Skidded vs Skidless Stylus

Reprinted with permission from The Surface Texture Answer Book.

Contact surface texture instruments can be skidded or skidless. A skidded instrument uses a probe with a radiused skid and a sharp stylus, both of which contact the surface. During a measurement the skid follows the general shape of the surface while the stylus detects the short wavelength roughness. This arrangement filters out the longer wavelength form and waviness because the movement of the stylus is always relative to the skid. It also protects the stylus from damage. The skidded system is commonly used with shop-floor instruments because of the protection it provides for the stylus, and because it is less sensitive to vibration and misalignment. It is also common on lower-priced devices.

Skidded surface texture measurement probe. . Courtesy The Surface Texture Answer Book.

A skidless probe only contacts the workpiece with the sharp stylus. An internal precision datum surface provides the reference for the stylus motion, rather than the skid. Therefore a skidless measurement captures all of the wavelengths of the surface—form, waviness, and roughness—and gives a truer representation of the surface. Skidless systems typically leave the stylus exposed and are more prone to experiencing damage to the stylus tip. Thus, they tend to be more appropriate for laboratory conditions. Skidless systems are typically more sensitive to vibration and workpiece alignment, and also tend to be more expensive.

Probe and stylus of a skidless system. Courtesy The Surface Texture Answer Book.

The choice of which system to use must primarily be driven by the characteristic being measured. If form or waviness are of interest, the skidless probe is the only option. If roughness data is the only requirement, then a skidded instrument may work. The environment in which the system is to be used is also a consideration—the rigors of shop floor measurements make skidded instruments very desirable—but this must be secondary to the needs of the product or process. Both skidded and skidless systems can create artifacts in the data.