Surface roughness is divided into Ra, Rz, Ry. The three most popular surface roughness parameters and their values are:
(Ra) - arithmetical mean deviation of the profile;
(Rz) - the point height of irregularities;
(Ry) - maximum height of the profile.
Ra-- in the sampling length L within the outline of the arithmetic mean of the absolute value of the offset
Rz-- within the sampling length and average value of the five largest profile peak height and the average of the five largest outline of a deep valley.
Ry-- between sampling length L within peak contour lines and contour line from the bottom.
 
The surface roughness, a smaller pitch and the tiny valley irregularities on machined surface. The distance between two peaks or two troughs (the pitch) is small (less than 1mm), it is indistinguishable with the naked eye. Therefore, it is the micro geometry error. The smaller surface roughness is, the more smooth the surface is. The surface roughness of size, have a great impact on the performance of mechanical parts, mainly in the following areas:
① wear of the parts. The rougher the surface, the smaller the effective contact area between the mating surfaces, the greater the pressure, the faster the wear.
② mating stability properties. For clearance fit, the rougher the surface, the easier to wear, so that the gap increases during operation; interference fit, because the assembly will squeeze microscopic convex peak level, reducing the actual effective interference, reduce the coupling strength.
③ the fatigue strength of parts. There is a big trough on the surface of rough parts, just like sharp corners, gaps and cracks, they are very sensitive to stress concentration, thus affecting the fatigue strength of parts.
④ corrosion resistance of parts. Rough surface will make corrosive gas or liquid to penetrate through the surface of microscopic metal inner valley, causing surface erosion.
⑤ the seal of the parts. Unable tight fit between the rough surface, gas or liquid will leakage between the contact surfaces.
Furthermore, the appearance and the measurement accuracy will be also influenced by surface roughness.