Runout referenced with a datum of pattern of holes

[Madhu454]

Hi All,

Recently I saw a drawing where the runout control is referenced with axis of pattern of holes as the datum. I doubt whether it is a legal specification for runout?

Please see the attached drawing.
The four holes dia 10-11 are the clearance holes, the disk will be mounted to a spinning part using the fasterners.

I understood that dia 9.8 gauge pins located on a PCD of 100mm can be used to varify the position of the 4 holes. Also I understood that using M modifier is a must when we use pattern of holes as a datum. I have no doubt on this.

The question is, the outer diameter of the disk is controlled with respect to the 4 hole pattern using runout , How is this possible?
because the datum B is the axis derived from the pattern of 4 fixed gauge pins of size 9.8. How to simulate the datum B here? will there not be any play between the holes and the datum simulator? obviously there will be a play - How to measure runout for this.

Can any one help me to understand this doubt.

 

[Belanger]

I was about to post this in the other thread about ISO vs. ASME, but this example of runout wrt a pattern of holes is a good example.

I am becoming a bigger proponent of the "extended principles" idea over that of the black & white viewpoint. This isn't because of a fundamental change in my thoughts about following rules, but rather because the more we discuss stuff here, the more I find some subtle inconsistencies, not just a lack-of-example situations.

Certainly, if ASME explicitly mandates or prohibits something, then that should be followed. But aren't there times when that might tie our hands in other areas?

Case in point: despite the standard "recommending" certain datum features for runout (para. 9.3.1), I do see the benefit of having a pattern of holes as a datum for runout. The problem is that the standard clearly requires the datum references to be RMB.

From the other thread it sounds like ISO tries to nail down more loose ends while still allowing flexibility. Either way, there are always going to be gray areas where contradictions come to light, in both standards.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[MechNorth]

quite right, J-P

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[Madhu454]

Thank you all for clarifying the doubt. We can conclude this way that, FOS pattren axis cannot be used as a datum for the runout control.

By going through the above posts I got one more doubt, I think I can better post that in the new thread.