There are 3 types of fit; clearance, transition and interference. For filing, the piece of metal would need to be made of bar stock and you would work to tolerances that are found in a zeus book (every engineer should have one of these). There are 4 grades of file; rough, *******, second cut and smooth. The general rule is, the more material that is required to be removed, the rougher the grade of file you would use. The rougher grades remove metal faster, where surface finish and accuracy is unimportant. You would finish your datum with a smooth file, first by cross filing, then by draw filing (this provides a better surface finish and removes bumps caused by cross filing). If you require a better surface finish, you would lap the face using a fine grit lapping compound, which removes any high spots caused from filing. This creates a flat, smooth finish.Scraping is a process usually used for finishing machine ways. Parts of the ways are scraped away to allow oil retention which helps to prevent seizure on moving mating parts.