Ra is the number on the drawing, and it is also the most over-specified number on the drawing. A blanket "Ra 0.8 all over" on a stamped bracket buys a polishing operation nobody needed and adds cost to every part in the programme, while leaving a sealing face with no requirement at all.
What is a typical Ra value?
Ra is the arithmetic mean roughness of the surface profile, measured in micrometres (µm) or microinches (µin). As a rule of thumb: hot rolled steel arrives at 3.2–12.5 µm, cold rolled sheet at 0.8–3.2 µm, a laser-cut edge at 3.2–12.5 µm, a turned or milled face at 0.8–3.2 µm, a ground face at 0.2–1.6 µm and a polished face at 0.05–0.4 µm. A drawn part inherits the die surface, usually 0.4–1.6 µm, and an electropolished surface typically improves Ra by 30–50% compared with the same surface before treatment. Coatings add roughness of their own: a powder coat reads about 1–3 µm because of its orange-peel texture.
What Ra does and does not measure
Ra averages the peaks and valleys over a sampling length, so it says nothing about the shape of the profile. A deeply scored surface and a finely textured one can report the same Ra. That is why the more complete callout includes the evaluation length and the cutoff, and why a critical sealing or bearing face is usually specified as Ra together with a machining process rather than as a number on its own. For sealing faces we agree the measurement method with the customer rather than pretending that one number describes the surface.
Ra by process
| Process | Typical Ra (µm) | Equivalent (µin) | Note |
|---|---|---|---|
| Hot rolled steel, as received | 3.2–12.5 | 125–500 | Mill scale; usually removed or coated |
| Cold rolled sheet, as received | 0.8–3.2 | 32–125 | The starting point for most stamped parts |
| Laser cut or blanked edge | 3.2–12.5 | 125–500 | Dross and striations on the cut face |
| Drawn surface (die transfer) | 0.4–1.6 | 16–63 | Depends on die polish and lubrication |
| Turning | 0.8–3.2 | 32–125 | Feed marks; 0.4 achievable with finish passes |
| Milling | 0.8–3.2 | 32–125 | Cusp height from step-over |
| Grinding | 0.2–1.6 | 8–63 | Directional lay |
| Honing | 0.1–0.8 | 4–32 | Used for bores and cylinder walls |
| Polishing | 0.05–0.4 | 2–16 | Manual; cost scales with area and access |
| Electropolishing | Improves the previous value by roughly 30–50% | - | Also deburrs and passivates |
| Zinc or nickel plating | Replicates the substrate with slight levelling | - | Not a route to a smoother surface |
| Powder coating | 1–3 | 40–120 | Orange-peel texture; smoothing is not its job |
| Passivation | No measurable change | - | Chemical cleaning, not a surface finish |
Ranges are the typical published figures for each process. The value you actually receive depends on the tool, the parameters, the material and the measurement settings, so a requirement that matters should name the process, the Ra value and the measurement convention.
How to specify it so you get what you need
- Mark the finish only where it functions - a sealing face, a sliding face, a contact face, a visible cosmetic face.
- Give the process together with the number: "ground, Ra 0.8" is enforceable; "Ra 0.8" on its own invites a polishing operation that costs more.
- State the evaluation length or cutoff if the value is critical, and where on the part it is measured.
- Separate cosmetic from functional requirements. A visible face judged by eye is a different specification from a face measured with a profilometer.
- Remember that measuring a curved or interrupted surface is not straightforward, and agree how it will be checked before first article.
Where the money goes
- As-received sheet - if the part only needs to look consistent, cold rolled sheet at 0.8–3.2 µm is usually acceptable and free.
- Die finish - a drawn part gets its finish from the die, so specifying a smoother surface often means polishing the die once rather than finishing every part.
- Grinding and polishing - the cost driver is area and access, not the Ra number, so confining the finish to a small face can cut a finishing operation entirely.
- Coating - a powder coat hides small surface defects but adds its own texture; if a smooth painted face matters, that is a coating system choice, not a metal finish choice.
Key point
Put the Ra callout on the faces that function, give the process with the number, and say how it will be measured. A blanket finish specification on a stamped part is one of the quietest cost additions in a programme, and it is usually invisible until someone compares two quotations.
Frequently asked questions
What does Ra mean?
Ra is the arithmetic mean of the surface profile deviations from the mean line, over a defined sampling length. It is expressed in micrometres or microinches and it averages peaks and valleys, so it does not describe the shape of the surface.
What is a good surface finish?
It depends on function: 3.2 µm is normal for a painted or non-functional face, 0.8 µm suits most sealing and mating faces, and 0.4 µm or better is for sliding or optical surfaces. A tighter number than the function needs is added cost with no benefit.
Does powder coating improve surface roughness?
No. Powder coating adds a film with its own texture, typically reading around 1–3 µm. It hides small defects and gives a uniform colour, but it does not make a rough surface smooth.
Can you measure Ra on a curved surface?
It can be done with the right fixture and a skidless pick-up, but it is not as straightforward as on a flat face. Where the value matters on a curved or interrupted surface, agree the measuring method before first article.