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Surface Roughness (Ra)

Ra is the arithmetic average surface roughness, measuring the mean absolute deviation of the surface profile from its centerline, expressed in micrometers or microinches.

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.

Related Guides

Surface Roughness Symbols Explained Surface Finish Conversion Chart Ra to RMS

Frequently Asked Questions

How do you convert Ra micrometers to microinches?
Multiply by 39.37. Ra 1.6 micrometers = approximately 63 microinches.
What is the difference between Ra and Rz?
Ra is the arithmetic average roughness. Rz is the mean peak-to-valley height over the sampling length. Rz is typically 4-7 times larger than Ra for the same surface.

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