Datum shift + Stack-Up

[prdave00]

I just finished looking at the latest Tec-Ease tip (

http://www.tec-ease.com/gdt-tips-view.php?q=253)

regarding datum shift and its message that the modifier applied to a datum feature does not change the tolerance on the feature or features being toleranced, but should (or can) the modifier be included in the tolerance stack up?

In the attachment I provided an incomplete drawing of a part with coaxiality controls. The primary datum is an external thread, although a cylindrical boss could have been just as pertinent. I also included a tolerance stack up to look at wall thickness using both worse-case and statistical models. The handling of bonus tolerances and datum shift was based on a series of papers written by Ngoi et al (affiliated with the Nanyang Technological University) in lieu of having access to any stack up analysis textbooks, etc. 2-D CAD drawings were used to check my work based on the guidance from the aforementioned papers.

Should I have excluded the datum shifts? Can anyone refer me to a good text that discusses inclusion of geometric tolerances in a stack up analysis?

 

[Belanger]

Dave, I agree that hiding the bonus in the same line as the size is a little harder, especially if you're trying to justify it to someone. But that's the inherent drawback of that template. I prefer a template that shows separate columns for "max" and "min" because you can then safely separate the bonus tolerance into its own line.

Peter, sorry that I was combining my reply to Dave and you together. I like your idea of using profile, because it tackles size, form, and location all together. But it doesn't allow bonus tolerance. Not sure what to do...

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

 

[prdave00]

J-P: Point taken about the min/max template. If I could do it all over I think I would have gone with that template over the nominal/plus/minus. Next time I will.