Machining Methods

The Methods For Improving Machining Accuracy

Spindle rotation error. The spindle rotation error refers to the variation of the actual rotation axis of the spindle at each instant relative to its average rotation axis. The main reasons for the radial rotation error of the main shaft are: the coaxiality errors of several sections of the main shaft journal, various errors of the bearings themselves, the coaxiality errors between the bearings, the main shaft winding, etc. Appropriately improve the manufacturing accuracy of the main shaft and the box, select high-precision bearings, improve the assembly accuracy of the main shaft components, balance the high-speed main shaft components, and preload the rolling bearings, etc., which can improve the rotation accuracy of the machine tool spindle.
Guide rail error. The guide rail is a reference for determining the relative positional relationship of each machine tool component on the machine tool, and is also a reference for machine tool motion. The precision requirements of the lathe guide rails are mainly in the following three aspects: straightness in the horizontal plane; straightness in the vertical plane; parallelism (twisting) of the front and rear guides. 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 that cause the error of the guide rail.
Transmission 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 internally connected transmission chain. Transmission errors are caused by manufacturing and assembly errors of various components in the transmission chain, as well as wear during use.
Geometrical error of the tool. During the cutting process of any tool, wear is unavoidable, and this causes the size and shape of the workpiece to change. Correct selection of tool materials and selection of new wear-resistant tool materials, reasonable selection of tool geometric parameters and cutting amounts, and correct use of coolant, etc., can minimize the size of the tool wear. If necessary, a compensation device can also be used to automatically compensate for tool size wear.
Positioning error. One is that the benchmark does not coincide with the error. The datum used to determine the size and position of a surface on a part drawing is called a design datum. The reference used to determine the size and position of the processed surface on the process drawing 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 datum during processing. If the selected positioning datum does not coincide with the design datum, a reference misalignment error will occur. The second is the inaccurate error of positioning pair manufacturing. The positioning elements on the jig cannot be manufactured with absolute accuracy according to the basic dimensions, and their actual dimensions (or positions) are allowed to vary within the respectively specified tolerance ranges. The positioning surface of the workpiece and the positioning element of the fixture together constitute a positioning pair. The maximum position variation of the workpiece due to the inaccurate manufacturing of the positioning pair and the matching gap between the positioning pairs is called the inaccurate error of the positioning pair manufacturing.
Error caused by deformation of the process system. One is the rigidity of the workpiece. If the stiffness of the workpiece in the process system is lower than that of the machine tool, tool, and fixture, under the action of the cutting force, the deformation of the workpiece due to insufficient rigidity will have a greater impact on the machining accuracy.
The second is the tool rigidity. The rigidity of the external turning tool in the normal (y) direction of the machining surface is very large, and its deformation can be ignored. When boring an inner hole with a small diameter, the rigidity of the shank is very poor, and the deformation of the shank’s force will greatly affect the accuracy of the hole processing.
Third is the stiffness of machine tool parts. Machine tool components are composed of many parts. There is no suitable simple calculation method for the stiffness of machine tool components. At present, the main method is to use experimental methods to determine the stiffness of machine tool components. The deformation is not linear with the load, the loading curve and the unloading curve do not coincide, and the unloading curve lags the loading curve. The area contained between the two curves is the energy lost in the loading and unloading cycle, which is consumed by the work of friction and contact deformation work; after the first unloading, the deformation cannot be recovered from the starting point of the first loading This indicates that there is residual deformation. After multiple 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.
Error caused by thermal deformation of the process system. The thermal deformation of the process system has a greater influence on the processing accuracy, especially in precision machining and large parts processing, the processing error caused by thermal deformation can sometimes account for 50% of the total error of the workpiece. Machine tools, cutters and workpieces are gradually heated by various heat sources, and they also radiate heat to surrounding materials and spaces through various heat transfer methods.
Adjustment error. In each process of mechanical processing, the process system must always be adjusted in one way or another. Since the adjustment cannot be absolutely accurate, an adjustment error occurs. In the process system, the mutual position accuracy of the workpiece and tool on the machine tool is guaranteed by adjusting the machine tool, tool, fixture or workpiece. When the original accuracy of machine tools, cutters, fixtures and workpiece blanks all meets the process requirements without considering the dynamic factors, the influence of adjustment errors plays a decisive role in machining accuracy.

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