Name: Surface Roughness Ra
Creator: MINATA

Question 1

What is Ra?

Accepted Answer

Ra is the arithmetic mean roughness of the surface profile, in μm; the smaller the Ra, the smoother the surface.

Question 2

How do the old ▽▽▽ triangle marks map to Ra?

Accepted Answer

The old triangle marks (more triangles = finer) are now replaced by the Ra value; see the corresponding grade in the table.

Question 3

Should I always specify the smallest Ra?

Accepted Answer

No. A smaller Ra costs more machining; specify fine finishes only on mating, sealing or fatigue-critical surfaces and leave the rest at a reasonable level.