True Position questions

[Hirxm]

Hello again.

Im trying to find the true position of these holes, they’re two and so far the program has already given me some results. This is the first time I’m looking for true position related to three different datums. What I did was to measure the datum features and then I used Xact dimensions to create the control frames for those two particular holes.
Is that correct?
Now, the results are what I can’t finish to understand. I attach the results I get. I don’t understand where does that Bonus come from?? And what do all those letters mean (D3, TP, RN)???

[bfire85]

First off, modify the tolerances to match the print. This will make sure the bonuses and datum shifts are applied correctly. The +/- TOL should match your print tolerances.

D1 = Datum 1, D2 = Datum 2, D3 = Datum 3, TP = True Position, RN = Form

In addition, in Xact Measure, you want to selected both diameters and measure them in the same Feature Control Frame, not in separate ones like you currently have.

[Hirxm]

Ok I think I'm starting to get it. Another question: bfire85 says that "in Xact Measure, you want to selected both diameters and measure them in the same Feature Control Frame, not in separate ones like you currently have." This means that there's no need to create two separate dimensions, one for each hole? I just need to select both holes on the same dimension?

[bfire85]

You will have two true position measurement values for each diameter, however it will be calculated correctly because of the datum shifts. Each feature will shift along the datum together as a set, which will simulate an attribute gage.

[louisd]

ASME y14.5m references simultaneous evaluation as applying since 1994 revision. So if your print is per 1994 or later, both true position tolerances must be evaluated at the same time, using one set of datum shift metrics to fit the pattern of features.