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Machining error refers to the degree of deviation between the actual geometric parameters (geometric dimension, geometric shape and mutual position) and the ideal geometric parameters. The degree of conformity between the actual geometric parameters and the ideal geometric parameters is the machining accuracy. The smaller the machining error is, the higher the degree of conformity is, and the higher the machining accuracy is. Machining accuracy and machining error are two ways to solve the same problem. Therefore, the size of CNC machining error reflects the level of machining accuracy.
Main Causes of CNC Machining Errors
- Manufacturing Error Of CNC Machining
Manufacturing errors of CNC machining mainly include spindle rotation error, guide rail error and transmission chain error.
Spindle rotation error refers to the change of the actual rotation axis at each moment of the spindle relative to its average rotation axis, which will directly affect the accuracy of the workpiece to be processed. The main causes of spindle rotation error are the coaxiality error of the spindle, the error of the bearing itself, the coaxiality error between the bearings, the spindle winding and so on. The guide rail is not only the benchmark to determine the relative position relationship of each machine tool component, but also the benchmark of 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 causing the guide rail error. The error of transmission chain refers to the relative motion error between the transmission elements at both ends of the 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.
- Geometric Error Of Tool
Any tool in the cutting process will inevitably produce wear, which will cause the change of the size and shape of the workpiece. The influence of tool geometric error on machining error varies with different kinds of tools: when using fixed size tools, tool manufacturing error will directly affect the machining accuracy of workpiece. However, for general cutting tools (such as turning tools for CNC turning service), the manufacturing error has no direct influence on the machining error.
- Geometric Error Of Fixture
The function of the fixture is to make the workpiece equal to the cutter 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).
- Positioning Error
The positioning error mainly includes the inaccuracy error of the datum and the manufacturing inaccuracy error of the positioning pair. When the workpiece is processed on the machine tool, several geometric elements on the workpiece must be selected as the positioning datum. 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 non coincidence error will occur.
The positioning surface of the workpiece and the positioning elements of the fixture constitute the positioning pair. The maximum position variation of the workpiece caused by the inaccurate manufacturing of the positioning pair and the fit gap between the positioning pairs is called the inaccurate manufacturing 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 method.
- Error caused by force 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 caused by the insufficient stiffness of the workpiece will have a greater impact on the machining error.
Tool stiffness: The stiffness 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 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 influence on the machining accuracy of the hole.
Stiffness of machine tool parts: machine tool parts are composed of many parts. So far, there is no suitable simple calculation method for the stiffness of machine tool parts. At present, the stiffness of machine tool parts is mainly determined by experimental methods. The factors that affect the stiffness of machine tool parts are the contact deformation of joint surface, the friction force, the low stiffness parts and the clearance.
- Error caused by thermal deformation of process system
Thermal deformation of process system has great influence on 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 workpiece.
- Adjustment error
In every process of machining, the process system must be adjusted one way or another. Because the adjustment can not be absolutely accurate, there will be adjustment error. 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 machine tool, cutter, fixture and workpiece blank meet the process requirements without considering the dynamic factors, the adjustment error plays a decisive role in the machining error.
- Measurement error
When measuring parts during or after machining, the measurement accuracy is directly affected by the measurement method, measuring accuracy, workpiece and subjective and objective factors.
- Internal stress
There is no external force and exists in the internal part of the stress, known as 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 instinctively transforms to a stable state of low energy level, accompanied by deformation, so that the workpiece loses its original machining accuracy.
