Ra Arithmetical mean roughness Ra
Arithmetical mean roughness Ra · Estado superficial · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
Ra 1.6 means the arithmetic mean roughness of the profile on the seating surface must not exceed 1.6 µm: the smaller the number, the smoother the finish.
What controls
Arithmetic mean of the absolute values of the profile deviations from the mean line, over the evaluated sampling length.
How to verify
Contact-stylus or laser roughness tester per ISO 4287/ISO 21920; check the sampling length (cut-off) against the expected Ra.
Workshop guideline values
- General
- Ra 3.2 to 6.3 µm on general-purpose milled and turned surfaces.
- Accuracy
- Ra 0.8 to 1.6 µm on seats, guideways and close-fitting bearing faces.
- High precision
- Ra 0.1 to 0.4 µm on fine grinding, honing and dynamic sealing surfaces.
Guidance only: the drawing's value always wins.
Common mistakes when interpreting it
- Measuring along the machining marks instead of across them: the value comes out artificially good.
- Using the instrument's default cut-off length without adjusting it to the expected finish, which changes the result without the part changing.
- Accepting the requirement on a single reading: measure in several representative areas of the face.
How to get it in the workshop / if it comes out of tolerance
The most frequently specified roughness parameter on general machining drawings: it is almost always the number under the √ symbol. A low Ra (0.4-0.8) is a fine, ground-type finish; Ra 3.2-6.3 is normal milling/turning; Ra 12.5+ is roughing or as-cast.
If Ra comes out worse than required, increase spindle speed or reduce feed per revolution before changing tools: in turning, finish depends directly on feed and nose radius (Ra ≈ f²/(8·r), so reducing the feed or using an insert with a larger nose radius improves the finish without changing anything else). In milling, reduce the stepover and increase spindle speed on the finishing pass. A worn edge or built-up edge (BUE) in soft or gummy materials worsens Ra even when the parameters are correct: change the insert before touching the program. Vibration or resonance (chatter) leaves irregular marks that the numerical Ra does not always reveal but the eye does: reduce tool stickout or spindle speed if visible waviness appears.
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