## Mathematical definition: multiple datum feature-primary on a cylindrical surfaces

## Mathematical definition: multiple datum feature-primary on a cylindrical surfaces

(OP)

What is the mathematical definition of A-B as multiple datum feature?

I do understand what is the axis of UAME for datum feature A and how to determine that. The same for datum feature B, but what is the axis of A-B (compound) and how to determine/ explain what means? –see fig. 4-25 /2009

I do understand what is the axis of UAME for datum feature A and how to determine that. The same for datum feature B, but what is the axis of A-B (compound) and how to determine/ explain what means? –see fig. 4-25 /2009

## RE: Mathematical definition: multiple datum feature-primary on a cylindrical surfaces

John-Paul Belanger

Certified Sr. GD&T Professional

Geometric Learning Systems

## RE: Mathematical definition: multiple datum feature-primary on a cylindrical surfaces

I don't think the ASME standards have a mathematically rigorous and general explanation for datums established from multiple features of size.

What does "smallest pair" mean? Should the sum of the diameters be minimized? Should the diameter difference be held fixed at 5? How would the answer differ if the two features had unequal total size tolerances?

pylfrm

## RE: Mathematical definition: multiple datum feature-primary on a cylindrical surfaces

And the "smallest pair" means that the two must close down until each makes high-point contact with the actual part. So differing size tolerances would not matter; just close down until your grab each diameter. But you have to close down while always keeping these envelopes coaxial.

John-Paul Belanger

Certified Sr. GD&T Professional

Geometric Learning Systems

## RE: Mathematical definition: multiple datum feature-primary on a cylindrical surfaces

Equal rate of closing? Absolute rate? Percentage of the total allowed variation per unit time?

If two cylindrical datum features have different nominal sizes and different tolerances, is the clamped-up position truly determinate in the fixture? If there's no prescribed "how", there are going to be multiple possibilities.

## RE: Mathematical definition: multiple datum feature-primary on a cylindrical surfaces

Yep, that's basically the point I was trying to make. Given OP's use of the word 'mathematical', I suspect they may be wondering along the same lines.

pylfrm

## RE: Mathematical definition: multiple datum feature-primary on a cylindrical surfaces

http://files.engineering.com/getfile.aspx?folder=a...

## RE: Mathematical definition: multiple datum feature-primary on a cylindrical surfaces

Interesting approach. It seems fairly robust at first glance. Always good to learn about the ISO side of things, so thank you for posting that.

pylfrm

## RE: Mathematical definition: multiple datum feature-primary on a cylindrical surfaces

That is very helpful.

And since some parts with the above A-B callout are measured on CMM which use by default ISO language then your posting is even more relevant to me.