Two tolerance zones to a single datum axis A-B 1

[greenimi]

I have a question for the group: the part I would like to talk about it’s a shaft with a weird thru irregular feature. The axis of one radius is datum A and the axis of the other radius is datum B.
If you have the centerline on the part:
Datum feature A, radius -left side from the centerline – will generate a datum axis on the other side of CL—on the right side.
And similar, datum feature B (radius, datum feature on the right side of CL) will generate a datum axis on the left side. See sketch with red and blue centerlines.
It’s a coaxial control which locates and orients two tolerance zones to a single datum axis.
I was told this is the design intent and reflects the functionality of the product (there is a mating part which go inside this irregular feature and the mating part has the same construction).
See the profile callouts depicted:
Datum feature A: profile .004 wrt A-B
Datum feature B: profile .004 wrt A-B
Datum feature C: profile .003 wrt A-B
Profile of bottom surface wrt A-B and C
The questions I have:
1.) Is those valid GD&T callouts? I think so, but I just want to double check.
2.) How we can measure the profile callout? How to establish DRF between A-B on the CMM, for example?
3.) Could be a common misunderstanding in the industry, but both datum feature A and B are referencing a location back to themselves. I am almost sure (but not convinced, and that’s the purpose of the question) that is not the case here, and the datum feature A and B are NOT referencing back to themselves. Any help to shoot some lights here?
Thank you

 

[pmarc]

greenimi,
If you introduced all the datum features just to control remaining surface of the pin by profile callout with relation to datums derived from the other three surfaces, I would say it is not the best choice. Also, I have a feeling that this does not really reflect the design intent, if pin's counterpart is of the same "weird" irregular shape. If the pin serves as a true datum feature and the other features of the shaft, that is dia. .669 and height .591, are going to be controlled relative to the pin, I would consider use of whole contour of the pin as datum feature. In other words, maybe it makes sense to define true profile of the pin with the use of basic dimensions, control it with profile FCF all around without any datum reference and assign it as primary datum feature. Maybe the whole contour of the pin could be a secondary datum feature with primary datum feature being one of the faces perpendicular to axis of the shaft?

Just some additional things to think about.

 

[greenimi]

Pmarc,
We have chosen the depicted DRF because of the following reasons:
1.) The “weird pin” (so to speak) is mating with the “weird hole" in question based on the “diameter” (those 2 radiuses). These two areas will orient the part in the assembly (since the axis of those two areas/radiuses are not located in the center of the part we have to call them separately and we cannot use a single axis). There is another set screw which located the “weird pin” axially into the “weird hole” so here it’s datum C (the pin is pushed against surface C by the set screw).
2.) On your proposal, the face perpendicular to the axis of the shaft is sitting in the air, so why using it as a primary datum as it does not orient the part into the assembly? I am not telling you that your proposal does not work, but I just put in words our thought process. Would that make any sense to you?

Thank you pmarc

 

[pmarc]

Okay, things have become clearer now.

1. In that case, I would say it is reasonable to choose both radial portions of pin as primary datum features without making one of them more important than the other. And no, it is not datum self-referencing.
There is however one thing that worries me, but before saying what it is, I would like to know how the radial portions of the pin mate with their counterpart. Is it press fit, loose fit or something in between?
Ah, and one more thing - your datum feature C is not able to constrain remaining translational DOF (translation along pin's axis). So probably there should be another datum feature that is capable of doing this, correct?

2. If surface perpendicular to datum axis does not stop translation of the shaft axially, then it indeed makes no sense to assign it as datum feature, neither primary, secondary nor tertiary. Your initial description just did not offer any info about, thus I said "maybe...".

3. Minor thing - though for the current state of the print it is rather clear that both |.004|A-B| profile callouts apply to radial surfaces, from standard point of view the callouts should not be associated with basic R.111 dimensions, but rather leaders should connect them with proper outlines.

 

[greenimi]

pmarc,
Let me answer your questions:
1.) "I would like to know how the radial portions of the pin mate with their counterpart. Is it press fit, loose fit or something in between?"
I would say it's an locational clearance fit (90% of assemblies) but could be also locational transition fit (10%). The pin will drive the part in question and a small torque is transmitted between the parts. How small is the torque? just like tightening an 1/2 inch screw by hand.
1.a. "your datum feature C is not able to constrain remaining translational DOF (translation along pin's axis). So probably there should be another datum feature that is capable of doing this, correct?"
Do I really need to constrain all six degrees of freedom? We intentionally didn't want to do it as in assembly the part maintain it's stability by the set screw and not by any datum feature on the part itself. Does it make sense to you what we were thinking?
2.) No comment
3.) We will detach the profile callout from the radius basic. Thank you.
The question still remain: if our proposal fit the functional criteria, how to measure on the CMM(for example) the profile callout. How to establish A-B (as a single axis)? If I go down to the inspector level, how can I explain to him/her to set up his coordinate system and/or to find the common axis A-B.
I can put on the drawing anyting I want (and I agree that's how the part functions), but how can I move the thinking a little bit forward and explain to somebody else what we just talked and decided. Thank you again for your help

 

[CheckerHater]

I am slow today. What exactly is common axis of A and B?

 

[greenimi]

CH,
Hmmmm, Good question, I don't know....I am just learning.......

Just reading from Y14.5-2009
"Where more than one datum feature is used to establish a datum feature simulator for a single datum, the appropriate
datum feature reference letters and associated modifiers, separated by a dash, are entered in one compartment
of the feature control frame. See para. 3.4.2 and Fig. 4-22. Since the datum features have equal importance, datum feature reference letters may be entered in any order within this compartment. Where the intent is clear, a datum feature reference letter may be used to define the multiple surfaces as a single datum feature." para 4.12

Also, I see fig 4.22 and Fig.4.24 / Fig. 4.25

That's the purpose of my thread.....

 

[CheckerHater]

The common application (and the way it’s illustrated in the book) for axis of two features is when your A and B are coaxial under perfect condition, say, you have shaft resting on two bearings.

In your case it’s more like a common axis of two shotgun barrels – we may try to imagine something, but I don’t think it is well supported in standard. Unfortunately your radial features are not even features of size, so they don’t make a pattern either.

You still can derive datum from something of irregular shape, using, for example, datum targets, but all that brings us back to the questions pmarc already asked: is there a need for so many datums and how part actually works.

Don’t forget that datum is actually a baseline you are taking your measurements from. Do you control other features in the part wrt datums you’ve created, or all of that is just to control the hole itself?

 

[greenimi]

CH,
Yes, we are controlling other feature on the other end of the part back to A-B and C. The "weird pin" is the mating part and the driving force and the other end (controlled back to A-B and C) is just driven.

 

[CheckerHater]

Another question: do your basic dimensions .0053 and .0895 specify location from some sort of a center point / axis?

Is that center / axis somehow related to DIA .669 and .591 dimensions?

 

[greenimi]

do your basic dimensions .0053 and .0895 specify location from some sort of a center point / axis?
these basic dimensions and the applicable profile tolerances are chosen in a way that the pin fit in the hole each and every time with clearance fit and/or intermediate/transition fit

Is that center / axis somehow related to DIA .669 and .591 dimensions?
I would say, NO. Is not.

 

[CheckerHater]

Normally your basic dimension is dimension between two features controlled with FCF, or feature and a datum.

[.0053] is dimension between what and what else?
Another [.0053] is dimension between what and what else?
[.0895] is dimension between what and what else?

Do you understand what I ask?

 

[greenimi]

CH,
Yes, now I understand what you ask, but I guess here is coming the confussion if it is not datum self-referencing. I guess is not.
.0053 is between the axis of one of the radiuses R.111 and the axis created by the co-datum A-B. In other words, (but again in don't know if I am right or not):
Left radius creates a datum axis A, right radius creates a datum axis B, the axis of the part is the middle (sic, what is that??) between the two axis which creates the co-datum axis A-B. So, .0053 is the basic between datum axis A and datum axis A-B. Hmmmmmm, I am lost too!!

 

[CheckerHater]

This is not so bad.
Your datum A-B is symmetry plane between radial surfaces A and B. Not everybody may agree, but it looks legal.

OK, now what is .0895?

Also, will you accept the part if .1675 dimension will not be parallel to .591 dimension?

How far off-center from .669 diameter your “ugly slot” can be?

 

[pmarc]

greenimi,
I would like to stop for a moment on fig. 4-22 referred by you in one of your previous replies. Why not to use this approach? I mean, to define true profile of both radial portions of the pin by basic radii R.111, then define basic relationship between centers of both radii with basic distance .0106 (twice .0053), and assign one radius as A and second as B. Profile FCF for both radii would be |prof.|.004|, that is without any datum references. Do you see analogy?

 

[greenimi]

CH,
.0895 is basic between the datum feature C (surface the pin is pushed against by the set screw) and "a plane" perpendicular to the symmetry plane to talked about in your reply.
And about the off-center from Ø.669 and .591 width, I know is not very well defined (missing GD&T callouts), but being shown on the same centerline it's kind of assuming the datum feature C (surface) is parallel to .591 (width). Sorta....

Pmarc,
I have attached a revision for your review to see. I hope I understood you correctly. Are those two datum schemes (the original one and the one you are proposing) equivalent with each other, in other words have the same effects and produce similar results?

And one more question for you: if we were in 2009 standard can we use A(MMB)-B(MMB) at the datum feature C profile callout, meaning profile .003 wrt A(MMB)-B(MMB) for datum feature C and the same thing for the opposite surface profile .003 wrt A(MMB)-B(MMB)and C. What do you think?
I know A and B are not feature of size, but I think in 2009 is acceptable to get MMB's. And I guess for our assembly MMB's are acceptable since it's a clearance/intermediate fit assembly. Am I right? Thank you pmarc for your input.

 

[CheckerHater]

This is why I am asking.

You dimension your C feature essentially from nothing.
Is “a plane perpendicular to the symmetry plane to talked about in your reply.” a real feature?
You do realize that you cannot take measurement from imaginary plane, right?

Do you think this plane may be the median plane of flats determined by .591 dimension?

 

[pmarc]

greenimi,
Yes, this is exactly what I meant in my previous post. With one remark though - I would clearly indicate which center is for which arc. The way it is shown now may be misleading.

Are the datum schemes equal? I would say geometrically they are the same (at least I can't visualize the difference), but I would call the latter one more intuitive.

And yes, you could use A(M)-B(M) if the print was according to Y14.5-2009. As a matter of fact, as you noticed, it would reflect assembly conditions much better. This is actually the reason why I asked for nature of the fit between the pin and its counterpart.

Coming back to your question on how to measure profile callouts on A & B with the use of CMM, if you decide for the second scheme, I would say the operator must simply probe two as-produced surfaces and check whether the points fall into profile boundaries defined by the drawing.

The procedure of establishing datum from datum features A & B is a different story.

 

[CheckerHater]

Pmarc,
If we set procedure aside, after all, methods are not our problem, how in your opinion datum [A-B] looks like? I mean, is it point, axis or plane? And is it good enough to be used as primary datum?

 

[pmarc]

CH,
In my opinion A-B datum defines a plane passing through the center established by datum feature simulators A and B which centers are spaced .0106 apart. Second datum plane is derived from datum feature C and third plane is missing since we see no tertiary datum feature on the print.

And I think features A & B are good enough for primary being datum features collectively. If only one of them (A or B) was selected as primary datum feature, it would not do this job properly IMHO.