Actual Part Center of a Pattern 2

[dtmbiz]

I am told this subject is a "can of worms", however I want to know... I think...

A number of our parts have a pattern of holes called out as a datum. The patterns are normally a functional interface which meet datum selection requirments. Our company uses CMM and has a philosophy of cost savings by eliminating hard gages.

I have as an example of an exaggerated part to hopefully show the question at hand, which is identifying the pattern center of the actual part.

After much head banging and mind torture, I plead ignorance and would appreciate input. For part setup on a CMM are pins for holes still required? or is it best fit? or what? And of course where is the pattern center of varying produced hole sizes and their produced locations.

Please see attached

 

[dtmbiz]

Jim,

I have wondered what one would use as the pattern center concerning a pattern of irregular holes. Thought maybe to pick the most extreme holes vertically and horizontally and use that center. We have decided here to only use 4 holes with a separate hole callout for the remainder. Never crossed my mind to use "arbitrary" point.

thanks for the thread references...

Belanger,
No asterick and I did scan the standard's examples for any referencing a datum with material condition. Doesnt show any. I do recognize that the examples are not neccessarily complete and certainly not exhaustive.

Uncle... enough... enough...

Want to ask question on new thread regarding feature of size defined by basic dimensions...

 

[Belanger]

I just looked in the 1994 standard, and you're right: there is no example or mention of using modifiers on datum references for profile. But I would say that since datum references are permissible for profile, then just about anything that's kosher in the datums chapter can carry over to the same usage in profile.

The new 2009 standard avoids this problem by explicitly saying that MMB/LMB are OK in a profile feature control frame (paragraph 8.5).


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

 

[MechNorth]

Oh, careful J-P. Carte-blancheing the use of datum modifiers to controls based on what the datum chapter indicates can get you in trouble; I've seen someone put an MMC modifier on a datum reference in a total runout control. Wasn't pretty, but was pretty funny when I showed him what it meant.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[Belanger]

Good point. I was somehow trying to justify "M" in a profile's FCF when the standard doesn't explicitly show it...

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

 

[axym]

I don't have the standard in front of me, but I think there's a figure that shows it. It's a gasket-type part with both position and profile tolerances, and simultaneous requirements. It might be in the position section.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[Belanger]

There is one as you describe with position invoking BOUNDARY in conjunction with profile all around. But it doesn't show the "M" modifier in the profile's FCF.

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

 

[MechNorth]

'09: Fig. 8-24 MMC Principle Used With Profile Controls? (or '94: Fig 6-19 Boundary Principle Used With Profile Controls?) It's establishing that a Position control at MMC, applied to a feature whose size is controlled by a profile control, is actually controlling the boundary of the irregular profile control rather than its specific location. In this example, there's no datums referenced on the profile control (as it's only controlling the size) and the position's datums aren't modified (that would just make for a big ol'keg of worms there!!)

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com