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Fun with GD&T! Datum Shift Revisited.

RobertoFlores
Ok, so it sounds like a simple question, do I get MMC/LMC bonus off a datum feature?
For example [Dia .005-M][A-M]
normal logic would state that I get bonus from my feature, and I get more bonus off my datum, that's how I have been taught to calculate it (with a calculator).
but in my little world I have the pleasure of programming in PC-Dmis and Calypso, and these programs don't always agree with each other (that's an understatement).
I've been up and down the internet, and this video is the best explanation I can find of why you don't get bonus from a datum.
I do believe the conundrum I am chasing is known as "Datum Shift / Datum Mobility".
You can search it on the internet, but reading that kind of text is difficult for me to digest, and what makes it even more difficult is that everyone refers to documents I do not have available.

But I still am left with the question of, Why does PCD give me bonus, and Calypso does not?

BTW I'm using PCD 2010 mr3, so I'm a little out of date. is this addressed in later versions?

https://www.tec-ease.com/gdt-tips-view.php?q=276
http://www.pcdmisforum.com/showthread.php?19372-Datum-Shift
http://www.pcdmisforum.com/showthread.php?21780-Pc-Dmis-Bonus-Tolerance-freak!!!-(MMC-Bonus-vs-Datum-Shift-discussion)

I would prefer simple answers like "Yes you get bonus, go ahead and add it all up" or "Stop taking that bonus, Only use the bonus from your feature".
Slight smile
Second question, what if the only modifier I have is on a datum?
Parents
  • As mentioned the material condition applied to a datum is not the same as MMC on a feature. For example, if your datum is 2.000 +.001/-.000 and it checks 2.0005 you do not get to add .0005 "bonus" to your tolerance zone. What you do get is the ability to shift the datum feature location around within that .0005 zone to 'best fit' the feature location. Xactmeasure does this shifting and reports how much it shifts the datum feature(s) and in which directions. Great if you trust Xactmeasure. However the only certain way I know of to make use of the material modifier on a datum is to build a hard functional gage per Y14.43-2003 DIMENSIONING AND TOLERANCING PRINCIPLES FOR GAGES AND FIXTURES.

    HTH
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  • As mentioned the material condition applied to a datum is not the same as MMC on a feature. For example, if your datum is 2.000 +.001/-.000 and it checks 2.0005 you do not get to add .0005 "bonus" to your tolerance zone. What you do get is the ability to shift the datum feature location around within that .0005 zone to 'best fit' the feature location. Xactmeasure does this shifting and reports how much it shifts the datum feature(s) and in which directions. Great if you trust Xactmeasure. However the only certain way I know of to make use of the material modifier on a datum is to build a hard functional gage per Y14.43-2003 DIMENSIONING AND TOLERANCING PRINCIPLES FOR GAGES AND FIXTURES.

    HTH
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