Analysis of thermal deformation of CNC machine tools - Database & Sql Blog Articles

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The main cause of thermal deformation of the machine tool is the temperature difference between the heat source and the various parts of the machine. The heat source usually includes cutting heat during processing, friction heat of the motion pair and heat generation of the power source, and other external heat sources such as radiation and the surrounding environment. In addition, the inconsistency in the material, structure, shape and size of machine tool parts is also an important factor in thermal deformation.
First, the cutting heat in the metal cutting process, from the mechanical energy to the deformation energy of the material to be cut, resulting in a large amount of heat, the amount of heat generated mainly depends on the nature of the material being cut and the amount of cutting. The heat generated by the cutting is mainly distributed to the tool, the workpiece and the chip by heat transfer, and the heat is distributed between them in a certain proportional relationship. The calculation and distribution of cutting heat varies for different types of processing.
In the turning process, a large amount of heat is carried away by the chips, and the heat transferred to the workpiece is second, generally less than 30%, and the heat transferred to the tool is second, generally not more than 5%; for milling and planing, The heat transferred to the workpiece is generally less than 30%; when cutting without coolant, a large amount of chips fall on the machine bed and the workbench, and its influence on the thermal deformation of the bed and the table is obvious. However, a large amount of chips are only available during roughing, and the influence of machine tool thermal deformation on roughing accuracy is generally negligible; for finishing, since the amount of cutting and depth of cut are generally small, the heat taken away by the chips is also small. The thermal deformation of the bed and the workbench caused by them is relatively small, and it needs to be considered, depending on the specific situation. However, in the machining center, the rough and fine machining of the parts is often processed on the same machine tool, and attention should be paid to the influence of the thermal deformation of the table.
Second, the friction heat of the sports pair and the heat source of the power source have various motion pairs, such as the rolling bearing of the spindle component, the table and the guide rail, the screw and the nut, etc., the mutual movement between the moving parts generates friction, thereby causing friction Heat forms a heat source. The heat of the motor itself and the heat of the hydraulic system also become a source of heat for a certain part of the machine tool. The transmission structure of modern machine tools, especially CNC machine tools, has been greatly simplified. The spindle components have become a major part of the thermal deformation of the machine tool, and the main heat source of the spindle components comes from the heat of the spindle bearings.
Third, radiation and the surrounding environment and other external heat sources Machine tools exposed to sunlight are usually single-sided or partial, so the temperature difference between the irradiated part and the unirradiated part, resulting in deformation of the machine tool. The temperature of the surrounding environment changes with the change of temperature and day and night temperature. The temperature of the machine tool and the workpiece also changes due to air convection, which not only has a certain influence on the accuracy of the machine tool, but also affects the machining accuracy of the part. For example, when rolling a large-diameter helical gear, it takes several days and nights of continuous processing, and the temperature difference between day and night will cause waviness error on the surface of the gear tooth.
Fourth, reduce the thermal deformation of CNC machine tools Research previously analyzed the cause of thermal deformation of CNC machine tools are mainly three major heat sources, therefore, to reduce the thermal deformation of CNC machine tools, it is necessary to reduce heat, control temperature rise and improve machine tool structure, Take measures such as thermal compensation.
1. Reducing the heat inside the machine tool is the main heat source for thermal deformation. The heat source should be separated from the main machine as much as possible, such as the motor, gearbox, hydraulic device and fuel tank. For heat sources that cannot be separated from the main engine, such as spindle bearings, screw nut pairs, high-speed motion guide pairs, etc., it is necessary to improve the friction characteristics and lubrication conditions to reduce the heat inside the machine.

The spindle component is a key component that directly affects the machining accuracy, and the bearing on the spindle is a large heat source. In addition to precision rolling bearings and oil mist lubrication of bearings on CNC machine tools, hydrostatic bearings can also be used. At the same time, the use of friction components such as friction clutches should be avoided in the headstock of precision CNC machine tools.

The chips generated during machine tool machining are also a heat source that cannot be ignored. CNC machine tools that generate large amounts of chips should be equipped with a complete chip evacuation device to remove heat as quickly as possible, or to install heat shields on the table or rails so that this heat is isolated from the machine. In addition, on CNC machines that use cutting fluids, the cutting fluid cools the tool and the workpiece, taking away the heat of the cut, and when it is scattered around the machine, it also produces a local temperature rise. Therefore, precision CNC machine tools should control the temperature of the cutting fluid and allow the cutting fluid to be quickly discharged from the machine through the shortest path.
2. Control temperature rise In addition to the above measures to reduce heat, it is necessary to control the temperature rise through good heat dissipation and cooling to reduce the influence of heat sources, such as forced cooling in the hot part of the machine tool. At present, for a multi-axis CNC machine tool, since it has high precision in several directions, it is difficult to use a compensation method to reduce the influence of thermal deformation. For this type of CNC machine tool, the method of forced cooling of the lubricating fluid by the refrigeration system can achieve good results. However, it should be noted that the cooling capacity of the refrigeration system must be appropriate. If the heat absorption is greater than the heat generated by the internal heat source of the machine tool, then this will inevitably cause the temperature of the machine tool to be lower than the ambient temperature, which will not only cause shrinkage, but also humid air will condense on the machine tool. The machine is rusted on the surface.

In addition to forced cooling, the temperature of the machine tools can be tempered by heating in the low temperature part of the CNC machine. This keeps the temperature field uniform and reduces the warpage caused by the temperature difference, such as some Large CNC machine tools are equipped with heating equipment to increase the machine's actual production rate by shortening the machine's warm-up time before machining.
3. Improve the structure of CNC machine tools Under the same heat conditions, different machine tool structures have different effects on thermal deformation. Therefore, improving the structure of CNC machine tools can effectively control the thermal deformation of CNC machine tools. Therefore, when designing a CNC machine tool, it is necessary to adopt a double-column structure instead of the currently popular unit column structure. Due to the left-right symmetry of the double-column structure, the axis of the heated spindle is less deformed in the other direction except for the translation in the vertical direction, and the movement of the axis in the vertical direction can be conveniently compensated by the correction amount of one coordinate. In addition, when designing the headstock of a CNC machine tool, the thermal deformation of the spindle should be made as far as possible in the vertical direction of the tool cutting. Thus, the deviation of the tool tip along the tangential direction of the workpiece has little effect on the change of the radial dimension of the workpiece. It can be ignored. Structurally, the distance between the center of the spindle and the bottom surface of the spindle head should also be minimized to reduce the total amount of thermal deformation. At the same time, the front and rear temperature rise of the headstock should be consistent to avoid tilting after the spindle is deformed.