The main reasons for machining errors
1, machine manufacturing error. Machine tool manufacturing errors include spindle rotation error, guideway error and transmission chain error. Spindle rotation error refers to the actual axis of rotation of the spindle at a moment relative to the average axis of rotation variation, it will directly affect the accuracy of the workpiece being machined. Spindle rotation error is the main reason for the spindle coaxial error, the bearing itself, the error between the coaxiality of the bearings, the spindle around the degree of tolerance. The guide rail is the basis of the machine tool to determine the relative position of the various machine parts, but also the basis for machine 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. Transmission chain error refers to both ends of the transmission chain transmission components relative motion error. It is caused by manufacturing and assembly errors in the various components of the drive chain, as well as wear and tear during use.
2, the geometric error of the tool. Any cutting tool in the cutting process are inevitable to produce wear and tear, and thus cause the workpiece size and shape change. The influence of tool geometry error on the machining error varies with the type of tool: When using the fixed-size tool machining, the manufacturing error of the tool will directly affect the machining accuracy of the workpiece. For a general tool (such as turning tool), the manufacturing error No direct impact on machining error.
3, the geometric error of the fixture. The function of the fixture is to make the workpiece equivalent to the correct position of the tool and the machine, so the geometric error of the fixture has a great influence on the machining error (especially the position error).
4, positioning error. The positioning error mainly includes the reference misalignment error and the inaccurate positioning error. When machining a workpiece on a machine tool, several geometrical elements of the workpiece must be selected as the positioning datum for machining. If the selected positioning datum and design datum (the datum used to determine a certain surface size and position on the part drawing) ) Do not overlap, it will produce a benchmark does not coincide error.
The workpiece positioning surface and the fixture positioning components together constitute the positioning pair. Due to the inaccurate positioning of the positioning assembly and the variation of the maximum position of the workpiece caused by the matching clearance between the positioning sub-assemblies, it is called inaccurate positioning error. Inaccurate positioning error is only produced when the use of adjustment method will not produce in the trial cut method processing will not produce.
5, the process of system deformation caused by the error. Workpiece rigidity: If the work piece stiffness is relatively low compared with the machine tool, the tool and the fixture in the process system, the deformation due to the lack of rigidity of the workpiece affects the machining error greatly under the action of the cutting force.
Tool Rigidity: The lathe has a very high stiffness in the normal (y) direction of the machined surface and its deformation is negligible. Boring diameter of the smaller hole, the arbor stiffness is poor, Arbor deformation of the hole machining accuracy will have a great impact.
Rigidity of machine tool components: The machine tool components are composed of many parts. To date, there is no suitable simple calculation method for the stiffness of machine tool components. At present, the rigidity of the machine tool components is mainly determined by experimental methods. Factors that affect the rigidity of machine tool components are the effects of contact deformation, the influence of friction, the influence of low-rigidity parts and the influence of clearance.
6, process system caused by thermal deformation error. The thermal deformation of the process system has a great influence on the machining error. Especially in the precision machining and large-size machining, the machining error caused by the thermal deformation sometimes accounts for 50% of the total workpiece error.
7, adjust the error. In each process of machining, the process system must be adjusted one way or another. As the adjustment can't be absolutely accurate, resulting in adjustment error. In the process system, the accuracy of the workpiece and the position of the tool on the machine tool can be guaranteed by adjusting the machine tool, the tool, the fixture or the workpiece. When the original precision of the machine tools, tools, fixtures and workpiece blanks meet the technological requirements without considering the dynamic factors, the adjustment error plays a decisive role in the machining error.
8, measurement error. Parts in processing or processing after measurement, due to measurement methods, measuring accuracy as well as the workpiece and the subjective and objective factors directly affect the measurement accuracy.
9, internal stress. The stress that exists inside the part without external force is called internal stress. Once the internal stress on the workpiece, it will make the workpiece metal in a high-energy instability, it instinctively to the low-energy position of a stable state, and accompanied by deformation, so that the workpiece loss of the original machining accuracy.