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Two tolerance zones to a single datum axis A-B

Two tolerance zones to a single datum axis A-B

Two tolerance zones to a single datum axis A-B

(OP)
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

RE: Two tolerance zones to a single datum axis A-B

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.

RE: Two tolerance zones to a single datum axis A-B

(OP)
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

RE: Two tolerance zones to a single datum axis A-B

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.

RE: Two tolerance zones to a single datum axis A-B

(OP)
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

RE: Two tolerance zones to a single datum axis A-B

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

RE: Two tolerance zones to a single datum axis A-B

(OP)
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.....

RE: Two tolerance zones to a single datum axis A-B

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?

RE: Two tolerance zones to a single datum axis A-B

(OP)
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.

RE: Two tolerance zones to a single datum axis A-B

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?

RE: Two tolerance zones to a single datum axis A-B

(OP)
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.

RE: Two tolerance zones to a single datum axis A-B

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?

RE: Two tolerance zones to a single datum axis A-B

(OP)
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!!

RE: Two tolerance zones to a single datum axis A-B

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?

RE: Two tolerance zones to a single datum axis A-B

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?

RE: Two tolerance zones to a single datum axis A-B

(OP)
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.

RE: Two tolerance zones to a single datum axis A-B

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?

RE: Two tolerance zones to a single datum axis A-B

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.

RE: Two tolerance zones to a single datum axis A-B

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?

RE: Two tolerance zones to a single datum axis A-B

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.

RE: Two tolerance zones to a single datum axis A-B

Thank you pmarc,

This looks pretty much like “symmetry plane between radial surfaces A and B” I was thinking about, so we are probably on the same page.

Now, back to establishing datum feature C.
What in your opinion [.0895] is measured from?

I feel uneasy because if you put enough pressure on OP, requirements are becoming more complicated smile.

Say if on 02-19 he was confident:
Is that center / axis somehow related to DIA .669 and .591 dimensions?
I would say, NO. Is not.


But on 02-20 he is not so sure:
being shown on the same centerline it's kind of assuming the datum feature C (surface) is parallel to .591 (width). Sorta....

And we all know, in GD&T world sharing centerline is not enough.

I have a feeling that puzzle cannot be really solved until we figure relation of slot to the rest of the part. Any opinions?

RE: Two tolerance zones to a single datum axis A-B

My opinion?...
I made a mistake saying that datum established by datum features A and B collectively is associated with a single plane. Two mutually perpendicular planes are there - just as if primary datum feature was a cylinder. The planes can freely rotate around their intersection line.

Datum feature C orients these two planes so that one of them is parallel to a plane formed by datum feature simulator C, and in consequence the second is perpendicular.

[.0895] is measured from the intersection line of these two planes.

RE: Two tolerance zones to a single datum axis A-B

Don't you think that two parallel axes would be enough to establish plane that is NOT free to rotate around the datum axis?

RE: Two tolerance zones to a single datum axis A-B

And another thing:
If your planes are free to spin around the axis, IN WHICH DIRECTION do you measure .0895 to establish plane C?

RE: Two tolerance zones to a single datum axis A-B

Think of it from different perspective.
When the pin is assembled with its counterpart hole of the very same shape, is it possible to rotate it around "an axis" or the rotation is stopped?. I think it is possible to rotate the pin. In the same way datum feature simulators A and B are not able to constrain this rotational DOF.

As for direction of measurement of [.0895], I think it does not matter. As long as as-produced datum surface C fully lies within profile tolerance zone, everything is OK. Look at fig. 4-30 from Y14.5-2009 - it is very similar case except that primary datum feature is derived from cylinder and not from pair of offset portions of cylinders.

RE: Two tolerance zones to a single datum axis A-B

By saying that it is possible to rotate the pin I meant it is possible to a certain extent until one or both radii will start interfering with flat portions of holes.

RE: Two tolerance zones to a single datum axis A-B

I am sorry, but I don’t buy it.

You say that “As long as as-produced datum surface C fully lies within profile tolerance zone, everything is OK.”
But you didn’t define that zone yet. You know it’s .0895 away from the axis in any direction. Until you find something it's parallel or perpendicular to, you don't have tolerance zone yet.
And the Fig.4-30 is completely different, because feature B is defined wrt perfectly round feature A, so direction really doesn’t matter, you CAN spin it around.
And we also know that “the datum feature C (surface) is parallel to .591 (width). Sorta....” – something you didn’t take into consideration … just yet.

I have some idea I am working on right now, so maybe tomorrow I’ll share.

RE: Two tolerance zones to a single datum axis A-B

You don't need to be sorry.

One more question:
What if in greenimi's example round surfaces were portions of a single cylinder, the flats stayed unsymmetrical relative to the axis, and the upper flat was assigned as secondary datum feature? How would this be different in terms of establishing a datum from this surface?

RE: Two tolerance zones to a single datum axis A-B

Quote (pmarc)

What if in greenimi's example round surfaces were portions of a single cylinder
If the slot is free to spin wrt outline of the part, then situation would be pretty much like Fig4-30.

You have something like D-shaft: no matter how you spin it, relationship between round and flat stays the same. I am just not comfortable with primary datum only restricting 1 DOF.

Now, if OP “forgot” to tell us that we still have to position slot wrt outline of the part, things are getting more complicated, even if radial features are portions of one single cylinder.

And two cylinders side-by-side just are asking to draw the plane thru their axes. Unfortunately it brings another problem: with primary datum being axis and two planes fixed in space we don’t really need secondary datum C as it will not restrict any more DOFs.

I am including a picture showing one possible approach. In my opinion it does what OP asked for and what he didn’t ask for… yet.

RE: Two tolerance zones to a single datum axis A-B

CH,
Why do you think that primary datum established by surfaces A & B constrains only 1 DOF? As far as I see 4 DOFs will be constrained - 2 translations (in x and y direction), and 2 rotations (around x and y axis). The unconstrained DOFs will be: translation along z and rotation around z. Thus datum plane C is used - to constrain the rotation around z. Am I missing something?

I do not think things will get more complicated due to the fact that the outline is still to be controlled somehow. These could be two separate postional callouts (one for cylinder, second for pair of of parallel planes) or profile all around or position with boundary approach or position/profile combo - all with relation to |A-B|C|. Dozen of possibilities.

As for your proposal, technically it looks fine. However, knowing what features are vital from assembly point of view, I am afraid it cannot be considered as a functional approach to dimensioning and tolerancing. Switching datum features just because smaller pin is more complex and it is harder to find a standard way of dimensioning the part is not a way to go, IMHO.

RE: Two tolerance zones to a single datum axis A-B

You said it yourself: "The planes can freely rotate around their intersection line" How is it "constrained"?

Also: "I think it is possible to rotate the pin. In the same way datum feature simulators A and B are not able to constrain this rotational DOF" If features A and B are not constraining mating part, how come they "are vital from assembly point of view"?

I see the problem with my scheme creating unnecessarily tight positioning requirement, but tolerance zone for the slot is exactly the same. And you know my opinion on "functional". Is it functional if slot (and pin inserted into it) will be way off-center?

RE: Two tolerance zones to a single datum axis A-B

The fact that two mutually perpendicular datum planes derived from datum features A and B can freely rotate around their intersection line does not make these features improper as primary datum features. Similar situation happens when a regular cylinder is used as primary datum feature. Datum axis derived from the cylinder is actually an intersection of two planes and these planes can freely rotate until another datum feature stops the rotation (if needed of course).

And one more comment to your scheme, since I previously said it is technically fine. There seems to be no control between datum features A and B, correct?

RE: Two tolerance zones to a single datum axis A-B

Exactly what kind of control do you expect to see?

RE: Two tolerance zones to a single datum axis A-B

First, I have no idea how the rest of the part looks like, so could we consider the drawing “intentionally incomplete”?

Second, imagine features A and B being controlled only by default general tolerances. It is absolutely irrelevant. It is still possible to center part wrt two flats and cylinder no matter how bad and inaccurate flats and cylinder are.

You are not saying it is my fault that dimensions .591 and .669 have no tolerances?

RE: Two tolerance zones to a single datum axis A-B

(OP)
Woww, interesting discussion and very instructive for somebody who’s learning GD&T, like myself. Took me awhile to read and comprehend.
“And we all know, in GD&T world sharing centerline is not enough.”
I understood that if a feature is shown on the centerline does not imply is fully defined and I agree with you:
.591 width and Ø.669 outside diameter are shown on the centerline, but are missing some geometrical controls. Was not intentionally left under defined or” intentionally incomplete”, but are just an oversight and are missing…….
Now, if we add for example,
on .591 position .030 wrt A-B and C
on .669 position Ø.050 wrt A-B and C
I think that portion of the part is well defined now, Right?
I just want to make sure to clarify (and you can see that by the tolerances) that both these features .591 width and Ø.669 are sitting in the air and have no connection to other mating parts and those feature have no functionality whatsoever. In other words, the geometry of these two features should be driven by the more important and functional features A-B and C. We don’t want the outside of the part to drive, but we want the “ugly hole” and consequently the “weird pin” to be the driving forces because, again, reflects the functionality.
If the “ugly hole” is not in the center it’s just okay.
If we add the above positional callouts then the .591 and Ø.669 features are aligned with the “ugly hole” by the simultaneous requirements. Right?
CH,
I am not saying that your sketch does not work, but I don’t see how the GD&T philosophy is applicable here (functionality)
Just reading the post, and force myself to repeat, we want A-B and C to drive and not the outside of the part.
“Is it functional if slot (and pin inserted into it) will be way off-center”
Yes, it’s acceptable. How much off? .030 / .050 (as per my addition)
Would this new addition, solve the coordinate system “stability” and fully definition? I understood from the thread, we are kind of struggling defining it’s validity. I am too……..

RE: Two tolerance zones to a single datum axis A-B

Thank you for the response greenimi,

I am not sure position will work with outline of the part, but we'll set it aside by now.

The reason I was trying to move datums away from features of the slot is some obscure rule which is not really a standard, but based on common sense:

http://files.engineering.com/getfile.aspx?folder=0...

And also we couldn't agree with pmarc what datum A actually is.

I will try to revise my suggestion according to your input, but cannot guarantee it will happen today.

RE: Two tolerance zones to a single datum axis A-B

I'm confused. To me, the primary datum A-B in op's draft establishes two mutual perpendicular planes intersecting at middle of the axis A and B. One of the planes passes through axis A and B. There is only 1 DOF not constrained by A-B, the translation along longitudinal axis. It is just like two parallel holes establish a single primary datum. Flat C is not necessary unless customized DRF is introduced to free rotation around longitudinal axis DOF, which I think is better because the distance between axis A and B is small.

RE: Two tolerance zones to a single datum axis A-B

I stand corrected. Primary datum derived from datum features A-B constrains 5 DOFs.

RE: Two tolerance zones to a single datum axis A-B

Thank you bxbzq,

Glad to see that I was not alone. Two parallel cylinders set side-by-side really establish enough of a datum framework, so we don't really need datum C.

Now, what's your opinion on "common sense" rule that states: "A profile control should not be applied to the surfaces it references as datum features"?

RE: Two tolerance zones to a single datum axis A-B

CH,
A while ago I read a thread in this forum discussing the self referencing profile control. From my fuzzy memory, the conclusion (?) was self referencing is legal, but not recommended. The profile tolerance zone applied to the datum feature is cut in half.

RE: Two tolerance zones to a single datum axis A-B

Yes, I guess it was this thread

Now I just have to convince myself that we need datum at all.

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