Ra is the arithmetic average of the absolute deviations of the roughness profile from the mean line.
Typical Ra by process:
Rough milling: Ra 6.3-12.5μm
Semi-finishing: Ra 3.2-6.3μm
Finishing: Ra 0.8-1.6μm
Grinding: Ra 0.2-0.4μm
Polishing: Ra 0.05-0.1μm
Mirror polish: Ra 0.01-0.025μm
Ra does not distinguish between peaks and valleys — a sharp peak and a wide valley can have the same Ra if average height matches.
Rz (ten-point height): average of 5 highest peaks and 5 deepest valleys. More sensitive to extreme values. Rmax: maximum peak-to-valley height. Used for sealing surfaces and fatigue-critical parts.
Practical conversion: Ra ≈ Rz/5 (machined) to Rz/7 (ground). This is approximate, not precise — if your drawing specifies Ra 0.8, don't be surprised when Rz measures 4-5μm.