

 

[fsincox]

JP,
I appreciate your pointing out "legality" when it if is clearly there, and taking time to make your case for it, when the views differ.
I also, agree that location is desired in both my (sphere) and Evan's (compound angle hole) examples. It is as a refinement of location to datums when orientation really becomes handy, and the “traditional hierarchy” becomes very useful to us.
Frank

 

[axym]

J-P,

I agree, of course a hole at an angle is controlled for location from the edges. But one could argue that this is different than controlling the location relative to the datums, as described in 7.2. None of the three datums can control the location of the hole on its own. It's the combination of the three, or something extracted from all three.

So when the standard states that "Position is the location of one or more features of size relative to each other or to one or more datums", what exactly does "the location of" mean? What does it mean to locate something? Perhaps this is the question we need to answer.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[Belanger]

Location = distance

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

http://www.gdtseminars.com

 

[axym]

J-P,

Ok, let's try location = distance. So a Position tolerance must control the distance between a feature of size and another feature of size, or the distance between a feature of size and one or more datums. I'm not sure that this works - the distance is not clearly defined if the nominal relationship is not parallel.

I think that location has more to do with controlling translation. The only difference between Perpendicularity zones and Position zones is that the Perpendicularity zone can translate relative to the datum reference frame, and the Position zone is fixed. So a Perpendicularity zone doesn't control translation but a Position zone does.

In your special-case example of the single hole in the plate, the geometry is such that the translation-controlling ability of Position does not apply. There is only one considered feature, and the only datum feature is perpendicular to it. The Position zone is located to the DRF, but the DRF can freely translate relative to the part (in the directions parallel to the datum plane). The end result is that the relationship between the tolerance zone and the considered feature is the same as it would be for a Perpendicularity. But the tolerance zone mechanics were different.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[Belanger]

I think you're making an issue where there is none.
You wrote that "the distance is not clearly defined if the nominal relationship is not parallel." What about profile of a surface, applied to a curved surface, with basic dimensions tying it back to datums? You are saying that location is not controlled simply because different points on the curve have different distances to the datum? C'mon

For the example of the angled hole: The tolerance zone at the entry point of a hole is at a certain distance from a datum, and the tolerance zone for the exit point of the hole is at a certain distance from the datum. It's just that they are different basic distances from the datum. But the key is that the distances are locked -- IOW, they can't translate.

I hope we can agree that preventing something from "translating" means that we are locking down its distance.

Or your other statement: a "perpendicularity zone can translate relative to the datum reference frame, and the Position zone is fixed."
What do you mean fixed? You mean that its distance doesn't change, right? Semantics, in my view.


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

http://www.gdtseminars.com

 

[MechNorth]

J-P, you already know my position on the use and the meaning of position as an orientation control exclusively. Your argument in support of your position which invokes the profile control as fundamentally a low-level control is not supported though. You take Position as a top-down control and Profile as a bottom-up control; that's too convenient for your argument. Profile is a top-level control which is the only way to locate surfaces. Within that control, there is orientation and form included depending on the datum references. I don't see position any differently. I still believe that position shouldn't be used for orientation alone, but to say that it is illegal by the standard is reading more into the statement.
The standard calls for a Position control to invoke a datum reference(s). Have you ever had to put a separate FCF for a group of features to effectively create a composite position control? What if that separate Position control doesn't have a datum reference? Perfectly functional, but is it illegal? Sometimes we have to look beyond.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[fsincox]

JP,
I was also curious about your profile statements, I have always thought of profile as the ASME's answer to the ISO's "true position of a surface" since the ISO has allowed that, and still does the last time I checked. So I had never thought of it the way you put it as a just fundamentally a form control, I understand it works that way without no datums. I never thought that was it's sole intent.
Jim commented, in a thread one time, that composite profile was not in 1982 and I guess he is right technically but I had always thought of it as a logical extension as I have always thought of profile as "true position of a surface".
Frank

 

[Belanger]

Thanks for the feedback, guys. Jim, given the statements of para. 7.2, how can you justify that position for perpendicularity alone is not prohibited?

In my view, what you're saying is like this: I can use parallelism to control a surface's orientation to be 34 degrees from the given datum. Yeah, I know it's not really controlling parallelism, but my basic dim of 34 tells everyone that I'm using parallelism in a "lower-level" sense of angularity.

Huh?

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

http://www.gdtseminars.com

 

[Belanger]

Whoops -- I also meant to address the profile question.
I'm baffled that you guys disagree that profile is a form control at heart.

Can I use profile to control form only? Yes.
Can I use profile to control orientation only? No.
Can I use profile to control orientation and form? Yes.
Can I use profile to control location only? No.
Can I use profile to control location and form? Yes.

Notice that the only way to use profile is to always have form be part of the equation. Thus, profile is inherently a FORM control that can (not must!) use steroids (datum refs and basic dims) to control higher-level qualities.

On the other hand, let's try position:
Can I use position to control location only? Yes.
Can I use position to control form? No.
Can I use position to control form and location? No.
Can I use position to control orientation only? No (given para. 7.2).
Can I use position to control orientation and location? Yes.

So position is a higher-level idea that must entail location. It's the opposite of profile: Profile does small things (form) but it's possible to "add" to it. Position does big things (location) but you can't "subtract" from that.

Jim, I think you are saying that profile is a higher-order control because it appears at the summit of the hierarchy or pyramid or whatever. That's true. But I'm saying that profile beomes a higher-order control by building from the lower levels. If profile wants to stop at a lower level and work there (i.e., form), that's fine. But position is a higher-order symbol that can't stop at lower levels and hide there; it has to have location as a fundamental element.

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

http://www.gdtseminars.com

 

[axym]

J-P,

Here's a question. Is it legal for a Profile of a Surface tolerance to be applied to a single planar surface, when the only datum feature referenced is a planar surface that is nominally perpendicular? From your last post it looks like your answer would be yes, but I just wanted to make sure.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[MechNorth]

J-P, I am able to make the distinction because I've helped write standards for about 15-plus years now. The statement does not preclude position's use as a lower-level control. Note that there must be an explicit statement precluding something for it to be technically against the rules. It's the same reason/means we apply to extend principles from sometimes scant bits randomly gleaned from thoughout the standard. That is how standards are developed (for better or for worse).

Not as a criticism, but as an opportunity, it's highly advantageous for people creating and providing technical training to participate in the standardization process. Not the most exciting, granted, but it gives you fantastic insight into the language and subtleties of technical writing for standards.



Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[Belanger]

Evan: yes.

Jim, para. 7.2 precludes that because it says that position controls location. Other qualities may come along too, but it controls location. I don't see how helping write standards can dance around that clear definition.

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

http://www.gdtseminars.com

 

[MechNorth]

JP,
Helping to draft the standards helps to develop an understanding of the meaning of specific wording; what is preferred and prohibited. In general, standards-makers like to avoid absolute statements, whether requirements or prohibitions, but they do use them occasionally. When they specifically want to preclude something, they use strong and direct language. The cited paragraph was not preclusive.


Interestingly, in '09, Position doesn't reference the hierarchical nature re orientation until the composite section (it is shown in Fig. 7-8). Does that mean that Position, outside of a composite control situation, does not control orientation? Of course not. The standard is (perhaps too frequently) not specific for requirements or prohibitions. That doesn't mean that you have to or cannot do something.

Paralleling your argument, if you have a profile control applied to a surface (with datum reference) which is part of a feature of size, which has a FOS toleranced dimension already locating the surface, then you have a conflict with the location ... after all, a profile control with a datum reference is first and foremost a location control. But in reality, not necessarily so; depending on the datums referenced, it may be just an orientation refinement.

I do see your side of the argument, J-P, but I do not see it as a preclusive statement. Perhaps you should do the exercise of trying to defend the opposite position. It's a method that I've used successfully to figure out alternate perspectives on several occasions.



Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[Belanger]

I think we're having a disconnect on the words themselves. Position is by definition a location tolerance. You are saying that it's permissible to undercut that definition, under the premise that there's no specific statement in the standard to overtly prohibit such undercutting.

I'm saying that there is a specific prohibition: it's called the word location! How can you control location but not control location?

You asked about composite position (Fig. 7-8): "Does that mean that Position, outside of a composite control situation, does not control orientation? Of course not."
Agreed. Position always controls orientation, so the question is irrelevant. The issue is whether position can control orientation without controlling location. That's not possible because it sacrifices the intrinsic meaning of position in order to get some lesser quality.
I realize I never answered your question about position occasionally omitting a datum reference. But the standard never says that position by definition references datums. The standard does, however, define position as controlling location.

Recall my example of using parallelism to control an angle of 34 degrees (rather than 0 degrees). Is that legal? Why or why not?

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

http://www.gdtseminars.com