When I run the true position for Circle 1 I get a MMC bonus of .0008" but when I run the true position for circle 2 in relation to circle 1 (datum C) it shows a bonus that is associated with circle 1 as .0190" Can anyone answer as to why it is showing different bonuses for the same circle? Please see attached pdf.
Well I don't use exact measure a lot and I am far from a GD&T expert but this is what I believe I know. In the first FCF you have a mmc (maximum material condition) on the feature itself. This is going to give you bonus depending on the feature of size (in your case your circle) and what tolerances you give on that feature and what it actually measures from nominal. On that second FCF that's a whole other story because you have a maximum material condition on the feature and a boundary on the datum, a mmb (maximum material boundary). This is where it starts to get complicated and I couldn't not even attempt to explain how pcdmis calculates mmb or lmb.
Whenever a modifier is applied to a datum feature, it is defining a boundary. For a datum that is an internal feature of size, it is essentially defined by the MMC size of the feature minus it's position tolerance, in this case, it would be "0.608". I say essentially because datum precedence must be taken into account being it is a tertiary datum (see ASME Y14.5.1M-1994 for the math).
The increased reported bonus is a result of the smaller boundary zone.
Below the area you have highlighted (not included in the attachment) is where the amount of Datum Shift is reported. Assuming the DRF constrains all 6 degrees of freedom, most of the values will either report "0.000" or say "Fixed" (depending on which version of PC-DMIS you are running), but it may show some rotation shift due to the smaller zone of Datum C.
It's like having in inspection gage with pins that allow the part to "best fit" onto it.
