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

 

[CheckerHater]

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 .

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?

 

[pmarc]

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.

 

[CheckerHater]

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

 

[CheckerHater]

Just look at the picture:
Coaxial cylinders will give you axis and infinite number of planes to spin on it "just as if primary datum feature was a cylinder".
Parallel cylinders set apart will establish axis and planes firmly fixed in space

 

[CheckerHater]

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

 

[pmarc]

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.

 

[pmarc]

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.

 

[CheckerHater]

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.

 

[pmarc]

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?

 

[CheckerHater]

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.

 

[pmarc]

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.

 

[CheckerHater]

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?

 

[pmarc]

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?

 

[CheckerHater]

Exactly what kind of control do you expect to see?

 

[CheckerHater]

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?

 

[pmarc]

A control that would prevent from something like this happening, for example:

http://files.engineering.com/getfil...-abda-47d1-8f70-119a64453801&file=Example.pdf

 

[CheckerHater]

I think I already answered, but I can provide picture as well.

OP didn't tell us what reationship between features is, so I didn't add anything of my own. Still think it doesn't matter

 

[greenimi]

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……..

 

[CheckerHater]

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/getfil...b450-e71412505c7f&file=profile_datum_rule.JPG

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.

 

[bxbzq]

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.