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Ra (arithmetical mean roughness) is the average of absolute profile deviations from the mean line. It gives a good general indication of surface texture.

Rz (maximum height) is based on the five highest peaks and five deepest valleys. It is more sensitive to individual scratches or peaks.

Ra is more commonly used internationally, while Rz is popular in European standards, especially for critical sealing surfaces.

A commonly used approximation is:

• Rz ≈ 4 × Ra for typical machined surfaces

However, the actual ratio depends on the surface profile. For ground surfaces it can be closer to 5–6×, while for turned surfaces 3–4× is typical. For critical applications, measure the parameter directly rather than converting.

N classes (N1 through N12) are roughness grade numbers per ISO 1302. Each grade doubles the Ra value from the previous grade:

• N1 = 0.025 μm, N2 = 0.05 μm, N3 = 0.1 μm, …
• Each step is 2× the previous Ra value

N classes simplify specification on drawings by using a single number instead of the actual Ra value.

Typical achievable Ra values:

• Lapping / superfinishing: 0.025–0.1 μm (N1–N3)
• Fine grinding / honing: 0.2–0.4 μm (N4–N5)
• Grinding / fine turning: 0.8–1.6 μm (N6–N7)
• Turning / milling: 3.2–6.3 μm (N8–N9)
• Rough machining: 6.3–12.5 μm (N9–N10)
• Casting / forging: 25–50 μm (N11–N12)

Standard bearing manufacturer recommendations:

• Shaft seat: Ra 0.8 μm (N6) for normal bearings, Ra 0.4 μm (N5) for precision bearings
• Housing bore: Ra 1.6 μm (N7) for normal, Ra 0.8 μm (N6) for precision
• Thrust faces: Ra 1.6 μm (N7)

Always check the specific bearing manufacturer's catalog for exact requirements.