Position (tolerance vs true vs error) - Let's fix our vocabulary

[MechNorth]

We debate terminology and applications all the time on this forum, however I keep seeing one glaring mistake that is glossed over most of the time. In short, the mis-use of "True Position" really irks me.

In the '09 Y14.5 standard;
"1.3.64 True Position
true position: the theoretically exact location of a feature of size, as established by basic dimensions."

"7.2 Positional Tolerancing
Position is the location of one or more features of size relative to one another or to one or more datums. A positional tolerance defines either of the following:
(a) a zone within which the center, axis, or center plane of a feature of size is permitted to vary from a true (theoretically exact) position
... (dropping (b) here for brevity)
Basic dimensions establish the true position from specified datums and between interrelated features. ..."

Then, to top things off, people commonly reference the Positional Error (not defined in the standard, by the way) as "position", as in "The hole is at a position of 0.5."

PLEEEEEEEASE reconsider your terminology before using these terms. Tks.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[Belanger]

Agreed, Jim. In a class, people often ask me when they changed the name of the symbol from "true position" to "position." (Cue Homer Simpson: doh!)
And you've probably seen the book that calls it TOP, tolerance of position. I'm not a fan of that, because we don't say TOA (tolerance of angularity), TOP (tolerance of profile), etc.

Thus:
true position = perfect location of a FOS, and it must have basic dims

position = the actual location of a FOS, and it must have an associated GD&T callout with the position symbol

location = general term for anything in the realm of position, concentricity, symmetery, etc.

I put "etc." in this last one because profile often controls location, and runout certainly does, but also to leave some wiggle room for non-GD&T methods for location, despite the inadvisability of such.

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

 

[CheckerHater]

It is not that I don't believe you guys, but could you point me to a standard book / paragraph, etc.with a definition of "location" for example?

 

[dingy2]

I absolutely agree with you on this one, Jim. How many times have I had to correct the terminology - too many to count.

I will always remember a very sharp Process Engineer who took both basic and advance GD&T with me and when he had a question, guess what the term was - True Position. I told him to get a elastic band over his wrist and each time he used the wrong term, he should pull the elastic and then let it go. That might help.

Dave D.

http://www.qmsi.ca

 

[dingy2]

Sorry CheckerHater:

I will use the newest standard ASME Y14.5 - 2009 here.

1.3.64 (page 7) True Position

true position: the theoretically exact location of a feature of size as established by basic dimensions.

7.2 (page 108) Positional Tolerances

Positional is the location of one or more feature of size relative to one another or to one or more datums. The positional tolerances defines either of the following:

(a) a zone within which the center, axis, or center plane of a feature of size is permitted to vary from a true (theoretically exact) position, that may not be violated by the surface or surfaces of the considered feature of size.

(b) (where specified on an MMC or LMC basis) a boundary, defined as the virtual condition, located at the true (theoretically exact) position, that my not be violated by the surface or surfaces of the considered feature of size.

Terminology is important for credibility.

Dave D.

http://www.qmsi.ca

 

[CheckerHater]

Dear Dingy,

Did you notice that I asked for the definition of the term "location"?

I believe Belanger's definition is made-up.

It's easy to answer if you make-up question as well.

 

[Belanger]

It appears made up because I don't recall Y14.5 giving a precise definition of location, other than using it as the name for the category of symbols consisting of position, concentricity, and symmetry. It always treats it in a broad, non-symbol-specific manner.

Since the other words have very narrow defintions, doesn't it make sense that "location" can be used for the more generic concept?


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

 

[CheckerHater]

JP,

You said it yourself: "location is a category"

It resides on different level. There is location and there is a position, which is also location.

When you say "It's not cat, it's dog" I expect you to have definitions for cat and dog somewhere.
When you say "It's not cat, it's mammal" I am not sure you know what you are talking about.

This is why I am asking if some standard book actually clarifies the terms, including broad ones.

 

[Belanger]

No, it's more than a category. I should have clarified that that's just one way that the standard uses the word location. But it also uses the word location very frequently when describing profile as it relates to a datum, and for runout, etc.

But it would never say "position" when describing the relationship between profile and its datum(s).

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

 

[CheckerHater]

Sorry,

I am seriously tired with this and want to quit.

But it looks like we now have Upper-lever Location, that includes Orientation, Run-ot and, well, Location; and then the Lower-level location including Position, Concentricity and Symmetry.

Are you comfortable with definition being THAT broad?

 

[Belanger]

Think of an analogy: Many people use the term "concentric" but don't really mean it in the GD&T sense. So most GD&T folks try to be more careful and use the general term "coaxial," because that is not married to a specific symbol.
It's similar with location: that's a broad term that doesn't nail down a specific symbol. But position does.

Heck with it. I'm tired too :0

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

 

[MechNorth]

No, the standards don't establish a specific definition for "location". The standards do, however, indicates means of controlling specific locational relationships; see 7.1, '09. "This Section establishes the principles of tolerances of location. Included are position, ocncentgricity, and symmetry used to control the following relationships: ..." and it goes on to list four "categories" for want of a better term.
Then 8.2, '09; "A profile is an outline of a surface, a shape made up of one or more features, or a two-dimensional element of one or more features. 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. ..."
I didn't find a specific reference to "location" in the runout controls section, BUT, it is locating the surface wrt the axis. It's unfortunate that runout is still defined by means of its inspection method.

Orientation, however, NEVER locates.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[axym]

I'll throw out some ideas.

Jim, I agree on the strict use of proper terminology with regards to position, true position, actual position, etc. The use of the term "true position" to refer to the characteristic or symbol is a pet peeve of mine as well. "True position tolerance" and all that. It's not just because I'm anal, it's because confusion starts when we're not all understanding the terms in the same way. The same thing goes for all things with the word "datum" in them.

One thing that I find myself doing when I write about GD&T in emails and forums is to use capital letters when referring to a Y14.5 geometric characteristics (Position, Flatness, Concentricity, etc.). This is to emphasize that I'm talking about the Y14.5 definition and all that carries with it, as opposed to the generic meaning of the word.

The discussion of the term "location" has been interesting. I agree that the word "location" is used frequently in Y14.5 but not precisely defined. Personally, I don't find the Y14.5 classification of tolerances to be very useful. We really can't look at the properties of the "Tolerances of Location" and figure out what location really is. Concentricity is a Tolerance of Location, but as Jim says the runout tolerances locate the feature wrt the axis in very much the same way as Concentricity. I would say that all Y14.5 tolerances locate the feature, except for form tolerances and orientation tolerances.

So what does "locate" mean then? The best that I can come up with is that to "locate" a feature means to constrain the feature's translation in some way. Orientation tolerance zones can freely translate relative to each other, and relative to the DRF. Form tolerance zones can freely translate relative to each other, and there is no DRF. All other tolerance zones have their translation constrained in some way and thus provide location control:

-Position zones cannot translate relative to each other, or relative to the DRF if present
-Concentricity zones cannot translate relative to the datum axis
-Symmetry zones cannot translate relative to the datum centerplane
-Circular Runout and Total Runout zones cannot translate relative to the datum axis
-Profile of a Surface zones cannot translate relative to each other, or relative to the DRF if present
-Directly toleranced dimensions constrain the translation of the features relative to each other, in a non-rigorous way
-The zones for lower tiers of composite FCF's cannot translate relative to each other, but can translate relative to the DRF

Does that make sense?

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca