Surface finish reference
Ra is the number most drawings use, but it is not the whole story. Rz, the process, and the direction of the lay all change how a surface looks and performs.
Ra vs Rz
Ra is the arithmetic average of the profile height over the sampling length. Rz is the average maximum peak-to-valley height. Two surfaces can have the same Ra and look different because Rz captures the peaks that cause leaks, wear and visible scratches. As a rough guide, Rz is 4–6× Ra for a machined surface; the exact ratio depends on the process.
| Ra (µm) | Rz (µm, approx.) | What it looks like | Typical process |
|---|---|---|---|
| 0.05–0.2 | 0.2–1.0 | Mirror | Polishing, lapping |
| 0.4–0.8 | 2–4 | Bright, fine tool marks | Fine turning, fine milling |
| 0.8–1.6 | 4–8 | Smooth machining marks | CNC milling/turning, good finish |
| 1.6–3.2 | 8–16 | Visible tool marks | Standard CNC milling |
| 3.2–6.3 | 16–32 | Coarse marks | Roughing, bead blasting |
| 6.3–12.5 | 32–63 | Rough | Sawing, coarse blasting |
Approximate values. The ratio depends on the process; specify Ra and Rz when both matter.
What each process gives you
| Process | Typical Ra (µm) | Notes |
|---|---|---|
| CNC milling, standard | 1.6–3.2 | Face and pocket milling; visible stepover |
| CNC milling, fine | 0.8–1.6 | Smaller stepover, sharper tool, slower feed |
| CNC turning | 0.8–3.2 | Insert geometry and feed rate drive the finish |
| Fine turning | 0.4–0.8 | Wiper insert or low feed; good for seal faces |
| Wire / sinker EDM | 0.8–3.2 | Recast layer; may need a polish after |
| Bead blasting | 1.6–6.3 | Uniform matte; hides tool marks |
| Polishing | 0.05–0.4 | Cosmetic; can round edges if overdone |
These are starting points, not guarantees. The machine, tool, coolant and part geometry all move the result.
How to call it out
- Mark the surface, not the whole part. A global Ra 0.8 callout can double the cost of a part that only needs a seal face at Ra 0.8.
- Use Ra for function, Rz for peaks. Sealing faces and optical parts often need both.
- Say “as-machined” when cosmetics do not matter. It gives the shop permission to choose the efficient route.
- Send a sample for cosmetic parts. A photo is not a finish specification; a physical sample is.
Common mistakes
Calling out Ra 0.4 on a deep pocket that cannot be reached with a polishing tool. Specifying a mirror finish on a part that will be painted. Using a bead-blasted finish on a sealing face. Marking a global tolerance and a global finish, then wondering why the quote is high. Each of these adds cost without adding function.
Send the drawing and tell us what the surface has to do — seal, slide, reflect, or just look good. We will propose the process and the callout. Surface finishing · CNC milling · EDM machining · Quality and inspection.
Written by the engineering team at GY Machining, Dongguan. We machine prototypes and production parts in aluminum, stainless, titanium, brass and engineering plastics.