Free tool
Surface finish converter
The surface finish converter turns an Ra in micrometers into microinches and back exactly (a microinch is 0.0254 micrometer), names its ISO 1302 roughness grade from N1 to N12, and gives Rz only as an approximate range, because no standard converts Ra to Rz.
Exact: micrometers and microinches
Ra 3.2 µm = 126 µin
| Ra, micrometers | 3.2 µm |
|---|---|
| Ra, microinches | 126 µin (written 125 µin on an inch drawing) |
| Roughness grade (ISO 1302) | N8 (Ra 3.2 µm, 125 µin) |
| Rz, approximately | 13 to 22 µm: 4 to 7 times Ra, a rule of thumb, not a conversion |
| Typically produced by | Forging, drilling, milling, turning and boring, investment casting, permanent mold casting, reaming, broaching, extruding, cold rolling and drawing |
Worth knowing
- Ra and Rz measure different things, and no standard converts one into the other. A common rule of thumb puts Rz at about 4 to 7 times Ra on machined surfaces; the real ratio depends on the surface's profile, so measure the parameter the drawing asks for.
How to read it
How to read a surface finish callout.
A surface finish callout is a check-mark symbol with the roughness written on it. The symbol says whether material may be removed; the number says how smooth.
The number. On an inch drawing to ASME Y14.36, a single number on the symbol is the largest Ra allowed, in microinches: 63 means Ra 63 µin at most, which is 1.6 µm. On a metric drawing it is in micrometers. An ISO 1302 symbol names the parameter outright, as Ra 1.6 or Rz 6.3, and older drawings may give a grade number instead: N7 is Ra 1.6 µm.
Ra is the average deviation of the surface's profile from its mean line, the most common roughness parameter. Rz is the profile's height from its highest peak to its deepest valley within a sampling length. Older standards used Rz for a ten-point height, so check which one an old drawing means.
Converting. Micrometers and microinches convert exactly: a microinch is 0.0254 micrometer. Ra and Rz do not: a common rule of thumb puts Rz at about 4 to 7 times Ra, and the converter shows it only as a range.
Sources: ASME B46.1 (surface texture), ASME Y14.36 (surface texture symbols), ISO 1302 (symbols, and in its 1992 edition the grade numbers), ISO 21920-2, formerly ISO 4287 (Ra and Rz).
Roughness grades
Roughness grades N1 to N12.
ISO 1302's grade numbers and their Ra, with the microinch value an inch drawing writes for each.
Roughness grade numbers and their Ra
| Grade | Ra, µm | Ra, µin, as drawings write it | Ra, µin, exactly |
|---|---|---|---|
| N12 | 50 | 2000 | 1969 |
| N11 | 25 | 1000 | 984.3 |
| N10 | 12.5 | 500 | 492.1 |
| N9 | 6.3 | 250 | 248 |
| N8 | 3.2 | 125 | 126 |
| N7 | 1.6 | 63 | 62.99 |
| N6 | 0.8 | 32 | 31.5 |
| N5 | 0.4 | 16 | 15.75 |
| N4 | 0.2 | 8 | 7.874 |
| N3 | 0.1 | 4 | 3.937 |
| N2 | 0.05 | 2 | 1.969 |
| N1 | 0.025 | 1 | 0.9843 |
Source: ISO 1302 (1992 edition; the 2002 edition dropped the grade numbers). On inch drawings the usual callouts are 250, 125, 63, 32 and 16 µin.
Typical finishes
The finish a process typically gives.
Typical ranges as commonly published for production methods: not limits from a standard. Tooling, feeds and speeds and the material move them both ways.
Typical Ra by production method
| Process | Typical Ra, µm | Typical Ra, µin | Grades |
|---|---|---|---|
| Flame cutting | 12.5 to 25 | 500 to 1000 | N10 to N11 |
| Sand casting | 12.5 to 25 | 500 to 1000 | N10 to N11 |
| Hot rolling | 12.5 to 25 | 500 to 1000 | N10 to N11 |
| Forging | 3.2 to 12.5 | 125 to 500 | N8 to N10 |
| Drilling | 1.6 to 6.3 | 63 to 250 | N7 to N9 |
| Milling | 0.8 to 6.3 | 32 to 250 | N6 to N9 |
| Turning and boring | 0.4 to 6.3 | 16 to 250 | N5 to N9 |
| Investment casting | 1.6 to 3.2 | 63 to 125 | N7 to N8 |
| Permanent mold casting | 1.6 to 3.2 | 63 to 125 | N7 to N8 |
| Reaming | 0.8 to 3.2 | 32 to 125 | N6 to N8 |
| Broaching | 0.8 to 3.2 | 32 to 125 | N6 to N8 |
| Extruding | 0.8 to 3.2 | 32 to 125 | N6 to N8 |
| Cold rolling and drawing | 0.8 to 3.2 | 32 to 125 | N6 to N8 |
| Die casting | 0.8 to 1.6 | 32 to 63 | N6 to N7 |
| Grinding | 0.1 to 1.6 | 4 to 63 | N3 to N7 |
| Honing | 0.1 to 0.8 | 4 to 32 | N3 to N6 |
| Polishing | 0.1 to 0.4 | 4 to 16 | N3 to N5 |
| Lapping | 0.05 to 0.4 | 2 to 16 | N2 to N5 |
| Superfinishing | 0.025 to 0.2 | 1 to 8 | N1 to N4 |
Typical ranges, not a standard's limits. The drawing's own value is the requirement; ask the shop what its process holds.
Questions
What people ask about surface finish.
How do you convert Ra from micrometers to microinches?
Divide by 0.0254, or multiply by about 39.37: a microinch is exactly 0.0254 micrometer. So Ra 3.2 µm is 126 µin, which inch drawings write as 125.
Can Ra be converted to Rz?
Not exactly. Ra is the average deviation of the surface's profile from its mean line; Rz is the height from its highest peak to its deepest valley. A common rule of thumb puts Rz at about 4 to 7 times Ra on machined surfaces, but the real ratio depends on the shape of the profile and no standard defines a conversion, so measure the parameter the drawing asks for.
What is an N8 surface finish?
N8 is roughness grade 8 of ISO 1302: Ra 3.2 µm, written 125 µin on an inch drawing. The grades run from N1 (Ra 0.025 µm) to N12 (Ra 50 µm), each roughly double the one before. The 2002 edition of ISO 1302 dropped the grade numbers, but older drawings still carry them.
What does 63 mean on a surface finish symbol?
On an inch drawing, a 63 over the surface texture symbol is an Ra of at most 63 microinches, which is 1.6 µm, roughness grade N7. On a metric drawing the number is in micrometers.
What surface finish does machining give?
Typically Ra 0.8 to 6.3 µm (32 to 250 µin) for milling, 0.4 to 6.3 µm (16 to 250 µin) for turning and boring, and 0.1 to 1.6 µm (4 to 63 µin) for grinding. These are typical ranges, not limits: the tooling, the feeds and speeds and the material move them.
From SpecCapture
Reading a whole drawing?
SpecCapture reads a drawing's surface roughness requirement from its title block, with the material, the finish and the heat treatment. SpecCapture reads the title block, the revision history, the bill of materials, the hole table, the application block, the tabulated dash numbers and the general notes off 2D engineering drawings, with a box on the drawing around every value it reads, so checking it takes a glance.