What is the difference between Ra and RMS?

Ra is the arithmetic mean deviation of the profile from its mean line. RMS (also written Rq) is the root-mean-square deviation, which weights peaks more heavily. For most machined surfaces RMS is roughly 1.1 to 1.5 times Ra, so the often-quoted 1.11 factor is an approximation rather than a fixed conversion.

Rz is the average of the peak-to-valley heights over the sampling length - the average of the five highest peaks plus the five deepest valleys in the older definition. It is more sensitive to isolated peaks than Ra, which is why a surface that passes on Ra can still fail on Rz where a seal or a fatigue life is involved.

What are the N-grade numbers?

N-grades are the ISO 1302 roughness grade numbers, a shorthand for Ra bands. N7 is Ra 1.6 µm, N6 is Ra 0.8 µm and so on, each step being a halving. A drawing that specifies N7 is asking for the same surface as Ra 1.6, just written the short way.

Which surface roughness can CNC milling achieve?

Standard CNC milling lands around Ra 3.2 to 1.6 µm. Fine finishing with a small stepover and a sharp cutter reaches about Ra 0.8 µm, and the calculators on this site work to Ra 0.8 µm as a standard surface roughness. Anything below that generally means grinding, lapping or polishing as a separate operation.

Surface Roughness Converter — Ra, RMS, Rz and N-Grades

Ra, RMS, Rz and the N-grade numbers all describe the same surface in different ways. Convert between them, and check what each machining process can realistically deliver.

Roughness value

You have

Value

Converted in your browser. RMS and Rz conversions are approximations — Ra, Rq and Rz are independent parameters and only a stylus trace gives all three exactly.

Result

Ra (µm)—Ra (µin)—RMS / Rq (µin, approx.)—Rz (µm, approx. range)—N-grade—Reachable by—

Ra(µin) = Ra(µm) × 39.37. RMS(µin) ≈ Ra(µin) × 1.11. Rz(µm) is shown as a range because the ratio to Ra varies by process, from about 3:1 on ground surfaces to 6:1 on turned ones.

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Reference

ISO 1302 roughness grades and what reaches them

N-grade	Ra (µm)	Ra (µin)	Typical route to get there
N12	50	2000	Rough sawing, flame cutting
N11	25	1000	Rough turning or milling, primary passes
N10	12.5	500	Standard rough turning and milling
N9	6.3	250	Normal CNC turning, coarse milling
N8	3.2	125	Standard CNC milling and turning finish
N7	1.6	63	Fine finishing pass, sharp tool, light stepover
N6	0.8	32	Fine milling or turning, or a light grind
N5	0.4	16	Precision grinding, fine boring
N4	0.2	8	Lapping, honing, polishing
N3	0.1	4	Fine lapping, superfinishing
N2	0.05	2	Superfinishing, optical work
N1	0.025	1	Mirror finishing, specialist processes

Every grade is half the previous one, so moving from N8 to N6 roughly doubles the finishing time and often needs a different tool, a smaller stepover and sometimes a second machine. If a drawing specifies a finish finer than the function needs, relaxing it is usually the cheapest change available.

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Basis & references: ISO 1302 — Indication of surface texture in technical product documentation · ISO 4287 — Surface texture: profile method, terms and definitions · NIST — Surface metrology

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