Surface Finish - 4: Predicting Surface Finishes WARNING: Machine tools present a safety hazard. Improper operation can result in severe injury. These topics are for non-laboratory study only and are not to be used in conjunction with the operation of any tool or machine described herein. Never use a machine tool without the supervision of a qualified instructor. A surface finish can be approximately predicted using common sense. Grinding will yield a better surface finish than milling and milling will produce a better surface finish than sawing. However, when more exact projections are required a few formulas can be applied. For a milling operation the following formula will produce the surface finish for a specific set of circumstances: Ra is the surface finish in microinches r is the radius of the tool nose in inches f is the feed rate in IPR for one tooth. For example: A milling cutter with a 1/16" tool nose radius running at .010 inches per revolution will produce, in theory, a surface finish value Ra of 100. For the lathe a similar formula can be applied: Ra is the surface finish in microinches r is the radius of the tool nose in inches f is the feed rate in IPR for the lathe bit For example: A lathe cutting tool with a 1/16" tool nose radius running at .010 inches per revolution will produce, in theory, a surface finish value Ra of 67. Note: The above calculations are highly theoretical. Finish is also effected by RPM, lubrication, temperature, and other considerations. label: surface finish, finish, symbols, ANSI/ASME B46.1, forumlas, measuring, costs