Surface Finish Ra Explained: What Roughness Numbers Actually Mean
A drawing that says "Ra 3.2" is ordering a specific surface roughness — and it costs money. This guide explains what Ra is, what values are realistic for cast vs machined parts, and how to specify it without overpaying.
What Does Ra Mean?
Ra (roughness average) is the arithmetic average of surface profile heights and valleys over a sampling length. It is measured in micrometres, so a lower number means a smoother, finer surface. Ra is the most common single number used to specify finish worldwide.
Think of it as the "average height of the scratches" on the machined or cast surface. Ra 6.3 and above is visibly coarse; Ra 0.8 and below is almost mirror-like.
Common Ra Values in One Table
| Ra (µm) | Roughness | Typical process | Feeling |
|---|---|---|---|
| Ra 25 – 12.5 | Very coarse | Rough sand casting, shot-blasting | Markedly rough |
| Ra 6.3 | Coarse | Investment casting as-cast, general turning | Visible machining marks |
| Ra 3.2 | Medium | Good machining, special casting finishing | Fine, clearly machined |
| Ra 1.6 | Fine | Precision machining, grinding base | Smooth, uniform |
| Ra 0.8 – 0.4 | Very fine | Grinding, honing, lapping | Near-polished |
What's Realistic for Castings vs Machined Parts?
- Investment casting (as-cast): a good foundry holds about Ra 6.3 on general surfaces, improving to Ra 3.2 with special finishing treatments — without machining.
- CNC machining: comfortably achieves Ra 1.6–3.2 in standard work; Ra 0.8 and finer requires grinding or a finishing pass, adding cost and time.
- Ground / honed surfaces: Ra 0.4 or better for sealing, bearing or mating faces.
A useful reference
Many drawings from China mark cast surfaces at ≤ Ra 6.3 and add ≤ Ra 3.2 on surfaces needing a finer finish after special treatment. Specify only the faces that truly need to be smooth.
How to Specify Finish on Your Drawing
Use the standard surface-finish symbol with the Ra value above the tick, and add the roughness symbol alongside the affected dimension. Two practical rules:
- Mark the finish on the specific faces that need it — not the whole part.
- State the required value as a maximum (≤ Ra 3.2) so it is unambiguous.
If no finish is marked, the supplier assumes the standard process default — which may be coarser than you expect. When in doubt, say so.
How Ra Affects Cost
- Finer finish = more operations. Going from Ra 3.2 to Ra 0.8 can mean switching from turning to grinding.
- More inspection. Smooth surfaces need surface-roughness measurement, adding QC time.
- Higher reject risk. Tighter finish requirements on cast surfaces can lower first-pass yield.
The most expensive mistake is marking the entire part Ra 0.8 when only a bearing seat needs it. Apply finish where function demands, and let the rest be standard.
Need a quote with the right finish?
Send your drawing with the finish notes. We'll confirm achievable Ra for the material and process, and quote transparently — as-cast, machined, or ground.
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