What Is the Bearing's Superfinishing Process? Is Superfinishing Important for Bearings?

The superfinishing process is not only used in the bearing industry but is also extensively utilized in engine manufacturing. Additionally, other precision machinery and instruments are beginning to adopt this technique.

What is superfinishing in bearings?

The superfinishing process for bearings is a type of finishing process that involves feed motion to achieve micro-grinding. Before superfinishing machining, the surface is generally finely turned and ground. Specifically, this process operates under good lubrication and cooling conditions, using fine-grained abrasive tools (like oil stones) to apply minimal pressure on the workpiece. The workpiece, which rotates at a certain speed, undergoes a rapid and short reciprocal oscillation movement in a direction perpendicular to the workpiece's rotation direction.

What is the purpose of superfinishing for bearings?

In the manufacturing process of rolling bearings, superfinishing is the final step in processing the bearing raceway. It plays a crucial role in reducing or eliminating circular deviations left over from grinding, correcting the shape errors of raceways, refining surface roughness, improving the physical and mechanical properties of the surface, reducing the vibration and noise of the bearings, and enhancing the service life of the bearings.

The specific roles can be reflected in the following three aspects:

1. It can effectively reduce waviness. During the superfinishing process, to ensure that the oil stone always acts on the wave peaks without contacting the wave valleys, the arc of the oil stone in contact with the workpiece is ≥ the wavelength of the surface waviness of the workpiece. This results in greater contact pressure on the wave peaks, cutting them off and thus reducing waviness.

2. It improves the profile error of the ball bearing raceway. Ultra-precision can effectively correct about 30% of the raceway profile error.

3. It can induce compressive stress on the superfinishing surface. During the superfinishing process, primarily cold plastic deformation occurs, resulting in residual compressive stress on the workpiece surface post superfinishing.

4. It increases the contact area of the working surface of the raceway. After superfinishing, the contact bearing area on the working surface of the raceway can be increased from 15%-40% post-grinding to 80%-95%.

The superfinishing process for bearings

1. When the cutting surface of the oil stone comes into contact with the peaks of the rough raceway surface of the bearing, due to the small contact area, the pressure per unit area is large. Under a certain pressure, the oil stone is first subjected to the "reverse cutting" action of the bearing workpiece, causing some of the abrasive grains on the oil stone surface to fall off or break, exposing new, sharp abrasive grains and edges. Simultaneously, the peaks on the surface of the bearing workpiece are quickly cut off, removing the peaks and the grinding-burnt layer on the workpiece surface through the combined effect of cutting and reverse cutting. This stage is known as the cutting phase, during which the majority of the metal allowance is removed.

2. During the semi-cutting phase of the bearing, as the processing continues, the surface of the bearing gradually becomes flat. At this stage, the contact area between the oil stone and the workpiece surface increases, the pressure per unit area decreases, the cutting depth reduces, and the cutting capability weakens. Simultaneously, the pores on the oil stone surface get clogged, and the oil stone enters a semi-cutting state. This phase is known as the semi-cutting phase of the bearing finishing, where the cutting marks on the workpiece surface become shallower, and a darker luster appears.

3. The final finishing phase can be divided into two steps: the transitional grinding phase and the post-cutting grinding phase.

Transitional grinding phase: The self-sharpening of abrasive grains decreases, the edges of the abrasive grains become smooth, and the cut oxide chips start to embed into the pores of the oil stone, clogging them. This causes the abrasive grains to perform weak cutting actions accompanied by squeezing and polishing effects. During this phase, the surface roughness of the workpiece decreases rapidly, and black cut oxide chips adhere to the oil stone surface.

Post-cutting grinding phase: The friction between the oil stone and the workpiece becomes smooth, greatly increasing the contact area, and reducing pressure. The abrasive grains can no longer break through the oil film to contact the workpiece. When the oil film pressure on the support surface balances with the pressure of the oil stone, the oil stone is lifted. An oil film forms, and no cutting action occurs. This phase is unique to superfinishing machining.