Drawings sometimes specify Ra and sometimes Rz values — and often finer surfaces than necessary. Yet the surface finish is one of the big cost drivers of a machined part. A short overview.
What Ra and Rz measure
Ra (arithmetic mean roughness) is the average of all deviations of the profile from the mean line. It is insensitive to individual scratches and is therefore the most widely used value internationally.Rz (mean roughness depth) is the average of the largest peak-to-valley heights over five sampling lengths. It reacts more strongly to individual peaks and scratches and is common in Germany.There is no fixed conversion, because the two values measure different things. As a rule of thumb for turned and milled surfaces: Rz is about four to seven times Ra.
Which roughness each process achieves
| Ra (µm) | Rz approx. (µm) | typically achieved by |
|---|---|---|
| 6.3 | 25 to 40 | roughing, pre-machining |
| 3.2 | 16 to 25 | standard turning and milling |
| 1.6 | 6.3 to 10 | finishing |
| 0.8 | 4 to 6.3 | fine finishing |
| 0.4 and finer | below 4 | grinding, honing, polishing |
By fine finishing on our lathes and milling machines we achieve surfaces down to Ra 0.8 (Rz 4); finer surfaces are produced by grinding.
Which surface finish do you really need?
- Visible faces without a function: Ra 3.2 is almost always enough.
- Sealing and contact faces: Ra 1.6 to 3.2, depending on the seal.
- Bearing seats and fits: usually Ra 0.8 to 1.6.
- Sliding surfaces and dynamic seals: Ra 0.4 and finer, often ground.
Specify fine surfaces only on functional faces, and a general value in the title block for the rest. Every step finer costs additional passes — across the whole surface of a part this can raise the price considerably.