Engineering Resource
Surface finish is easy to ask for and easy to over-ask for. This guide explains what the number means, what drives it and where it actually earns its cost.
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 on metals, ceramics and graphite — not on plastics. 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.
Allowance for surface treatment
Anodizing, plating and coating build material onto a surface. That thickness has to be planned for before the part is machined, not discovered afterwards — and it is one of the most common reasons a finished part lands outside the drawing tolerance.
There are two ways to handle it, and which one applies depends on the finish and on how tight your tolerance is:
- Machining allowance on the drawing — the part is machined undersize by the finishing thickness, and the finisher brings it back to the drawing dimension. Your supplier has to know the allowance before cutting.
- Finished dimension on the drawing — the part is machined to the final size and the finish is applied on top. The drawing then has to carry the coating thickness explicitly, because the delivered part is larger than the machined one.
We confirm which route applies to your specific finish and part during the DFM review, and state it in the quote — so nothing about the finished dimension is left to guesswork between you and the finisher.