Ra
Ra (roughness average) is the arithmetical mean of absolute height deviations of a surface profile from the mean line over a sampling length. Lower Ra means smoother surface. It is the most common surface-finish callout on engineering drawings, typically reported in µm or µin.
On the shop floor, Ra determines whether a part can be left as machined, needs a finer toolpath, or must go to grinding and polishing. Most standard CNC machined parts land in the 1.6–3.2 µm Ra range, while functional surfaces like bearing seats and seals often require 0.8–1.6 µm Ra. A profilometer measures the average deviation of peaks and valleys over a set evaluation length, so the reading reflects the overall statistical texture rather than isolated defects. This matters because Ra directly affects friction, sealing, wear, and appearance. When a drawing specifies Ra, the machinist can adjust feed, chip load, insert geometry, and finishing passes to hold the surface callout. For sub-1.6 µm Ra work, secondary operations such as grinding or polishing are typically required, as normal milling or turning will not reliably achieve that finish.
- Tight Ra callout on a rough-milling operation: Running a high feed and chip load leaves visible tool marks that fail the Ra spec even when dimensions are good. Ra measures surface texture, not size, so a separate finish pass is required.
- Confusing Ra with peak-to-valley height: A surface can pass with a low average roughness while a few deep scratches remain because Ra hides isolated peaks and valleys. Severe defects need Rz or a visual inspection to catch.
- Inconsistent stylus measurement: Taking a reading across directional tool marks, with the wrong cutoff length, or with a worn stylus can misrepresent the finish. A good surface may look bad or a bad surface may pass; repeatable technique is essential.
Is Ra the only roughness parameter that matters?
No. Ra is the most common but does not fully describe peak severity or valley depth; other parameters such as Rz are used when maximum profile features matter.
What does a typical CNC finish spec look like?
General machined parts are often specified around 3.2 µm Ra (125 µin), with tighter functional surfaces at 1.6 µm Ra or 0.8 µm Ra depending on the application. Finishing passes and stable tooling are needed to hold these values.
Why can two parts with the same Ra behave differently?
Ra is a statistical average, so two surfaces can have the same arithmetic mean roughness but different peak shapes, lay direction, and valley distribution. That changes sealing, friction, and wear performance even though the Ra number is identical.