Machining error refers to the degree of deviation between the actual geometric parameters (geometric dimensions, geometric shapes and mutual positions) and the ideal geometric parameters after machining.
The degree of conformity between the actual geometric parameters and the ideal geometric parameters is the machining precision.The smaller the machining error, the higher the degree of conformity and the higher the machining accuracy.
The main causes of machining errors are as follows:
1. Machine tool manufacturing error
Machine tool manufacturing errors mainly include spindle rotation error, guide rail error and transmission chain error.
Spindle rotation error refers to the spindle at each moment of the actual axis of rotation relative to its average axis of rotation change, it will directly affect the accuracy of the workpiece to be processed.The main causes of spindle rotation error are spindle coaxiality error, bearing itself error, bearing between coaxiality error, spindle winding, etc.The guide rail is the reference to determine the relative position relation of each machine tool part on the machine tool, and it is also the reference to the machine tool movement.
The manufacturing error of the guide itself, the uneven wear of the guide and the installation quality are the important factors causing the error of the guide.Transmission chain error refers to the error of relative motion between transmission elements at the beginning and the end of the transmission chain.It is caused by the manufacturing and assembling errors of each component in the transmission chain, as well as wear and tear in the process of use.
2. Geometric error of cutting tool
It is inevitable that any tool will wear out during cutting, which will change the size and shape of the workpiece.The influence of geometric error of cutting tools on machining error varies with different kinds of cutting tools: when using fixed size cutting tools, the manufacturing error of cutting tools will directly affect the machining accuracy of the workpiece.For general tools (such as turning tools, etc.), the manufacturing error has no direct impact on the machining error.
3. Geometric error of fixture
The function of jig is to make the workpiece equal to the cutter and the machine have the correct position, so the geometric error of jig has a great influence on the machining error (especially the position error).
4. Positioning error
The positioning error mainly includes datum 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 shall be selected as the positioning datum during the processing. If the selected positioning datum and the design datum (the datum used to determine the size and position of a surface on the part drawing) do not coincide, the datum miscoincidence error will occur.
The workpiece positioning surface and the fixture positioning element together constitute the positioning pair. The maximum position change of the workpiece caused by the inaccurate manufacturing of the positioning pair and the mating gap between the positioning pair is called the positioning pair manufacturing inaccuracy error.The inaccurate error of positioning pair manufacturing can only occur in the adjustment process, but not in the trial cutting process.
5. Errors caused by stress 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 great impact on the machining error.
Tool stiffness: The rigidity of the cylindrical turning tool in the direction of normal (Y) on 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 cutter rod is very poor, the stress deformation of the cutter rod has a great influence on the machining accuracy of the hole.
Stiffness of machine tool components: 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, mainly by experimental methods to measure the stiffness of machine tool components.The factors that affect the stiffness of machine parts include the influence of contact deformation, friction, low stiffness parts and clearance.
6. Errors caused by thermal deformation of the process system
The thermal deformation of process system has a great influence on the machining error, especially in precision machining and bulk machining, the machining error caused by thermal deformation sometimes accounts for 50% of the total workpiece error.
7. Adjust errors
In every process of machining, there is always an adjustment of the process system in one way or another.Because the adjustment cannot be absolutely accurate, the adjustment error is generated.In a process system, the accuracy of the workpiece and the tool's mutual position on the machine tool is ensured by adjusting the machine tool, tool, fixture or workpiece.When the original precision of the machine tool, tool, fixture and workpiece blank meets the process requirements without considering the dynamic factors, the adjustment error plays a decisive role in the machining error.
8. Measurement error
When parts are measured during or after processing, the measurement accuracy is directly affected by the measuring method, measuring precision, 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 internal stress occurs on the workpiece, it will make the workpiece metal in an unstable state at a high energy level. It will instinctively transform to a stable state at a low energy level, accompanied by deformation, thus making the workpiece lose its original machining accuracy.