position tolerance

[dho]

can a feature's position tolerance having itself as a datum?
i like to control the hole location real tight ref to main axis and end plane, but it could be loose relative to flange orientation.
if the part axis A, end surface B, the hole in concern C, the position tol of the hole "POS dia .002 S A B C".
or i must generate the line connecting two flange holes as D, then specify "POS dia .002 S A B D"?
thanks.

 

[drawoh]

dho,

That makes no sense. If a feature is tagged as datum[ ]A, its location to datum[ ]A is perfect, by definition. Also, please do not say "axis". You cannot apply a datum to an axis.

You can specify datum[ ]A as the end face, and datum[&bnsp;]B as your critical diameter. On that diameter, a positional tolerance will have no meaning, but perpendicularity to[ ]A is perfectly reasonable. I would not call up a diameter as a primary datum unless it was long enough to accurately define an axis. By specifying a face and diameter in that order, you specify a face for perpendicularity, and a locating feature.

--
JHG

 

[pmarc]

Question to dho: Exactly which of the holes shown on the picture is your hole C? Which features are datum features A and B?

Question to drawoh: Can we really specify the end face and the critical diameter (whatever it is in this case) as datums A and B respectively, or rather as datum features A and B?

 

[dho]

 

[greenimi]

Are we really sure it is functioning like shown, or maybe the face parallel with the shown datum feature B should be the primary datum feature?
Then A should be secondary and everything else should be positioned / profiled to B primary and A secondary.
No need for clocking. And the simultaneous requirement will take care of the clocking.
If the part is measured on the CMM and the clocking is needed, then the inspector is free to pick any of the flange holes, but will make the inspection more stringent than the designer wants/allow, in other words will reject some good parts due to an artificial alignment imposed by the CMM and not by the function.

 

[dho]

A as primary, so, the .312 hole intersects the main bore.
if B as primary, only keep the location from B right on.

 

[greenimi]

Could you tell us how this part you are trying so hard to define interacts with its mating parts? How fit in the assembly?
What would be the physical reality of this assembly?

 

[dho]

just envision a ball valve.
even a loose fit assembly, i think it still needs proper GEO tol callout.
as CMM measurement wise, the machine can setup A first, B second and finally line up C.
thanks.

 

[greenimi]

dho,
I guess you did not answer my question, did you?
"Could you tell us how this part you are trying so hard to define interacts with its mating parts? How fit in the assembly?
What would be the physical reality of this assembly? "


even a loose fit assembly, i think it still needs proper GEO tol callout.

I agree!!

 

[dho]

i answered. if you envision a ball valve.
thanks,

 

[greenimi]

Okay.

The follow up question: do you need help or you are trying to make sure everybody on this form will have some imagination of the product you are designing?

 

[dho]

i just have a different opinion as you have.
here i was asking (for help) to properly specify a position tolerance. whatever this part is used in a ball valve, or anything, precise or coarse, does NOT matter.

 

[greenimi]

I agree. But what DOES matter is how the product FUNCTIONS.

 

[dho]

that will be the design issue. a good product or bad.
it has NOTHING to do with correct way of GEO tol.
thanks.

 

[JNieman]

If you think the product function has nothing to do with proper geometric tolerance of location, then you need more help than you realize.

It is very difficult to suggest the best solutions without knowing the part well enough. You have called out obviously incorrect or inferior constraints, but in order to suggest something better, more information is required, as asked.

 

[greenimi]

Wowww! I learn something new:

"the design issue. a good product or bad.
it has NOTHING to do with correct way of GEO tol."

The design has nothing to do with the correct way of GEO tol.

The question is: then WHO has ? (to do with the correct way of Geo tol). Manufacturing? Purchasing? Quality?

 

[greenimi]

THANK YOU JNieman !!

 

[dho]

you have to read the whole threads. not jump into conclusion.
does anyone here know how to proper GEO the .312 holes which intersect the c.l. of 1.147 bore and XX distance away from B?
have a good day.

 

[gabimo]

dho,
What standard are you using to define the print?

 

[dho]

ASME Y14.5.
i still would like to ask. would anyone specify the GEO tol for this part properly. so, it has a tight control on the .312 hole intersecting the main bore at 90 degrees and away from the end surface XX distance, but could be off some relative to flange orientation?