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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BTW I'm using PCD 2010 mr3, so I'm a little out of date.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?
  • 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.
  • yes you do. datum shift is not the same as bonus tolerance though. a hard gage or xact measure are your options probably..
    +1

    as big of a deal as hexagon makes xact measure i doubt calypso has anything that "robust" in it. .

    I program with both PC-DMIS and Calypso and IMNSHO Calypso is much better when it comes to applying GD&T correctly than PC-DMIS is. I've been hosed by PC-DMIS's GD&T output before when using MMC/LMC from datums and still will find myself verifying the reported results. I will note that I do not use XactMeasure very often as our production CMMs are still running V3.7MR4.

    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
    So.... you don't trust the numbers completely???
  • 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
  • +1



    I program with both PC-DMIS and Calypso and IMNSHO Calypso is much better when it comes to applying GD&T correctly than PC-DMIS is. I've been hosed by PC-DMIS's GD&T output before when using MMC/LMC from datums and still will find myself verifying the reported results. I will note that I do not use XactMeasure very often as our production CMMs are still running V3.7MR4.


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


    Legacy does not handle material modifiers on datum properly in all cases. Supposedly Xactmeasure does. I am not any near mathmatically inclined enough to be able to say one way or the other.
  • “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.


  • Legacy does not handle material modifiers on datum properly in all cases. Supposedly Xactmeasure does. I am not any near mathmatically inclined enough to be able to say one way or the other.


    I don't use datum modifiers unless I absolutely have and then I tell people to use caution and will add the actual coordinate locations to the report as REFERENCE only. Has saved my backside many times.

    Sent from my PC36100 using Tapatalk 2
  • 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.
  • 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.


    Legacy and XactMeasure ?
  • 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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