Refinement to positional tolerance

[prdave00]

I have a differing of opinions on a tolerance scheme best described in the attached file. Basically it is over some nuances regarding what a colleague calls a "composite" feature control frame (I think incorrectly), and what I would simply call 2 single segment control frames.

The part has a post that I want to control location and orientation in general, and then refine the location & orientation in one direction.

We are using ASTM Y14.5M-1994, but I'm curious if it breaks any rules of the 2009 version.

 

[dtmbiz]

Evan, Jim, John-Paul,


I believe I understand all points of view that you have presented. They are affirmation of my understanding.

My point of view regarding a single hole for example, with a positional FCF to only control orientation, is
that a positional FCF should have locating dimensions to be a positional tolerance, therefore I would use an orientation FCF
in that case. My opinion.

Even though as Jim presents that it would be "technically" legal to use the composite FCF; I changed the FCF
for the sake of clarity. My point of view these days is that the less explaining required, the better the callout.
Clarity with agreement, while not compromising integrity, is better for me than being technically legal.

One thing that does "bug me", is when Evan responds to the PLTZF as being a general guideline. It is a technical standard
and in my opinion should have weight to the terms is uses. If one considers certain terms as guidelines and not meaning specifically
the term used, and add to that the "extension of principles"; the communication at that point can become way too messy for me.
I understand the need for extension of principles, however if a term is intended as guideline, then say so.

I value all of your responses and appreciate your time to respond.

 

[axym]

dtmbiz,

Please keep in mind that my opinions on Y14.5 terminology are just that, my own opinions.

I sometimes question the usefulness of the terms PLTZF and FRTZF to describe the upper and lower segments of a composite FCF. The PLTZF (Pattern Locating Tolerance Zone Framework) does more than locate, it also orients. The FRTZF (Feature Relating Tolerance Zone Framework) does more than relate the features, it also orients them and in some cases can also locate them. So to me the terms FRTZF and PLTZF are oversimplifications that gloss over some of the intricacies of composite FCF's.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[Belanger]

I feel obligated to push back, Jim
Position by definition is used to control location. Oh by the way, orientation often comes along for the ride. Therein lies the "hierarchy."

To use the position symbol when the only quality to be obtained is perpendicularity is akin to using parallelism when all you want is flatness; you're putting the hierarchy out of order. Think about it: Parallelism by definition is meant to control orientation, but you're saying that we aren't really controlling orientation. Non sequitur!

Maybe I'm being too literal with the standard. But from a logic point of view, it doesn't make sense to point to a single hole and "position" it to a plane that intersects that hole.

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

 

[MechNorth]

I'm not disagreeing with you, JP. In fact, I quite agree that it shouldn't be used that way. Unfortunately, "shouldn't" isn't provided in the standard, so from a strictly "legal" interpretation, it would stand. I've argued both sides before on numerous occasions, both to learn and to teach (I do like playing Devil's Advocate!), and from a strictly technical perspective, it is valid. Inappropriate/confusing/unfriendly/ill-advised, but ...

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[axym]

I can understand JP's point of view, and I can understand Jim's point of view. The standard provides evidence to support both. The fact that orientation tolerances can only be applied to individual planar surfaces, cylinders, or parallel-plane slots kind of muddies the waters. There is no universal "orientation" tool in Y14.5. If I want to control the combined orientation of a pattern of coaxial holes, I have to use Position.

So I don't put too much weight in classifications or hierarchies for geometric tolerances in terms of controlling size, form, orientation, and location. What gets controlled depends heavily on the geometry of the considered feature and datum feature(s), and the nominal relationship between the two. It's very case-specific and defies simple categorization, as evidenced by the awkward groupings in Y14.5. Surface Profile is the only geometric tolerance that can locate irregular surfaces, but it is not included as a tolerance of location. Size tolerances also control form. Position is lumped in with Concentricity and Symmetry as location controls. The runout tolerances are "composite controls" even though they control location to a datum axis in very much the same way as Concentricity. Position can sometimes control orientation only. It's a mess.

Here's an example that bent my mind a bit when I first thought of it. Picture a cylindrical shaft with a diameter tolerance of 1.000 +/- 0.005 and a Straightness tolerance of 0.020 at MMC. The cylindrical surface would have to conform to a virtual condition boundary of diameter 1.025. Now let's add an o-ring groove halfway along the shaft, splitting the cylindrical surface into two sections. We can't use the same Straightness tolerance, because form tolerances can only be applied to individual features. To get the same virtual condition boundary as we had with the Straightness tolerance on the single cylindrical surface, we would have to specify a Position tolerance of 0.020 at MMC (with no datums) on what is now a pattern of 2 cylindrical surfaces. So in this case Position (a tolerance of location) applies very much the same control as Straightness (a tolerance of form). This flies in the face of the "hierarchy" in a big way.

Here's a question. Would it be legal to apply a Surface Profile control, with no datum features, to a single planar surface? Or are we obligated to use Flatness because it exists?

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[Belanger]

Evan -- here's my two cents on your scenarios:

The new standard has a "CF" modifier, so I'd say we are now allowed to use straightness of 0.020 MMC on those two surfaces interrupted by the groove. But you're right that in 1994, we would be forced to use position.

However, I would still say that using position in that case qualifies as a "location" control, because we are locating one cylinder to the other (take your pick as to which one becomes the "datum"). So it doesn't really mess with the hierarchy.

Position doesn't "sometimes control orientation only." It often controls orientation along with location. But I agree with your other statements about runout, profile, and the other jumble of categories.

To your last question: yes, it is legal to apply profile of a surface to a single planar surface. Profile's main job in life is to control form (usually it has datum references to do other things as well). So this case would be equivalent to flatness. (And I think flatness would have been better, but profile is legal.)

Yet this is still different from position's "orientation only" usage because in this case profile is fulfilling its main duty; in the other case position does not even fulfill its main duty of location.

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

 

[MechNorth]

hmmm... Isn't a surface profile's primary purpose to locate a surface; orientiation & form control are freebies within the location tolerance zone when datums are applied. Form is sort of the last thing it does ... on a hierarchy basis (i.e. the lowest functionality of the control). Don't get me wrong, I still LOVE profile because it's the most robust tool we have.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[Belanger]

No, if its primary purpose is to locate a surface, then you are saying that a datum reference is always required.

See paragraph 8.2 of the standard: "Profile tolerances are used to define a tolerance zone to control form or combinations of size, form, orientation, and location..."

Notice that "form" is always an integral element of profile.

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

 

[MechNorth]

Certainly, you always get form for free with a profile control because it's at the lowest end of the functionality. The hightest functionality for a profile control is location.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[axym]

I agree with you that the position tolerance I described qualifies as a location control. It's relative location, with neither surface being the datum for the other. Perhaps, then, form is really the relative location of the points within a feature.

I'm not sure that I agree with you about the role of orientation in position tolerances. For features of size whose orientation can be controlled (cylinders, slots), a position tolerance will always control orientation (perhaps relative orientation). The lower tier of a composite position FCF allows the datum features to constrain rotational degrees of freedom only - this is the example I was thinking of when I said that position sometimes controls orientation only. I will admit that the relative location of the features in the pattern would be controlled by the lower tier as well. So you are right that a position tolerance always has an aspect of location. Except in the case of a single cylinder ;^). But I would say that a position tolerance always has an aspect of orientation as well. Except in the case of a single sphere ;^).

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[Belanger]

Roger that. I think we're all on the same page, but had different terms flying around (hierarchy, primary function, orientation-only, etc.) that made it a little hard to see each others' perspectives.

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

 

[MechNorth]

Yup. Same hymn, different language. Still the same gospel.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com