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Glossary

Ra vs Rz - Surface roughness

Average Roughness (Ra) and Mean Roughness depth (Rz) are two different but associated methods to measure roughness. Roughness on a machined surface results from the machining process itself. Type of tool, speed of the tool, machine geometer, and the conditions of the machining environment can all cause roughness. Ra is calculated using an averaging process. Taking all the peaks and valleys across a surface, and neutralizing extreme outliers, the Ra algorithm is perfect for watching the texture of a surface and guaranteeing consistency in measurement. Rz, on the other hand, averages the 5 highest and 5 lowest peaks on a surface. Rz calculations are done through 3 different equations, and they generally are more impacted by extreme outliers. There are ways in which to convert Ra surface roughness measurements to Rz surface roughness measurement, dependent upon the acceptance criteria used by the manufacturer.

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