Machining accuracy refers to the degree of conformity between the actual geometric parameters (dimensions, shape, and position) of a machined part and its ideal geometric parameters. In mechanical machining, errors are inevitable, but they must be kept within permissible limits. Through error analysis, one can grasp the basic patterns of error variation and thereby take corresponding measures to reduce machining errors and improve machining accuracy.
I. Main Causes of Errors in Mechanical Machining
(i) Spindle Rotation Errors
Spindle rotation error refers to the variation between the actual instantaneous rotation axis of the spindle and its average rotation axis. The main causes of radial spindle rotation errors include: coaxiality errors of the spindle’s journal sections, various errors of the bearings themselves, coaxiality errors between bearings, spindle deflection, etc. Appropriately improving the manufacturing accuracy of the spindle and its housing, selecting high-precision bearings, enhancing the assembly accuracy of spindle components, balancing high-speed spindle components, and preloading rolling bearings can all improve the rotation accuracy of the machine tool spindle.
(ii) Guideway Errors
Guideways serve as the reference for determining the relative positional relationships of various machine tool components and also as the benchmark for machine tool motion. The accuracy requirements for lathe guideways mainly involve three aspects: straightness in the horizontal plane; straightness in the vertical plane; and parallelism (twist) of the front and rear guideways. In addition to manufacturing errors in the guideways themselves, uneven wear and installation quality of the guideways are also important factors causing guideway errors.
(iii) Transmission Chain Errors
Transmission errors in a transmission chain refer to the relative motion errors between the first and last transmission elements in an internally linked transmission chain. Transmission errors are caused by manufacturing and assembly errors of the various components in the transmission chain, as well as wear during use.
(iv) Tool Geometric Errors
Any tool inevitably undergoes wear during the cutting process, which in turn causes changes in workpiece dimensions and shape. Correctly selecting tool materials, adopting new wear-resistant tool materials, reasonably choosing tool geometric parameters and cutting parameters, and properly using coolant can all minimize tool dimensional wear. When necessary, compensation devices can also be used to automatically compensate for tool dimensional wear.
(v) Locating Errors
a) Datum mismatch error:On a part drawing, the datum used to determine the dimensions and position of a surface is called the design datum. On a process sheet, the datum used to determine the dimensions and position of the machined surface after processing is called the process datum. When machining a workpiece on a machine tool, certain geometric features of the workpiece must be selected as locating datums. If the selected locating datum does not coincide with the design datum, a datum mismatch error occurs.
b) Inaccurate manufacturing error of locating feature pairs:The locating elements on a fixture cannot be manufactured with absolute accuracy according to the nominal dimensions; their actual dimensions (or positions) are allowed to vary within specified tolerance ranges. The workpiece locating surface and the fixture locating elements together form a locating feature pair. The maximum positional variation of the workpiece caused by inaccurate manufacturing of the locating feature pair and the clearance fit between them is called the inaccurate manufacturing error of the locating feature pair.
(vi) Errors Caused by Deformation of the Technological System Under Force
a) Workpiece rigidity:In the technological system, if the rigidity of the workpiece is relatively low compared to that of the machine tool, tool, and fixture, the deformation of the workpiece due to insufficient rigidity under the action of cutting forces will have a relatively large impact on machining accuracy.
b) Tool rigidity:An external turning tool has high rigidity in the direction normal (y) to the machined surface, and its deformation can be neglected. When boring a small-diameter inner hole, the boring bar has very low rigidity, and its deformation under force significantly affects hole machining accuracy.
c) Machine tool component rigidity:Machine tool components consist of many parts. To date, there is no simple and suitable calculation method for the rigidity of machine tool components; currently, experimental methods are mainly used to measure the rigidity of machine tool components. Deformation is not linearly related to load; the loading curve and unloading curve do not coincide, with the unloading curve lagging behind the loading curve. The area enclosed between the two curves represents the energy dissipated during the loading and unloading cycle, consumed by frictional work and contact deformation work. After the first unloading, the deformation does not return to the starting point of the first loading, indicating the presence of residual deformation. After multiple loading-unloading cycles, the starting point of the loading curve coincides with the end point of the unloading curve, and the residual deformation gradually decreases to zero.
(vii) Errors Caused by Thermal Deformation of the Technological System
Thermal deformation of the technological system has a relatively large impact on machining accuracy. Especially in precision machining and large-part machining, machining errors caused by thermal deformation can sometimes account for 50% of the total workpiece error. Machine tools, tools, and workpieces are subjected to various heat sources, causing their temperatures to gradually rise. At the same time, they dissipate heat to surrounding materials and space through various heat transfer methods.
(viii) Adjustment Errors
In each step of mechanical machining, some kind of adjustment work must be performed on the technological system. Since adjustment cannot be absolutely accurate, adjustment errors occur. In the technological system, the mutual positional accuracy of the workpiece and the tool on the machine tool is ensured by adjusting the machine tool, tool, fixture, or workpiece. When the original accuracies of the machine tool, tool, fixture, and workpiece blank all meet process requirements and dynamic factors are not considered, the influence of adjustment errors plays a decisive role in machining accuracy.
(ix) Measurement Errors
When parts are measured during or after machining, the measurement method, measuring tool accuracy, workpiece conditions, and subjective and objective factors all directly affect measurement accuracy.
II. Measures to Improve Mechanical Machining Accuracy
(i) Reducing Original Errors
Directly reducing original errors includes improving the geometric accuracy of the machine tools used for part machining, enhancing the accuracy of fixtures, measuring tools, and cutting tools themselves, and controlling deformation of the technological system under force and heat, tool wear, deformation caused by internal stresses, and measurement errors. To improve machining accuracy, it is necessary to analyze the various original errors that cause machining errors and take different measures to address the main original errors causing machining errors according to different situations. For precision part machining, the geometric accuracy, rigidity, and thermal deformation control of the precision machine tools used should be improved as much as possible. For machining parts with formed surfaces, the main focus is on reducing the shape error of the forming tool and the tool installation error.
(ii) Error Compensation Method
For some original errors in the technological system, error compensation methods can be adopted to control their influence on part machining errors.
a) Error compensation method: This method artificially creates a new original error to compensate for or cancel out the inherent original errors in the original technological system, thereby reducing machining errors and improving machining accuracy.
b) Error cancellation method:This method uses one type of original error to partially or completely cancel out the original error or another type of original error.
(iii) Differentiation or Equalization of Original Errors
To improve the machining accuracy of a batch of parts, methods of differentiating certain original errors can be adopted. For part surfaces requiring high machining accuracy, the method of gradually equalizing original errors through successive trial cutting processes can also be used.
a) Differentiation (grouping) method for original errors: Based on the law of error reflection, the dimensions of blanks or workpieces from the previous operation are measured and divided into n groups according to size, reducing the dimension range of each group to 1/n of the original. Then, according to the error range of each group, the accurate position of the tool relative to the workpiece is adjusted separately, so that the center of the dimensional dispersion range of each group of workpieces is basically consistent, thereby greatly reducing the overall dimensional dispersion range of the entire batch of workpieces.
b) Equalization method for original errors: This process involves continuously reducing and averaging the original errors on the machined surface through machining. The principle of equalization is to identify differences between closely related workpiece or tool surfaces through mutual comparison and inspection, and then perform mutual correction machining or reference machining.
(iv) Transferring Original Errors
The essence of this method is to transfer original errors from the error-sensitive direction to the error-non-sensitive direction. The degree to which various original errors are reflected in part machining errors is directly related to whether they lie in the error-sensitive direction. If measures are taken during machining to transfer them to the error-non-sensitive direction, machining accuracy can be greatly improved. Transfer original errors to other aspects that do not affect machining accuracy.
III. Conclusion
In mechanical machining, errors are inevitable. Only by conducting a detailed analysis of the causes of errors can appropriate preventive measures be taken to reduce machining errors and improve mechanical machining accuracy.
