How to check CMM touch probe accuracy

Testing CMM probe accuracy - a sanity check

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? It is the complex device you put on the business end of your CMM, with many internal components that you can’t see which 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 which 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, like a qualification sphere or ring gage.

For instance, if your CMM was calibrated to the B89 specification, you’ll have one number for repeatability. 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. 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, to measure that same sphere with at least 25 points and our rows with the iso-cert-smallsame A0B0 (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 is 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 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/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, re-run 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 non-interchangeable modules, like a TP-2 or HP-T Probe, first check the connection to the probe head is tight, then confirm it is not sticking (when triggered) or excessively leaking oil. You can also check and adjust the trigger force with a strain gage, but if either of the two previous symptoms (sticking or leaking) is present, you’re best advised to replace the probe and move on.

HP-TM for probes with interchangeable magnetic modules, like a Renishaw TP-20, 200, SP-25 or HP-TM Probe, 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, 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 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 backward 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, like adding more tip angles and measuring the sphere with more points and so on to achieve your desired level of confidence.