pattren of holes - datum

[Madhu454]

From my previous post on runout I got this doubt.

Is it mandatory to use M modifier if I want to use a pattern of holes as Datum?
If I use M modifier then I can use a fixed gauge and the axis of the pattern can be considered as datum.


Is it possible to have a pattern of holes as datum at RFS condition, If possible how to simulate the datum in this case? Here also the datum will be the axis of the hole-pattern only, not the axis of individual holes.

Can anyone help me to understand this?

 

[drstrole]

Jim,
I went back an reread the tip and looked at the illustration. I appologize it does recommend not to do that, it eludes that it could be done, but then shows an example using one hole as secondary and then another as teriary datum features. I appologize for the misrepresentation from my posting.

Drstrole
GDTP - Senior Level

 

[MechNorth]

Basically it repeats / paraphrases what's in the standard, and strongly guides the reader not to use RFS because it's not supported (as of that release).

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[pmarc]

By the way, imho there is a mistake in the middle picture of Tec-Ease example. There should not be '3X' preceding dia. 6 for size of upper hole of the pattern.

 

[MechNorth]

Pmarc,
the 3X ties the three holes together as a pattern, thus the position control relates the 3 features to each other as well. Which way is used depends on the desired functionality.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[pmarc]

Jim,
Not sure if we are talking about the same figure. I am thinking about the one in which the bottom left hole has perpendicularity callout to A and is assigned as datum feature B, while the bottom right one with position callout to A and B is assigned as datum feature C.

I do not think 3 holes in this figure can be considered as a pattern since geometrical tolerances applied to each of the hole is different, so simultaneous requirement does not apply. In fact one may read this in a way that all 3 holes are controlled by the positional tolerance referencing A, B and C which would lead to self-referencing datums situation.

I agree that last scheme (at the very end of the tip) is defining pattern of holes, but not the middle one.

 

[MechNorth]

I think there is a problem with the drawing, but not necessarily what you see. In this particular case, the easiest fix is to remove the "3X", then all's good. However, consider this alternative; The sizes are removed from the datum-B and datum-C features and left on the third hole callout (with the 3X callout).
In this way, the 3X establishes a pattern and the positional tolerance relates all three features to each other as well as to the datum reference frame. Arguably, the relationship of the two datum features is already established to higher precedent datums. As this is only 3 holes, it's not really useful to have the same tolerance value. Picture if the quantity had been 8X instead; then the inter-feature relationship is necessary/of value.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[GDTcoach]

pmarc is correct. The 3X should be removed from the middle drawing on the Tip. Cut and paste will get me every time.

This has been a great thread. Until the 2009 Standard there was no support for a pattern of features RFS/RMB. As Evan said, referencing a datum feature on an RMB basis is a "can of worms". If your simulators are expanding to fill the holes in a pattern, once one simulator fills one hole do the other simulators stop expanding and set up a candidate datum set situation or do the other simulators continue expanding until the part is fully constrained? I believe in applying extensions of principle provided the building blocks of the principle are well defined. If you extend a principle into uncharted territory, yoyo (you're on your own).

I will remove the 3X from the middle drawing. As Daddy said: "If you never do anything--you never do anything wrong."

 

[fsincox]

J-P, thanks, for standing up.

So some of you have not heard of books and tests being wrong before?

drstrole, thank you,
(2) Press fit dowel pins was the example I encountered way back when, I heard you. IMHO, If it functions RFB it is RFB whether some book knows it or not.

So, why do you suppose they changed the standard, then? My experiance is that change is generally resisted unless, and sometimes even when, it is just plain wrong.
Frank