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. After machining, the degree of conformity between the actual geometric parameters and the ideal geometric parameters is the machining accuracy. The smaller the machining error, the higher the degree of conformity, the higher the machining accuracy. Machining accuracy and machining error are two kinds of formulation of a problem. Therefore, the degree of machining error reflects the level of machining accuracy.
The main cause of machining error
1. Machine tool manufacturing error
Machine tool manufacturing errors mainly include spindle rotation error, guide rail error and transmission chain error.
The spindle rotation error refers to the variation of the actual rotation axis of the spindle relative to its average rotation axis at each moment, which will directly affect the accuracy of the workpiece to be processed. The main causes of spindle rotation error are the coaxiality error of spindle, the error of bearing itself, the coaxiality error between bearings and the winding degree of spindle. The guide rail is the datum to determine the relative position relationship of each machine tool component on the machine tool, and it is also the datum of the machine tool movement. The manufacturing error of the guide rail, the uneven wear of the guide rail and the installation quality are the important factors causing the guide rail error. Transmission chain error refers to the relative motion error between transmission elements at both ends of transmission chain. It is caused by the manufacturing and assembly errors of each link in the transmission chain, as well as the wear in the process of use.
2. The geometric error of the cutting tool
The geometric error of the cutting tools is inevitably worn in the cutting process, and the size and shape of the workpiece will be changed. The influence of geometric errors of cutting tools on machining errors varies with the types of cutting tools: when fixed size cutting tools are used, the manufacturing errors of cutting tools will directly affect the machining accuracy of workpieces; while for general cutting tools (such as turning tools), their manufacturing errors have no direct impact on the machining errors.
3. The geometric error of fixture
The function of fixture is to make the workpiece equivalent to the tool and machine tool have the correct position, so the geometric error of fixture has a great influence on the machining error (especially the position error).
4. The positioning errors
The positioning error mainly includes the misalignment error of datum and the manufacturing inaccuracy error of positioning pair. When machining the workpiece on the machine tool, it is necessary to select some geometric elements on the workpiece as the positioning datum during machining. If the selected positioning datum does not coincide with the design datum (the datum used to determine the size and position of a certain surface on the part drawing), the datum misalignment error will occur.
The positioning surface of the workpiece and the positioning element of the fixture constitute the positioning pair together. The maximum position variation of the workpiece caused by the inaccuracy of the positioning pair and the matching clearance between the positioning pairs is called the manufacturing inaccuracy error of the positioning pair. The inaccuracy error of positioning pair can be produced only when the adjustment method is used, but not in the trial cutting process.
5. Error caused by stress deformation of process system
Workpiece stiffness: in the process system, if the workpiece stiffness is relatively low relative to the machine tool, cutting tool and fixture, under the action of cutting force, the deformation caused by the insufficient stiffness of the workpiece has a greater impact on the machining error.
Cutting tool stiffness: 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. The boring diameter of the inner hole is small, the rigidity of the cutting tool is very poor, the stress deformation of the cutter arbor has a great influence on the machining accuracy of the hole.
Stiffness of machine tool components: machine parts are composed of many parts, so far there is no suitable simple calculation method for the stiffness of machine tool components. At present, the main method is to measure the stiffness of machine tool components by experimental methods. The factors that affect the stiffness of machine tool components include contact deformation of joint surface, friction force, low stiffness 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 the precision machining and large workpiece processing, the processing error caused by thermal deformation sometimes accounts for 50% of the total error of the workpiece.
7. Adjustment error
In every process of machining, the process system must be adjusted in one way or another. Because the adjustment cannot be absolutely accurate, there will be adjustment error. In the process system, the mutual position accuracy of workpiece and cutting tool on the machine tool is ensured by adjusting the machine tool, cutting tool, fixture or workpiece. When the original accuracy of machine tool, cutting tool, fixture and workpiece blank meet the process requirements without considering the dynamic factors, the adjustment error plays a decisive role in the machining error.
8. Measuring error
When parts are measured during or after machining, the measurement accuracy is directly affected by the measurement method, measuring accuracy, workpiece and subjective and objective factors.
9. Internal stress
The stress in the part without external force is called internal stress. Once the internal stress is generated on the workpiece, it will make the workpiece metal in an unstable state of high energy level. It will instinctively transform to the stable state of low energy level, accompanied by deformation, so that the workpiece will lose its original processing accuracy.