Surface roughness Ra

Roughness Ra is the arithmetic mean of the surface profile deviations from the mean line, measured in micrometers (µm) with a profilometer. It is the most used finish parameter on drawings because it is simple to measure and compare across processes.

How it works

Each process has a typical range: regular milling sits around Ra 3.2 µm, finish turning Ra 1.6, grinding Ra 0.8 to 0.4 and lapping below 0.2. Asking for a lower Ra almost always requires more passes, another process or another machine, which raises cost. That is why finish should be called out only on functional surfaces (sealing, sliding, bearing seats), with a general value for the rest of the part.

Practical example

On a shaft, the bearing seat needs Ra 0.8 (ground), the seal area Ra 0.4 and the rest Ra 3.2 (turned). The drawing carries a general Ra 3.2 symbol near the title block and calls out the lower values only on the faces that need them.

Common mistakes

• Putting Ra 0.8 on the whole part just to be safe.
• Using old triangle symbols with no numeric value.
• Ignoring that heat treatment or coating changes the final finish.