A process for precision machining of e.g. sliding faces. Unlike grinding, lapping is carried out with abrasive particles loosely distributed in a paste or a liquid, with a film of this lapping suspension located between the workpiece and a shaping counterpart (e.g. a lapping disc). Preferred types of lapping grain are silicon carbide, boron carbide or diamond in grain sizes from 2 to 100 mm. The lapping liquid is available as practice-proven mixtures (e. g. petroleum and lubricating oil in the ratio 1:1). For optimum stock removal it is necessary to coordinate the workpiece, abrasive, lubricant and carrier material in a carefully matched system.

SEM photo of a lapped surface

A distinction is drawn between external cylindrical lapping, internal cylindrical lapping, spherical lapping and face lapping. The most important process for mechanical seals is face lapping performed on single-wheel flat lapping machines to obtain plane sliding and mating faces of utmost precision. DIN 4761 (December 1978) gives the following definition of the nature of surfaces produced by lapping: "A scored surface, … clusters of scores in random arrangement, … , curved, intersecting at irregular points."

Attainable roughness to DIN 4766 T1/T2: R_Z 0.04–10 mm R_a 0.006–1.65 mm sliding faces are usually prelapped to R_a 0.1–0.3 mm and then finished by polishing.