Pc-Dmis GD&T class summary by Wes Cisco
First I want to say that I have previously attended the Pc-Dmis Basic and Advanced courses. While I thought both of those were good, I felt this class taught me more, and was more important.
I tried to begin using Xactmeasure a few months ago and ran into issues. I appealed to the forum communities for help, but was unable to resolve most of my issues. My employer was receptive to the idea of sending me to the course, so I put off using Xactmeasure pending the class.
This is a three day class. The scope of this document is too narrow to fully cover that much material. I do not recommend anyone consider it a substitute for the class. I am trying to hit the “flags” that anyone attempting to use Xactmeasure without having had the class will need to know. Most of this is NOT in the help files. I requested it be added more than once during the class.
The class will NOT teach you GD&T. The purpose of the class rather is to teach you how properly use Pc-Dmis to dimension part programs compliant with ASME Y14.5M-1994.
The class focused on using Xactmeasure properly while acknowledging there are areas it does not perform properly and legacy dimensions should be used in those places instead. It is possible to switch back and forth between Xactmeasure and legacy in one program.
1) It was stressed several times that nominals must be correct. If they are not your report is GIGO.
2) Perhaps the biggest single thing I took away is the knowledge that in Xactmeasure Pc-Dmis takes the first datum in the FCF you build in Xactmeasure and levels Z+, then the second datum in the FCF is used to rotate X+ to.
3) Obviously this will yield proper results in only a small percentage of cases. The solution is in the advanced tab of the Xactmeasure window you select “current alignment” rather than “Datum Reference Frame”. When you do, Pc-Dmis will use the current alignment, exactly as you created it, rather than try to create a new alignment from the Datum Reference Frame you create in the FCF. However if your current alignment does not use the Datums in the Datum Reference Frame of the FCF you will get GIGO.
4) I have spent a little bit of time playing with this and in some cases using Datum Reference Frame will give the correct position value, but the axis will be flipped. The safe way is to always create an alignment in your program that represents the DRF and always choose current alignment in the advanced tab.
5) Multiple Datum Features. Also called “A-B” datums, where there is a Datum A on the part and a Datum B and then in some DRFs there is a Datum A-B listed. The help files of Pc-Dmis say that in Xactmeasure it is possible to type in A-B, even though it is not in the drop down menu. This DOES NOT WORK PROPERLY. That is straight from one of the two guys who wrote the training manual for the Pc-Dmis GD&T course.
6) The solution is to construct a feature that represents Datum A-B. If both are diameters on either end of a cylindrical part, (this is what I see most often), then construct a 3D line between them. The same advice as above about creating an alignment and using the current alignment rather than the DRF in the advance tab applies.
7) In the Advance tab of Xactmeasure on the right side is a box labeled GD&T standard. There are three options in the drop down. ASME Y14.5, ISO 1101, and Custom. Choosing current alignment will not change this. However if you check or uncheck Fit to Datums or Deviation Perp to Centerline it can change this. If you have ASME selected and it changes to Custom that means your results will not be compliant with ASME. This is supposed to work properly and to the best of my knowledge it does.
8) Minimum Circumscribed and Maximum Inscribed does not work properly on a radius. Use least squared if less than 270 degrees or funky results.
9) Ensure you take a sufficient number of hits. The instructor pointed out that in the Basic and Advanced classes, examples use barely enough hits to define the features for the sake of teaching the concepts, but not nearly enough hits for most real world applications. I was quite fond of this quote: “Things we taught you in the past were not incorrect, just insufficient.” He said their rule of thumb is for .0005” tolerance for size on a diameter requires at least 30 points. (total, not per level of a cylinder. 3 levels of 11 would be fine.)
10) Xactmeasure does not support modifiers for straightness. Use legacy.
11) For many form checks, Xactmeasure will only report the worst case or maximum deviation, however all hits can be reported by turning textural analysis on in the advance tab.
12) Remember #1 back up there at the top? Nominals must be correct. In legacy if your nominals are not correct when you create the dimension, you can change them in the edit window and the popup box asks if you want to update the nominals in the feature. In Xactmeasure you correct or change the nominal values in the advances tab and on the right hand side there is a tick box for the “update feature” column. If you tick that box it will carry the new nominal value back to the feature. WARNING: This will only work as advertised if you are creating the dimension in the same alignment you measured the feature in. For example if you measured the feature aligned to A/B/C and then you change alignments to B/D/F and try to use this pc-dmis will not properly update the nominals of the feature and in most cases it will crash.
13) Opposing planes. If you have a tang protruding from the part and it’s location is controlled by position, how do you account for MMC? The only way in pc-dmis for both legacy and Xactmeasure is to create a generic cylinder using a variable to assign the diameter based upon the width of the feature (distance between the opposing points or planes). The word is they are trying to come up with an autofeature that is essentially an inverse of a slot, but they have not gotten there yet.
14) Concentricity. They say the best way to check it with pc-dmis is to take multiple circles along the length and apply concentricity to each individually. Pc-Dmis uses the center of the circle, not opposing median points as the standard requires.
15) Profile. Make sure min/max is checked if using legacy. In Xactmeasure when checking profile of a surface you must choose least squared for the Best Fit algorithm, min/max is currently not available for 3D fitting.