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How do you measure the quality of a surface finish?

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Quick Answer

Measure the quality of surface finish using two types of measuring systems, skidded and skidless evaluations. The skidded method measures roughness only, whereas the skidless method measures the form, roughness and waviness of a surface.

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Full Answer

Skidded gages have sensitive diamond-tipped styli contained within a probe, which has a skid that rests on the work piece. Therefore, skidded gages use the work piece as the reference surface. Skidless gages can be used to measure the waviness, form and roughness of a finished surface because they use an internal precision surface as a reference.

The use of the diamond stylus profilometer is the most common method, in which the stylus is run perpendicular to the lay of the surface. The probe traces in a circular arc around a cylindrical surface or along a straight line on a flat surface, forming the sample measurement length.

The sampling length is the wavelength of the lowest frequency filter for analyzing the data, and the measurement length should be at least seven times longer than the sampling length and at least two times longer than the wavelength. The length of the data to use for analysis is the evaluation length. Usually, one sampling length is discarded from each end of the evaluation length. The profilometer can also make three-dimensional measurements by scanning over a two-dimensional area on the surface.

The analysis begins by filtering the raw data to remove very high-frequency data, allowing you to bring closer the roughness evaluation made using profilometers with varying stylus ball radius. Then use reference lines, fractals, digital methods or other techniques to separate the data into waviness, roughness and form. Finally, summarize the data using a graph. Alternatively, store the data on a computer and analyze it using techniques from signal analysis and statistics.

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