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Measures to reduce thermal deformation of CNC machine tools

1. Reduce fever

The heat generated inside the machine tool is the main heat source that causes thermal deformation. The heat source should be separated from the main machine as much as possible. At present, most CNC machine tools have external motors, gearboxes, hydraulic devices and fuel tanks. For heat sources that cannot be separated from the host machine, such as spindle bearings, screw nut pairs, high-speed motion guide rail pairs, etc., their friction characteristics and lubrication conditions must be improved to reduce heat inside the machine tool.

The spindle component is a key component that directly affects the machining accuracy, and the bearing on the spindle is usually a large internal heat source. In addition to using precision rolling bearings and oil mist lubrication on CNC machine tools, static pressure bearings can also be used. These measures are beneficial to reducing the temperature rise of the spindle. In the spindle box of precision CNC machine tools, the use of heating components such as friction clutches should be avoided as much as possible. The chips generated during machine tool processing are also a heat source that cannot be ignored. CNC machine tools that generate a large amount of chips must be equipped with a complete chip removal device to take away the heat as quickly as possible. Heat shields can also be installed on the workbench or guide rails to isolate this heat from the machine tool. In CNC machine tools that use cutting fluid, after the cutting fluid cools the tool and workpiece, it takes away the cutting heat. When it is scattered throughout the machine tool, it will also produce a local temperature rise. Precision CNC machine tools should control the temperature of the cutting fluid and quickly discharge the cutting fluid from the machine tool through the shortest path. The lubricating oil flows between the transmission parts, taking away the friction heat, causing the temperature of the lubricating oil pool to gradually increase, becoming a secondary heat source. Therefore, in some precision CNC machine tools, the lubricating oil pool has been moved out of the machine tool. Of course, the temperature of the oil pool can also be controlled. The hydraulic transmission system and its oil sump are another source of heat on the machine tool.

In addition to moving the oil pool out of the machine tool as much as possible, the oil supply volume of the oil pump must be appropriately selected, otherwise a large amount of excess oil will flow through the overflow valve, which will waste energy and generate a lot of heat. Therefore, for hydraulic systems that require frequent changes in oil supply, variable pumps should be used as much as possible.

2. Control temperature rise

After taking a series of measures to reduce heat sources, the thermal deformation situation will be improved, but it is usually very difficult or even impossible to completely eliminate the internal and external heat sources of the machine tool, so it must be controlled through good heat dissipation and cooling. temperature rise to reduce the impact of heat sources.

A more effective method is to perform forced cooling on the hot parts of the machine tool. At present, for multi-axis CNC machine tools, because it requires high precision in several directions, it is difficult to use compensation methods to reduce the impact of thermal deformation.

For this type of machine tool, the method of using a refrigeration system to force-cool the lubricating fluid can achieve good results. However, the cooling capacity of the refrigeration system must be appropriate. If the heat absorption is greater than the heating capacity of the internal heat source of the machine tool, the temperature of the machine tool will be lower than the ambient temperature, which will not only cause shrinkage, but also the moist air will condense on the surface of the machine tool and cause the machine tool to erode. rust. In addition to forced cooling, heating can also be used in the low-temperature part of the machine tool to make the temperature of each point of the machine tool consistent. This can maintain the uniformity of the temperature field and reduce warpage deformation caused by temperature differences. Some larger CNC machine tools are equipped with heaters, which shorten the warm-up time of the machine tool by heating before processing to increase the actual productivity of the machine tool.

3. Improve machine tool structure

Under the same heating conditions, the machine tool structure also has a great influence on thermal deformation. At present, CNC machine tools designed according to the principle of thermal symmetry have achieved good results. Therefore, the single column structure used in CNC machine tools in the past may be replaced by a double column structure. Due to the left-right symmetry of the double-column structure, the main axis axis after heating has very little deformation in other directions except for vertical translation. The vertical axis movement can be easily compensated by a coordinate correction amount. For the spindle box of CNC lathes, try to make the thermal deformation of the spindle occur in the vertical direction of the cutting tool. This is because the offset of the tool tip along the tangential direction of the workpiece has very little effect on the change in the radial size of the workpiece and can almost be ignored. Structurally, the distance between the spindle center and the bottom surface of the spindle box should also be minimized as much as possible to reduce the total amount of thermal deformation; at the same time, the temperature rise of the front and rear of the spindle box should be consistent to avoid tilting of the spindle after deformation.

The ball screw in CNC machine tools operates under the conditions of large preload, high speed and poor heat dissipation, so the screw is extremely easy to heat up. The consequences of thermal elongation of the ball screw are serious, especially in open-loop systems, where it will cause the feed system to lose positioning accuracy. Currently, some machine tools use pre-tensioning methods to reduce thermal deformation of the screw. This method is to make the pitch slightly smaller than the nominal value when processing the ball screw, and pre-stretch the screw during assembly to make the pitch reach the nominal value. When the screw is heated while working, the tensile stress in the screw compensates for the thermal stress, which not only reduces the influence of thermal elongation, but also increases the stiffness of the screw. For thermal deformation that cannot be eliminated after taking the above measures, the CNC system can send out compensation pulses to correct it based on the measurement results.

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