Datums Required for Hole Position
Datums Required for Hole Position
(OP)
I have what I hope is a pretty straight forward question. When placing a hole position tolerance, are 2 or 3 datum references required?
Take the drawing below:

If the hole is a through hole, does the call-out need to reference Datum A? A primary to B and a secondary to C leaves one DOF free for the hole to move (translation in and out of the screen).
Take the drawing below:

If the hole is a through hole, does the call-out need to reference Datum A? A primary to B and a secondary to C leaves one DOF free for the hole to move (translation in and out of the screen).





RE: Datums Required for Hole Position
A may be of use for perpendicularity but the nature of the positional tolerance ensures that both ends of the hole are within tolerance, therefore controling perpendicularity as a matter of course.
(Based on ISO understanding anyway)
Designer of machine tools - user of modified screws
RE: Datums Required for Hole Position
Datums represent your mating parts.
If you just hang your part on a stick using DIA 2 hole, then 2 datums are enough.
If the datum feature A of the part is leaning against flat mating part, then you need all 3 datums: datum A will control perpendicularity of your hole to the flat.
And TED7, you need to learn some basics about “ISO understanding”. I suggest reading this book:
http://www.iso.org/iso/home/store/publications_and...
RE: Datums Required for Hole Position
John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: Datums Required for Hole Position
RE: Datums Required for Hole Position
“Know the rules well, so you can break them effectively.”
-Dalai Lama XIV
RE: Datums Required for Hole Position
I agree with your statement if it is assumed that B and C are perfectly perpendicular to A. I was not making that assumption though when I posted. Any amount that A is out of perpendicularity with B and/or C is induced into the hole's perpendicularity to A. Considering the part is 5 inches thick, I don't think it's a stretch to imagine that condition.
John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: Datums Required for Hole Position
“Know the rules well, so you can break them effectively.”
-Dalai Lama XIV
RE: Datums Required for Hole Position
RE: Datums Required for Hole Position
your explanation is same as like the below mentioned ?..
When your referencing B and C datum with .01 dia zone, drilling starting point must be within dia.01.
Later the drill bit is free to move out of dia.01 zone. In Otherwords the hole axis can tilt out of tolerance zone after the starting point. Because the Depth refrence (A Datum) is absent in the GD&T call out.(PFA).
Any one pleae let me know whether my understanding is right or wrong ?
RE: Datums Required for Hole Position
No, you are not interpreting my post correctly. The axis of the hole must stay within the tolerance zone. Reference to datum A is not a depth reference, it is an orientation control. It means that the tolerance zone must be perpendicular to A. If you remove reference to A then B becomes primary and then the tolerance zone must be parallel to B. Fundamental rule 1.4(m) in the 1994 standard says that the tolerance zone applies to the entire length of the feature being toleranced so no depth specification is required. CH's drawing is correct per his explanation. If you look at it you will see the the hole is not perpendicular to A but is parallel to B. If you add reference to datum feature A the hole now becomes perpendicular to A and ignores any orientation to B.
John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: Datums Required for Hole Position
RE: Datums Required for Hole Position
John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: Datums Required for Hole Position
RE: Datums Required for Hole Position
I was trying to draw something like this, but screwed-up in a hurry.
RE: Datums Required for Hole Position
BTW, where did your post go that asked me about that to begin with?
John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: Datums Required for Hole Position
I am not the one who should be asked about it.
CH,
Now it is much better.
RE: Datums Required for Hole Position
John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: Datums Required for Hole Position
Apparently moderators decided to remove pmarc's response as well to reduce confusion.
RE: Datums Required for Hole Position
“Know the rules well, so you can break them effectively.”
-Dalai Lama XIV
RE: Datums Required for Hole Position