Nine causes of machining errors

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. 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 is, the higher the degree of conformity is, and the higher the machining accuracy is. Machining accuracy and machining error are two formulations of a problem. Therefore, the size of machining error reflects the machining accuracy.

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 moment, which will directly affect the accuracy of the workpiece to be processed. The main causes of spindle rotation error include the coaxiality error of spindle, the error of bearing itself, the coaxiality error between bearings, the winding degree of spindle and so on. The guide rail is not only the benchmark for determining the relative position relationship of each machine tool component on the machine tool, but also the benchmark for the movement of the machine tool. 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. Transmission chain error refers to the error of relative motion between transmission elements at both ends of the transmission chain. It is caused by the manufacturing and assembly errors of each component link in the transmission chain and the wear in the process of use.

2、 Geometric error of tool

Any cutting tool will inevitably be worn in the cutting process, which will cause the change of workpiece size and shape. The influence of tool geometric error on machining error varies with the type of tool: when machining with fixed size tool, the manufacturing error of tool will directly affect the machining accuracy of workpiece; For general cutting tools (such as turning tools), the manufacturing error has no direct impact on the machining error.

3、 Geometric error of fixture

The function of the fixture is to make the workpiece equivalent to the tool and machine tool have the correct position, so the geometric error of the fixture has a great impact 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, several geometric elements on the workpiece must be selected 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 surface on the part drawing), the datum non coincidence error will occur.

The workpiece positioning surface and the fixture positioning element together constitute 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 inaccurate manufacturing error of the positioning pair. The manufacturing inaccuracy error of positioning pair will occur only when the adjustment method is used, and will not occur in the trial cutting method.

5、 Error caused by stress and deformation of process system

Workpiece stiffness: in the process system, if the workpiece stiffness is relatively low compared with the machine tool, cutting tool and fixture, the deformation of the workpiece caused by insufficient stiffness will have a greater impact on the machining error under the action of cutting force.

Tool stiffness: the rigidity of the cylindrical turning tool in the normal (y) direction of the machining surface is very large, and its deformation can be ignored. Boring the inner hole with small 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 machining accuracy.

Machine tool component stiffness: machine tool components are composed of many parts. So far, 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 experimental methods. The factors affecting the stiffness of machine tool parts include the contact deformation of joint surface, friction, low stiffness parts and clearance.

6、 Error caused by heating 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 workpiece error.

7、 Adjustment error

In each process of machining, the process system must be adjusted in one way or another. Since the adjustment cannot be absolutely accurate, there is an adjustment error. In the process system, the mutual position accuracy of workpiece and cutting tool on the machine tool is guaranteed by adjusting the machine tool, cutting tool, fixture or workpiece. When the original accuracy of machine tools, cutting tools, fixtures and workpiece blanks 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 machining, the measurement accuracy is directly affected by measurement methods, measuring tool accuracy, workpiece and subjective and objective factors.

9、 Internal stress

The stress that exists inside the part without the action of external force is called internal stress. Once the workpiece is in the unstable state of high-energy machining, it will be in a state of unstable machining, so that the workpiece will be in a low-energy state.