Surface roughness is a measure of how smooth (or rough) a machined surface is.
It is also called "surface finish."
Viewed closely, a machined surface has tiny peaks and valleys; surface roughness expresses their size numerically.
The level required varies with the application.
A Closer Look
Surface roughness affects both the function and the appearance of a product.
Parts that need fluid to flow smoothly, parts that must slide freely, and parts that need a premium appearance all require a smooth surface (good surface roughness).
Surface roughness depends on feed, cutting speed, the tool's nose radius, tool wear, built-up edge, and more.
In general, a smaller feed, a suitable nose radius, and prevention of built-up edge produce a smooth surface.
Where a still higher surface quality is required, a finishing operation such as grinding may be added.