Understanding Ra surface roughness values in mass finishing

A clear explanation of Ra measurements, how they relate to your process outputs, and how to achieve consistent surface roughness targets across production runs.

What is Ra?

Ra (Roughness Average) is the most widely used parameter for measuring surface roughness. It represents the arithmetic mean of the absolute deviations of the surface profile from the mean line, measured in micrometres (µm). A lower Ra value means a smoother surface.

Typical Ra values by process

• After machining (turned/milled): 0.8–6.3 µm
• After light vibratory deburring: 0.4–1.6 µm
• After vibratory polishing (ceramic): 0.2–0.8 µm
• After burnishing (steel media): 0.05–0.4 µm
• After fine polishing (multi-stage): <0.1 µm

How vibratory finishing affects Ra

Vibratory finishing generally reduces Ra — it smooths peaks and removes sharp edges. The extent of reduction depends on:

• Starting surface condition of the component
• Media type and grade (coarser = more cut, finer = smoother finish)
• Cycle time (longer = lower Ra)
• Compound used

Achieving a consistent Ra target

To hit a specific Ra target consistently across production runs:

1. Establish your process parameters through trial (media grade, load ratio, amplitude, cycle time)
2. Document those parameters and fix them — don't vary them between runs
3. Monitor incoming component condition — variations in machining can shift your Ra result
4. Replace media regularly — worn media cuts less aggressively, raising Ra over time
5. Check compound concentration — diluted compound reduces cut rate

Our team can help you establish a validated process with documented parameters. Contact us to discuss your Ra requirements.