How to choose and use styli

Source：By Dennis Bobo, Senior Applicati Release Date：2012-08-06 443

Metalworking

When collecting measurement data on machine tools the selection and uss of stylus becomes critically important.

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 – such as kinematic, strain gauge, scanning – 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, and it is easy to lose as much as 10 percent of a CMM's 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, making 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, such as the SP25M CMM probe, 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 accumNike Converse Shoes