Question 1

What is the difference between Ra and Rz?

Accepted Answer

Ra (arithmetic average roughness) is the mean absolute deviation of the profile from the mean line over the sampling length. Rz (ten-point mean roughness or maximum height) is the average of the five highest peaks plus five deepest valleys. The typical relationship is Rz ≈ 4 to 8 × Ra. Ra is statistically more stable and is the most widely specified parameter in engineering drawings.

Question 2

What is Rq (RMS roughness) and when should I use it?

Accepted Answer

Rq is the root-mean-square roughness, the square root of the mean of squared profile deviations. It is particularly useful for evaluating optical properties such as scatter and reflectivity, since scattered intensity is proportional to (Rq/λ)². For a random Gaussian surface, Rq ≈ 1.25 × Ra. Use Rq when specifying optical surfaces or tribological contact models that require RMS input.

Question 3

Which manufacturing process achieves the finest surface finish?

Accepted Answer

Lapping and polishing achieve the finest surfaces. Grinding typically reaches Ra 0.1–1.6 μm, lapping Ra 0.01–0.1 μm, and super-finishing polishing can achieve Ra below 0.001 μm for precision optical components. EDM produces Ra 0.4–6.3 μm depending on discharge energy. The choice depends on target roughness, geometry, material, and cost constraints.

Question 4

How do I read an ISO surface finish symbol?

Accepted Answer

Per ISO 1302, the basic symbol (√) is annotated with roughness value, machining method and lay direction. The Ra value in μm is written above the bar, e.g. √Ra1.6 specifies arithmetic average roughness of 1.6 μm. A bar through the symbol means material removal is required; a circle means no material removal. ANSI N grades map as follows: N6 ≈ Ra 1.6 μm, N7 ≈ Ra 3.2 μm, N5 ≈ Ra 0.8 μm.

Surface Roughness Converter · Ra / Rz / Rq

Theory