Machined surface finishes are usually measured with "Surface Profilometers." But for any given Rotor surface you can use a Relative Machined Surface (RMS) measurement. For example:
· A # 125 finish = fingernail finish, your fingernail will drag on this finish.
· A # 63 finish = a smoother finish, your fingernail will not drag, however you would be able to detect lines in the finished surface.
With the introduction of Anti-Lock Braking Systems, brake rotor finish tolerances are more critical. Manufacturer's are now providing surface finish specifications. They will specify, for example, a brake rotor surface finish of 60 Ra or Less" where Ra equals "Roughness Average."