Why do cores get lapped?   What is the surface finish?
Lapping is an additional production process used to improve the mating surface. It Is typically done on cores with material permeability of 5000 and greater in order to achieve the maximum AL value for a given material. A mirror-like finish is the result. The surface finish for a normally flat-ground surface is 0.5 to 1.0 microns and for a lapped core is 0.1 to 0.2 microns