Frequently Asked Questions

A CMM stylus (also called a CMM probe or touch probe) is a precision measurement component used with Coordinate Measuring Machines (CMMs). It consists of a ball tip (typically ruby), a stem (ceramic, steel, or carbon fiber), and a threaded mounting base. The stylus physically contacts the workpiece surface, allowing the CMM to capture precise 3D coordinate data for dimensional inspection and quality control.

CMM styli typically use a genuine ruby ball for the contact tip due to its extreme hardness (Mohs 9), low friction coefficient, and excellent wear resistance. The stem is available in three common materials: tungsten carbide (rigid, for short styli), ceramic (lightweight, good stiffness-to-weight ratio), and carbon fiber (ultra-lightweight, minimal bending deflection for long extensions). The mounting base is usually stainless steel with M2, M3, M4, or M5 threads.

Selecting the right CMM stylus depends on four key factors: (1) Ball diameter — larger balls bridge surface roughness better, smaller balls access tight features; (2) Stem length — use the shortest stylus that reaches the measurement point to minimize deflection; (3) Stem material — ceramic for general use, carbon fiber for lengths over 50mm, tungsten carbide for very short rigid styli; (4) Thread size — must match your probe head (M2, M3, M4, or M5). Always ensure the stylus is properly qualified on your CMM before use.

Ceramic stems offer excellent rigidity and thermal stability at an economical price, making them ideal for standard-length styli (up to 50mm). Carbon fiber stems are significantly lighter (about 40% lighter than ceramic) with superior stiffness-to-weight ratio, which reduces bending deflection and measurement uncertainty on long styli (50mm+). Carbon fiber also has excellent vibration damping characteristics.

CMM styli should be replaced when you notice visible scratches or flats on the ruby ball, a measurable change in qualification results, a bent or damaged stem, or a loose ball-to-stem connection. Under normal use with proper contact force settings, a quality ruby ball stylus can last 6-12 months or longer. High-volume production inspection may require replacement every 2-3 months.

CMM styli are available in standard metric thread sizes: M2 (2mm, common on small/tactile probes), M3 (3mm, the most widely used standard for Renishaw and most mainstream CMM probe heads), M4 (4mm, used on some larger probe systems), and M5 (5mm, used on heavy-duty or special application probes).

CMM styli with standard M3 threading are compatible with the vast majority of CMM probe heads including Renishaw TP20, TP200, SP25, TP2, and equivalent systems from Hexagon, Zeiss, Mitutoyo, and Wenzel. Always verify: (1) thread size matches your probe head, (2) stylus length and ball diameter are within your probe head qualification range, and (3) total stylus weight does not exceed the probe head maximum load specification.

A star stylus (also called a multi-tip or cluster stylus) features multiple ruby ball tips arranged in a radial star pattern — typically 3, 4, or 5 tips at different angles. Star styli are used when a single-tip stylus cannot access all measurement surfaces without re-qualifying or re-orienting the probe head. They are ideal for measuring inside bores, undercuts, dovetails, and complex geometries, reducing cycle time by eliminating multiple stylus changes.