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What is the difference between roughness, waviness and form?

We analyse three main elements of surface texture - roughness, waviness and form.

Roughness ? this is usually the process marks or witness marks produced by the action of the cutting tool or machining process, but may include other factors such as the structure of the material. 

Waviness ? this is usually produced by instabilities in the machining process, such as an imbalance in a grinding wheel, or by deliberate actions in the machining process. Waviness has a longer wavelength than roughness which is superimposed on the waviness. 

Form ? this is the general shape of the surface, ignoring variations due to roughness and waviness. Deviations from the desired form can be caused by many factors. For example: 

  • the part being held too firmly or not firmly enough
  • inaccuracies of slides or guideways of machines
  • stress patterns in the component
Surface, Waviness and Form are rarely found in isolation. Most surfaces are a combination of all three and it is usual to assess them separately. 

Please note: There is no set point at which roughness becomes waviness or vice versa as this depends on the size and nature of the application. For example, the waviness element on an optical lens may be considered as roughness on an automotive part.