CMM Programming with GD&T


Applied GDT CMM and GD&T

There is a lot of experience, skills, and knowledge needed for a good CMM programmer. Indeed, as many manufacturing companies would attest, a good CMM programmer is hard to find. And when inspection equipment is seen as pure overhead, many companies have a difficult time paying a high salary for the level of skilled personnel needed to program their CMM's (after all CMM's aren't cutting chips!).

One skillset that is critical for CMM programming is an in-depth knowledge of Geometric Dimensioning and Tolerancing (GD&T).

It may seem obvious that a CMM programmer needs to understand GD&T but the depth of GD&T expertise necessary may not be realized. In reality, a good CMM programmer needs to understand GD&T as well or better than a designer. A designer needs to know enough about GD&T to apply datums and tolerances that ideally reflect the function of the part following the applicable standard (ANSI or ISO). The CMM programmer needs to interpret the GD&T and then apply measurement strategies to correctly evaluate the part's conformance. If the CMM programmer doesn't understand GD&T as well or better than the designer, the implementation of GD&T will logically fail.


A significant challenge arises due to the varied "styles" of GD&T that is used on different prints. The CMM programmer needs to not only understand the GD&T standards (e.g. ASME Y14.5-2009) but also how it is applied by designers with varying GD&T "philosophies". Frequently GD&T is applied poorly or simply in an invalid way, but the programmer still needs to inspect the part and provide accurate results. This means that the CMM programmer needs to get "into the mind" of the designer by evaluating the print and observing how the designer uses datums and different geometric tolerances. And it may be necessary to interpolate the gaps or make the best judgment on what to do with an invalid or poor callout.

In many cases, the only way to properly work through poor/invalid GD&T callouts is to get clarification from the designer. But, oftentimes that is simply not an option. So, CMM programmers need to make the best decisions possible and their level of GD&T knowledge and experience makes a huge difference. And of course, they also should clearly document the methods used especially when the GD&T is not clear or invalid. The often-unspoken reality is that, depending on things like datum alignments and feature calculations, there can be significantly varied results on any given part. 

The last thing anyone wants is a CMM that simply produces numbers. Once a CMM gets a reputation for being a random-number-generator, it can be hard to restore its place of value for manufacturing.

Making decisions for CMM programming with GD&T can be quite difficult. The more the CMM programmer understands GD&T and how it is applied in industry, the more likely they will be able to create a CMM program that properly determines part conformance. It's also important that the CMM programmer can provide results that give clear feedback to those in manufacturing to monitor and improve processes. The numbers that manufacturing needs for process feedback may be quite different than those that determine part conformance. If implemented well, CMM's can provide manufacturing significant value.

GD&T expertise is just one of the critical components for good CMM programming, if you have a CMM with PC-DMIS and you need some programming help, let me know. Whether it's bringing your PC-DMIS programmers up to speed or outsourcing your PC-DMIS programming, Applied GD&T is here to help.

For more information on CMM programming or GD&T training, just contact Applied GD&T.