Stylus ball considerations

The ruby ball is most commonly used for the stylus tip. It is exceptionally hard and smooth, with excellent compressive strength and abrasion resistance. 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 measurement uncertainty, and it is possible to lose as much as 10% of a CMM’s accuracy as a result of this error. It is important to remember that the more accurate a CMM is the more significant the effect of ball grade. The two most common ball specifications are grade 5 and 10 (the lower number is better). Grade 5 (sphericity of 0.13 micron) is ideal for most applications.

For higher accuracy CMMs, a grade 3 ball (with sphericity of just 0.08 micron) is recommended. Styli equipped with these balls utilize a special manufacturing technique because research has shown the form of the ball can be distorted by drilling and gluing—the standard way of mounting the ball on the stem.

The high-accuracy stylus uses an undrilled ball, bonded to a spherical cup. The amount of glue applied is critical to ensure maximum bond strength. A certificate of roundness is supplied with each of these styli, and they are coated with titanium nitride (gold color) for easy identification.

Surface scanning with a probe results in a more aggressive type of interaction with the part that can ultimately affect the ball’s shape and accuracy, so it introduces a variable in the selection of ball materials.

Scanning produces 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. Debris can include metal oxide particles on the surface or airborne debris, such as coolant mist or paper dust. Debris 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. Adhered material cannot be removed with normal cleaning. Thus, as surface material from the workpiece adheres to the ball and remains in contact with the part surface, like materials will attract and further buildup can occur. Adhesion of material will eventually degrade the form of the stylus ball and compromise measuring results.