Self-Referencing datums; all around profile (REF: GDT Question thread) 4

The confusion arose because the ASME standard didn't envision a situation where someone would wrap a profile tolerance around some surfaces that are the very datum features referenced in that profile tolerance. (Notice that the figures you reference don't have datum references.)

John-Paul Belanger

To follow up on my previous comment, attached is a graphic I created that shows this goofy thing about profile referencing datums unto itself. The second page of the graphic shows three interpretations and which one the other discussion seemed to favor.

The reason is that the datum is a perfect plane around which the profile is bilateral (by definition). Well, if the datum is created from the high points only, and the tolerance zone is bilateral, then the result is that half of that zone isn't really usable because there can't be any points beyond the three highest points.

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

Thank you for your time to respond John-Paul

dtmbiz,
I already said it in the original thread:
"This concept has not been explicitly pictured and described in the Y14.5-2009, but most likely will be covered in the future version of the Y14.5. From existing standards the closest is fig. 4-16 from Y14.8-2009. It shows datum target application, but the basic tenet is the same - profile tolerance zone applied to datum features from which datum plane is derived is equally disposed about that datum plane. This means that in case of datum targets the entire profile tolerance zone is usable, but in case of datum feature being entire surface only half of the profile tolerance zone is usable."

If you have access to Y14.8-2009, check out para. 4.5, where it has been stated that for datum target areas application...:
"Allowable variation of the actual workpiece surface coincident with the target area is one-half the specified profile tolerance for an equal bilateral distribution, or the amount toward material specified in an unequal bilateral control."

The "halving" concept, as described above, can easily be extended to a situation where the entire surface, not just some areas of that surface, is datum feature.

As for your questions 3 and 4, technically there is no need to have perpendicularity callouts on datum widths B and C (in the initial post of the referenced thread). Their relationship to each other, as well as to higher order of precedence datum feature A, is controlled by the all-around profile callout. Take a look at fig. 4-33 in Y14.5-2009. On that figure there is no need for a perpendicularity callout on datum feature B relative to datum plane A, because relationship between both widths, A and B, is controlled by the 0.2 profile between points M and N.

Profile Datum Rule Quote from Alex Krulikowski "Advanced Concepts of GD&T" page 22-12.

Or use an unequal profile zone.

Overall ASME Y14.5's concept of defining and controlling DRF's as a 1st step in application of GDT, and then defining the features and their relationships to a particular DRF leaves me to believe that "referencing datum features" in a FCF that includes them in that FCF's geometric control is a bad idea at best.

Even though the standard does not specifically prohibit this practice that I can find, the standard does not appear to support this practice in the text, figures or intent relative to the DRF concept. IMO


SeasonLee

Glad to see your post.

My interpretation is as #1 states.

I have not agreed with the consensus position as number #2 states.


Pmarc

"Halving" or "not halving" is getting in the weeds and asking for trouble IMO.

No need to confuse the geometric control by referencing the datum features in a profile all around control for instance, when the referenced datum features are included in the considered profile. Just adds potential confusion to interpretation.

As for your extension of principal in the case of Target datums carrying over to an entire datum feature;
and your view that the profile control covers the control of the DRF datum features control to each other; neither are practices that that I would recommend doing. Not saying conclusions are wrong. Just do not see how either adds any clarity to an interpretation.

Controlling the datum features relative to each other and then applying geometric controls to the features are two separate functions in my understanding.

You know full well that Y14.5 has a disclaimer relative to all figures being "complete". (Ref 4-33)

If you consider that the DRF is set up and controlled before the features are related to it and then go down the path of having the feature relationship actually define the DRF controls simultaneously seems like a can of worms to me.

I do appreciate your comments and the Y14.5 references.

dtmbiz,

I think I owe you a reply here.

First, before I refer to some of your statements, I would like to say that I agree that the tolerancing scheme shown in OP sketch in the referenced thread has been overcomplicated and most likely could have been modified/simplified (by removing datum feature B and C references from the all-around profile callout) to look much more traditionally and without risking too much that function of the part would have been grasped improperly.

In general, I also agree with you that referencing datum features in a FCF that includes them in that FCF's geometric control should be avoided (wherever possible), however you statement:

dtmbiz said:

Even though the standard does not specifically prohibit this practice that I can find, the standard does not appear to support this practice in the text, figures or intent relative to the DRF concept. IMO

Click to expand...

is not fully correct. There are at least two examples in the Y14.5-2009 showing otherwise - figs. 4-25 and 9-7.

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As for SeasonLee's example taken from AK's 'Advanced Concepts of GD&T' I am guessing it comes from a pre-2009 edition of the book. I am curious if today, in 2016, the author would wrote the same knowing that in 2009 the Y14.8 gave the interpretation (using datum targets example). So it is your full right to agree with option #1, but it is the option #2 that actually has been documented in any ASME standard. Unfortunately not in the Y14.5, which I think is the main issue here.

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dtmbiz said:

As for your extension of principal in the case of Target datums carrying over to an entire datum feature;
and your view that the profile control covers the control of the DRF datum features control to each other; neither are practices that that I would recommend doing. Not saying conclusions are wrong. Just do not see how either adds any clarity to an interpretation.

Click to expand...

Actually, I have used the "datum features self-referencing" scheme with datum targets quite often and with success for cast parts, because there is one big advantage behind doing this. It guarantees part-to-datum feature simulator repeatability during inspection of a part.

Using example, please take a look at fig. 4-48 from Y14.5-2009 (which, by the way, is unfortunately intentionally incomplete because we do not know how exactly the datum features A, B and C are controlled relative to each other), and picture what could happen if the secondary and tertiary datum surfaces B and C were controlled in a traditional way, that is, if B was controlled wrt to A only, and C was controlled wrt to A and B.

If someone was to set up the part only on 3 datum target flat-tipped pins A spaced apart as defined by basic dimensions in order to qualify both datum feature surfaces B, there would actually be infinite ways of doing it. In case of parts with datum surface A irregularities of significant magnitude (like cast components), it means that the results of datum feature B check could vary a lot between different inspected parts. That is why, in order to assure part-to-datum feature simulator repeatability, it is reasonable to bring the part in contact with the datum feature simulators (targets) A, B and C first, and then qualify all datum features and other toleranced features relative to a datum reference frame derived from the datum target simulator.

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dtmbiz said:

You know full well that Y14.5 has a disclaimer relative to all figures being "complete". (Ref 4-33)

Click to expand...

Okay, I agree that Fig. 4-33 is not complete. So what about tolerancing scheme shown under the link below? If I understand your standpoint correctly, you are going say it is not good practice, because width B is not controlled by perpendicularity callout wrt datum A, and width C is not controlled by perpendicularity callout wrt to datums A and B. Am I right?