What Ra actually measures
Drag a stylus across a machined face and it rides over peaks and valleys the eye can barely see. Ra is the arithmetic mean of those deviations from the centre line, in micrometres. Bigger number, rougher surface. It is the most commonly specified roughness parameter, measured with a profilometer, and it is what the tick symbols with numbers on your drawing are asking for.
Ra is an average, which means it hides character: a surface with occasional deep scratches can share an Ra with an evenly textured one. For most engineering purposes that does not matter. Where it does, sealing against pressure, bearing surfaces, additional parameters like Rz exist, and the drawing should say so explicitly rather than hoping.
The grades, in practice
| Ra (µm) | How it is produced | Look and feel | Typical use |
|---|---|---|---|
| 6.3 | Roughing cuts, heavy machining | Visible tool marks you can feel | Non-critical faces, surfaces to be machined again |
| 3.2 | Standard milling and turning | Clean machined texture, fine marks visible | The general-purpose default for most faces |
| 1.6 | Fine machining: sharper tools, lighter cuts | Smooth to the touch, faint marks | Mating faces, gasket lands, better cosmetics |
| 0.8 | Very fine machining or grinding | Near-matt sheen, no felt texture | Sealing faces, bearing seats, sliding fits |
| 0.4 | Grinding, honing | Smooth sheen | Dynamic seals, precision journals |
| 0.2–0.1 | Honing, lapping, polishing | Mirror territory | Hydraulic sealing bores, optical and special cases |
The values are the widely used reference points; individual shops and processes shade them either way. Two things follow from the table. Each step down roughly changes the process, not just the effort: 3.2 is a normal cut, 1.6 is a careful cut, 0.8 and below usually means a second operation on that face. And the cost is per face, which is the whole trick of specifying finish well.
When finish actually matters
- Sealing. O-ring grooves and gasket faces need controlled roughness in a band: too rough leaks past the peaks, too smooth can starve a dynamic seal of lubrication. Follow the seal maker's stated range rather than guessing smoother is better.
- Sliding and rotating. Bearing journals, bushes, guide faces: roughness becomes wear rate and friction. This is classic 0.8 and finer territory, on those diameters only.
- Fatigue. Rough surfaces seed cracks. Highly stressed, cyclically loaded features earn a finer finish, and the surface texture of the cut matters alongside the number.
- Appearance and cleaning. Visible faces, food and medical surfaces where cleanability is specified. Note that coatings change the game: paint hides texture, while anodising faithfully reproduces it, so cosmetic anodised faces need the finish before the tank.
Everything else on the part is happy at the as-machined default. A drawing that puts Ra 0.8 on every face, like a blanket-tight tolerance, turns ordinary faces into second operations nobody benefits from.
Specifying it well, and proving it happened
Three habits make finish callouts work. Put the symbol on the specific faces that need it, with the general finish stated once in the title block. Use the standard your industry expects, Ra in microns for most UK work, and state the band where a seal demands one. And say what the face does in your RFQ pack, because "sealing face for hydraulic oil" tells the machinist more than the number alone.
Finish is also an inspection item: a specified Ra is a measurable promise, checked with a profilometer, not judged by thumb. TrueNorth treats it that way. Callouts get sanity-checked at the DFM review, the right process route gets chosen per face, and specified finishes are verified before dispatch along with the dimensions, as part of the same documentation pack.