Machining error refers to the degree of deviation between the actual geometric parameters (geometric size, geometric shape and mutual position) and the ideal geometric parameters of a part after processing. The degree of conformity between the actual geometric parameters and the ideal geometric parameters after part machining is the machining accuracy. The smaller the machining error and the higher the degree of conformity, the higher the machining accuracy. Machining accuracy and machining error are two ways to address the same problem. Therefore, the size of processing error reflects the level of processing accuracy. The main causes of machining errors are as follows.
1. Manufacturing error of machine tool
The manufacturing errors of machine tools mainly include spindle rotation error, guide rail error and 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 precision 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 bearings, and the spindle winding. The guide rail is the benchmark for determining the relative position of each machine tool component on the machine tool, and also the benchmark for machine tool movement. The manufacturing error of the guide rail itself, uneven wear of the guide rail and installation quality are important factors causing the guide rail error. Transmission chain error refers to the relative motion error between transmission elements at both ends of the transmission chain. It is caused by the manufacturing and assembly errors of each component of the transmission chain, as well as the wear in the use process.
2 Geometric error of tool
Any tool in the cutting process will inevitably produce wear, which will cause changes in the size and shape of the workpiece. The influence of geometric errors of cutting tools on machining errors varies with the types of cutting tools: the manufacturing errors of cutting tools will directly affect the machining accuracy of workpieces when using fixed size cutting tools; For general tools (such as turning tools), their manufacturing errors have no direct impact on machining errors.
3 Geometric error of fixture
The function of the fixture is to make the workpiece have the correct position equivalent to the cutter and machine tool, so the geometric error of the fixture has a great influence on the machining error (especially the position error).
4 Positioning error
Positioning error mainly includes datum misalignment error and positioning pair manufacturing inaccuracy error. When machining the workpiece on the machine tool, a number of 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 (the datum used to determine the size and position of a surface on the part drawing), the datum misalignment error will occur.
The workpiece locating surface and the fixture locating element together constitute the locating pair. The maximum position variation of the workpiece caused by the inaccuracy of the manufacturing of the locating pair and the fit clearance between the locating pairs is called the manufacturing inaccuracy error of the locating pair. The manufacturing inaccuracy error of the positioning pair will only occur when the adjustment method is used for processing, and will not occur when the trial cutting method is used for processing.
5. Error caused by stress deformation of process system
Workpiece stiffness: In the process system, if the workpiece stiffness is relatively low compared with the machine tool, tool and fixture, under the action of cutting force, the deformation of the workpiece due to insufficient stiffness will have a greater impact on the machining error.
Tool stiffness: the external turning tool has a large stiffness in the normal (y) direction of the machining surface, and its deformation can be ignored. For boring the inner hole with smaller diameter, the rigidity of the cutter bar is very poor, and the force deformation of the cutter bar has a great impact on the hole processing accuracy.
Stiffness of machine tool components: machine tool components are composed of many parts. Up to now, there is no suitable simple calculation method for the stiffness of machine tool components. At present, the stiffness of machine tool components is mainly measured by experiment. The factors that affect the rigidity of machine tool components include the influence of contact deformation of joint surface, friction force, low rigidity parts and clearance.
6. Error caused by thermal deformation of process system
The thermal deformation of the process system has a great influence on the machining error, especially in precision machining and large workpiece machining. The machining error caused by thermal deformation sometimes accounts for 50% of the total error of the workpiece.
7 Adjustment error
In each process of machining, the process system must be adjusted in one way or another. Because the adjustment cannot be absolutely accurate, adjustment errors are generated. In the process system, the mutual position accuracy of workpiece and cutter on the machine tool is guaranteed by adjusting the machine tool, cutter, fixture or workpiece. When the original accuracy of the machine tool, cutter, fixture and workpiece blank meet the process requirements without considering dynamic factors, the adjustment error plays a decisive role in the machining error.
8 Measurement error
When parts are measured during or after processing, the measurement accuracy is directly affected by measurement methods, measuring tool accuracy, workpiece and subjective and objective factors.
9 Internal stress
Internal stress refers to the stress that exists inside the part without the action of external force. Once the 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 will lose its original processing accuracy.