CNC Machining Surface Finish: The Ra Chart, Explained
Ra values in μm and μin, N1–N12 grades, what each process actually achieves, and what finer finishes cost — the reference we wish every drawing came with.
IsoCNC Engineering Team
CNC Machining & DFM Engineers · Updated · 8 min read
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The 30-Second Answer
Standard CNC machining delivers Ra 3.2 μm (125 μin, grade N8) out of the box — and most parts need nothing finer. Ra 1.6 μm is a fine machined finish, Ra 0.8 μm is precision work, and anything at 0.4 μm or below is no longer milling or turning — it is grinding, honing, or lapping, with the process-change cost that implies. The chart below is the whole system; the sections after it are how to use it without overpaying.
The Surface Finish Chart
| Finish level | Ra (μm) | Ra (μin) | N grade | Process | Use it for |
|---|---|---|---|---|---|
| As-machined (standard) | 3.2 | 125 | N8 | Standard CNC milling / turning | Default for most parts — functional surfaces, non-cosmetic |
| Fine machined | 1.6 | 63 | N7 | Fine milling / turning | Visible surfaces, press-fit prep, cosmetic-ish parts |
| Precision machined | 0.8 | 32 | N6 | Controlled finishing pass, sharp tooling | Bearing seats, gasket and sealing faces |
| Ground | 0.4 | 16 | N5 | Grinding | Precision fits, shafts, bearing journals |
| Fine ground / honed | 0.2 | 8 | N4 | Grinding / honing | Hydraulic bores, sliding surfaces |
| Lapped / polished | 0.1–0.025 | 4–1 | N3–N1 | Lapping, polishing, superfinishing | Gauges, optical surfaces, dynamic seals |
Two entries deliberately missing from the chart: bead blasting and tumbling. Both are cosmetic treatments — they produce a uniform matte or polished look but do not hold a controlled Ra. Specify them by appearance (or better, by a reference sample), not by roughness number. For choosing between the two blasting processes, see sandblasting vs bead blasting.
What Ra Actually Measures (and When Rz Matters)
Ra is the arithmetic mean of the profile's deviation from its centerline — an average roughness, measured with a profilometer over a defined evaluation length. It answers "how smooth is this surface, on average," which is why it became the default callout on both ISO (ISO 1302) and US (ASME B46.1) drawings.
Rz measures peak-to-valley height, and it tells a different story: a surface can average smooth while still carrying deep scratches that leak or initiate fatigue cracks. Specify Rz for sealing faces, wear tracks, and fatigue-critical contact surfaces. On the same surface Rz typically runs 4–7× the Ra value — a drawing that accidentally swaps one for the other tightens the requirement by that factor, and the quote will notice.
What Controls Finish on the Machine
In turning, the theoretical finish is set by geometry: Ra ≈ f² ÷ (32 × r), where f is the feed per revolution and r is the tool nose radius. Halve the feed and the theoretical Ra quarters; double the nose radius and it halves. Milling follows the same logic per tooth. That formula is the idealized floor — real finishes add the effects of material behavior, built-up edge, tool wear, coolant, and chatter.
The practical levers, in order of impact: a finishing pass with a fresh, sharp tool; larger nose radius; reduced feed (with its cycle-time cost); rigid fixturing to kill vibration; and material choice — free-cutting alloys finish cleanly, gummy ones smear. A shop that understands this gets to Ra 1.6 μm on the machine; a shop that does not reaches for the polishing bench and charges you for it.
Finish is set by the machining strategy — see our roughing vs finishing guide for the passes that decide it.
What Finer Finishes Cost
There is no universal percentage — the cost of a tighter finish depends on geometry and whether a second process is needed — but the structure is predictable. Moving from 3.2 to 1.6 μm usually means a dedicated finishing pass at lower feed: a modest cycle-time increase. At 0.8 μm you are paying for controlled finishing: fresh tooling, measured verification, maybe a second setup. At 0.4 μm and below, the mill or lathe leaves the equation — grinding, honing, or lapping enters it, with new machines, new setups, and new inspection. Each step down the chart roughly doubles the attention the surface gets.
The design-side takeaway: apply fine finishes surgically. Call out Ra 0.8 μm on the bearing bore, not on the whole part. A blanket "all surfaces 1.6" on a housing can move the quote more than the material does.
Below Ra 0.4 µm, machining gives way to grinding — see our CNC grinding guide.
The Mistakes That Cost Money
Over-specifying. If Ra 3.2 μm functions — and on most surfaces it does — a tighter callout buys nothing but cycle time. The most expensive sentence on a drawing is a finish requirement nobody needed.
Treating bead blasting as roughness control. It hides machining marks beautifully and holds no Ra at all. If the surface is functional, blast after verifying, not instead of verifying.
Forgetting coatings move the number. Anodize, plating, and powder coat all add thickness and change texture. State whether Ra applies before or after coating — especially on sealing faces.
Mixing Ra and Rz. They differ by a factor of 4–7 on the same surface. One wrong symbol and the part is suddenly four times smoother than anyone intended — or four times rougher.
How to Specify Finish on a Drawing
Use the ISO 1302 surface-texture symbol (the check-mark) with the Ra value, and place it on the specific surface — not in general notes unless it truly applies everywhere. State the parameter (Ra or Rz), the value, and where coatings are involved, whether the requirement applies before or after finishing. If you work in N grades, Ra 3.2 μm is N8, 1.6 is N7, 0.8 is N6 — the chart above maps them.
And when in doubt, describe the function. "Sealing face for O-ring" tells our engineers more than a bare number does — and if the finish you specified costs more than the function needs, we will say so in the DFM review. Related reading: CNC machining tolerances and the DFM design guide.
For two of the most-specified steel finishes, see our black oxide vs zinc plating and electropolishing vs passivation comparisons — and for outdoor steel parts, hot-dip galvanizing is the heavy-duty coating route.
For the cosmetic finishes — brushed, bead blasted, and more — see our brushed metal finish and bead blasting finish guides.
FAQ
What is the standard surface finish for CNC machining?
Ra 3.2 μm (125 μin, N8) — the as-machined finish any competent CNC shop hits by default with standard tooling and feeds. If your drawing has no finish callout, this is what you get. Most functional parts need nothing finer, which is why adding tighter callouts casually is one of the most common ways to inflate a machining quote.
Should my drawing specify Ra or Rz?
Ra in most cases — it measures average roughness and is the default on US and ISO drawings. Use Rz (peak-to-valley) when the peaks matter functionally: sealing surfaces, wear surfaces, fatigue-critical contact. The two are not interchangeable; Rz typically runs 4–7× the Ra value on the same surface, so mixing them up on a drawing changes the requirement dramatically. The full breakdown is in Ra vs Rz.
Does bead blasting improve surface finish?
It improves appearance, not precision. Bead blasting produces a uniform matte texture that hides tool marks and makes parts look consistent — but it does not hold a controlled Ra the way grinding or lapping does. Specify it as a cosmetic finish, not as a roughness requirement.
Does anodizing change the surface finish?
Yes — anodizing adds an oxide layer (typically 5–25 μm for Type II, up to 50+ μm for hard coat) and slightly alters texture. If the Ra requirement is functional, state on the drawing whether it applies before or after coating. A sealing face measured after hard anodize is not the same surface you machined.
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Finish Questions on a Real Part?
Upload the drawing. Every quote includes a DFM review — if a finish callout is costing you money without buying function, we will flag it. Typically within 1–2 business days.