compiled & edited by Bernard Martin
RMS and Ra are based on different methods of calculating the roughness. Both are done with a profilometer, but the profilometer calculates the roughness differently for Ra and RMS.
Ra is the arithmetic average of surface heights measured across a surface. Simply average the height across the microscopic peaks and valleys.
Ra and RMS are both representations of surface roughness, but each is calculated differently.
Ra is calculated as the Roughness Average of a surfaces measured microscopic peaks and valleys.
RMS is calculated as the Root Mean Square of a surfaces measured microscopic peaks and valleys. Each value uses the same individual height measurements of the surfaces peaks and valleys, but uses the measurements in a different formula.
A single large peak or flaw within the microscopic surface texture will effect (raise) the RMS value more than the Ra value, which is why Ra is more commonly used today as a measurement.
RMS: What is Root Mean Squared?
Ra roughness average is the main height as calculated over the entire measured length or area. It is quoted in micrometers or micro-inches. For 2 dimensional computation: Ra = 1/n * SUM(ABS[Zi-Zmean] from i = 1 to n
RMS is a Root Mean Squared calculation. That means you:
The Root Mean Square (RMS) average is precisely that: the square root of the average height deviations from the mean line/surface squared.
RMS = SQRT[ 1/n* SUM(Zi-Zmean)^2 ] from i=1 to n
Technical Support Blog
At Next Generation Tool we often run into many of the same technical questions from different customers. This section should answer many of your most common questions.
We set up this special blog for the most commonly asked questions and machinist data tables for your easy reference.
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Our technical section is written by several different people. Sometimes, it's from our team here at Next Generation Tooling & at other times it's by one of the innovative manufacturer's we represent in California and Nevada.