Surface finish chart
Typical surface roughness for common machining and finishing processes, with Ra in micrometers and microinches and the matching ISO N grade. Use it to pick a process for a required finish or to read a finish callout on a drawing.
Roughness grades
| N 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 |
Typical Ra by process
| Process | Ra range (µm) | Ra range (µin) |
|---|---|---|
| Sand casting | 12.5-25 | 500-1000 |
| Hot rolling | 12.5-25 | 500-1000 |
| Sawing | 3.2-25 | 125-1000 |
| Drilling | 1.6-6.3 | 63-250 |
| Milling | 0.8-6.3 | 32-250 |
| Turning | 0.4-6.3 | 16-250 |
| Reaming | 0.8-3.2 | 32-125 |
| Grinding | 0.1-1.6 | 4-63 |
| Honing | 0.05-0.8 | 2-32 |
| Lapping | 0.012-0.4 | 0.5-16 |
| Polishing | 0.05-0.4 | 2-16 |
Ra, the arithmetic average roughness, is the most common finish measure: a smaller Ra means a smoother surface, and the N grade is an ISO shorthand for the same value. Finer finishes cost more and take longer, so specify only the finish a part actually needs.
Choosing a drill or machining setup?
See the Drill Size Chart for hole sizes, or browse the engineering calculators for related work.
What Ra means
Ra is the average height of the tiny peaks and valleys left on a machined surface, measured from the mean line, in micrometers or microinches. A turned shaft might be 1.6 micrometers Ra, while a ground bearing seat could be 0.4 or finer. Lower Ra is smoother but more expensive to produce.
Surface texture symbols
On a drawing, the required surface finish is called out with a checkmark-style symbol that points to the surface. The base symbol can be modified to require or forbid material removal, and the Ra value is written above the long leg.
| Symbol | Name | Meaning |
|---|---|---|
| Basic symbol | Surface may be produced by any method. Material removal is allowed but not mandatory. | |
| Material removal required | The surface must be machined. A value on the added bar gives the minimum machining allowance. | |
| Material removal prohibited | The surface must be left as produced (cast, forged, rolled) with no machining. |
A lay symbol can be added at the lower right to control the direction of the dominant tool marks relative to the surface the symbol points to:
| Lay | Direction of tool marks |
|---|---|
| = | Parallel to the edge the symbol points to |
| ⊥ | Perpendicular to that edge |
| X | Crossed (angular in two directions) |
| M | Multidirectional |
| C | Circular relative to the center of the surface |
| R | Radial relative to the center of the surface |
| P | Particulate, non-directional, or pitted |
Matching finish to function
Sealing faces, bearing seats, and sliding surfaces need fine finishes to seal and wear well. Non-critical surfaces can be left rough to save time and cost. Over-specifying finish is a common and expensive mistake, so call out tight roughness only where the function demands it.
FAQ
What is a good surface finish for a sealing surface?
Often around 0.4 to 0.8 micrometers Ra (16 to 32 microinches), produced by fine turning or grinding, so the seal seats without leaking.
What is the difference between Ra and N grades?
They describe the same roughness. Ra is the numeric average in micrometers or microinches; the N grade is an ISO code where N6 equals 0.8 micrometers Ra. Lower N is smoother.
Does a finer finish always cost more?
Yes. Finer finishes need slower cuts, extra operations like grinding or lapping, and more inspection. Specify a fine finish only where it is functionally needed.