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Chapter 11: E. Specialized Inspection Equipment 99
Hinge
Surface
texture
Stylus
Skid
Drive unit
(a)
Stylus
Drive unit
Surface
texture
(b)
Reference
surface
Part II.E.2
Figure 11.2
(a) Skid-type or average surface finish measuring gage, (b) skidless or profiling gage.
Reprinted with permission of the Society of Manufacturing Engineers, Manufacturing Processes and
Materials, 4th edition, copyright 2000.
rides the work surface in close proximity to a fairly large skid that also contacts
the work surface. The skid-type instruments usually have inductive transducers
and are used predominantly for averaging measurements of surface roughness,
but not waviness. The skid filters out waviness. Most portable or handheld
instruments are the skid type and they are reasonably accurate for roughness
measurements in the range of 0.30–0.51 mm (12–20 min.) R a .
The skidless type of instrument illustrated in Figure 11.2b has a built-in reference
surface that permits the probe to sense both long- and short-wavelength
variations in surface conditions. Thus, these can be used to measure both waviness
and roughness, as well as surface inclination (straightness). These instruments
are often referred to as profiling gages and they usually generate a profile
chart on paper or on a computer screen. The international standard for the assessment
of surface texture, ISO/R468, defines three parameters: R a (CLA), R z , and
R max , all measured relative to a straight mean line:
1. R a (centerline average) value is the arithmetic mean of the departures
of a profile from the mean line. It is normally determined as the mean
result of several consecutive sample lengths L.
2. R z (10-point height) is the average distance between the five height
peaks and five deepest valleys within the sampling length and
measured perpendicular to it.
3. R max is the maximum peak-to-valley height within the sampling
length.