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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Stuart over 18 years ago

FIT TO DATUMS.... means if you have MMC on your datums, PCDMIS will 'shift, translate, rotate' your coordinate system using the BONUS on the datums to get the best possible True Position result. PCDMIS 3.7 does this by default. Using this option, it is possible to get a 0.000 True Position frequently, as it reduces the BONUS to improve the TP.

If you uncheck FIT TO DATUMS, then the coordinate system stays where it is (i.e. the true CENTERS of the datum holes don't move) and you get a True Position value, with a TOTAL available bonus.

The latter is what 99% of the industry is looking for.

Stu
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Stuart over 18 years ago

FIT TO DATUMS.... means if you have MMC on your datums, PCDMIS will 'shift, translate, rotate' your coordinate system using the BONUS on the datums to get the best possible True Position result. PCDMIS 3.7 does this by default. Using this option, it is possible to get a 0.000 True Position frequently, as it reduces the BONUS to improve the TP.

If you uncheck FIT TO DATUMS, then the coordinate system stays where it is (i.e. the true CENTERS of the datum holes don't move) and you get a True Position value, with a TOTAL available bonus.

The latter is what 99% of the industry is looking for.

Stu
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