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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BTW I'm using PCD 2010 mr3, so I'm a little out of date.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
  • yes you do. datum shift is not the same as bonus tolerance though. a hard gage or xact measure are your options probably.. as big of a deal as hexagon makes xact measure i doubt calypso has anything that "robust" in it. its gotten a lot better in 2012-2013 but watch your numbers carefully and report basics using distance dimensions as a check being that you have 2010.

    if your summary dimensions dont match a linear dimension of the same basic it should make sense how the datum shift allowed for the small change.. i can't remember if they had the 'datum shift' in the report in 2010 or not.
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  • yes you do. datum shift is not the same as bonus tolerance though. a hard gage or xact measure are your options probably.. as big of a deal as hexagon makes xact measure i doubt calypso has anything that "robust" in it. its gotten a lot better in 2012-2013 but watch your numbers carefully and report basics using distance dimensions as a check being that you have 2010.

    if your summary dimensions dont match a linear dimension of the same basic it should make sense how the datum shift allowed for the small change.. i can't remember if they had the 'datum shift' in the report in 2010 or not.
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