Nowadays, more and more machine tools have realized intelligence and automation, and machining accuracy and stability have made great progress. However, sometimes it is inevitable that processing errors will occur, resulting in economic losses. What are the causes of processing errors?
1. Machine tool error
Machining error includes spindle rotation error, guide rail error and transmission chain error.
Spindle rotation error refers to the change of the actual rotation axis of the spindle relative to the average rotation axis at each instant, which directly affects the accuracy of the workpiece. The main causes of spindle rotation error include spindle coaxiality error, bearing error, coaxiality error between bearings, spindle winding, etc.
The guide rail is the reference for determining the relative position of various machine tool parts on the machine tool, and also the reference for 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.
The transmission chain error refers to the relative motion error between the transmission elements at the end of the transmission chain. This is caused by the manufacturing and assembly errors of various parts of the drive chain and the wear during use.
2. Tool error
The tool inevitably wears during the cutting process, changing the size and shape of the workpiece. The influence of tool geometry error on machining error depends on the type of tool. When using fixed size tools, the tool manufacturing tolerance directly affects the machining accuracy of the workpiece. In addition, for general tools (such as turning tools), the manufacturing error does not directly affect the processing error.
3. Fixture error
The function of the fixture is to make the workpiece and the tool equivalent, so that the machine tool is in the correct position, so the geometric error of the fixture has a great impact on the processing error (especially the position error).
4. Positioning error
Positioning error mainly includes datum misalignment error and positioning pair manufacturing error. When machining a workpiece with a machine tool, it is necessary to select some geometric elements on the workpiece as the positioning data during machining. If the positioning datum and design datum (the datum used to determine the specific surface size and position on the part drawing) are selected, if they do not match, the misalignment error will occur in the datum.
The workpiece positioning surface and the fixture positioning element jointly constitute a positioning pair. The maximum position change of the workpiece caused by the inaccurate manufacturing of the positioning pair and the fit clearance between the positioning pairs is called the inaccurate manufacturing error of the positioning pair. The inaccurate manufacturing error of the positioning pair only occurs when the adjustment method is used, not in the trial cutting method.
5. Error caused by force deformation of process system
Workpiece rigidity: in the process system, if the workpiece rigidity is relatively low compared with the machine tool, tool and fixture, the deformation of the workpiece due to insufficient rigidity will have a greater impact on the deformation of the workpiece under the action of cutting force. Machining error. Here.
Tool rigidity: the rigidity in the normal (y) direction of the machined surface of the cylindrical turning tool is very high, and the deformation can be ignored. When drilling small diameter internal holes, the rigidity of the tool bar is very poor, and the force and deformation of the tool bar have a great impact on the machining accuracy of the hole.
Rigidity of machine tool parts: machine tool parts are composed of many parts. So far, there is no suitable simple calculation method for the rigidity of machine tool parts. At present, the test method is mainly used to determine the rigidity of machine tool parts. The factors that affect the rigidity of machine tool parts include the influence of contact deformation, friction, low rigidity parts, clearance, etc.
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 parts machining. The processing error caused by thermal deformation can account for 50% of the total error of the workpiece.
7. Adjustment error
Each processing process needs to adjust the process system in some way. The alignment error occurred because the alignment was not completely accurate. The process system ensures the mutual position accuracy of workpiece and tool on the machine tool by adjusting the machine tool, tool, fixture or workpiece. When the original accuracy of the machine tool, tool, fixture and workpiece blank all meet the process requirements without considering the dynamic factors, the adjustment error plays a decisive role in the processing error.
8. Measurement error
When measuring parts during or after machining, the measuring accuracy is directly affected by the measuring method, measuring tools, workpiece accuracy and subjective and objective factors.
9. Error caused by internal stress of workpiece
The stress existing inside the part without external force is called internal stress. When the internal stress of the workpiece occurs, the metal of the workpiece will be in a high-energy unstable state, and will instinctively change to a low-energy stable state, accompanied by deformation, so that the workpiece will lose its original processing accuracy.