When an engineer includes a surface finish spec on a print, the intent is usually not just to make the part look good. Surface finish affects how a part will fit, wear, reflect light, transmit heat, distribute lubrication, and accept coatings.
There are two basic principles of operation for probes used to measure surface finish— skidded and non-skidded. With "skidded" probes, the sensitive, diamond-tipped stylus is supported by a metal skid that rests on the workpiece. Thus, the workpiece itself is used as the reference surface.
Gage calibration is a routine process followed by most users of precision gages. How regular a gage needs to be checked and its performance documented is usually defined step by step in a documentation procedure.
As the manufacturing of parts continues to become internationalized, it is not uncommon for different standards developed in various countries to become blended into new international standards. And once new international standards become available it is expected that the world will embrace them and begin using them.
Surface roughness measurement of valve seats on cylinder heads is a challenging application. The land areas are short in length and the roughness values are typically high. Normally valve seats require very basic and simple roughness parameter analysis, typically done by a skidded measuring system.
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.