Probe applications for machine tools and measurement systems come in many varieties. Although seemingly simple, a lot of science and design is used to produce the styli for these probes. Choosing the correct one for an application doesn’t have to be as complex, though.
A stylus is mounted to kinematic, strain gage connected to the scanning probes, and acts as a point of contact with the workpiece. For traditional kinematic inspection probes used on machine tools, ceramic stems and ruby ball styli are the first choice, according to “How to Choose and Use Styli.” Ceramic stems are lighter than tungsten carbide, have stiffness comparable with steel and are thermally stable—especially useful in a machining environment. Ruby balls are exceptionally hard and smooth, with compressive strength and resistance to abrasion. They are manufactured to various levels of precision defined by their grade, which relates to the maximum deviation of the ball from a perfect sphere. The two most commonly used ball grades are 5 and 10 (the lower the number, the better).
For high accuracy strain gage probes, particularly those used on machine tools, carbon fiber styli are recommended. These styli, whether hollow or solid, have low mass, making them best suited for sensitive strain gage probes.
The ruby ball is best for most scanning applications, including stainless steel and titanium workpieces, but it can suffer adhesive wear on aluminum under extreme conditions.
Straight styli are typically used for simple features where the probe directly contacts the part, but many alternatives are available for specialized applications, which you can read about here. This article also gives more detailed information about choosing the correct styli.