Superfinishing: How a Micron-Level Process Creates Quiet, Long-Lasting Bearings

Superfinishing, also known as "super-finishing," is the most critical finishing step at the end of rolling element manufacturing. It's not simply re-grinding. Instead, under good lubrication and cooling conditions, a fine-grained oilstone is used to remove a small amount of material (0.5-2 µm per side) from the already finely ground raceway or rolling element surface over a period of 3-8 seconds using a micro-pressure of 0.15-0.6 MPa and a short stroke (f≈10-25 Hz) with high speed.

The process consists of three stages:

① Cutting—sharp abrasive grains rapidly remove peaks;

② Semi-cutting—the abrasive grains become blunt, clogging the oilstone pores and reducing cutting power;

③ Finishing—the oilstone forms an oil film on the workpiece, primarily through friction polishing, resulting in a mirror-like finish. The physical effects of ultra-finishing are reflected in "three reductions and three increases":

Roughness reduction (Ra from 0.08 µm to 0.01 µm);
Waviness reduction (reduction by over 30%);
Groove shape error reduction (ovality and convexity can be improved by 20-30%);
Contact area increase (from 15-40% after grinding to 80-95%);
Reduced residual compressive stress (cold plastic deformation produces a compressive stress layer ≥ -200 MPa, 5-15 µm deep);
Improved lubricant film retention (the surface support index Spk is reduced, and oil film formation is 2-3 times faster). Ultimately, bearing vibration is reduced by 1-2 levels, noise levels can be reduced by 3-6 dB, life is increased by 20-40%, and rated dynamic load is increased by 5-8%. Modern superfinishing machines integrate CNC oscillation, servo pressure closed-loop control, online acoustic emission monitoring, and automated loading and unloading. They can complete a set of 6201 model outer ring raceways in just 15 seconds, achieving a Cpk ≥ 1.67 and achieving "zero-defect" production. Superfinishing is essential for high-end bearings used in wind power, high-speed rail, and electric spindles. Superfinished 2.3 MW spindle bearing raceways have an Ra ≤ 0.05 µm and a roundness ≤ 1 µm, with no premature spalling after 1 billion cycles of fatigue testing. It can be said that without superfinishing, there would be no truly "silent, long-life" bearings.