Surface Roughness Comparison Table for CNC Parts

• 1
  Surface Roughness Comparison Table: Ra, N Grade and Process
• 2
  Tighter finish vs looser finish: what changes on the quote
• 3
  What each column in a surface roughness comparison table actually means
• 4
  Which CNC process reaches which surface roughness grade
• 5
  How to read a roughness callout on a drawing without over-tightening it
• 6
  Where secondary finishing changes the roughness number

Ra is the arithmetic mean deviation of the profile from its center line. It is an average, so a single deep scratch and a field of shallow chatter can report the same Ra. That is the first thing to know before you trust any surface roughness comparison table. The number describes the general texture, not the worst defect.

RMS is the root mean square of the same profile. It weights peaks and valleys more heavily than Ra does. For a sine-like profile, RMS runs about 11 percent higher than Ra. For a random machined surface the ratio can reach 1.3 or more. If a drawing calls out RMS, converting back to Ra with a fixed factor is a rough guess, not a measurement.

The N grade system runs from N1 to N12 and is still common on older European drawings. N7 corresponds to roughly Ra 1.6 μm, N6 to Ra 0.8 μm, N8 to Ra 3.2 μm. The grade is a band, not a single value, which is why two suppliers can both meet N7 and still look different under light.

When you compare Ra, Rz and RMS side by side, remember they answer different questions. Ra tells you the average. Rz, the ten-point height, tells you how tall the tallest peaks are. For a gasket face or a press fit, Rz is often the number that matters, because a few tall peaks will crush the seal or eat the interference.

Standard CNC milling with a sharp carbide insert lands in the Ra 0.8–1.6 μm range when the setup is rigid and the feed is moderate. Push the feed per tooth up and you slide to Ra 1.6–3.2 μm. That is still a fine functional surface for most brackets and housings, and it costs less spindle time.

Turning tends to beat milling on finish, because a single-point tool leaves a regular helical pattern. With a wiper insert and a light finish pass, a lathe can hold Ra 0.4–0.8 μm on 6061 or 303 stainless without any secondary operation. This is why shaft diameters and threaded bosses are usually turned, not milled.

Reaching Ra 0.2–0.4 μm means grinding, lapping or polishing. Those are separate operations with separate setup, and they cannot fix a part that was machined with the wrong geometry. Leave 0.05–0.1 mm of stock on the surface if you plan a grind. A finished hard surface cannot be ground down to size later.

At GreatLight we run 127 high-precision CNC machines across 3 wholly-owned plants, including 16 simultaneous 5-axis centers and 16 mill-turn centers. That mix lets us hold ±0.005 mm and Ra 0.8–1.6 μm as a routine as-machined result, then send parts out for anodizing, bead blasting or polishing when the function asks for more.

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  As-machinedRa 1.6–3.2 μm, no secondary operation, lowest cost per part.
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  Fine machinedRa 0.8–1.6 μm, needs a rigid setup and a light finish pass.
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  Ground or polishedRa 0.2–0.8 μm, separate operation, leave stock on the drawing.

A surface roughness symbol on a drawing names a limit, not a target. If you write Ra 0.4 μm across a whole part, every surface must be measured and every surface must pass. That drives grinding, extra fixturing and inspection time on faces that may never touch anything. Put the tight callout only where the function lives.

The usual mistake is specifying the same finish everywhere because it feels safer. A better pattern is one tight callout on the sealing or sliding face, a general note for the rest of the part. Something like Ra 1.6 μm unless otherwise stated, with Ra 0.4 μm on the two bores that carry bearings.

Measurement matters as much as the number. A handheld stylus gauge reads a 4–5 mm cut-off length, so it averages over that window and misses a short scratch. For a critical bore, ask for a traceable report with the cut-off length written down. Otherwise two labs can disagree on the same part and both be correct.

Material changes the achievable finish too. Soft aluminum 6061 smears and builds up on the tool edge, so it can be harder to hit Ra 0.4 μm than 17-4PH stainless. Plastics like POM and PEEK cut clean but scratch easily after machining, so handling and packaging decide whether the surface arrives as specified.

Bead blasting produces a uniform matte texture that reads around Ra 1.6–3.2 μm regardless of the machining marks underneath. It hides tool paths well, which is why it is common on visible aluminum housings. It also rounds sharp edges slightly, so keep it away from a sealing land.

Anodizing adds a thin oxide layer that follows the existing profile. It does not smooth a rough surface, and on a very rough face the coating can look patchy. Hardcoat anodizing builds more thickness and can raise the measured Ra by 0.2–0.4 μm. If the drawing locks Ra after coating, say so on the purchase order.

Polishing and tumbling both remove material. Tumbling is fine for deburring and a soft sheen, but it will not hold a flatness callout. Polishing can reach Ra 0.2 μm or better on a small area, though it is a manual operation and the result depends on the operator as much as the machine.

Plating is a different case. Electroless nickel and zinc plate deposit evenly and largely preserve the underlying texture. If you need a low-friction or wear surface, the important question is the base finish plus the coating thickness, not the coating alone.