Engineer Q&A

Q: What is controlled force grinding of bearing rings?

A:When grinding on a general internal surface grinder, the feed speed and feed time are predetermined (called controlled speed grinding or constant feed grinding). Therefore, the final size, surface roughness and geometric shape deviation of the workpiece are affected by the sharpness of the grinding wheel, the hardness of the workpiece material, the machining allowance and the grinding parameters. The selection of feed speed and feed time is always based on the worst blank to be processed, so the efficiency of the machine tool and the grinding wheel cannot be fully utilized, resulting in very low productivity.

The controlled force grinding method can effectively eliminate the above-mentioned shortcomings of ordinary grinding.When grinding bearing rings, the force exerted by the grinding wheel on the workpiece can be decomposed into three components: axial, tangential and radial. Generally, the axial grinding force is very small and can be ignored; the radial grinding force is 2-3 times the tangential grinding force, so it is more valuable to control the radial grinding force. Controlled force grinding is to control the size and change law of the radial force during the grinding process.

The figure below is a simplified diagram of the principle of controlled force grinding.

The grinding wheel frame is installed on the transverse feed carriage supported by the hydrostatic guide rail. Given the static radial grinding force P0, the applied feed force F0 should be equal to P0 as a constant value, and the friction in the transverse feed system must be reduced to a negligible level. Since the static grinding force P0 is constant, the deflection of the grinding wheel shaft will also remain unchanged, so the instantaneous displacement of the transverse feed carriage can be used to determine the inner diameter of the workpiece during the grinding process. At the same time, according to the grinding force P0, the deformation deflection of the grinding wheel shaft is known in advance, and the grinding wheel frame can be rotated to a certain appropriate angle a to compensate for the bending deformation of the grinding wheel shaft. In this way, the reciprocating direction of the workpiece headstock can be kept parallel to the grinding wheel surface, and the inner hole after processing will not produce errors such as taper. In fine grinding or channel grinding, the angle a can be made zero.

The lateral feed force F0 is generated by the hydraulic system and its size can be adjusted. Tests show that when grinding medium-sized outer raceways, the control force is generally 6-15N per millimeter of the workpiece surface width. There are generally two feed methods for grinding the inner surface: lateral feed of the grinding wheel frame and the carriage and lateral feed of the workpiece head frame. Tests have shown that both feed methods are feasible. But in theory, lateral feed of the grinding wheel frame is better than lateral feed of the workpiece head frame.