Reference Chart

Surface Finish Chart

Q: What is the difference between Ra and RMS?

Ra is the arithmetic average of surface height deviations. RMS (root mean square) gives more weight to large peaks and valleys. RMS is about 11% higher than Ra for most machined surfaces. Multiply Ra by 1.11 to get an approximate RMS value.

Convert between Ra, Rz, and RMS surface roughness values. See which machining process gives the finish you need, from mirror polish to rough mill.

Ra (μin) ▲	Ra (μm) ▲	Rz (μin) ▲	RMS (μin) ▲	N Grade ▲	Machining Process ▲	Typical Application ▲
1	0.025	4	1.1	N1	Superfinish / Lapping	Optical mirrors, gauge blocks
2	0.05	8	2.2	N2	Lapping / Polishing	Precision bearings, seals
4	0.1	16	4.4	N3	Fine grinding	Hydraulic valve spools, piston rods
8	0.2	32	8.9	N4	Grinding	Bearing journals, crankshaft pins
16	0.4	63	17.8	N5	Fine grinding / Honing	Cylinder bores, sealing surfaces
32	0.8	125	35.5	N6	Fine turning / Fine milling	Mating surfaces, close-fit shafts
63	1.6	250	69.9	N7	Standard milling / Turning	General machined surfaces, brackets
125	3.2	500	138.8	N8	Milling / Turning	Structural parts, non-critical faces
250	6.3	1000	277.5	N9	Rough milling / Rough turning	Rough-machined castings, forgings
500	12.5	2000	555	N10	Rough milling / Sawing	Raw stock surfaces, non-functional faces
16	0.4	63	17.8	N5	Fine turning	Precision OD/ID fits, valve stems
63	1.6	250	69.9	N7	Standard turning	Shafts, bushings, spacers
32	0.8	125	35.5	N6	Fine wire EDM	Mold cavities, precision dies
63	1.6	250	69.9	N7	Wire EDM	Stamping dies, extrusion dies
125	3.2	500	138.8	N8	Sinker EDM	Deep ribs, complex mold shapes
250	6.3	1000	277.5	N9	Rough sinker EDM	Roughing cavities, electrode burns

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Notes on This Chart

Ra is arithmetic average roughness. RMS is approximately Ra x 1.11 for typical machined surfaces.
Rz measures peak-to-valley height over a sample length. It is more sensitive to deep scratches.
N grades follow ISO 1302. N1 is the smoothest. N12 is the roughest.
Actual finish depends on material, tooling, speed, feed, and coolant. Always verify with a profilometer.

Frequently Asked Questions

What is Ra in surface finish?

Ra is the arithmetic average roughness. It measures the average height of peaks and valleys on a surface. A lower Ra means a smoother finish. Ra is the most common way to call out surface finish on drawings.

What is the difference between Ra and RMS?

Ra is the arithmetic average. RMS (root mean square) gives more weight to large peaks and valleys. RMS is about 11% higher than Ra for most machined surfaces. Multiply Ra by 1.11 to estimate RMS.

What surface finish can CNC milling achieve?

Standard CNC milling gives 63 to 125 microinch Ra. Fine milling with sharp tools and light cuts can reach 32 microinch Ra. With grinding or polishing, you can get below 16 microinch Ra.

How do I specify surface finish on a drawing?

Use the surface finish symbol (a check mark with a number). Write the Ra value in microinches or micrometers next to it. Place the symbol on the surface that needs it. Only call out finishes tighter than standard when needed. Tighter finishes cost more.

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Surface Finish Chart

Notes on This Chart

Frequently Asked Questions

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