Surface Measurement Gaging Tips
Proper care of contact points is one of the basic considerations in gaging.
Many columns in these pages have talked about the importance of surface finish. What we haven't talked much about are the holes in surface finish gaging equipment.
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.
Computer aided design has dramatically facilitated the process of getting ideas into the marketplace (not to mention sending a lot of drafting boards and T-squares to the junk heap).
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.
In designing gages to make dimensional measurements in a production environment, speed and high precision are the names of the game.
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.
When an engineer includes a surface finish spec on a print, the intent is usually not just to make the part look good.
The irregularity of a machined surface is the result of the machining process including the choice of tool, feed and speed of the tool, machine geometry and environmental conditions.