Why use surface finish charts? Reason three

Ra and other surface finish units

A type of metal surface finish chart may describe surface finish parameters, that is, different units of measurement and arithmetic calculations used to describe surface finish. Such a surface finish chart may include the following parameters:

The most commonly used parameter is the average roughness (Ra). This is the average length (or average height) between all the crests and troughs of the waves calculated from the average line of the surface. Because it can neutralize any obvious outfield point, Ra is insensitive to occasional spikes and grooves.

The root mean square roughness (RMS) is similar to Ra, but is generally considered to be an approximation and therefore less accurate than Ra. RMS is calculated using an algorithm that looks for the square root of the square mean of values. Basically, RMS converts the surface profile into a sine wave and measures the average deviation of the curve from the average line.

Maximum roughness depth (Rmax) measures the vertical distance from the highest peak to the lowest trough within the sampling length and selects the maximum value of the measurement. As a highly sensitive method for estimating surface finish, Rmax is susceptible to burrs or scratches that result in higher readings indicating a rougher overall surface.

Mean roughness (Rz) is calculated by averaging the heights of the five highest peaks and the depths of the five lowest valleys. Because Rz only looks at extreme cases, the values it generates tend to be high and may not accurately reflect average surface finish.

In metal cutting, we usually test Ra, which is a requirement of most of our customers. Although Ra and RMS are sometimes used interchangeably, as the approximate conversion factor is RMS = Ra x 1.11, customers are encouraged to use Ra. It is considered more accurate and is widely accepted in the industry.

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