Surface finish affects how a part seals, slides, wears and looks. The most common way to specify it is the Ra value. Here is what Ra means, what CNC turning and VMC milling typically achieve, and how to call it out correctly.
What is Ra?
Every machined surface has tiny peaks and valleys left by the cutting tool. Ra (roughness average) is the arithmetic average height of these peaks and valleys from a mean line, measured over a set length. It is usually given in micrometres (µm). A smaller Ra means a smoother surface.
You may also see Rz, which looks at the average peak-to-valley height. Rz values are larger than Ra for the same surface, so always check which one the drawing uses.
Typical Ra values and uses
| Ra (µm) | N grade | Typical description | Common uses |
|---|---|---|---|
| 6.3 | N9 | Rough machined | Non-functional surfaces, clearance faces |
| 3.2 | N8 | Standard machined | Most general machined surfaces |
| 1.6 | N7 | Fine machined | Mating faces, light sliding surfaces, many fits |
| 0.8 | N6 | Very fine machined | Bearing seats, seal faces, precision fits |
| 0.4 and below | N5 and finer | Usually ground or polished | High-precision sealing and running surfaces |
These are general guides. What can be achieved depends on the material, tooling, machine condition and the feature itself.
What affects surface finish
- Feed rate – a slower feed on the finishing pass leaves smaller tool marks.
- Tool nose radius and sharpness – worn or chipped tools tear the surface.
- Cutting speed and coolant – correct parameters reduce built-up edge and chatter.
- Material – aluminium and brass finish easily; some stainless steels and soft materials are harder to finish well.
- Rigidity – long, thin parts or deep features can vibrate and leave chatter marks.
CNC turning vs VMC milling finish
CNC turning leaves a fine spiral feed mark around the diameter and can produce very smooth diameters with a controlled finishing pass. VMC milling leaves a different pattern depending on the cutter and tool path – face mills give a uniform pattern on flat faces, while pockets and walls show the path of the end mill. Both can achieve fine finishes on functional faces when planned for it.
How to specify finish on your drawing
- Add a general finish note, for example “Unless otherwise stated, surface finish Ra 3.2 µm”.
- Mark finer finishes only on functional surfaces using the surface texture symbol with the Ra value.
- State whether the finish applies before or after any plating or coating.
- Mention any appearance requirement separately – for example “no visible tool marks on face A”.
Finish and cost
Every step finer in surface finish usually means a slower finishing pass, more frequent tool changes and sometimes an extra operation. Like tolerances, finish should match the function. Read more in machining tolerances explained and what decides CNC machining cost.
Need a specific finish on a CNC or VMC part? Send us your drawing and we will confirm what is achievable for your material and feature.
Frequently Asked Questions
What is a typical as-machined surface finish?
A standard as-machined finish is commonly around Ra 3.2 µm. Finer finishes such as Ra 1.6 or Ra 0.8 µm are achievable with finishing passes on suitable materials, at extra time and cost.
Is a lower Ra value always better?
No. A lower Ra means a smoother surface, but it also costs more. Only sealing, sliding, bearing or appearance surfaces normally need a fine finish.
What does N7 mean on a drawing?
N7 is a roughness grade from the ISO N-scale and corresponds to Ra 1.6 µm. N8 is Ra 3.2 µm and N6 is Ra 0.8 µm.
CNC and VMC machining specialists in Metoda GIDC, Rajkot. Questions about your part? Send us your drawing.
