Zhongshan, Guangdong, China · GT 8 (GMT+8) evelyn_lin@machiningservice.cn  |  Request a quotation
Boyang Hardware Precision CNC Machining

Home › Ra surface finish chart

Ra Surface Finish Chart: Every Machining Band, What It Means, When It's Used

Quick reference for reading machining drawings · Boyang Hardware, Zhongshan, Guangdong, China

Direct answer For CNC machined parts, the working Ra ladder is: Ra 6.3 µm rough machined / non-functional, Ra 3.2 µm standard machined face, Ra 1.6 µm good finish-milled or turned face, Ra 0.8 µm fine finish / anodise-ready, Ra 0.4 µm sealing-quality, Ra 0.2 µm bearing-grade (grinding). Each band roughly halves the height of the surface irregularities; each step down costs process effort. The chart below maps every band to the process that produces it and the applications that ask for it.

On this page

The chart

Table 1 — Ra bands on machined parts: process that produces each, typical application, and rough appearance. Values are for milled/turned metal surfaces in normal production.
Ra (µm) Roughness grade Process that produces it Typical application Surface character
6.3 N9 Rough milling/turning, heavy cuts Non-functional faces, inside pockets, clamp areas Clearly visible tool marks
3.2 N8 Standard finish milling/turning General machined faces, bracket pads, housings Tool marks visible, edge clean
1.6 N7 Controlled finishing pass, sharp tooling Fitted faces, threads adjacent, most cosmetic surfaces Fine lay, marks barely visible
0.8 N6 Fine finishing, light passes Anodise-ready faces, precision fits, fatigue-conscious surfaces Smooth, lay visible under light
0.4 N5 Very fine turning/milling, start of grinding range Static sealing faces, O-ring glands, sliding fits Reflective, lay faint
0.2 N4 Grinding, fine honing Bearing journals, dynamic seals, gauge surfaces Near-mirror

Context for specific industries: aerospace application bands are tabulated in aerospace surface finish requirements, and the measurement side — how any of these numbers is actually verified — is in surface finish measurement (Ra, Rz).

N-grade equivalents

Older and some European drawings specify finish as N-grades rather than Ra values. The mapping is linear in Ra: N4 = 0.2, N5 = 0.4, N6 = 0.8, N7 = 1.6, N8 = 3.2, N9 = 6.3 µm — each step doubles the Ra. If a drawing carries both, treat the Ra number as governing; if only an N-grade appears, convert once and confirm at quotation to avoid a factor-of-two surprise.

Choosing a band without over-specifying

Three questions decide it. Does the face seal, bear or slide? Then 0.4–0.8 µm or finer, from the sealing/bearing rows. Does it take a cosmetic finish? Anodising and plating read well at 0.8–1.6 µm — finer substrates also show every fingerprint. Otherwise it is a standard face: Ra 3.2 µm costs nothing extra and says "machined" on arrival. Every step below the standard band buys process time, so hold tight callouts for faces that earn them. Where tight Ra combines with tight dimensions — a ground bore at Ra 0.4 µm and ±0.005 mm, for example — that is the grinding route described in CNC machining tolerances.

Honest limits: Ra is an average — it says nothing about individual deep scratches (that is Rz's job, and the distinction matters on sealing faces), lay direction, or waviness. A chart is a starting point, not a specification; the drawing's parameter, value and note govern what we produce and measure. And a practical boundary: below Ra 0.2 µm you are leaving machining for superfinishing, lapping and polishing processes with their own cost curves — we will quote those honestly rather than pretend a mill can reach them economically.

FAQs

What Ra can CNC machining achieve as standard?

Ra 1.6–3.2 µm from standard milling and turning; Ra 0.8 µm with controlled finishing passes; Ra 0.2–0.4 µm via grinding rather than milling.

Is a lower Ra always better?

No — beyond the functional requirement, finer finish only adds cost, and some seals and coatings actually prefer a controlled texture. Specify the band the function needs.

How do N-grade numbers relate to Ra?

N5 = 0.4, N6 = 0.8, N7 = 1.6, N8 = 3.2, N9 = 6.3 µm — each step doubles the Ra. When both appear on a drawing, the Ra value governs.