THE TIP OF THE SPEAR: HOW TO CHOOSE AND USE STYLI

When collecting measurement data with a probe, the selection and use of the stylus becomes critically important because of the proliferation of probes in manufacturing – on machine tools, CMMs, portable arms – and the many different types of probes available, such as kinematic, strain gauge, scanning, etc. Here are some core points that you need to know before making that investment.

In military terms, the stylus is the “tip of the spear” when collecting measurement data with a probe, and with the proliferation of probes in manufacturing – on machine tools, CMMs, portable arms – and the many different types of probes (kinematic, strain gauge, scanning, etc.) selection and use of the stylus becomes critically important. This topic deserves more depth and detail than we can give in this space, but these core points provide an overview.

For traditional kinematic inspection probes used on machine tools, ceramic stems and ruby ball styli are the first choice. For CMMs, steel and tungsten carbide are fine for shorter stems, with ceramic or carbon fiber recommended for longer stems. Ceramic stems are lighter than tungsten carbide, have stiffness comparable to steel, and are thermally stable – ideal for use in a machine tool environment. The ruby ball is exceptionally hard and smooth, with excellent compressive strength and resistance to abrasion.

Ruby balls are manufactured to various levels of precision defined by their grade, which relates to the maximum deviation of the ball from a perfect sphere. Any error in sphericity will increase CMM measurement uncertainty . . . in fact, it is easy to lose as much as 10 percent of CMM accuracy this way. The two most common ball specifications are grade 5 and 10 (the lower number is better). The more accurate the CMM is, the more significant the effect of ball grade. We recommend standardizing on grade 5, and offer grade 3 (with sphericity of just 0.08 micron) for the most demanding applications.

For strain gauge probes, particularly those used on machine tools, carbon fiber styli are recommended. Carbon fiber styli, whether hollow or solid, have low mass that makes them best suited for use with sensitive strain gauge probes. Carbon fiber is the most common material for long styli and extensions, because it is stiff, light and thermally stable. Where metals are needed – for joints, knuckles, etc. – titanium provides the best combination of light weight, strength, stability and density.

Styli for portable arm CMMs require robust design and construction. They utilize Grade 5 Zirconia balls that are highly fracture resistant. The balls are bonded to the high strength tungsten carbide stem with impact resistant adhesive, and special construction techniques have been incorporated to ensure the joint is extremely rigid and virtually indestructible.

Scanning probes introduce other variables that affect the choice of ball materials. Scanning produces a more aggressive type of surface interaction than touch probing, resulting in three interactive phenomena: debris accumulation, adhesive wear and abrasive wear.

Debris is practically unavoidable, and is independent of the stylus ball or part surface material. It can be removed with a dry lint-free cloth. Adhesive wear involves the permanent transfer of material from one surface to the other through local welding or transfer of minute particles from one surface to the other. Adhesion of material will eventually degrade the form of the stylus ball and compromise measuring results. Abrasive wear involves the removal of material from the ball and part surfaces.

Based on extensive testing in scanning applications, ruby is the best ball material for most applications, including stainless steel and titanium, but it can suffer adhesive wear on aluminum under extreme conditions. Silicon nitride is a substitute for ruby in extreme aluminum applications, but can suffer abrasive wear on stainless steel or cast iron. Zirconia is the best choice for scanning cast iron parts, though tungsten carbide performs well also.

A few simple rules for stylus use can maximize accuracy for most probing applications:

• Keep styli short and stiff – The minimum stylus length will give optimum results. A longer stylus amplifies errors and introduces the potential for excessive bending.

• Minimize the number of joints – Joints and extensions introduce potential bending and deflection points.

• Keep the stem diameter as large as possible – Thicker stems increase the stiffness of the stylus. If a stem with small diameter is required, try to use an M4 extension with only a short stylus of the smaller diameter stem on the end.

• Keep the stylus ball as large as possible – This maximizes the ball/stem clearance, reducing the chance of false triggers caused by “shanking out” on the stylus stem, and increasing the effective working length. A larger ball also allows a larger stem diameter, increasing the stiffness of the stylus. Finally, the larger ruby ball reduces the influence of a part’s surface finish on measurement results.