The most commonly used—and perhaps the simplest—measure of surface finish is the Ra parameter, or roughness average. But 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 (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.
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 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.
Under high magnification, some workpieces appear to have wavy surfaces of very short wavelength. 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. Surfaces of this type can cause problems for gages with button-type skids that can “ride” the waves. 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. Thus, the contact’s vertical travel is measured against a 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 are used on the probe itself. Basically there are three different radii that are commonly available today: 2 µm/80 µ", 5 µm/0.0002" or 10 µm/0.0004", all on a 90-degree conical cone. In Europe and throughout much of the world, the 2-µm/80-µ" probe is the most common. In the United States, many applications call for the 5-µm/0.0002" probe, although for rougher surfaces a 10-µm/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 the required gaging force. Too much probe force with a small-radius probe can scratch the surface. For a 2-µm/80-µ" probe, a 0.7-mN force is typically used. For a 5-µm/0.0002" or a 10-µm/0.0004" radius, a 15-mN force is standard.
For gaging sharp edges or small ODs where the probe is aligned with the axis of traverse (in 180 degrees, or closed position), a “chisel” probe is used. This incorporates a 90-degree sapphire chisel with a 10-µm/0.0004" radius. The chisel is 1.3-mm/0.050" wide to allow easy alignment to the fine edge or small diameter being measured.
A similar stylus uses the same chisel, but is designed for probes that are being used perpendicular—either at 90 degrees or 270 degrees—to the part. It is also used for gaging sharp edges or ODs smaller than 6.35 mm/0.25".
Most standard size probes are good for IDs down to 7.9 mm/0.312". For smaller holes, special “small-hole probes” can be used. These allow gaging IDs as small as 3.2 mm/0.125" and up to a depth of 19 mm/0.75".
For measuring the bottoms of grooves, recesses and small holes, a “groove bottom” probe is used. These reach depths of 6.35 mm/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.