Surface Roughness (Ra) Explained
What Ra measures, typical values by process, and how to specify a roughness callout that gets machined correctly the first time
Ra (arithmetic mean roughness) is the most common surface finish measurement, expressed in micrometres (µm), and describes the average height deviation of a surface's peaks and valleys from its centreline. A lower Ra means a smoother surface; a milled aluminium surface at standard feed rates typically reads Ra 1.6-3.2 µm, while a ground and polished surface can reach Ra 0.1-0.4 µm or lower.
This page gives typical Ra values by manufacturing process, explains how to read and specify an Ra callout on a drawing, and states when a tighter roughness is functionally necessary versus cosmetic.
Typical Ra by process
Every manufacturing process leaves a characteristic surface texture, and the achievable Ra range without extra finishing steps varies widely: laser cutting leaves a striated edge texture from the cutting process itself, milling leaves tool-mark ridges spaced by the feed rate, turning leaves a helical texture from the tool path, and grinding or polishing progressively removes those marks to reach a smoother result.
Specifying a roughness tighter than the process naturally achieves forces an additional finishing step (grinding, lapping, polishing, or electropolishing), which adds cost and lead time; specifying looser than needed leaves cost on the table if a smoother surface was actually required for the application.
| Process | Typical Ra achieved | Ra range with extra finishing |
|---|---|---|
| Laser cutting (edge) | 3.2-6.3 µm | Not typically improved by finishing alone |
| CNC milling (standard feed) | 1.6-3.2 µm | 0.4-0.8 µm with fine finishing pass |
| CNC turning (standard feed) | 1.6-3.2 µm | 0.4-0.8 µm with fine finishing pass |
| Surface grinding | 0.4-0.8 µm | 0.1-0.2 µm with fine grinding |
| Polishing (mechanical) | 0.1-0.4 µm | below 0.1 µm with extended polishing |
| Electropolishing (stainless) | 0.2-0.4 µm | below 0.1 µm achievable on some grades |
Reading and writing an Ra callout
An Ra callout on a drawing is typically written as 'Ra 1.6' or 'Ra 1.6 µm', sometimes with a checkmark-style surface finish symbol pointing to the specific surface it applies to, distinct from the general surface finish that applies to the rest of the part. A callout with a single number means that value is the maximum permitted Ra unless stated as a range; a callout of 'Ra 0.8 max' or simply a target number both mean the surface must measure at or below that value.
Not every surface on a drawing needs an explicit Ra callout; specify it only on functional surfaces (bearing seats, sealing faces, sliding contacts, mating surfaces) and leave the rest to the process's natural finish, called out generally in the title block if needed (e.g. 'unless otherwise noted, Ra 3.2 µm').
When roughness matters functionally
A rougher surface increases friction and wear on sliding or rotating contacts, so bearing bores, shaft journals and slideways typically need Ra 0.4-1.6 µm depending on speed and load. Sealing faces (O-ring grooves, gasket surfaces) need a controlled Ra, usually 0.8-1.6 µm, because a surface too smooth can fail to retain lubricant film and one too rough leaks past the seal.
Fatigue-loaded parts benefit from a smoother surface because rough machining marks act as stress concentrators where cracks initiate; aerospace and high-cycle fatigue components often specify Ra 0.8 µm or better on critical radii even where the part is not otherwise a sealing or bearing surface.
Worked example
A CNC-machined shaft with a bearing journal and a keyway: the bearing journal surface is called out at Ra 0.8 µm because the bearing manufacturer's fit tables require it for correct running clearance and seal life, achieved with a fine turning pass or light grinding. The keyway and the rest of the shaft carry the general drawing note of Ra 3.2 µm, achievable directly from standard turning without an extra finishing operation, since neither surface is a rotating or sealing contact.
Design tips
- Specify Ra only on functional surfaces (bearing seats, seals, slideways); leave the rest to the process's natural finish and note it generally in the title block.
- Match the Ra target to what the process naturally achieves where possible; specifying Ra 0.4 µm on a milled pocket that does not need it forces an unnecessary finishing pass.
- For sealing faces, specify a range rather than only a maximum; too smooth a surface can fail to hold lubricant or sealant film just as too rough a surface can leak.
- State the measurement direction if it matters (Ra can differ across versus along a turned or ground texture); most drawings leave this to shop convention unless critical.
- Remember Ra is an average value; a surface can have the correct Ra but still contain isolated deep scratches. For critical sealing surfaces, also specify no visible scratches or defects.
- Group parts needing the same tight Ra into one order where possible, since grinding and polishing setups are more efficient run together than mixed with standard-finish parts.
- For stainless parts needing both a smooth Ra and corrosion resistance, specify electropolishing rather than mechanical polishing alone; it improves both simultaneously.
Applications
| Industry | Example parts | Typical Ra requirement |
|---|---|---|
| Machinery and OEM parts | Bearing housings, shafts | Ra 0.8-1.6 µm on running surfaces |
| Hydraulic and pneumatic components | Cylinder bores, seal grooves | Ra 0.2-0.8 µm |
| Medical and lab equipment | Instrument surfaces, trays | Ra below 0.4 µm, electropolished |
| Food and beverage equipment | Contact surfaces, mixing vessels | Ra below 0.8 µm |
| General sheet metal enclosures | Panels, brackets | Ra 3.2-6.3 µm, process default |
| Automotive aftermarket | Machined brackets, shafts | Ra 1.6-3.2 µm general, tighter on bearings |
| Aerospace and precision parts | Fatigue-critical radii | Ra 0.4-0.8 µm on critical features |
Pricing and lead time
Cost is driven by how far the target Ra is from the process's natural finish (a smoother target than standard milling or turning achieves forces a separate grinding or polishing step), by how many surfaces carry a tight callout (each individually specified surface adds inspection and processing time), and by the finishing method chosen (electropolishing and fine grinding cost more per surface than a standard finishing pass). Parts machined to the process's natural Ra follow standard lead times; parts needing grinding or polishing to a tighter Ra typically add 1-3 working days, and electropolishing adds 2-3 days as a separate step.
Frequently asked questions
Typically Ra 1.6-3.2 µm on aluminium and mild steel at standard feed rates; a finer feed or finishing pass can bring this down to 0.4-0.8 µm without a separate grinding operation.
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