Machining error refers to the degree of deviation between the actual geometric parameters (geometric size, geometric shape, and mutual position) of the part after machining and the ideal geometric parameters. The degree of agreement between the actual geometric parameters and the ideal geometric parameters after the part is machined is the machining accuracy.
The smaller the machining error, the higher the degree of conformity and the higher the machining accuracy. Machining accuracy and machining error are two formulations of a problem. Therefore, the size of the machining error reflects the level of machining accuracy. The main reasons for machining errors are as follows.
1. Manufacturing errors of machine tools
The manufacturing error of the machine tool mainly includes the spindle rotation error, the guide rail error, and the transmission chain error. Spindle rotation error refers to the variation of the actual rotation axis of the spindle relative to its average rotation axis at each instant, which will directly affect the accuracy of the workpiece to be processed.
The main reasons for the spindle rotation error are the coaxiality error of the spindle, the error of the bearing itself, the coaxiality error between the bearings, and the rotation of the spindle. The guide rail is the benchmark for determining the relative positional relationship of each machine tool component on the machine tool, and it is also the benchmark for the machine tool movement. The manufacturing error of the guide rail itself, the uneven wear of the guide rail, and the installation quality are the important factors that cause the guide rail error. Transmission chain error refers to the relative motion error between the transmission elements at the beginning and end of the transmission chain. It is caused by manufacturing and assembly errors of each component in the transmission chain, as well as wear during use.
2. The geometric error of the tool
Any tool will inevitably wear during the cutting process, which will cause changes in the size and shape of the workpiece. The influence of tool geometric error on machining error varies with the type of tool: when a fixed-size tool is used for machining, the manufacturing error of the tool will directly affect the machining accuracy of the workpiece; for general tools (such as turning tools, etc.), its manufacturing error It has no direct effect on machining errors.
3. The geometric error of the fixture
The function of the fixture is to make the workpiece equivalent to the tool and the machine tool have the correct position, so the geometric error of the fixture has a great influence on the machining error (especially the position error).
4. Positioning error
The positioning error mainly includes the reference misalignment error and the inaccurate manufacturing error of the positioning pair. When processing the workpiece on the machine tool, several geometric elements on the workpiece must be selected as the positioning datum during processing. ) does not coincide, and a reference misalignment error will occur.
The workpiece positioning surface and the fixture positioning element together form the positioning pair. The maximum position variation of the workpiece caused by the inaccurate manufacturing of the positioning pair and the matching gap between the positioning pairs is called the manufacturing inaccuracy error of the positioning pair. The inaccurate manufacturing error of the positioning pair will only occur when the adjustment method is used for processing, and will not occur in the trial cutting method.
5. Error caused by force deformation of process system
Workpiece stiffness: If the workpiece stiffness in the processing system is relatively low compared to machine tools, tools, and fixtures, under the action of cutting force, the deformation of the workpiece due to insufficient stiffness will have a greater impact on machining errors.
Tool rigidity: The rigidity of the cylindrical turning tool in the normal (y) direction of the machined surface is very large, and its deformation can be ignored. When boring an inner hole with a small diameter, the rigidity of the toolbar is very poor, and the force deformation of the toolbar has a great influence on the machining accuracy of the hole.
The rigidity of machine tool components: 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 stiffness of machine tool components is mainly determined by experimental methods. The factors affecting the rigidity of machine tool components include the influence of contact deformation of the joint surface, the influence of friction, the influence of low-rigidity parts, and the influence of clearance.
6. Errors caused by thermal deformation of the processing system
The thermal deformation of the processing system has a great influence on the machining error, especially in precision machining and large-scale machining, the machining error caused by thermal deformation can sometimes account for 50% of the total workpiece error.
7. Adjustment error
In each process of machining, there is always one way or another adjustment to the processing system. Since the adjustment cannot be absolutely accurate, an adjustment error occurs. In the processing system, the mutual positional accuracy of the workpiece and the tool on the machine tool is guaranteed by adjusting the machine tool, tool, fixture, or workpiece. When the original precision of machine tools, tools, fixtures, and workpiece blanks all meet the technical requirements without considering dynamic factors, the adjustment error plays a decisive role in the machining error.
8. Measurement error
When the part is measured during or after processing, the measurement accuracy is directly affected by the measurement method, the accuracy of the measuring tool, the workpiece, and subjective and objective factors.
9. Internal stress
The stress that exists inside the part without external force is called internal stress. Once internal stress is generated on the workpiece, the workpiece metal will be in an unstable state of high energy level. It will instinctively transform to a stable state of low energy level, accompanied by deformation, so that the workpiece loses its original machining accuracy.
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