Surface Finish (Ra) Reference Table

The Ra (roughness average) scale ranks machined surfaces from very rough to mirror-smooth, with the operation that typically produces each level and a common application. Lower Ra is smoother, and each step finer usually adds a grinding, lapping, or polishing pass, so Ra is a cost dial as well as a finish spec. This table covers the full ISO 21920-2 range from Ra 25µm down to Ra 0.05µm.

How to read the Ra scale

Ra is the arithmetic average of surface height deviations, reported in micrometres (µm) or microinches (µin); lower means smoother.

The scale range and what each row shows

The scale spans more than two orders of magnitude, from Ra 25µm on a saw-cut or flame-cut edge down to Ra 0.05µm on an electro-polished medical surface. Each row pairs a level with the operation that normally reaches it (saw, mill, grind, lap, electro-polish) and a typical use, so you can read both what a surface will look like and what it will cost to get there. The reading is local, so a critical surface is measured in more than one spot.

Matching Ra to function

Match the callout to what the surface does, not to a generic fine target. A non-mating exterior is fine at Ra 6.3 or 3.2, the standard as-machined finish. A locating face or a tapped bore is comfortable at Ra 3.2 or 1.6. Bearing journals, seal seats, and hydraulic surfaces need Ra 0.8 or finer, which calls for grinding. Sliding or precision-guideway surfaces move to Ra 0.4, and gauge, optical, or implant surfaces need lapping, honing, or electro-polishing at Ra 0.2 down to 0.05. Specifying Ra 0.4 on a bracket exterior that only needs Ra 3.2 adds lapping cost for no functional gain.

A worked example

A worked example makes the choice concrete. A hydraulic cylinder body might call for Ra 0.4 on the bore (ground and honed for the piston seal), Ra 1.6 on the port faces (a clean milling finish for the fitting seat), and Ra 3.2 everywhere else (standard as-machined). Three callouts on one part, each tied to a function, instead of a blanket fine spec. The bore is the only surface that earns the honing cost; the rest stay at general finish and the part costs less to make.

Operation to finish range

Each machining operation has a natural Ra band, which sets the finish a part gets before any secondary work. Sawing and rough milling sit around Ra 6.3 to 25µm; face and end milling land at 1.6 to 6.3µm; a finishing end-mill or reaming pass reaches 0.8 to 1.6µm. Grinding spans 0.2 to 1.6µm depending on whether it is a rough or finish grind. Lapping, honing, and electro-polishing take a surface from 0.4µm down to 0.05µm. If a drawing asks for finer Ra than the chosen operation can reach, plan a secondary process and its cost.

Cost and specifying Ra

Treat Ra as a cost ladder. Moving from Ra 3.2 to 1.6 may need only a slower finishing pass, but reaching 0.8 or below adds a grinding setup, and 0.2 or below adds lapping or honing, each a separate operation with its own fixturing and inspection. Call out Ra only on the surfaces that need it, such as bearing, seal, and sliding interfaces, and let the general as-machined finish govern the rest. For sealing work, also consider Rz, which catches extreme peaks that Ra averages away.

Coatings and plating effects

Coatings and plating also change the surface you measure. Powder coat adds 60 to 120µm and masks fine machining marks, so a powder-coated face should not carry a fine Ra callout. Anodizing removes only 10 to 15µm from an aluminum surface and largely preserves the underlying finish, so the machined Ra matters underneath. Hard chrome can build up 10 to 50µm and is often used to restore an oversize surface back to size. Plan the Ra callout together with any finish or coating the part will receive, or the measured result after finishing will not match the drawing.

Limitations

Ra values are the ISO 21920-2 standard levels with typical producing operations, not a promise for a specific tool or material. The finish a given operation reaches moves with tool sharpness, feed and speed, coolant, and the material itself; free-machining brass and aluminum 6061 finish cleaner than gummy stainless or work-hardening titanium at the same settings. Confirm the achievable Ra for the specific material and process with the shop, and verify critical surfaces with a profilometer, before locking a specification.