how to establish datums at MMB using CMM

[bxbzq]

Hi,
A very simple question but I don't have answer. Let's take fig. 4-18 in '09 standard as an example. I have a hard gage simulating datam A, B and C. If I use CMM to inspect all gd&t, do I establish datums by probing surfaces on actual part or datum feature simulators on gage?
If the latter approach, how do I achieve good R&R? I mean you can't repeat the datum shift that other inspector gets during his inspection process.

Thanks in advance.

 

[axym]

bxbzq,

You could establish the datums and DRF either by probing the gage surfaces or the actual part surfaces. In theory, the result should be the same either way.

In reality, it is very difficult to get the same result either way. If you probe the gage surfaces, the DRF is easy to set up but then you are allowed to shift the part around on the gage. If you probe the part surfaces, the DRF is difficult to set up because the software must perform "soft gage" calculations. Most software is not able to do this, at least with the default functionality.

In order to achieve good R&R, you would need to find the "optimal" datum shift. This would be the shift that results in the smallest measured value for the position tolerance.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[Belanger]

Here is an older thread that discussed a similar question:

Link

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

 

[greenimi]

https://www.linkedin.com/groups/1781397/1781397-52911848

Gage R&R and Datum Shift discussion on linkedin

https://www.linkedin.com/groups/1781397/1781397-187183877

Assembly shift and Datum shift