The spindle usually uses a cooling oil circulation system, where the cooling oil flows through an oil temperature control system and then circulates inside the spindle to cool the entire spindle. At the same time, when the spindle rotates at high speed, the cooling oil is subjected to centrifugal force, entering small holes on the inner ring of the spindle bearings to directly lubricate the inside of the bearings. This cooling and lubrication method is called spindle center cooling and internal pressure lubrication, ensuring that the spindle can operate continuously at high speeds for a long time. However, the quality of the spindle cooling oil will directly affect the cooling and lubrication effect and the lifespan of the spindle, so it is particularly important to choose high-quality spindle cooling fluid.

Spindle cooling fluid is a type of coolant containing special additives, which can be used in liquid-cooled engine cooling systems operating under low-temperature conditions (below 0°C). Automotive engine coolants have four main functions: cooling, antifreeze, anti-boil, and anti-corrosion. Based on the composition of additives: inorganic and organic; based on base liquid: inorganic salt type, methanol type, ethanol type, propylene glycol type, ethylene glycol type, diethylene glycol-ethylene glycol type, propylene glycol type, dimethyl sulfoxide type; based on engine load (reinforcement coefficient): light load engine coolant and heavy load engine coolant; based on whether it is regenerated: non-regenerative and regenerative; other categories: hard water resistant, anti-corrosion cooling water, emulsified oil coolant, etc.

The ideal state of spindle cooling fluid is to be free of any solid impurities. However, in actual use, the coolant will inevitably be mixed with chips generated during cutting, especially noticeable when processing cast iron and plastic parts with powdered chips. Impurities in the coolant will act on the cutting area through the cooling holes of the tool, so impurities will also be repeatedly cut. The larger the volume of the impurities, the more impurities will be repeatedly cut in the entire circulation of the coolant, which will more significantly affect the wear of the tool and the processing effect of the workpiece. This is closely related to the filtering function of the filter.

If the filter can remove impurities larger than 5 microns in diameter, then the impurities in the coolant acting on the cutting area will not exceed 5 microns; if the filter holes are larger, for example, 106 microns, then impurities with a diameter less than 106 microns will be mixed in the coolant throughout the entire circulation. Therefore, the quality of the coolant used in the cutting area largely depends on the filter; the smaller the filter holes, the smaller the volume of impurities in the coolant, and the fewer the number of impurities.