Keeping the Renishaw TP20 and TP200 in fighting trim
Even though contact “drag” scanning and noncontact laser scanning are prevalent in our industry, there are many manufacturers who still use kinematic touch trigger probes. Questions often come up regarding when a probe should be replaced. Is there a best practice for troubleshooting?
Of course, touch trigger probes are like any probe—mechanical and subject to damage through accidental means or just the daily wear and tear. In order to determine the best time to replace them, we should first understand how these probes work.
The Renishaw TP20 is a kinematic resistive probe. This means the TP20 has a spring and mechanical contacts inside the module consisting of three rods sitting against three pairs of spheres, which provide a total of six points of contact. Once the stylus touches the surface of the part, the force is applied to the spring, and the mechanical contacts become separated. As the separation increases, so does the electrical resistance. The probe’s body monitors the build-up of resistance and senses the change, resulting in a trigger.
The Renishaw TP200 uses strain gauge technology—i.e., micro-strands of silicon mounted within the probe—which is quite different to its resistive cousin the TP20. Although the TP200 still uses a kinematic mechanism inside the module to hold the stylus and protect it from over-travel, the body contains micro-silicon strain gauges that are able to detect when a force is applied. The strain gauges are able to consistently sense low force in any direction; therefore, they do not exhibit the lobing characteristics of the resistive probes. However, because they do trigger at low force, the TP200 is prone to trigger from machine vibrations.
Before any troubleshooting is done, clean the magnets between the body and module.
TP20: The internal mechanics of the probe module is prone to a weakening of the spring tension, which will cause an increasing amount of false triggers during the machine’s traverse. Another indicator of a worn module is that the six-point seating will appear to be sticking and needs the user to assist it in resetting the stylus. Users may also notice a decrease in the accuracy during probe qualifications.
TP200: This probe’s body must be handled carefully. You can observe the LED for inconsistent flickering or no light at all. Once the probe modules begin to wear, the user will notice no points being taken at all. Of course, you may notice a loss of accuracy as well.
The only test I can recommend is to test by substituting the components. To that point, it is in your best interest to keep a master set consisting of a probe body and a module in order to carry out testing.
With both probes, it’s important to monitor and track their consistency. You can do that with simple repeatable tests like the ISO 10360-5 sphere center test or a single-point repeatability test. Whichever test you choose, it’s important to track your data. We’ve seen TP20 and TP200 probes that have surpassed their advertised milestones without showing signs of wear, and we’ve had probes that came nowhere close in durability. If you perform these tests, common sense will be your guide.