Sorry for the dump question (and draft paint image) but I am a bit confused:
I have a TP callout for the hole (center of image) on a part like that. Datum A is the Upper plane (basic dimension value not important), Datum B is the vertical Ø through the part as displayed in dashed lines and Datums C as per picture. The callout is A/B/C and I was wondering since C is controlling the rotation in this datum structure, it would be wrong to control this hole as a circle right ? It would need to be a cylinder...
Thanks in advance for your help
You can't use C for the rotation if B is listed second with A being Z+.
B is capable of constraining K (rotation about Z) and therefore, being listed second, must do so.
There is a method an engineer can use to state that the tertiary datum rather than secondary is constraining something, but you don't show that in your notation for the position.
So, A constrains I and J rotations and Z translation.
B constrains K rotation and X translation.
C constrains nothing.
Y translation is not constrained by anything.
This is how it is written, and how it must be done. In your sketch, there is nothing for Y no matter what you use to halt the third rotation.
If B and C were perpindicular to each other, rather than parallel, then you'd be fully constrained.
Without special instructions, no datums yields to a subsequent datum.
YOU can't SAY A just does the rotations because B is more important and holds Z. The standards are not written that way.
The ENGINEER must WRITE if that is the case.
Apparently, the brain trust making Y14.5 couldn't use I, J and K like EVERYONE in the industry (I wonder if this is U.S. only and that's why I think this?), I looked this up to get it right, they use U spinning around X, V spinning around Y and W spinning around Z.
So, your FCF (I made up a tolerance since you didn't give one) of POS | Ø .014 RFS | A | B | C |
Diametral tolerance zone of .014 regardless of feature size with
A constraining U, V and Z
B constraining W and X
nothing for Y
What you are suggesting in your words, B is more important for translation but C controls the spin would be written: POS | Ø .014 RFS | A [u, v, z] | B [x] | C [w] |
Diametral tolerance zone of .014 regardless of feature size with
A constraining U, V and Z
B constraining X
C constraining W
nothing for Y
This is Y14.5-2009, sec 4.23, pg 83, with a figure 4-46 on pg 84. You (the inspector) don't get to skip B in favor of C, the engineer has to write it. Of course, that doesn't stop the engineer from wanting it and not writing it because the engineer doesn't fully understand GD&T... but I'm being crazy, engineers ALWAYS understand GD&T perfectly, that's why they make threaded holes datums and apply MMB to them, I'm being silly. (HEAVY sarcasm if you couldn't read my tone in to that lol)
B cannot control the K rotation as its vector is K. A constrains 3 DOF as you wrote, B does 2 (but translation in Y is not needed in this case) and C is left to constrain Rotation. For me nothing wrong with the callout just that it get a bit complicated and can confuse a bit (you and me both apparently :P )
B cannot control the K rotation as its vector is K. A constrains 3 DOF as you wrote, B does 2 (but translation in Y is not needed in this case) and C is left to constrain Rotation. For me nothing wrong with the callout just that it get a bit complicated and can confuse a bit (you and me both apparently :P )