The machining accuracy refers to the degree of conformity of the actual geometric parameters (size, shape and position) of the parts with the ideal geometric parameters. In machining, errors are unavoidable, but errors must be within the allowable range. Through the error analysis, grasp the basic law of its change, so as to take corresponding measures to reduce the processing error and improve the processing accuracy.
⒈The main cause of the error in machining
⑴Spindle rotation error. Spindle rotation error refers to the change of the actual rotation axis of the spindle at each moment relative to the average rotation axis of the main reasons for the spindle radial rotation error are: the coaxial error of the spindle neck, the various errors of the bearing itself, the coaxial error between the bearings, the spindle winding, etc. Properly improving the manufacturing accuracy of the spindle and box, selecting the high-precision bearing, improving the assembly accuracy of the spindle parts, balancing the components of the high-speed spindle parts, pre-tightening the rolling bearing, etc., can improve the rotation accuracy of the spindle of the machine tool.
⑵ Guide rail error. Guide rail is the reference to determine the relative position relationship of each machine tool component on the machine tool, and also the reference of machine tool movement. The accuracy of the lathe rails requires the following three aspects: straightness in the horizontal plane, straightness in the vertical plane, and parallelism (distortion of the front and rear rails). In addition to the manufacturing error of the rail itself, the uneven wear and installation quality of the rail are also important factors causing the rail error.
⑶Transmission chain error. The transmission error of the transmission chain refers to the error of the relative movement between the first and last two ends of the transmission elements in the internal transmission chain. Transmission error is caused by the manufacturing and assembly error of each component of the transmission chain, as well as the wear in the use process.
⑷Geometric error of the tool. Any tool in the cutting process, will inevitably produce wear, and thus change the size and shape of the workpiece. The correct selection of tool materials and new types of wear-resistant tool materials, reasonable selection of tool geometric parameters and cutting amount, the correct use of coolant, can minimize the size of the tool wear. When necessary, the compensation device can also be used to automatically compensate for the tool size wear.
⑸Positioning error. First, the datum does not overlap the error. The reference on which a surface size and position is called a design reference. The reference used on the process diagram to determine the size and position of the process after the machining surface is called the process reference. When processing the workpiece on the machine tool, several geometric elements on the workpiece must be selected as the positioning benchmark during processing. If the selected positioning benchmark does not coincide with the design benchmark, the benchmark will be inconsistent error. Twenty positioning secondary manufacturing inaccurate error. The positioning elements on the fixture cannot be manufactured absolutely accurately in the base size, and their actual dimensions (or positions) are allowed to vary within the separately specified tolerances. The workpiece positioning surface and the fixture positioning element jointly constitute the positioning auxiliary. The maximum position change of the workpiece caused by the inaccuracy of the positioning auxiliary manufacturing and the coordination gap of the positioning accessory is called the inaccurate error of the positioning auxiliary manufacturing.
⑹the error caused by the force deformation of the process system. One is the workpiece stiffness. In the process system, if the stiffness of the workpiece is relatively low compared to the machine tool, cutting tool and fixture, under the action of the cutting force, the deformation caused by the insufficient stiffness of the workpiece has a greater impact on the machining accuracy. The second is the tool stiffness. The outer circular knife has great stiffness in the normal direction of the machining surface, and its deformation is negligible. The inner hole with a small boring diameter has a poor stiffness of the knife rod, and the force deformation of the knife bar has a great impact on the processing accuracy of the hole. Three is the stiffness of the machine tool parts. Machine tool parts consof many parts. There is no suitable simple calculation method to calculate the stiffness of machine tool parts, but the test method is mainly used to determine the stiffness of machine tool parts. The deformation and the load are not linear, the loading curve and the unloading curve do not coincide, and the unloading curve lags behind the loading curve. The area between the two curve lines is the energy lost in the load and unloading cycle, which consumes the work done by the friction and the contact deformation work. After the first unloading, the deformation cannot recover the starting point of the first loading, indicating that there is residual deformation. After repeated loading and unloading, the starting point of the loading curve coincides with the end point of the unloading curve, and the residual deformation gradually decreases to zero.
⑺Error caused by the thermal deformation of the process system. The thermal deformation of the process system has a great impact on the processing accuracy, especially in the precision machining and large piece machining, and the processing error caused by the thermal deformation can sometimes account for 50% of the total error of the workpiece. Machine tools, cutting tools and workpieces are affected by a variety of heat sources, the temperature will gradually rise, and they also emit heat to the surrounding material and space through various kinds of heat transfer methods.
⑻Adjustment error. In each process of mechanical processing, the process system should always be adjusted in this way or another. Because the adjustment cannot be absolutely accurate, the adjustment error occurs. In the process system, the mutual position accuracy of the workpiece and the cutting tool on the machine tool is guaranteed by adjusting the machine tool, the cutting tool or the workpiece. When the original accuracy of machine tools, cutting tools, fixture and workpiece blank all meet the process requirements without considering the dynamic factors, the influence of adjustment error plays a decisive role in the processing accuracy.
⑼ Measurement error. When the parts are measured during or after processing, the measurement method, measuring accuracy, workpiece and subjective and objective factors all directly affect the measurement accuracy.
