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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BTW I'm using PCD 2010 mr3, so I'm a little out of date.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
  • Good to know, I guess I just haven't ran into that situation yet

    In my experience, Maximum Material Boundary (MMC on a datum feature of size) is performed correctly in PC-DMIS as long as the datum feature is an auto/measured feature and not constructed.

    PC-DMIS XactMeasure actually makes the datum shift not from reported feature size but by calculating Related Mating Envelope size: if it's a hole picture the largest gage pin fixed perp to primary plane that will fit in it, if it's a boss picture the smallest ring gage fixed perp to primary plane that will fit over it. The gap between the RME and the MMB is the allowed datum shift which reduces axial error. Also, the RME is calculated from hit data so take more points or move up to scanning for best results.

    Note that at this time (v2013 SP1) Constructed features of size do not work correctly as datums with MMB, as XactMeasure cannot access their 'ingredient' features hit data to calculate RME. I personally put in a request to have this changed for future versions.

    Also, the last time I ran Calypso I could never get circle-M on a datum to work correctly.


    So.... you don't trust the numbers completely???

    I trust nothing completely until I verify it.. even a hard gage. If the datum shift is incorrect its going to be noticeable and the problem is usually between the monitor and the chair.

    As with everything else in this software, there are 5 different ways to do something; 2 that will crash the program, 1 that will work about 60% of the time, 1 that will be disabled in the next version and 1 that is hard to find and impossible to remember
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  • Good to know, I guess I just haven't ran into that situation yet

    In my experience, Maximum Material Boundary (MMC on a datum feature of size) is performed correctly in PC-DMIS as long as the datum feature is an auto/measured feature and not constructed.

    PC-DMIS XactMeasure actually makes the datum shift not from reported feature size but by calculating Related Mating Envelope size: if it's a hole picture the largest gage pin fixed perp to primary plane that will fit in it, if it's a boss picture the smallest ring gage fixed perp to primary plane that will fit over it. The gap between the RME and the MMB is the allowed datum shift which reduces axial error. Also, the RME is calculated from hit data so take more points or move up to scanning for best results.

    Note that at this time (v2013 SP1) Constructed features of size do not work correctly as datums with MMB, as XactMeasure cannot access their 'ingredient' features hit data to calculate RME. I personally put in a request to have this changed for future versions.

    Also, the last time I ran Calypso I could never get circle-M on a datum to work correctly.


    So.... you don't trust the numbers completely???

    I trust nothing completely until I verify it.. even a hard gage. If the datum shift is incorrect its going to be noticeable and the problem is usually between the monitor and the chair.

    As with everything else in this software, there are 5 different ways to do something; 2 that will crash the program, 1 that will work about 60% of the time, 1 that will be disabled in the next version and 1 that is hard to find and impossible to remember
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