Simultaneous position and establishing a clocking datum 1

[sendithard]

I've got so many questions on this callout that it is better to ask questions over several posts Apologies for the rambling. The part I have is somewhat like the below picture. It is two less than half spherical cutouts inside a part. Disregard the idea of a feature of size as everything under the sun here is a fos.

The callout is simultaneous as the spheres are located basic to each other(and with same DRF) and then to the bore axis(datum A) and the bottom plane(Datum B):
0.05mm A|B so this is a spherical position callout located to the bore axis and a basic dimension to the bottom plane.

Our engineering team requires two positional outputs separately. So if the spheres are 6mm basic apart...We in essence are measuring them 3mm from the axis.

1) If you measure them separately...and they pass, does that mean they will pass simultaneously?

a) I played around in CAD with a 4 hole pattern and I couldn't really come up with a scenario where if they all pass individually, they would somehow fail together. This is where my lack of experience makes me question all my thoughts on this​

2) The clocking datum is not arrested so I am thinking about using the midpoint of the spheres as the clocking XY datum plane creation.

a) this would mean the spheres could be crooked compared to the rectangle nature of the part, but does this strategy creatively measure them simulteneously while still outputting them individually?​

3) Alternatively, if I use a midplane of the part as the clocking datum this would keep the sphere more true to the overall part, but then does that violate the simultaneous theory of the standard?

I'm a little confused here so I appreciate the guidance. I will be discussing with our overall leader soon, but I wanted to get your thoughts before I ask for a meeting. Functionality, and eliminating assembly errors trumps everything, but I'm asking for a technical take on the GDT and perhaps personal opinions.

 

[3DDave]

Burunduk is sure that his statements are correct.

What he meant to say is separately they individually control both perpendicularity and flatness, but would not provide a mutual location tolerance. Some other tolerance would be required and, as a feature of size, it could be a +/- tolerance.

No need for parentheses about the basic dimension.

 

[greenimi]

I am thinking a small clarification on Burunduk's statement is needed (for my own understanding or .....misunderstanding). If the length of the pin is basic and with the usage of SEP REQT the length of the pin is still controlled (by the combination of both profiles of the end faces to datum feature A).
So, I think the length of the pin is still fully defined regardless if simultaneous requirement is defaulted or not (SEP REQT is used) on the pin.
I am not sure orientation /perpendicularity has much to do with what the OP is asking because orientation controls are not location controls hence not subject to the simultaneous requirements.

 

[3DDave]

If the length of the pin is basic and with the usage of SEP REQT the length of the pin is still controlled (by the combination of both profiles of the end faces to datum feature A).

What he meant to say is separately they individually control both perpendicularity and flatness, but would not provide a mutual location tolerance.

Profile which may control form, orientation, and location. Making them separate eliminates mutual orientation and location, leaving form and individual orientation.

 

[Burunduk]

If the length of the pin is basic and with the usage of SEP REQT the length of the pin is still controlled (by the combination of both profiles of the end faces to datum feature A).
So, I think the length of the pin is still fully defined regardless if simultaneous requirement is defaulted or not (SEP REQT is used) on the pin.

With SEP REQT profile tolerances, if there is no directly toleranced dimension for the length of the pin, the length would be uncontrolled. Even if you assume that the profile tolerance zones for the two faces are still separated by the basic dimension, the actual faces don't have to fall into those tolerance zones simultaneously. Which means you could approve one face, then utilize the available degree of freedom in the axial direction to translate the part until the second face falls within its tolerance zone. It is the same principle as shown in figures 7-47 and 7-48 - the 2 keyslots are basically aligned, but with the lack of a clocking datum reference and the simultaneous requirement overridden in figure 7-48, rotation about the datum axis can take place between one position evaluation and the other, and this way alignment is no longer controlled.

 

[3DDave]

With SEP REQT profile tolerances, if there is no directly toleranced dimension for the length of the pin, the length would be uncontrolled. Even if you assume that the profile tolerance zones for the two faces are still separated by the basic dimension, the actual faces don't have to fall into those tolerance zones simultaneously.

Because that is what separate requirement means.

 

[3DDave]

Also, I was right about what he meant to say; did not expect a rotational example for a translational tolerance control requirement.

There would be no clocking requirement for nominally perpendicular end faces.

 

[Burunduk]

greenimi asked me for a clarification, and I gave him one.
The additional example, which he is probably familier with, was meant to emphasize the principle. Not to be an exact duplicate of the first example.

 

[3DDave]

It was the only example you could copy/paste. The reason being that this is an absurd application of profile to control only form and orientation:

Imagine a simple cylindrical pin with two end faces normal to the axis. You could use the diameter of the pin as datum feature A and profile each end face with ref. to A. The relationship of each face to datum axis A is only perpendicularity, and that is what you would be controlling IF you overrode the default by SEP REQT. But with the simultaneous requirement default, you are also essentially controlling the length of the pin (when the length dimension is basic).

It muddies the waters of SEP REQT which is why a position tolerance on a width was in the standard considering the original problem centered on location, not creating an undefined feature width.

 

[3DDave]

It's really worse - it's an example of how to not use SEP REQT, changing a default interpretation into an uncontrolled one with a basic dimension; from valid to invalid.

 

[Burunduk]

It was not an example for how to use SEPT REQT. It was an example for how the simultaneous requirement rule works and for what it would mean not to have that default rule intact.
Meant to answer on OP's "and again if you didn't know the rule and you measured them separate you are also good. So why the rule?". Quit spreading your misunderstandings to others.

 

[3DDave]

"It was not an example for how to use SEPT REQT."

Then why did you make it, because it doesn't seem like a good idea to make a counter example of how it should be used.

That was promoting a misunderstanding of how it is to be used.

 

[sendithard]

Dave,

The real senior exam is here on this forum rubbing shoulders with the likes of older grumpy crazy experienced professionals like yourself that I dearly love, reviewing customer prints, discussing concepts with mentors, etc. Why would I rely on a certification test to tell me what is right?

The test didn't fail me by no means, it was a self imposed Curriculum that I pursued to improve my knowledge base and skillset. You look at it as if it is a defining moment in my understanding. It was simply a part of my personal growth.

I think you are hung up on the exam b/c you are a little grumpy about the politics and all that nonsense that you think is wrong with the exam. Maybe you are right, but, I don't care about the exam, it was a personal course of study, I paid for it myself, and I would have honestly not read the entire standard had I not paid for the test. I actually almost ran out of the 6 month window, b/c I kept a full read of the standard off until the last month. If you want me to hand in my certification because I'm a complete loser let me know Maybe in 30 years I'll be half as strong as you

I still love you.

 

[3DDave]

I have read 5 versions - the quality has generally declined, the price has gone up, and the fundamental concepts have been buried under an avalanche of special cases that can only be evaluated by CMM - a device that is rarely found at the assembly line to fit parts together and often not in realizable mechanisms of parts and assemblies.

Writing a standard that fails to make obvious a core and primary concept to someone dedicating time to its study is a systemic failure. Perhaps that was part of the 10% incorrect?

If you missed it, there are certainly thousands more who also missed it and I want it to be fixed for them as well.

It is as puzzling as a student in Algebra asking what multiplication means and suggests a problem in earlier teaching.

 

[Burunduk]

Then why did you make it, because it doesn't seem like a good idea to make a counter example of how it should be used.

I didn't realize you were expecting spoon-feeding of the obvious. Just for you then, a correction:

Imagine a simple cylindrical pin with two end faces normal to the axis. You could use the diameter of the pin as datum feature A and profile each end face with ref. to A. The relationship of each face to datum axis A is only perpendicularity, and that is what you would be controlling IF you overrode the default by SEP REQT. IN THAT CASE, THE DIMENSION BETWEEN THE TWO END FACES WOULD NEED TO BE DIRECTLY TOLERANCED!!! But with the simultaneous requirement default, you are also essentially controlling the length of the pin (when the length dimension is basic).

 

[3DDave]

Too late - I got there a long time before you and made the correction. You need to be precise. Having to create that additional condition really spoils the clarity in contrasting the two cases.

What he meant to say is separately they individually control both perpendicularity and flatness, but would not provide a mutual location tolerance. Some other tolerance would be required and, as a feature of size, it could be a +/- tolerance.

 

[greenimi]

Thank you guys for the explanation. I definitely learn more from this discussion.
Interesting fact I noticed is that if SEP REQT is used on Burunduk’s example (with the cylindrical pin) then one might also have to “adjust” the basic length of the pin as per the replies from 3dDave and Burunduk

separately they individually control both perpendicularity and flatness, but would not provide a mutual location tolerance. Some other tolerance would be required and, as a feature of size, it could be a +/- tolerance.

The relationship of each face to datum axis A is only perpendicularity, and that is what you would be controlling IF you overrode the default by SEP REQT. IN THAT CASE, THE DIMENSION BETWEEN THE TWO END FACES WOULD NEED TO BE DIRECTLY TOLERANCED!!! But with the simultaneous requirement default, you are also essentially controlling the length of the pin (when the length dimension is basic).

Well live and learn.
If that’s the “correct” ( read intended by the standard) usage of SEP REQT that’s another can of worms. That's a good leason for me. So, Thank you.

 

[Burunduk]

If that’s the “correct” ( read intended by the standard) usage of SEP REQT that’s another can of worms.

As I mentioned, that was not intended as a recommendation for an application of SEP REQT. It was only to explain what it would mean not to have the simultaneous requirement rule in force - a problem would be created which can be partially* solved by changing the length from basic to directly toleranced.

*The issue that would remain:
Using two perpendicularity tolerances instead of SEP REQT profiles would be equivalent and much more direct and straightforward in this case.