Surface Roughness by Process: What CNC Can Actually Reach
Short answer: CNC milling and turning reach about Ra 3.2 µm as a routine finish, around Ra 1.6 µm with care, and roughly Ra 0.8 µm on a good machine with light finishing passes. To go finer — Ra 0.4 µm or below — you need grinding, honing, lapping, or polishing, which are separate operations with separate cost. Specifying an Ra you cannot machine is one of the most common ways buyers pay for a process they did not need. At BQUQ we machine ±0.005 mm and quote within 12 working hours, so ask us what your surface call actually costs.
Surface roughness is measured as Ra, the arithmetic average deviation of the surface profile from its mean line, in micrometres. It describes fine-scale texture, not the larger waviness or form of a surface. A part can be dimensionally perfect and still fail on roughness, so the call belongs on the drawing only where it matters — sealing faces, bearing surfaces, sliding contacts, and optical or cosmetic surfaces.
What Each Process Actually Delivers
| Process | Typical Ra (µm) | Notes |
|---|---|---|
| Sawing / rough cut | 12.5–25 | Stock prep only |
| Rough milling/turning | 6.3–12.5 | Heavy passes |
| Standard milling/turning | 3.2 | Routine production finish |
| Fine milling/turning | 1.6 | Lighter passes, sharp tool |
| Precision turning/milling | 0.8 | Good machine, finishing pass |
| Grinding | 0.4–0.8 | Separate operation |
| Honing | 0.2–0.4 | Bores, cylinders |
| Lapping | 0.05–0.2 | Flat sealing faces |
| Polishing | 0.025–0.1 | Cosmetic/optical |
| EDM (fine) | 0.4–0.8 | Hard/tricky geometry |
| Bead blasting | Adds texture | Cosmetic, masks tool marks |
The line between Ra 1.6 and Ra 0.4 is the line between "machining" and "surface finishing as a second process." That is where cost and lead time step up.
Why Going Finer Costs More
Reaching a finer Ra means slower feeds, smaller depth of cut, a sharper or fresh tool, and often a dedicated finishing pass. Each of those increases cycle time. Below Ra 0.4 µm, CNC alone usually cannot get there — the geometry of the cutting edge and the machine's vibration floor limit you — and you move to grinding, honing, or lapping, which add setups, equipment, and inspection.
There is also a dimensional risk. A fine finishing pass removes very little material but can drift a tight dimension. On a ±0.005 mm feature, the finishing strategy has to be planned, not improvised.
Anisotropy: Direction Matters
Milled surfaces are directional; they carry tool marks aligned with the feed direction. Ra measured across the marks differs from Ra measured along them. A turned surface is typically better circumferentially. If your application cares about a specific direction — a seal that slides, a gasket that presses — call it out, and specify the measurement direction. Part-to-part roughness variation also rises as you push finer, so the drawing call should reflect what the function tolerates.
Roughness, Waviness and Form Are Different Things
| Characteristic | Scale | Typical call | Measured by |
|---|---|---|---|
| Roughness (Ra) | Fine texture | Ra 0.8 µm | Profilometer |
| Waviness | Longer waves | Varies | Profilometer, filter |
| Form | Overall shape | Flatness, roundness | CMM, indicator |
A shiny surface can hide poor flatness; a flat surface can have coarse texture. Specify the characteristic you actually need. A sealing face usually cares about both flatness and Ra, while a cosmetic housing panel cares mostly about visual appearance, which is better expressed as a finish class than a bare Ra number.
How to Specify Finish Without Over-Paying
Default your drawings to a general note such as "Ra 3.2 µm unless otherwise specified" and then upgrade only the surfaces that need it. Anodizing, plating, and bead blasting change the surface and can either help or hurt Ra, so specify finish and roughness together, not separately. Our guide to surface finish and Ra walks through the callouts, and our notes on anodizing finishes and secondary finishing show how post-processing interacts with roughness.
One more practical point: roughness costs are usually quoted separately from machining. If a quote does not mention an Ra requirement, assume it is not priced in. State it at inquiry so the number you get is the number you need.
Frequently Asked Questions
Q: What Ra can a CNC mill realistically achieve?
A: Routine milling holds about Ra 3.2 µm, fine milling about Ra 1.6 µm, and a well-set-up machine with a light finishing pass reaches roughly Ra 0.8 µm. Finer than that normally requires grinding, honing, or polishing.
Q: Can I specify Ra 0.4 µm on a milled surface?
A: It is possible on some geometries with special tooling, but it is usually more economical to mill to Ra 1.6 µm and then grind or lap only the surfaces that need Ra 0.4 µm. Blanket-calling it across a part drives cost sharply.
Q: Does anodizing change surface roughness?
A: Yes. Anodizing grows a thin oxide layer that generally increases surface roughness slightly and can round sharp texture. Specify finish and roughness together so the final surface meets your function.
Q: How is Ra measured and reported?
A: With a surface profilometer that traces the surface and filters out waviness. Reports usually give Ra in micrometres over a stated cutoff length and direction. Ask for the measurement plan if the call is strict.
Q: Does BQUQ hold specific Ra values?
A: Yes, we machine to specified roughness and verify with a profilometer. Send the drawing to sc@bquq.com or WhatsApp +86 13713157787; we return a quote with any finish-related notes within 12 working hours.
Related Resources
- Surface finish and Ra guide: how to read and specify roughness callouts.
- CNC turning parts: turned surfaces where circumferentially uniform finish is achievable.
- About BQUQ: an ISO9001-certified source factory in Dongguan running CNC, stamping, springs, and heat sinks under one roof.
- Contact us: finish questions and quotes within 12 working hours.
Authored by the BQUQ Engineering Team. BQUQ is an ISO9001-certified source factory in Dongguan, China, running CNC machining, metal stamping, custom springs, heat sink and collet lines under one roof. Send drawings to sc@bquq.com or WhatsApp +86 13713157787 for a quote within 12 working hours. www.bquq.com
