Question 1

When should I specify Ra versus Rz on drawings?

Accepted Answer

Ra is the global default and suitable for most applications. Specify Rz when surface function depends on peak heights (e.g., sealing surfaces, paint adhesion), because Ra can hide occasional deep scratches that Rz would capture. In automotive and German engineering practice (VDA), Rz is more commonly specified than Ra.

Question 2

What surface roughness can each machining process achieve?

Accepted Answer

Typical achievable Ra values: Lapping/polishing 0.012-0.05 μm, Grinding 0.1-1.6 μm, Turning (finish) 0.4-3.2 μm, Milling (finish) 0.8-3.2 μm, Drilling 1.6-6.3 μm, Sawing 6.3-25 μm. These are achievable ranges — actual results depend on tool condition, rigidity, and cutting parameters.