Machining error refers to the degree of deviation between the actual geometrical parameters (geometrical size, geometrical shape and mutual position) and the ideal geometrical parameters after machining a part. The machining precision is the degree of agreement between the actual geometric parameters and the ideal geometric parameters after machining. The smaller the machining error, the higher the degree of conformity, the higher the machining accuracy. Machining accuracy and machining error are two formulations of a problem. Therefore, the size of the machining error reflects the machining accuracy.
The main cause of machining error
- Manufacturing errors of machine tools
The manufacturing errors of machine tools mainly include spindle rotation errors, guide errors and transmission chain errors.
Spindle rotation error refers to the actual rotation axis of each moment of the spindle relative to its average rotation axis of variation, it will directly affect the accuracy of the workpiece. The main reasons of spindle rotation error are the coaxiality error of spindle, the error of bearing itself, the coaxiality error between bearings, spindle winding and so on. Guideway is the benchmark to determine the relative position relationship of each machine tool component on the machine tool, and it is also the benchmark of machine tool movement. The manufacturing error of guide rail itself, the non-uniform wear of guide rail and the installation quality are the important factors that cause the error of guide rail. Transmission chain error refers to the error of relative motion between transmission components at the beginning and end of the transmission chain. It is caused by the manufacturing and assembly errors of each component link in the transmission chain, as well as wear and tear during use.
2, the geometric error of the tool
Any tool in the cutting process will inevitably wear, and resulting in the size and shape of the workpiece change. The influence of tool geometry error on machining error varies with the type of tool. When using fixed size tool, the tool manufacturing error will directly affect the machining accuracy of the workpiece. But for general tool (such as turning tool, etc.), its manufacture error has no direct influence on machining error.
3. Geometric error of the fixture
The role of the fixture is to make the workpiece equivalent to the tool and the machine tool has 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 errors mainly include reference miscoincidence error and positioning pair manufacturing inaccuracy error. When the workpiece is processed on the machine tool, a number of geometric elements on the workpiece must be selected as the positioning datum during processing. If the positioning datum selected and the design datum (the datum used to determine the size and position of a surface on the parts diagram) do not overlap, there will be datum mismatch error.
The positioning surface of the workpiece and the positioning element of the fixture jointly constitute the positioning pair, and the maximum position variation of the workpiece caused by the inaccurate manufacturing of the positioning pair and the fit gap between the positioning pair is called the manufacturing inaccuracy error of the positioning pair. The manufacturing inaccuracy error of the positioning pair can only be produced when the adjustment method is used, but not in the trial cutting method.
5. Error caused by force and deformation of the 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 outer turning tool in the normal (Y) direction of the machining surface is very large, and its deformation can be ignored. Boring small diameter of the inner hole, the tool bar stiffness is very poor, the tool bar force deformation has a great impact on the accuracy of hole processing.
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 main method is to measure the stiffness of machine tool parts by experiment. The factors that affect the stiffness of machine tool components include the contact deformation of the joint surface, the friction force, the low stiffness parts and the clearance.
6. Errors caused by thermal deformation of the process system
The thermal deformation of the process system has a great influence on the machining error, especially in precision machining and large machining, the machining error caused by thermal deformation sometimes accounts for 50% of the total error of the workpiece.
7. Adjust the error
In each process of machining, the process system must be adjusted in one way or another. Because the adjustment cannot be absolutely accurate, resulting in adjustment error. In the process system, the mutual position accuracy of the workpiece and the cutting tool on the machine tool is guaranteed by adjusting the machine tool, the cutting tool, the fixture or the workpiece. When the original accuracy of machine tools, cutting tools, fixtures and workpiece blanks all meet the process requirements without considering the dynamic factors, the adjustment error plays a decisive role in machining error.
8. Measurement error
When the parts are measured during processing or after processing, the measuring method, measuring tool accuracy and workpiece and subjective and objective factors have a direct impact on the measurement accuracy.
9. Internal stress
The stress that exists inside a part without the action of external force is called internal stress. Once the internal stress is produced on the workpiece, it will make the workpiece metal in an unstable state of high energy, it instinctively to the stable state of low energy, and accompanied by deformation, so that the workpiece lost the original processing accuracy
