Specify CNC Surface Finish Ra Without Overpaying

CNC surface finish specification is one of the fastest ways to add cost to a drawing without adding function. Most machined faces need only a general note; sealing, bearing, and sliding surfaces are the ones that justify a tight Ra callout.

Start from what the process delivers by default. A standard 3-axis milled aluminum face in 6061-T6 usually lands around Ra 3.2 µm; a fine stepover with a sharp carbide end mill holds Ra 1.6 µm, and Ra 0.8 µm is realistic on flat faces with a dedicated finishing pass and rigid workholding. Below roughly Ra 0.4 µm you are into grinding, lapping, or burnishing, which means a secondary operation and extra lead time. Keep in mind that Rz typically runs three to five times the Ra number, so a drawing quoting Rz 6.3 is not the same as Ra 6.3.

Tolerances and finish interact. ISO 2768-mK covers most non-critical features, but a seal groove at ±0.05 mm combined with Ra 0.8 demands a defined finishing strategy and often a fixture. Material choice matters too: 7075-T6 and 303 stainless finish cleanly, while 316 stainless and POM/Delrin tend to tear or smear and need slower feeds, sharp tooling, and coolant control.

Practical rule: apply a general surface finish note, then add Ra callouts only where a mating surface, O-ring, or bearing requires it. State the inspection method as well, since profilometer or vision measurement reads differently from a visual check and adds inspection cost.

Need a machine shop that can hold these requirements? Explore our CNC machining services or request a quote for your part at CNCdoing.

For more guidance on related work, see: CNC Machining Thin-Wall Parts: Tolerance and Fixturing Tips.