Surface Finish - 4: Predicting Surface Finishes
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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.
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