Lapping

http://beamsandbobbins.co.uk/product/thunderbolt-cloud-eco-friendly-bamboo-earrings Lapping is a machining process, in which two surfaces are rubbed together with an abrasive between them, by hand movement or by way of a machine.

inappreciatively Lapping can be used to obtain a specific surface roughness; it is also used to obtain very accurate surfaces, usually very flat surfaces. Surface roughness and surface flatness are two quite different concepts.