Surface Roughness (Ra)

Ra is the most widely used surface roughness parameter on aerospace drawings worldwide. European drawings express Ra in micrometers (Ra 1.6). American drawings express Ra in microinches (63 microinches). Both refer to the same surface: Ra 1.6 micrometers equals approximately 63 microinches (1.6 x 39.37 = 63).

Common Ra values on aerospace machined parts: Ra 0.4 (16 microinches) requires grinding or honing. Ra 0.8 (32 microinches) requires fine turning or grinding. Ra 1.6 (63 microinches) is achievable with standard CNC milling. Ra 3.2 (125 microinches) is standard machining. Ra 6.3 (250 microinches) is rough machining.

Other roughness parameters include Rz (mean roughness depth, typically 4-7x Ra value) and RMS (root mean square, approximately 1.11x Ra). Confusing Ra with Rz or RMS is a common error that leads to incorrect surface finish verification.