The three most important surface finish parameters and why should you should understand them.

Before we start talking about the three surface parameters it is important to understand these parameters exist in both English and Metric units.  In the English units they are typically referred to as a function of micro inches and Metric units are listed as micro meters.  This sometimes creates confusion as the unit stated in micro inches are typically much larger values than those listed in micro meters.

The first surface finish parameter commonly known to 90% of the individuals working with products is Ra or roughness average.  Ra is simply an average height of the peaks and valleys in a surface finish.  The smaller the number the smoother the surface.  Typically values of 5 Ra or less approach a mirror finish.

One of the lesser known parameters that is highly useful is Rz.  This is an indication of the maximum distance from the tallest peak to the lowest valley.  This parameter is often used to provide an aesthetic continuity to surfaces.  For example an Ra indication may give a general representation of how rough a surface should be, however you may also notice scratches within that surface.  An Rz parameter can eliminate the appearance of scratches in an otherwise acceptable finish.  That apparent scratch is just a large Rz value that appears on the surface.

Finally, if you are designing components that have sliding fits or require bearing properties, parameters like Rpk can provide an indication as to how the surface will hold up to pressure and wear.