Surface finish Ra, Rz, N
What an N7 actually is, what Ra to aim for with a given process, and why Ra and Rz don't convert into one another.
Your requirement
The two fields are linked: picking a class fills in the Ra.
Enter an Ra, or pick a class
The N classes are defined by their Ra value, exactly. The Rz figures shown are usual orders of magnitude, never to be written on a drawing in place of a measurement.
Full table of classes
From the finest surface to the coarsest.
| Class | Ra µm | Ra µin | Typical Rz | Processes |
|---|---|---|---|---|
| N1 | 0.025 | 1.0 | 0.15 to 0.4 | Lapping, optical polishing |
| N2 | 0.05 | 2.0 | 0.3 to 0.8 | Lapping, superfinishing |
| N3 | 0.1 | 3.9 | 0.6 to 1.5 | Fine grinding, lapping |
| N4 | 0.2 | 7.9 | 1 to 2.5 | Fine grinding |
| N5 | 0.4 | 16 | 1.6 to 4 | Grinding, fine boring |
| N6 | 0.8 | 32 | 3 to 7 | Grinding, fine turning |
| N7 | 1.6 | 63 | 6 to 12 | Finish turning and milling |
| N8 | 3.2 | 126 | 12 to 25 | General turning and milling |
| N9 | 6.3 | 248 | 25 to 45 | Fine roughing, drilling |
| N10 | 12.5 | 492 | 45 to 90 | Roughing, sawing |
| N11 | 25 | 984 | 90 to 160 | Heavy roughing, flame cutting |
| N12 | 50 | 1,969 | 160 to 300 | As-cast, as-forged |
Ra and Rz measure two different things
Ra is the arithmetic mean of the profile's deviations from its mean line. It's a stable value, little affected by an isolated accident, and that's what makes it the reference parameter: two labs measuring the same part will find close Ra values.
Rz is the height between the highest peaks and the deepest valleys, averaged over several sampling lengths. It states the amplitude, where Ra states the tendency. A single scratch sends Rz jumping while barely touching Ra.
This is why there's no exact conversion between the two. The Rz/Ra ratio typically runs from 4 to 7 depending on the process: a ground surface and a turned surface at the same Ra won't have the same Rz, because their asperities aren't the same shape. The ranges in the table above are there to give you a sense of scale, not to fill in an inspection report.
The N classes, and why you still meet them
Classes N1 to N12 come from the old ISO 1302. Each corresponds to a precise Ra value, in a geometric progression with a ratio of 2: N6 is 0.8, N7 is 1.6, N8 is 3.2. They no longer appear in recent versions of the standard, which ask for the Ra value to be written directly.
They remain very common on drawings in circulation, because a drawing has a long life and a drawing archive doesn't get redone. Knowing that N7 means Ra 1.6 is still useful every day.
What finish to aim for, by function
Too fine a surface costs money for nothing. A few markers. A seating face or a non-functional surface is happy with N9 to N10, which is ordinary machining. A bearing seat or a joint face wants N6 to N7. A dynamic sealing surface, a lip seal on a shaft for instance, goes down to N5 or below.
Below N4 you leave ordinary machining for fine grinding, lapping or superfinishing, and the cost climbs very fast. It's a requirement to impose only when a specific function calls for it.
Reading the symbol on a drawing
The basic symbol is an open chevron. Closed by a horizontal bar, it requires material removal. Completed with a circle, it forbids it and the surface must be left as produced. The value written to the left is the maximum Ra allowed. A note under the bar states, when present, the required process or the lay direction.
Frequently asked questions
- Can Ra be converted to Rz?
- Not exactly. Ra and Rz measure two different things about the same profile: Ra is an average, Rz a height between peaks and valleys. Their ratio depends on the shape of the asperities, so on the process. The Rz values given here are an order of magnitude, never a conversion to write on a drawing.
- What is N7?
- Ra 1.6 microns. That's the finish left by careful finish turning or milling, and the most common requirement on ordinary functional surfaces.
- Are the N classes still standardised?
- They come from the old ISO 1302 and no longer appear in recent versions, which ask for the Ra value to be stated directly. They remain very common on drawings in circulation, which is why they're still worth reading.
- Should a drawing state Ra or Rz?
- Ra in the great majority of cases: it's the most stable and most reproducible parameter. Rz is used when the peaks and valleys are what matter, for a seal or a coating for instance.
Surface finishes captured along with the dimensions
CoteCote reads the roughness symbols on the drawing, with their value and unit, and folds them into the inspection table.