Surface roughness Ra: which value belongs on your drawing?
A part can be dimensionally correct and still be rejected, because the surface is not what the drawing needed. Surface roughness decides how the part behaves — whether it seals, how it wears, whether a coating holds — and it moves cost as much as tolerance does.
What Ra actually measures
Ra is the arithmetic mean of the absolute deviations of the surface profile from the mean line. It summarises in one number how rough a surface is. That is why it is everywhere: it is easy to measure, it repeats well, and it reflects the general quality of the machining.
But Ra is an average, and averages hide things. A single deep scratch barely moves the average — yet on a sealing face that one scratch is a leak. This is why Ra alone is not enough on critical surfaces.
Rz, and how it differs
Rz is the average of the largest peak-to-valley heights within the evaluation length. Where Ra looks at the average, Rz looks at the extremes. The single deep scratch that Ra misses is exactly what Rz catches.
One common mistake belongs here: there is no fixed conversion between Ra and Rz. Factors such as "Rz = 4 × Ra" circulating online are rough assumptions that change with how the surface was produced. Two surfaces with the same Ra can have clearly different Rz. If Rz matters, the drawing must say Rz; you cannot specify Ra and expect Rz.
- Ra is enough whenDescribing general machining quality, appearance, typical mating faces.
- Rz is needed whenSealing and gasket faces, fatigue-critical parts, surfaces to be coated — anywhere a single deep scratch is a failure.
N grades (N1–N12) and their Ra values
Older drawings carry grade codes such as "N7" instead of an Ra value. These come from ISO 1302:1992; the 2002 edition dropped them. They still appear on a great many working drawings, so the equivalence is worth knowing. Each grade is double the previous one.
| Grade | Ra (µm) | Where it appears |
|---|---|---|
| N12 | 50 | Rough cutting, flame or plasma cut edge |
| N11 | 25 | Rough machining, as-cast surface |
| N10 | 12.5 | Heavy roughing |
| N9 | 6.3 | Standard roughing |
| N8 | 3.2 | General finish — the default for milling |
| N7 | 1.6 | Fine machining — the standard for turning |
| N6 | 0.8 | Fine turning, grinding |
| N5 | 0.4 | Precision grinding |
| N4 | 0.2 | Fine grinding, honing |
| N3 | 0.1 | Honing, lapping |
| N2 | 0.05 | Lapping |
| N1 | 0.025 | Superfinishing |
Which Ra each process can hold
The ranges below are typical; they shift with material, tool condition, feed and nose radius. Before writing a value, ask which process will actually create that surface.
| Process | Typical Ra (µm) | Note |
|---|---|---|
| Milling | 0.8 – 6.3 | As-milled default is 3.2 (N8) |
| Turning | 0.8 – 6.3 | Standard turning 1.6 (N7); 0.4 reachable with a fine finishing pass |
| Drilling | 1.6 – 6.3 | Bore wall as drilled |
| Reaming | 0.8 – 3.2 | To correct hole quality |
| Grinding | 0.1 – 0.8 | When tight tolerance and low Ra are wanted together |
| Honing | 0.1 – 0.8 | Cylindrical bores |
| Lapping | 0.05 – 0.4 | Where size and surface are both critical |
The practical consequence: asking a machine shop for Ra 1.6 is normal and costs nothing extra. Ra 0.8 requires a fine finishing pass. Ra 0.4 and below usually means grinding or honing — the part moves to a second machine, and both lead time and price change.
Measurement: without a cut-off, the result is arguable
This is the point most drawings leave out. Ra depends on the wavelength up to which the instrument counts the surface as "roughness". That limit is the cut-off (λc). Too short a cut-off leaves real roughness out; too long a cut-off lets waviness into a roughness measurement. The same surface gives two different Ra values under two different cut-offs.
| Expected Ra (µm) | Cut-off λc (mm) | Evaluation length (mm) |
|---|---|---|
| 0.006 – 0.02 | 0.08 | 0.4 |
| 0.02 – 0.1 | 0.25 | 1.25 |
| 0.1 – 2 | 0.8 | 4.0 |
| 2 – 10 | 2.5 | 12.5 |
| 10 – 80 | 8.0 | 40.0 |
As a rule the evaluation length is five times the cut-off. Most machined parts fall in the 0.1–2 µm band, so 0.8 mm is the cut-off used in practice — but that is an assumption, and if the drawing is silent the two sides can measure differently and disagree.
Putting it on the drawing
- Do not give every face the same valueA face with no function needs only a general value. Putting Ra 0.8 on the whole part is not precision, it is cost.
- State the lay when it mattersThe direction of the machining marks affects sealing and wear; show it with the symbol when it is relevant.
- If you need Rz, write RzDo not specify Ra and assume the surface will be free of defects; Ra does not catch a single scratch.
- State the cut-offBelow 0.4 µm especially, leaving the measuring condition out invites a dispute at acceptance.
- Before or after coating?If the part is plated or anodised, the drawing must say which state the Ra applies to.
At Q TECH we read incoming drawings this way before production: which face genuinely needs a low Ra, which one is fine with a general value, and under what condition it will be measured. Because inspection is in-house, that conversation happens before the part ships.
- Ra
- Rz
- surface roughness
- N grades
- ISO 4288
- technical drawing
- precision machining
- Ankara Ostim
Frequently asked questions
Is there a conversion between Ra and Rz?
There is no fixed conversion. Factors such as "Rz is about 4 × Ra" are rough assumptions that change with how the surface was produced; two surfaces with the same Ra can have clearly different Rz. If Rz matters, the drawing has to state Rz.
How many microns is N7?
N7 = Ra 1.6 µm. The N grades come from ISO 1302:1992 and each is double the one before: N6 = 0.8, N7 = 1.6, N8 = 3.2 µm. The 2002 edition dropped the grades, but they still appear on many working drawings.
What is the lowest Ra a machine shop can hold?
Milling and turning typically reach 0.8 µm, and 0.4 µm is possible with fine finishing passes. Lower values usually require grinding, honing or lapping, which means an extra operation and therefore a different lead time and price.
Why does a low Ra raise the price?
A low Ra means a separate finishing pass, smaller feed, more frequent tool changes and often a second machine such as a grinder or hone. It also changes inspection: a surface that could have been accepted by eye now has to be measured with a roughness tester and reported.
Does the drawing have to state the cut-off?
It is not mandatory, but below 0.4 µm it is necessary in practice. Ra depends on the cut-off length used; if it is not stated, the manufacturer and the customer can measure under different conditions and get different results. For most machined parts the default is 0.8 mm.
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