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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BTW I'm using PCD 2010 mr3, so I'm a little out of date.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
  • “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] ”
    No, you don’t get bonus from MMB/LMB (maximum material boundary/least material boundary when on datum features) What you get is datum shift


    “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).”
    That “normal logic” is wrong, you do get direct bonus from your feature. But datum shift depends on many factors, you should think in terms of hard gage.
    Let’s say you have a part where datum –A- is a plane and datums –B- and –C- are .250” +/-.005” holes at MMB with 5.000” spacing.
    Example 1
    Both holes measure .255” and spacing measure 5.000”, you have .010” datum shift
    Example 2
    Both holes measure .255” and spacing measure 4.990”, you don’t have any datum shift
    Example 3
    Datum –B- measure .255”, datum –C- measure .245” and spacing measure 5.000”, you get rotational datum shift about datum –C- of .1146 deg.
    You see where it can get complicated when you add holes perpendicularity of datums –B- and –C- to datum –A-, and you have bunch of features with same FCF to those datums.
Reply
  • “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] ”
    No, you don’t get bonus from MMB/LMB (maximum material boundary/least material boundary when on datum features) What you get is datum shift


    “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).”
    That “normal logic” is wrong, you do get direct bonus from your feature. But datum shift depends on many factors, you should think in terms of hard gage.
    Let’s say you have a part where datum –A- is a plane and datums –B- and –C- are .250” +/-.005” holes at MMB with 5.000” spacing.
    Example 1
    Both holes measure .255” and spacing measure 5.000”, you have .010” datum shift
    Example 2
    Both holes measure .255” and spacing measure 4.990”, you don’t have any datum shift
    Example 3
    Datum –B- measure .255”, datum –C- measure .245” and spacing measure 5.000”, you get rotational datum shift about datum –C- of .1146 deg.
    You see where it can get complicated when you add holes perpendicularity of datums –B- and –C- to datum –A-, and you have bunch of features with same FCF to those datums.
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