XactMeasure $#!*&$#!

neil.challinor & Matthew D. Hoedeman sorry to highjack feel free to move to a new thread if need be.



so why does xact measure automatically do this? (V2020R1)



When datum math changed to lsq also matches xact and legacy (V2022.2)

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2022.2 datum math changed back to default is just a few micron different but also calculates from the start point vs the end.



2022.2 legacy (cmm alignment is not to the DBC but to ABC) use datums on, fit to datums on, perpen to CL on, from worst end of axis.

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Ncogneto

This is a huge topic and best addressed via training. Our GD&T for PC-DMIS course covers all of this in detail and talks about the differences between legacy, XactMeasure and Geometric Tolerance commands.

Legacy dimensioning was added to PC-DMIS nearly 30 years ago and as such, is quite limited. You can only dimension a SINGLE considered feature at a time (no support for patterns) and it does not support simultaneity. The document I shared earlier explains how you can get different datum fitting behaviour, depending on the combination of settings, and applies to LEGACY DIMENSIONING only. Legacy Dimensioning relies on the user to interpret the drawing and to create the appropriate alignments, features and constructions to verify requirements. As such, it does not claim to be fully compliant to any standard.

XactMeasure was introduced in version 4.1 of PC-DMIS (2006) and was really our first attempt to properly follow the standards. It attempted to support both ASME and ISO but was heavily biased towards ASME (only real difference for ISO was the single value for profile). The reason XactMeasure applies "FIT TO DATUMS=ON", "DEVIATION PERPENDICULAR TO CENTRELINE=ON" and " USE AXIS=WORST" by default is because that is the correct way to evaluate position according to the rules defined in ASME Y14.5 - that's why you have to change the selected standard to "custom" in order to override those settings. Although it was an improvement on the older, legacy dimensioning method, it was by no means perfect. There were a number of things it didn't support, it still used traditional alignment concepts when constructing datum reference frames and there was no real intelligence or error checking. Simultaneity was supported but required all of the commands to be unmarked and then grouped together in a new, simultaneous evaluation command.

Geometric Tolerance commands were introduced in 2020 R2, COMPLETELY replacing Xactmeasure and employing an entirely new GD&T library. Although the dialog may look similar to XactMeasure, the two commands are entirely unrelated. Geometric Tolerance commands are the first for which we claim to be fully compliant to both ASME Y14.5 and ISO1101 - there are now significant differences between the two standards. Datum reference frames are built using a simulated hard gauge approach rather than traditional alignments. For the first time, we now have robust validity checks preventing the creation of invalid feature control frames. Simultaneity is fully supported and has been made much more user friendly. One other significant difference is that the Geometric Tolerance command recalculates based on the math types selected whereas XactMeasure and Legacy Dimensioning required you to select the appropriate fitting algorithm ​when measuring the features.


The first two examples you've created are roughly equivalent because you have overridden the Geometric Tolerance math to use Least Squared. The fourth example gives a similar measured value to examples 1 & 2 but is based on a different alignment. Only the third example could be claimed to be truly "compliant" because it is using DEFAULT math (constrained L2 for ASME Y14.5).

Ncogneto

This is a huge topic and best addressed via training. Our GD&T for PC-DMIS course covers all of this in detail and talks about the differences between legacy, XactMeasure and Geometric Tolerance commands.

neil.challinor . That's kind of a understatement. I have taken the the Applied GD&T for PC-DMIS – ASME (21150-21-231-1) earlier this year and have access to the online presentation. Nowhere in chapter 8 is legacy used for any of the location / positional tolerances nor is there any discussion of it. In the training the instructor literally went page by page and did not want to discus anything that was not in black and white per say. We in the class could tell right away that the instructor was not comfortable teaching a GD&T class to 10 - 20 year experienced inspectors. I do not recall covering any topics on the differences besides chapter 3 slide 11 which mentions the usage of the "use datums" but doesn't describe why or why not, and Chapter 4 slide 11. (I bookmarked them in the Hexagon book I bought for quick reference) I will go back and take an in depth look at the online training. Can you provide and insight as to what chapters / slides discuss this besides the aforementioned? As we also discovered in training, chapter 5 slides 13 & 14 if you construct a Primary Datum A Constrained_L2 as per chapter 4 slide 13 it will not show up in Geotol when the the flatness is required to be dimensioned. Then there is a note on chapter 5 on slide 13 that "Flatness on a constructed primary datum plane is not a valid requirement" so for those of us that used the constructed method, we had to use legacy to dimension the flatness for a constructed primary L2 primary datum plane. This seemed a little contradictory to all of us in the class since we selected used the L2 constrain algorithm, and then the question was asked "if we use an "auto feature plane" The algorithm options are lsq or min sep. So when dimension distance (non geotol) after defining it as Datum A what math is being used and does this meet the standard" but again the instructor did not want to go beyond what was in the documented training. Again sorry for the hijack and feel free to move this as necessary.

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