Skids and Probes for Surface Finish

Skids and Probes for Surface Finish
George Schuetz, Mahr Federal Inc.

The most commonly used—and perhaps the simplest measure of surface finish—is the Ra parameter, or roughness average. However, like ordering a coffee at one of today's deluxe cafes, it's not exactly simple. There are other things to consider in order to ensure the best results. For example, Ra can be measured with two types of contact gages, which are distinguished by the nature of the probe or contact that traverses 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. These skidded gages use the workpiece itself as the reference surface. This is a relatively simple, inexpensive approach to surface measurement.

More complex skidless gages use an internal precision surface as a reference. This enables skidless gages to be used for the measurement of waviness and form parameters, in addition to surface roughness.

With skidded gages, it is important to understand how the design of the skid itself may affect your measurements. 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 may 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 better suited. But surfaces of this type can cause problems for gages with button type skids which can "ride" the waves. As shown in Figure 1, 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 produces 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 Figure 2. Thus, the contact's vertical travel is measured against a far more constant reference height.

On the other hand, because probes with donut-type skids require substantial structure ahead of the stylus, they cannot reach certain features, such as surfaces next to shoulders. For this reason there are a number of specifically designed probes to meet virtually every application.

It is also important to decide which probe to use, or more specifically, which radii for the diamond stylus that is used on the probe itself. Basically there are three different radii that are commonly available today: 2µm/80µ", 5µm/0.0002" or 10µm/0.0004", all on a 90˚ conical cone. In Europe and throughout much of the world, the 2µm/80µ" probe is the most common. In the U.S., many applications call for the 5µm/0.0002" probe, although for rougher surfaces a 10µm/0.0004" probe may also be specified.

Probe radius is a big determinant of contact force. The smaller the radius of the probe, the lighter gaging force required. Too much probe force with a small radius probe can scratch the surface. For a 2µm/80µ" probe, a 0.7mN force is typically used. For a 5µm/0.0002" or a 10µm/0.0004" radius, a 15mN force is standard.

For gaging sharp edges such as knife/razor edges, or small ODs where the probe is aligned with the axis of traverse (in 180°, or closed position), a "chisel" probe is used, as shown in Figure 3. This incorporates a 90° sapphire chisel having a 10µm/0.0004" radius. The chisel is 1.3mm/0.050" wide to allow for easy alignment to the fine edge or small diameter being measured.

A similar stylus (Figure 4) uses the same chisel, but is designed for probes that are being used perpendicular—either at 90° or 270°—to the part. It is also used for gaging sharp edges or ODs smaller than 6.35mm/0.25".

Most standard size probes are good for inside diameters down to 7.9mm/0.312". For smaller holes, special "small hole probes" can be used as shown in Figure 5. These allow gaging IDs as small as 3.2mm/0.125" and up to a depth of 19mm/0.75".

For measuring the bottoms of grooves, recesses and small holes, a "groove bottom" probe is shown in Figure 6. These can get to depths of 6.35mm/0.25" and also allow for measuring short lands and shoulders.

So, if you are having difficulty with a roughness measurement or you have an unusually placed surface to measure, a different skid or a special probe may be required to get the results you need.