Engineering Resource

What Ra means

Ra is the arithmetic average roughness of a surface profile — effectively the average height of its peaks and valleys in micrometres. A lower Ra means a smoother surface. It says nothing about waviness or surface defects, which is why a good finish is specified alongside other requirements, not instead of them.

What determines the finish

• Tooling: a sharp, well-geometry cutter produces a better finish than a worn one.
• Parameters: lower feed per tooth generally smooths the surface while risking heat.
• Material: aluminium finishes easily; stainless, titanium and plastics each behave differently.
• Stability: a part or a setup that vibrates will show chatter marks no parameter change can hide.

Specifying finish sensibly

Put a roughness value only where the function needs it — sealing faces, sliding surfaces, optical seats, and surfaces where friction or cleanliness matters. A global fine-finish requirement costs money on every face while improving the few that matter.

Achieving and verifying

Surface finish down to Ra 0.2 µm is achievable. Where a drawing specifies a value, it is measured on the machine with a surface instrument rather than assumed from the cut. Scratches and handling marks are controlled by packaging and process discipline, not by the roughness parameter alone.