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An Operator’s Guide to CMM Probe Accuracy

Use this three-step procedure to diagnose measurement issues

Published: Friday, January 13, 2012 - 12:45

A Quality Digest reader wrote in asking for techniques to evaluate touch-trigger probes for accuracy. Specifically, he asked how someone can evaluate touch probes and magnetic modules for wear, or to determine if they have been damaged as the result of a collision or being dropped. He wanted to be able to recognize when to replace worn equipment when necessary, but also avoid replacing equipment that is still good. Finally, he asked how to do all this without specialized equipment or expertise.

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Drawing on our many years of collective experience in application engineering and customer training, we'll tackle this question in this brief guide for the coordinate measuring machine (CMM) user.

How do you know if the tool you're using is working properly? With a tool like a hammer, it's easy: If the nail goes in, then it's working. But what about a high-tech tool like a CMM probe? The probe is the complex device you put on the business end of your CMM, with many internal components that you can't see that may be damaged, worn, or out of adjustment. Where to start? Well, let's start by thinking of it as a hammer and keep it simple.

Like the hammer, a CMM probe will provide the expected results based on the product specifications: a baseline. A new hammer is tight and balanced with a flat head and firm grip and also certified by the manufacturer against defects. A new CMM and its probe form a measurement system that is also tight and balanced, and its accuracy and repeatability are certified by the manufacturer and presented for your records in the form of a calibration certificate. That certificate is your baseline—a simple set of numbers that gives you an expected range of where your own measurements should fall when measuring an artifact of known size, such as a qualification sphere or ring gauge. Most CMMs are calibrated to one of two specifications, either the ASME B89 from The American Society of Mechanical Engineers (ASME), or ISO 10360–2 from the International Organization for Standardization (ISO).

Figure 1: Calibration test report for ASME B89 specification

For instance, if your CMM was calibrated to the B89 specification, you'll have one number for repeatability (see figure 1). Let's say that number is 0.004 mm, or 4 microns; this means when you measure a certified sphere, its location measurement should repeat within 0.004 mm. On the other hand, if your CMM was calibrated to ISO 10360–2, you have a number for the "Probing Test," such as the 0.0025 mm value shown in figure 2. This value is your baseline probing uncertainty of .0025 mm, or 2.5 microns, and it states that the probing error of 25 points, measured on a certified sphere, does not exceed this value. This is similar to sphericity and size deviation from the certified diameter. So, for this example, if your CMM has been certified to ISO 10360–2, when you measure a certified sphere, it's diameter deviation and roundness (or sphericity) should be below 2.5 microns.

Figure 2: Calibration test report for ISO 10360–2

Now that you know where you should be, let's find out where you are.

A simple test on any CMM is a check of your probing setup. Simply qualify your probe setup as you normally would for any given inspection project. Then create an inspection routine, in Automatic Mode if your CMM is equipped for that, to measure that same sphere with at least 25 points and four rows with the same A0B0 (i.e., straight down) probe angle. Repeat this measurement three or more times; the results from this test should lie within the baseline range discussed earlier. If not, further investigation and testing are required. Should your test results fall outside the acceptable limits, work your way backward through the probing setup to diagnose the problem.

Begin your diagnosis with the qualification sphere, and confirm that the diameter you've specified in your CMM software matches the certified diameter of the sphere. Then give it a good wipe down with a lint-free cloth and some denatured alcohol. Next, double-check the stylus. Confirm that it is tight, clean, and again matches the diameter and length you've specified in your CMM software. These simple steps are actually the most common, and thankfully the most inexpensive, fix to most CMM accuracy and repeatability issues, so don't feel bad if you've been chasing a loose or chipped stylus for the last two hours, it happens even to the most seasoned CMM programmer. Once you've confirmed everything, rerun the sphere program to see if your issue is resolved.

If following the previous steps didn't resolve the problem, things can get potentially expensive if you're chasing damaged or worn-out probing. If you're working with a probe with noninterchangeable modules, like a TP-2 or TESAStar-p, first check that the connection to the probe head is tight, then confirm it is not sticking (when triggered) or excessively leaking oil, both signs of a worn-out probe. You can also check and adjust the trigger force with a strain gauge, but if either of the two previous symptoms (sticking or leaking) is present, you're best advised to replace the probe and move on.

For probes with interchangeable magnetic modules, like a Renishaw TP-20, 200, SP-25, or TESAStar-mp, the probe is in two parts: the module, which is the part the stylus screws into and contains the triggering mechanism—and the body, which screws into the probe head. The body is basically just a dumb on-off switch, and it either works or it doesn't; it has no effect on accuracy. To test a body, attach a module that is known to work, and make sure the contacts are clean and the module is seated properly. If it doesn't trigger at all, the body is probably bad.

If the body works, then it is a matter of testing your modules. Again, start by making sure the contacts are clean and the module is seated properly, and also that you have a good stylus seated in it properly. If after running the previously described test procedure the measurements don't repeat within specification, you may have a worn-out module and unfortunately, the only further test is to swap it with a known good one and confirm acceptable results.

Following this simple, three-step procedure—baseline, qualification check, and work backwards—will help you quickly diagnose measurement issues and give you more confidence and traceability in your measurement results. Of course, you may need to modify the steps depending on your setup, such as adding more tip angles and measuring the sphere with more points, to achieve your desired level of confidence.

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About The Authors

Brian Gudauskas’s picture

Brian Gudauskas

A 20-year CMM industry veteran, Brian Gudauskas is the national applications manager for Hexagon Metrology in Quonset Point, Rhode Island, where he leads his engineers providing support to Hexagon customers. Gudauskas has taught hundreds of courses on coordinate metrology and has given numerous presentations on subjects such as airfoil inspection, and metrology. He holds a bachelor of science in mechanical and materials engineering from the University of Connecticut. Before joining Hexagon Metrology, he was an engineer for United Technologies - Pratt and Whitney where he helped develop prototype jet engines.

Shaun Wissner’s picture

Shaun Wissner

Shaun Wissner is a software specialist for Hexagon Manufacturing Intelligence, developer and manufacturer of measurement instruments and software. Wissner’s background is in quality engineering, product life-cycle management, and laboratory management. Wissner joined Hexagon Manufacturing Intelligence in 2000 as an applications engineer, and currently provides new product training and communications for the Hexagon Manufacturing Intelligence software division.