Machining accuracy refers to the degree to which the actual geometric parameters (size, shape and position) of the part after machining are in accordance with the ideal geometric parameters. In machining, errors are unavoidable, but the error must be within the limits allowed. I analyzed and summarized the errors in machining, and according to some basic rules of its changes, we took corresponding measures to reduce machining errors to improve machining accuracy. Now I will discuss it with you. 1 Machining produces errors Main factors 1.1 Positioning error. First, the benchmark does not coincide with the error. The basis on which the surface dimensions and locations are used to determine a surface is called the design basis. The reference on which the size and position after processing the surface to be machined in this process are determined on the process drawing is called the process reference. When machining a workpiece on a machine tool, several geometric elements on the workpiece must be selected as the positioning reference during machining. If the selected positioning reference does not match the design basis, a reference misalignment error will occur. Second, the positioning sub-manufacturing inaccuracy error. The positioning elements on the fixture are not absolutely accurate in their basic dimensions, and their actual dimensions (or positions) are allowed to vary within the specified tolerances. The workpiece positioning surface and the fixture positioning component together form a positioning pair. Due to the inaccurate positioning of the positioning pair and the maximum position variation of the workpiece caused by the matching gap between the positioning pairs, the positioning sub-manufacturing inaccuracy error is called. 1.2 Spindle rotation error. The spindle rotation error refers to the amount of variation of the actual rotation axis of the spindle at each instant relative to its average rotation axis. The main reasons for the spindle radial rotation error are: the coaxiality error of the spindles of the spindle, various errors of the bearing itself, the coaxiality error between the bearings, and the spindle winding. Appropriately improve the manufacturing precision of the spindle and the box, select high-precision bearings, improve the assembly accuracy of the spindle components, balance the high-speed spindle components, and pre-tighten the rolling bearings to improve the rotation accuracy of the machine tool spindle. 1.3 Drive chain error. The transmission error of the transmission chain refers to the error of the relative motion between the first and last transmission elements in the internal linkage. Transmission errors are caused by manufacturing and assembly errors in the various components of the drive train and wear during use. 1.4 Track error. The guide rail is the reference on the machine tool to determine the relative positional relationship of each machine tool component, and is also the benchmark for machine tool motion. The accuracy requirements of the lathe guide are mainly reflected in three aspects: 1 straightness in the horizontal plane; 2 straightness in the vertical plane; 3 parallelism (distortion) of the front and rear rails. In addition to the manufacturing errors of the guide rail itself, the uneven wear of the guide rail and the quality of the installation are also important factors in the guide rail error. 1.5 Geometric error of the tool. Any tool in the cutting process will inevitably wear out, causing a change in the size and shape of the workpiece. Proper selection of tool materials and selection of new wear-resistant tool materials, reasonable selection of tool geometry parameters and cutting amount, proper use of coolant, etc., can minimize tool size wear. If necessary, compensation devices can also be used to automatically compensate for tool size wear. 1.6 Errors caused by deformation of the process system. 1 is the workpiece stiffness. In the process system, if the workpiece stiffness is relatively low relative to the machine tool, tool, and fixture, the deformation caused by insufficient rigidity of the workpiece under the action of the cutting force has a greater influence on the machining accuracy. 2 is the tool stiffness. The external turning tool has a large rigidity in the normal direction of the machined surface (y), and its deformation is negligible.内The inner diameter of the smaller diameter, the rigidity of the arbor is very poor, and the deformation of the arbor has a great influence on the machining accuracy of the hole. 3 is the rigidity of the machine tool components. The machine tool components are composed of many parts. There is no suitable simple calculation method for the machine component stiffness. At present, the experimental method is mainly used to determine the machine component stiffness. The deformation is not linear with the load. The loading curve and the unloading curve do not coincide, and the unloading curve lags behind the loading curve. Only when the starting point of the loading curve coincides with the end point of the unloading curve, the residual deformation is gradually reduced to zero. 1.7 Error caused by thermal deformation of the process system. The thermal deformation of the process system has a great influence on the machining accuracy. Especially in precision machining and large-piece machining, the machining error caused by thermal deformation can sometimes account for 50% of the total error of the workpiece. 1.8 Adjustment error. In each process of machining, the process system is always adjusted in one way or another. Since the adjustment cannot be absolutely accurate, an adjustment error is generated. In the process system, the mutual positional accuracy of the workpiece and the tool on the machine is ensured by adjusting the machine tool, tool, fixture or workpiece. When the original precision of machine tools, tools, fixtures and workpiece blanks meets the process requirements without considering dynamic factors, the influence of adjustment errors plays a decisive role in the machining accuracy. 1.9 Measurement error. When the part is measured during or after machining, the measurement accuracy is directly affected by the measurement method, the accuracy of the gage, and the workpiece and subjective and objective factors. 2 Measures to improve machining accuracy 2.1 Reduce the original error. Improve the geometric accuracy of the machine tool used for machining parts, improve the accuracy of the fixture, gage and tool itself, control the error caused by the force and deformation of the process system, reduce the deformation error caused by tool wear and internal stress, and minimize the measurement error. Belongs to directly reduce the original error. In order to improve the machining accuracy, it is necessary to analyze the original errors that cause machining errors, and take corresponding solutions to the main original errors that cause machining errors according to different situations. For the machining of precision parts, the geometric precision and rigidity of the precision machine tools used and the thermal deformation of the machining should be improved as much as possible. For the machining of parts with forming surfaces, how to reduce the shape error of the forming tools and the installation error of the tools. ? 2.2 Error compensation method. For some original errors of the process system, an error compensation method can be adopted to control its influence on the machining error of the part. 2.2.1 Error compensation method: This method artificially creates a new original error, which compensates or cancels the original error inherent in the original process system, and achieves the purpose of reducing machining error and improving machining accuracy. 2.2.2 Error cancellation method: Use the original original error to partially or completely offset the original original error or another original error. 2.3 Differentiating or homogenizing the original error. In order to improve the processing accuracy of a batch of parts, a method of differentiating some original errors can be adopted. For the surface of the part with high processing precision, it is also possible to adopt a method of gradually homogenizing the original error during the continuous trial cutting process. 2.3.1 Differentiation original error (grouping) method: According to the error reflection law, the workpiece size of the blank or process is divided into n groups according to the size, and the size range of each group is reduced to the original 1 n. Then, according to the error range of each group, the exact position of the tool relative to the workpiece is adjusted, so that the center of the size dispersion range of each group of workpieces is basically the same, so that the size dispersion range of the whole batch of workpieces is greatly reduced. 2.3.2 Homogenization Original Error: The process of this method is a process of continuously reducing and averaging the original error of the surface being machined by machining. The principle of homogenization is to find out the difference between the closely related workpieces or tool surfaces by comparing and inspecting them, and then perform mutual correction processing or reference processing. 2.4 Transfer original error. The essence of the method is to shift the original error from the error-sensitive direction to the error-insensitive direction. Transfer the original error to a non-sensitive direction. The extent to which various original errors are reflected in the machining error of the part is directly related to whether it is in the direction of error sensitivity. If you try to transfer it to the non-sensitive direction of the machining error during the machining process, the machining accuracy can be greatly improved. Transfer the original error to other aspects that have no effect on the machining accuracy. In short, it is inevitable that errors will occur during machining . Only a detailed analysis of the causes of the error can take the corresponding preventive measures to reduce the machining error as much as possible, thus effectively improving the accuracy of machining. Teach you to improve machining accuracy |
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