VDI 3400 Surface Finish, EDM surface finish, Vdi surface Finish
VDI Surface Finish – is a scale often used by German machine manufacturers and quite widely used in Spark and Wire Erosion (Verein Deutscher Ingenieure, the Society of German Engineers)
N Surface Finish – New ISO (grade) scale numbers ISO 1302.
Ra is calculated by an algorithm that measures the average length between the peaks and valleys and the deviation from the mean line on the entire surface within the sampling length. Ra averages all peaks and valleys of the roughness profile and then neutralizes the few outlying points so that the extreme points have no significant impact on the final results.
Rz is calculated by measuring the vertical distance from the highest peak to the lowest valley within five sampling lengths, then averaging these distances. Rz averages only the five highest peaks and the five deepest valleys—therefore, extremes have a much greater influence on the final value.”