Surface finish: the cheapest line to write and the most expensive to hold
Ra 0.8 µm takes four characters to write on a drawing and can add half again to the price of a nickel alloy part. It is the most common unforced cost on the drawings we see, ahead of tolerance and well ahead of material choice.
What each finish costs
Roughly, holding everything else constant, on titanium or a nickel superalloy:
| Ra | How it is reached | Relative cost |
|---|---|---|
| 6.3 µm | As-milled, roughing pass only | 0.8× |
| 3.2 µm | Normal finishing pass | 1.0× baseline |
| 1.6 µm | Careful finishing, sharp tooling | 1.1–1.3× |
| 0.8 µm | Fine finishing pass, controlled tool wear | 1.4–1.8× |
| 0.4 µm | Very light passes, or grinding | 2–3× |
| 0.2 µm | Grinding | 3–5× |
| 0.05 µm | Lapping or polishing | 5–10× |
The steps are not linear, and the jump from 0.8 to 0.4 is where the process usually has to change rather than just slow down.
On work-hardening alloys there is a second penalty. Reaching a fine finish means light passes, and light passes in nickel alloy risk burnishing rather than cutting, which hardens the surface and makes the finish worse — see work hardening in superalloys. So a fine finish on Inconel is not merely slower; it is technically harder to achieve consistently, and that uncertainty gets priced.
What each process reaches
| Process | Practical Ra range |
|---|---|
| Milling, roughing | 3.2–12.5 µm |
| Milling, finishing | 0.8–3.2 µm |
| Turning, finishing | 0.4–1.6 µm |
| Hard turning, CBN | 0.2–0.8 µm |
| Reaming | 0.8–1.6 µm |
| Cylindrical grinding | 0.1–0.8 µm |
| Surface grinding | 0.2–1.6 µm |
| Honing | 0.05–0.4 µm |
| Lapping / polishing | 0.012–0.1 µm |
| Wire EDM | 0.8–3.2 µm |
Turning reaches finer finishes than milling for the same effort, because the tool is in continuous contact rather than sweeping scallops. If a feature can be turned rather than milled, its finish comes cheaper.
The mistake: a global callout
A title block that says “all surfaces Ra 0.8 µm unless otherwise noted” applies a fine finish to every face of the part — including the back, the sides, the clamping surfaces and the features nobody will ever touch.
On a prismatic titanium housing with twenty faces, that can mean a dozen unnecessary finishing passes. The part works identically at Ra 3.2 µm on eighteen of them.
Write it the other way round: a coarse general callout with fine finishes marked on the faces that need them. The default should be what is cheap, and the exception should be what is expensive.
Where a fine finish actually earns its cost
Sealing surfaces. O-ring grooves and gasket faces have real finish requirements, and they are usually specified in a standard the seal manufacturer publishes. Use that number, not a guess.
Sliding and bearing surfaces. Where a shaft runs in a bore or a face wears against another.
Fatigue-critical surfaces. Surface roughness concentrates stress. On a cyclically loaded titanium part this is a genuine engineering requirement, not a cosmetic one — and it is also where surface integrity, not just roughness, matters.
Corrosion-critical surfaces. A rough surface on a duplex or nickel alloy part holds deposits and initiates pitting.
Anything that gets coated. Most coatings specify a substrate finish.
Visible cosmetic surfaces, if the part is seen — but say “cosmetic” so the shop knows it can be achieved by finishing rather than by machining.
What to specify beyond Ra
Ra is an average, and two surfaces with the same Ra can behave very differently. For critical surfaces consider:
Rz — mean peak-to-valley height. Better than Ra at catching isolated deep scratches, which is what usually fails a seal.
Lay direction. Circumferential or axial lay matters for seals and for fatigue. Specify it where it matters.
Surface integrity. On hard-turned or heavily machined superalloy parts, the subsurface condition can matter more than the roughness. If white layer or residual tensile stress would be a problem, say so — and say how it will be verified. Hard turning or grinding covers the white layer risk.
A practical drawing pattern
General surface finish: Ra 3.2 µm. Bore Ø40 H7 and face A: Ra 0.8 µm, circumferential lay. O-ring groove per seal supplier drawing, Ra 0.4 µm, Rz 3.2 µm max. Remaining surfaces: as machined.
Four lines, and the part costs what it should. Compare that with a single global Ra 0.8 µm, which costs more and communicates less.
Reference: surface roughness.
Send the drawing — if a finish callout is costing you money without buying anything, it will be in the first reply.