Stylus Profilometry

A stylus profilometer (stylus profiler) determines the shape and roughness of a surface by dragging a fine diamond tip across the sample. The 2D measurement trace can be used to calculate various surface roughness and waviness parameters, with high precision.

Stylus profilometers vary from small, handheld gages to large, highly automated systems for production measurement.

Stylus Geometry

Stylus tips come in several shapes and sizes. For a common, conical stylus, two, five, and ten micron radii are the most widely available tips. For the majority of surfaces a 2 µm tip radius is used. The included tip angle is also important. A 90° tip angle has historically been the most common, but tips with a 60° angle can be more durable and are now more prevalent, particularly for the 2 µm radius.

A “skidless” stylus tip. Courtesy The Surface Texture Answer Book.

Skidded vs Skidless

A stylus based instrument can be “skidded” or “skidless.” A skidded instrument, as in uses a probe with a radiused skid and a sharp stylus, both of which contact the surface. During a measurement the skid follows the shape of the surface while the stylus detects the smaller roughness features. This arrangement filters out the longer wavelength shape and waviness because the movement of the stylus is always relative to the skid. It also protects the stylus from damage. Skidded systems are 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.

A skidded stylus. 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, however, 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.