One reason that makes it difficult to implement GDT standard 100% in CMM world is that many things seem to be black and white from GDT standard point of view (e.g. ASME Y14.5) are often "grey" in real CMM practice. Below image shows a very typical alignment "puzzle": offset alignment.

I only extracted a small portion of the drawing so we can focus on this area. So let's say datum A is the hatched flat surface, datum B is the small hole and datum C is the large hole. To establish this DRF according to ASME Y14.5 is no big deal but when you create an alignment in CMM software will be a different story because we all know that there's no perfect part so datum C hole will never be perfect on both basic dimension 42.36/46.49 the same time. A cmm programmer can easily figure out that the angle offset from datum C hole to the main axis is 42.338645 deg. (or 47.661355 deg.). Theoretically using the angle offset or co-ordinates offset should define same co-ordinate system, but because there's deviation on datum C hole position, so these 2 alignment will present 2 different reports even the part is same! If we all agree that we should use the angle offset (i.e. the 2 way diamond pin on the gauge is using the same angle), then any cmm software will work out the same alignment. But if say 42.36 is required to be locked then do you know that your cmm software will do the job correctly?

You'll surprised that these "little" things are quite different in different software. For example, below is the screen shot of the offset alignment in CMM Manager and Open Dmis:

Both are offset alignment tool in the software but they work in a different way:

1.      CMM Manager applies both basic dimensions to set the rotation (so the software actually calculates the offset angle in the background then do the rotation), which will leave deviations in both directions on datum C hole;

2.      Open Dmis allows you to "lock" one basic dimension then leave all the deviations to the other direction so in this case datum C will be dead -42.36 after the alignment done;

Therefore don't assume that just because it's called offset alignment it will be the same in any cmm software. Some other software (especially old ones) don't even have a particular offset alignment function at all but there's always way to resolve this. For example, in pc-dmis you can create an offset line from those basic dimensions then use that line for rotational alignment. Some people will use variables to capture the real offset angle (which is different for each part) then set it back. But I'd like to present a method that will work in any cmm software (even in really old ones such as USoft, WinCmes, etc.)

The concept is very simple, using this example, all you need to do is these steps:

1.      Set origin at datum B hole;

2.      Create an offset point from datum C by simply moving it along the axis by 42.36 (e.g., create offset point, move point, etc.);

3.      Connect datum B hole and this offset point as a line and use it to set your rotation alignment (e.g. -Y axis);

4.      This is the key step: LOOP above 3 steps for a few times and your datum C hole will be at exactly 42.36.

Many people will question step 2 because obviously the offset direction is really not "correct". That's why you need to loop these steps (pre-condition is you have an rough manual alignment that's close enough). I also created a video to show you how datum C hole is getting closer and closer to the target 42.36 basic dimension in each loop. The video is free to view before Apr.30 and here's the link:

http://www.rxmetrology.com/demo-zone

I re-designed my on-line teaching website and now there're 4 categories: PC-Dmis, Open-DMIS, CMM Manager and GDT-n-Math. Videos in each category is in HD format covering various most discussed subjects in Metrology world. All these videos can be purchased individually (with unlimited access for 180 days), or you can purchase yearly subscription for each category. Right now there's a special offer to subscribe all 3 categories at as low as US\$199! All subscription purchased gives you unlimited access to all videos within the subscription year (videos will be added on a regular basis). This can be your low cost training source, so don't miss it.

If you need help or training to create accurate and reliable cmm programs for your cmm system, don't hesitate to contact me at:  This email address is being protected from spambots. You need JavaScript enabled to view it.

Currently I can offline program on following software:

Open Dmis, Camio, Modus, Mcosmos, CMM Manager, PC-Dmis

Ray Xing
GDTP S-0605

CGP Certified (ITAR equivalent)

CMM Application Specialist

### CMM Training

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