Surface Finish

Surface Finish When manufacturing components that fit together with mating faces it is not only size and form that can affect the function, the surface finish is also a factor which must be considered. If the surface finish is poor it will cause wear very rapidly and effect the function of the component. A good finish however will allow it to function correctly and prolong the life of the component. Surface finish is effected by a number of factors such as cutting speed, feed rate, type of cutter, vibration of machine and spindle or slide wear and varies according to the process.

Measuring Surface finish If a machined component is examined under a powerful magnifier the surface will have the appearance of peaks and troughs in the plane along which it was machined. The depth, height and width of these peaks and troughs is measured over a given length and an average is calculated this is known as the Ra Value (arithmetic average of the peaks and troughs) The diagram opposite shows this and how it is calculated

Measuring Surface finish The measurement of the peaks and troughs on the surface is measured using a machine called a surface roughness tester, usually with a diamond probe/stylus. The movement of the probe up and down as it moves across the surface is recorded electronically by the machine and the Ra value is then calculated from these readings. The diagrams opposite show a typical probe used to measure surface roughness

Surface Roughness Comparators As surface roughness testers are relatively expensive it is not always practical to own and use one therefore alternative methods are required. One other method is the use of surface comparison plates. These consist of a series of plates machined using different processes and calibrated using a surface roughness tester to indicate the roughness value. These are held against the component for visual comparison or feel by running a finger nail across the plate and the surface to compare the finish.