Surface Finish - 5: Measuring Surfaces
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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.
There are two typical ways to measure, or gauge, a surface finish.

This first method is very inexpensive but is subject to the perceptions of the machinist. It is a subjective method. The machinist will use his finger and his eyes to compare his work piece to a representative sample. In this instance the machinist is comparing surface finish with a surface finish tablet that has various finish standards imbedded. The drawback of this method is that one machinist will feel that the surface finish is 16 and the next machinist will feel that the surface finish is 32 on a particular work piece.

This second method is more expensive as one needs to purchase a surface tracing machine to make the measurement. This machine automatically pulls a stylus across the surface and records roughness and waviness. Older surface tracing machines produce a printout that needs interpretation but the newer surface tracing machines use a digital display to tell you the surface finish value. Be aware of your surroundings as those machines are sensitive to the levelling of the shop floor and/or surface plate, surrounding vibrations (such as trucks driving by), and calibration.

There are a group of methods using optics that can range from subjective to objective depending on the method used. Many instruments are available to perform the surface measurement using optics. The instrument chosen depends on the industry where it is used. One of these instruments, the most basic one, works on the principle of interferometry. A polished glass surface is used that comes in contact with the work piece. By observing the variations of light bands reflected on the glass surface because of the imperfection of the finish, you are able to make a measurement of the surface finish as well as over all flatness.

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label: surface finish, finish, symbols, ANSI/ASME B46.1, forumlas, measuring, costs