The Ra parameter is the most commonly used measurement for surface roughness.
Until recently, in fact, it was the only parameter recognized by ANSI, although new ANSI and ISO standards include many different parameters from which to choose. And while these additional parameters are useful in many applications to ensure or enhance functionality, Ra is still included in most specs as a good starting point and a basic benchmark of process consistency.
Ra can be measured with two types of contact gages. These gages are distinguished from one another by the nature of the probe or contact that traverses over the part's surface. In "skidded" gages, the sensitive, diamond-tipped contact or stylus is contained within a probe, which has a metal skid that rests on the workpiece. Thus, skidded gages use the workpiece itself as the reference surface. This is a relatively simple, inexpensive approach to surface measurement. Skidded gages sell for as little as $1600, and some are small enough to fit into a shirt pocket.
Skidless gages use an internal precision surface as a reference. This enables skidless gages to be used for measurements of waviness and form parameters, in addition to roughness. The drive unit is larger and more complex, and a computer is required to handle the complex algorithms for numerous parameters. Skidless gages are indispensable for complex surface analysis but, at a cost of ten to twenty times that of skidded systems, they are impractical if Ra is the only parameter required.
Getting back to skidded gages, it is important to look at the design of the skid itself. Some probes have a simple button-like skid, which may be located either in front of, or behind, the stylus. Others have a donut-shaped skid, with the stylus extending through the hole in the middle. In most applications, both types perform equally well, but occasionally, one or the other might be required to obtain accurate results.
Under high magnification, some workpieces appear to have wavy surfaces of very short wavelength; this is especially so of EDM parts. While the inclination may be to measure these surfaces using a waviness parameter, the pattern is really a tool mark, so a roughness parameter like Ra is required. Surfaces of this type may cause problems for gages with button-type skids. As shown in diagram "A", if the distance between the skid and the contact is roughly half the wavelength of the surface waviness, then the skid and contact will trade places at the tops and bottoms of the waves as the probe traverses the surface. This has the effect of nearly doubling the vertical travel of the contact relative to the reference, which will produce results that may be unreliable or non-repeatable.
The donut-type skid avoids this problem, because it remains at or near the tops of the waves as it traverses, as shown in diagram "B." Thus the contact's vertical travel is measured against a far more constant reference height.
But because probes with donut-type skids require substantial structure ahead of the stylus, they cannot reach certain features, such as surface next to shoulders. Probes with button skids mounted behind the stylus require little or no leading structure, and thus have the advantage of increased access. Probes with button skids are even available to reach into groove bottoms several millimeters deep.
Some pocket-type roughness gages offer users the ability to switch probes. This can extend the capabilities of the gage, allowing the user to select a probe with a donut skid for use on short-wavelength EDMed surfaces, and a trailing-button skid for use where access is restricted.