Machining accuracy refers to the degree of conformity between the actual geometric parameters (size, shape and position) and the ideal geometric parameters after machining. In machining, the error is inevitable, but it must be within the allowable range. Through error analysis, we can grasp the basic law of its change, so as to take corresponding measures to reduce machining errors and improve machining accuracy.
First, the main reasons for the machining error (a) spindle rotation error. Spindle rotation error refers to the variation of the actual rotation axis of the spindle relative to its average rotation axis at each moment. The main causes of radial rotation error of spindle are: coaxiality error of several journals of spindle, various errors of bearings themselves, coaxiality error between bearings, spindle winding and so on. Appropriately improve the manufacturing accuracy of spindle and box, select high-precision bearings, improve the assembly accuracy of spindle components, balance high-speed spindle components, preload rolling bearings, etc. , can improve the rotary accuracy of machine tool spindle.
(2) Guide rail error. Guide rail is the benchmark for determining the relative position relationship of machine tool parts on the machine tool, and it is also the benchmark for machine tool movement. The accuracy requirements of lathe guide rail mainly include the following three aspects: straightness in horizontal plane; Straightness in vertical plane; Parallelism (twist) of front and rear guide rails. In addition to the manufacturing error of the guide rail itself, the uneven wear and installation quality of the guide rail are also important factors causing the guide rail error.
(3) The transmission chain is wrong. The transmission error of transmission chain refers to the relative motion error between the first and last transmission elements in the interconnected transmission chain. The transmission error is caused by the manufacturing and assembly errors of each link in the transmission chain and the wear and tear during use.
(4) Tool geometric error. In the process of cutting, any tool will inevitably wear, which will cause changes in the size and shape of the workpiece. Correct selection of tool materials and new wear-resistant tool materials, reasonable selection of tool geometric parameters and cutting parameters, and correct use of coolant can minimize the dimensional wear of tools. If necessary, the compensation device can be used to automatically compensate the tool size wear.
(5) Positioning error. One is the datum misalignment error. The datum used to determine the surface size and position in the part drawing is called the design datum. The benchmark used to determine the size and position of the machining surface of the process on the process diagram is called the process benchmark. When machining a workpiece on a machine tool, several geometric features on the workpiece must be selected as positioning benchmarks. If the selected positioning datum is inconsistent with the design datum, datum dislocation error will occur. Second, the positioning pair produces inaccurate errors. The positioning elements on the fixture cannot be made absolutely accurate according to the basic dimensions, and their actual dimensions (or positions) are allowed to change within their respective tolerance ranges. The workpiece positioning surface and fixture positioning element * * * form a positioning pair, and the maximum position change of the workpiece caused by the manufacturing inaccuracy of the positioning pair and the fit clearance between the positioning pairs is called positioning pair manufacturing inaccuracy error.
(6) Error caused by deformation of process system. One is the rigidity of the workpiece. In the process system, if the rigidity of the workpiece is lower than that of the machine tool, cutter and fixture, the deformation of the workpiece due to insufficient rigidity will have a great influence on the machining accuracy under the action of cutting force.
The second is the tool stiffness. The rigidity of cylindrical turning tool in the normal (Y) direction of the machined surface is very large, and its deformation can be ignored. When boring small diameter inner holes, the rigidity of the cutter bar is very poor, and the deformation of the cutter bar has a great influence on the machining accuracy of the holes.
The third is the rigidity of machine tool parts. Machine tool parts are composed of many parts. So far, there is no suitable simple calculation method for the stiffness of machine tool parts. At present, the stiffness of machine tool parts is mainly measured by experiments. There is no linear relationship between deformation and load, the loading curve and unloading curve do not coincide, and the unloading curve lags behind the loading curve.