Fit To Datums

Can someone give a good explination of the difference in true position when the fit to datums check box is on or off?
When I have the check box on all holes match nominals perfectly. (NO DVIATION AT ALL).
Check box turned off shows deviation that matches the actuals in the measured features.
when the check box is on the part looks to good and questions arise in regards to the accuracy of the gage and or the programer "ME".
thanks for the help.

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Jan d. over 18 years ago

In V4.1, PC-DMIS allows you to actually put in the TP exactly as it is specified on the print. But I share Matt's concerns.

I ended up outputting TP in 3 different ways:
1) No MMC on anything. In other words, RFS.
2) Only MMC on the TP tolerance.
3) MMC on TP call-out AND datums.

1 gives me more process control information. 2 can sometimes behave like you describe and there is no way to get any process info from that.

I think that PC-DMIS evaluates 1 and 2 correctly. After staring at the results of hundreds of holes, I am quite convinced about that. However, I am still not convinced that PC-DMIS always evaluates number 3 right. I have not finished that debate with Hexagon yet.

Jan.
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Jan d. over 18 years ago

In V4.1, PC-DMIS allows you to actually put in the TP exactly as it is specified on the print. But I share Matt's concerns.

I ended up outputting TP in 3 different ways:
1) No MMC on anything. In other words, RFS.
2) Only MMC on the TP tolerance.
3) MMC on TP call-out AND datums.

1 gives me more process control information. 2 can sometimes behave like you describe and there is no way to get any process info from that.

I think that PC-DMIS evaluates 1 and 2 correctly. After staring at the results of hundreds of holes, I am quite convinced about that. However, I am still not convinced that PC-DMIS always evaluates number 3 right. I have not finished that debate with Hexagon yet.

Jan.
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