Surface Finish Guide
Ra values, N-grades, typical machining processes, and cost indicators. Click any two rows to compare them side-by-side.
| Ra (µm) | N-Grade | Profile | Typical Process | Application | Rel. Cost |
|---|---|---|---|---|---|
| 0.1 | N1 | Lapping, superfinishing | Gauge blocks, optical flats, precision bearings | ||
| 0.2 | N2 | Diamond turning, fine grinding | Precision shafts, optical mounts, high-speed spindles | ||
| 0.4 | N3 | CNC milling (fine), precision grinding | Sealing surfaces, bearing races, high-precision fits | ||
| 0.8 | N4 | CNC turning (fine), standard milling | Hydraulic cylinders, precision shafts, instrument housings | ||
| 1.6 | N6 | Standard CNC milling/turning | General machined parts, assembly interfaces | ||
| 3.2 | N8 | Rough milling, standard turning | Non-critical surfaces, structural parts | ||
| 6.3 | N9 | As-machined (rough) | Rough structural parts, non-contact brackets | ||
| 12.5 | N10 | Rough machining, sawing | Raw stock, non-functional surfaces |
Lapping, superfinishing
Gauge blocks, optical flats, precision bearings
Diamond turning, fine grinding
Precision shafts, optical mounts, high-speed spindles
CNC milling (fine), precision grinding
Sealing surfaces, bearing races, high-precision fits
CNC turning (fine), standard milling
Hydraulic cylinders, precision shafts, instrument housings
Standard CNC milling/turning
General machined parts, assembly interfaces
Rough milling, standard turning
Non-critical surfaces, structural parts
As-machined (rough)
Rough structural parts, non-contact brackets
Rough machining, sawing
Raw stock, non-functional surfaces
How to specify surface finish
On engineering drawings, surface finish is specified as the Ra value in µm (e.g., Ra 1.6). For CNC machined parts, Ra 1.6 µm is considered the default "standard" finish unless otherwise specified. Always call out critical surfaces with explicit Ra requirements — our team will select the appropriate process accordingly.
Why Surface Finish Matters in CNC Machining
Surface finish is not cosmetic — it directly affects how a part performs. Sealing faces leak if they are too rough. Bearing journals wear prematurely. Fatigue life drops as surface scratches become crack-initiation sites, which is why aerospace and medical drawings call out finish as strictly as dimensional tolerance.
The finish you can expect depends on the process: standard CNC milling and turning produce Ra 0.8–3.2 µm, fine finishing passes reach Ra 0.4 µm, and grinding, honing, or lapping take surfaces below Ra 0.2 µm. Every step smoother adds machining time and cost, so the engineering skill is matching the finish to the function — and no finer.
At OrangeSea CNC every drawing callout is verified with profilometer measurement before shipment, and a surface finish report is included whenever Ra is specified. See our quality inspection service for the full measurement capability, or request a quote with your finish requirements.
Surface Finish FAQ
What is Ra in surface finish?+
Ra (Roughness average) is the arithmetic mean of the absolute deviations of the surface profile from the mean line, measured in micrometres (µm). It is the most widely used surface roughness parameter on engineering drawings. A lower Ra value means a smoother surface — Ra 0.4 µm is a fine ground finish, while Ra 6.3 µm is a standard rough-machined surface.
What surface finish is standard for CNC machining?+
Ra 3.2 µm is the typical default for general CNC machined surfaces, and Ra 1.6 µm is the common standard for finished functional surfaces. Without extra operations, CNC milling and turning achieve Ra 0.8–3.2 µm. Finishes below Ra 0.4 µm usually require grinding, honing, lapping, or polishing as a secondary process.
How does surface finish affect part cost?+
Each step smoother roughly increases surface-related cost: tighter finishes require slower feeds, finer stepovers, newer tooling, and often secondary operations like grinding or polishing. Specifying Ra 0.2 µm on a surface that functions fine at Ra 1.6 µm can add 30–100% to machining time on that feature. Specify tight finishes only on surfaces that need them — sealing faces, bearing journals, and optical or sliding surfaces.
What is the difference between Ra and Rz?+
Ra is the average roughness across the whole sampled profile, while Rz is the average of the five highest peak-to-valley heights. Rz is typically 4–7× larger than Ra for the same surface. European drawings often specify Rz; converting between them is approximate, so when a drawing mixes both we measure and report each explicitly.