Reference

Surface finish (Ra)

Ra is the arithmetic average of how far the surface sits from its mean line. It is one number. It is not the whole specification.

Units

1 µm = 39.37 µin, so microinches ≈ micrometers × 39.37. The shop names (32, 63, 125) are that product rounded to the familiar sequence. 0.8 µm × 39.37 is 31.5, which everyone calls 32 µin.

A callout that says only "32" is ambiguous. 32 µin is a normal milled finish. 32 µm is a rough sawn one. Write the unit.

Rz, Rq, and Rt are different parameters. There is no honest fixed multiplier from Ra to Rz. If the drawing says Rz, measure Rz.

ISO N grades

The old ISO 1302 N scale steps by about 2×. The microinch column is the customary rounded equivalent, not the raw product.

ISO N grades and customary Ra equivalents
Grade	Ra µm	Ra µin
N12	50	2000
N11	25	1000
N10	12.5	500
N9	6.3	250
N8	3.2	125
N7	1.6	63
N6	0.8	32
N5	0.4	16
N4	0.2	8
N3	0.1	4
N2	0.05	2
N1	0.025	1

What a process can usually hold

These are typical as-machined bands on a decent setup. A worn insert, a dull end mill, or a chatter-prone part will not get there. A careful grind will beat the middle of the band. The drawing, not this table, is the requirement.

Typical as-machined Ra
Process	Ra µin	Ra µm	About
Sawing, snagging	250–2000 µin	6.3–50 µm	N9–N12
Rough turning or milling	125–500 µin	3.2–12.5 µm	N8–N10
Finish turning or milling	32–125 µin	0.8–3.2 µm	N6–N8
Reaming	16–63 µin	0.4–1.6 µm	N5–N7
Grinding	8–32 µin	0.2–0.8 µm	N4–N6
Honing or lapping	1–16 µin	0.025–0.4 µm	N1–N5

What Ra leaves out

Lay: the direction of the marks. A seal often cares which way they run.
Waviness: a longer wave than the roughness cutoff. Ra will not show a bowed face.
The cutoff length used to measure it. Two gauges can disagree if the cutoff differs.

Gear tooth finish and a gear rating are calculated work. Use gearcalculators.com for the numbers, and Evolvent Design when you need the gears made.