Datums Required for Hole Position

[shaun8567]

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).

 

[TED7]

I would say no, datum A is of no use. The hole is positioned in X and Y and has no Z dimension so you wouldn't need to measure / position it against datum A.

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

 

[CheckerHater]

It was discussed several times:
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_e-products/publication_item.htm?pid=PUB100296

 

[powerhound]

I agree with CH. Reference to datum A ensures perpendicularity to it, otherwise the hole axis it only needs to be parallel to datums B and C simultaneously within the .010 tolerance zone.

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[fsincox]

Function, Function, Function If it actually sits on "A", it is the most important surface!

 

[ewh]

Intuitively, "the hole axis it only needs to be parallel to datums B and C simultaneously within the .010 tolerance zone" seems to ensure .010 perpendicularity by default. What is missing is the relationship between datums B and C, and including A eliminates this issue. I agree with the others of tolerancing by function.

“Know the rules well, so you can break them effectively.”
-Dalai Lama XIV

 

[powerhound]

ewh,

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

 

[ewh]

I hope you agree... after all, I was agreeing with you, just explaining my thought process! ;-)

“Know the rules well, so you can break them effectively.”
-Dalai Lama XIV

 

[bxbzq]

To put it in another angle, reference datum A, your primary datum is derived from 3 highest points on datum feature A. Omitting datum A in your FCF, your primary datum is derived from 3 highest points on datum feature B. Feature A is irrelevant.

 

[Vimalmechs]

PowerHound,
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 ?

 

[powerhound]

Vimalmechs,

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

 

[Vimalmechs]

Thank you all. Especially CH and powerHound....

 

[powerhound]

I'm talking about the one you didn't like. I think it shows the implication of eliminating datum A as primary pretty well. It's incomplete but I think it gets the idea across. Did I miss something blatant?

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[pmarc]

Since it was mentioned that the hole is controlled to B, the cylindrical tolerance zone of DIA. .010 shall be parallel to B and at basic 5 from B. I did not see the parallelism, thus my comment.

 

[CheckerHater]

I apologize for creating confusion.
I was trying to draw something like this, but screwed-up in a hurry.

 

[powerhound]

If B is primary then the tolerance zone is oriented to B first, thus parallel in this case.

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

 

[pmarc]

powerhound,
I am not the one who should be asked about it.

CH,
Now it is much better.

 

[powerhound]

Aha! Yes, I completely missed that the zone was not initially shown parallel to B. I looked at that drawing again and again trying to find something wrong with it since CH seemed to acknowledge an issue with it. It sticks out like a sore thumb now.

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[CheckerHater]

Pmarc made me ashamed about myself, so I red-flaged my own post for removal.
Apparently moderators decided to remove pmarc's response as well to reduce confusion.

 

[ewh]

Though I understood the intent of your earlier pic, it is much clearer this time around CH.

“Know the rules well, so you can break them effectively.”
-Dalai Lama XIV