How To Choose A Vitrified Grinding Wheel For Hard Grinding Of Rolling Bearing Steel Balls?

The purpose of steel ball hard grinding is to remove the surface defects of the previous process and the decarburization, oxide layer and surface troostite structure produced during heat treatment, further improve the dimensional accuracy, spherical accuracy and surface quality, and make a contribution to the subsequent grinding process. Prepare. Hard grinding is a grinding method in which the cast iron disc is used as the fixed disc and the grinding wheel is used as the abrasive tool to process the surface of the steel ball, as shown in Figure 1.

During the hard grinding process, the steel ball is guided by the annular groove on the working surface of the fixed disc, and the steel ball is pressed on the working surface of the grinding wheel, and the grinding process of the steel ball is realized under the action of pressure. After the continuous rolling of steel balls on the working surface of the grinding wheel, grooves will gradually be formed on the working surface. During the grinding process of the steel ball, since the linear velocity at the outer edge of the grinding wheel groove is greater than the linear velocity at the inner edge of the grinding wheel groove, sliding friction will inevitably occur at the contact position between the grinding wheel and the steel ball during the operation of the grinding wheel; The increase of the sliding friction will also increase; the sliding friction at the bottom edge of the grinding wheel groove is small, but the rolling effect is the largest; under this complex friction, the abrasive grains on the surface of the grinding wheel The ball acts as a grinder. The motion state of the steel ball during the grinding process is also complex, including the revolution of the steel ball around the axis of the grinding wheel and the rotation of the steel ball around its own center. The rotation can continuously change the direction, so that any point on the surface of the steel ball has the opportunity to be ground; the revolution cooperates with the circular grinding method of the machine tool, and finally realizes the hard grinding of the steel ball.

Grinding wheel is an important tool in the hard grinding process of steel balls. Combined with the characteristics of hard grinding, in order to better ensure the accuracy and surface quality of steel balls, the following selection principles should be followed for vitrified grinding wheels for steel ball hard grinding:

1. Grinding wheel specification and size
The main specifications and dimensions of the grinding wheel include: outer diameter, aperture and thickness, which can be reasonably selected according to the technical performance of the machine tool. The common dimensions and specifications of vitrified grinding wheels for steel ball hard grinding are shown in Table 1.

Table 1 Common dimensions and specifications of vitrified grinding wheels for steel ball hard grinding

2. Abrasive type
Abrasives are an important part of abrasive tools, mainly for grinding. The choice of abrasive type in the design of abrasive tools mainly depends on the material of the grinding object. Bearing steel balls are mostly made of bearing steel, and corundum-based abrasives can be preferred. Brown corundum has strong crush resistance and high toughness, and is most commonly used in the production of steel ball hard grinding vitrified grinding wheels. In the grinding process, when the abrasive grains are passivated, due to the relatively high toughness of the corundum series abrasives, the abrasive grains cannot be broken by themselves under the action of grinding to expose new edges, which will affect the grinding efficiency. Therefore, the combination of steel ball heat treatment process and grinding According to the cutting characteristics, silicon carbide abrasives can be properly introduced, that is, the vitrified grinding wheel for hard grinding of steel balls uses a mixed abrasive of corundum and silicon carbide.
3. Abrasive particle size
When selecting the abrasive particle size, it is mainly based on the surface roughness requirements of grinding and grinding efficiency. First of all, it is necessary to ensure that the surface roughness of the workpiece after grinding meets the requirements. If the grain size is too coarse, the coarse grain abrasive will cut a large amount during grinding and generate more heat, but the heated area is small, and small spots are prone to appear on the surface of the steel ball. If the particle size is too fine, it will affect the grinding efficiency on the one hand, and on the other hand, due to the small cutting force of the fine particle size, the steel ball and the arc surface of the groove mostly exist in the form of friction, resulting in a large amount of frictional heat. Susceptible to burns. On the basis of meeting the roughness requirements of the machined surface, try to use coarse-grained abrasives to improve the grinding efficiency of the abrasive tool. Refer to Table 2 for the particle size selection of vitrified grinding wheels for steel ball hard grinding.

Table 2 The particle size of vitrified grinding wheel for different steel balls

4. Grinding wheel hardness
Hardness is one of the important indicators to measure the quality of the grinding wheel. For vitrified grinding wheels for steel ball hard grinding, if the hardness is too soft, the abrasive will fall off prematurely during the grinding process, exposing new abrasive grains. These new abrasive grains have more The sharp grinding edge has a strong grinding ability and generates a lot of heat. When the cooling is not timely, it will cause spot burns on the surface of the steel ball; at the same time, because the hardness of the grinding wheel is too soft, the groove will be reduced during the grinding process. The constraint of the surface of the groove on the steel ball can easily cause excessive variation in the diameter of the steel ball. If the hardness is too high, the self-sharpness is poor, as the diameter of the steel ball is continuously ground by the groove arc surface, the diameter of the steel ball becomes smaller, and the fit between the groove arc surface and the steel ball surface is poor, reducing the impact of the groove on the steel ball. The binding force will easily cause the steel balls to connect and roll together, and in severe cases, linear burns will appear on the surface of the steel balls.
Refer to Table 3 for the hardness selection of vitrified grinding wheels for steel ball hard grinding.

Table 3 Hardness of vitrified grinding wheel for hard grinding with different steel balls

In summary, the various characteristics of the grinding wheel have an important relationship with the grinding quality, so it is necessary to choose carefully.