Why Place MMC in Reference Datum when tolerance is in RFS? 3

[dingy2]

I see this so often and I wonder the purpose of placing a reference datum at MMC when the tolerance is shown in RFS.

Let's say, as an example, we have a profile of a surface tolerance of 0.2 mm referencing primary datum A, secondary datum B (RMB) and tertiary datum C (in MMB). What is the purpose of having a datum in MMB when one must actually measure the profile tolerance on a CMM? I know that it is legal (nothing in the standard stating otherwise) but what is the value or purpose?

I can see when one has a profile of a surface tolerance of 3 mm and the secondary and tertiary datums are at MMB, one can utilize a checking fixture with the outer and inner profile tolerance boundaries shown on a checking fixture but a tolerance of 0.2 mm??

In another situation, one has a positional tolerance of a diametrical tolerance zone of 0.3 in RFS while the secondary and tertiary datums are at MMB. Shouldn't the positional tolerance also be reflected at MMC or should both the positional and reference datums be reflected at RFS/RMB?

I have always been blown away with the application of MMB on the reference datums while the tolerance is in RFS but this may have something to do with my background in measuring rather than designing.

Maybe some of the Designers here could help me out on this one.


Dave D.

http://www.qmsi.ca

 

[dingy2]

Evan:

I gave you a star because you proved that one can confirm Profile of a Surface referencing secondary and tertiary datums at MMB with a high degree of confidence.

We simply would use a hard gauge for the datum structure and pick up on the gauge pins rather than the part. The feature would be measured over and over moving the part on the datum gauge pins each time. One would report the best results but this is time consuming and extremely expensive. I hope that the application was a vital component of the part's function and mating relationship rather than a perimeter that had no functional importance.

I have seen the use of optical comparitors or a checking fixture with scribed lines reflecting the inner and outer boundaries but we can not confirming Profile of a Surface this way. This is a 2 dimensional check and we are confirming Profile of a Line at the intersection of the primary datum and the contour. I guess that I do challenge the Tec-Ease tip previously mentioned. If your company has design review and this example is present on the drawing, someone should ask for it to be changed to Profile of a Line as long as it does not harm the design intent.

Most CMMs do not handle datum shift well and I would also set the part up with the datums in RMB. We, both, would be confirming this requirement incorrectly. The software that attempts to handle the datum shift should be proven. We assume that the software designer is extremely proficient in GD&T but are they? Do they take the average size of the datum hole or is it the inner inscribed point? mmmmmmm

There are examples of the use of Profile of a Surface with the secondary and tertiary datums at MMB shown in both the 94 and 2009 standard. This application certainly does meet the standard but no thought is given on confirmation methods.

There are limited uses of applying a tolerance in RFS and having the datum structure in MMB but some do apply. Otherwise, mmmmmmm??

Thanks for all the input.



Dave D.

http://www.qmsi.ca