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How many of the nine major errors in machining do you know?

Oct 10-2020     Views: 159


This article analyzes the nine major errors of machining, avoiding these errors, you can go further in machining.


Machining error refers to the degree of deviation between the actual geometric parameters (geometric size, geometric shape, and mutual position) of the parts after machining and the ideal geometric parameters.


The degree of agreement between the actual geometric parameters and the ideal geometric parameters of the part after machining is the machining accuracy. The smaller the machining error, the higher the degree of compliance, and the higher the machining accuracy.


Machining accuracy and machining error are two formulations of the same problem. Therefore, the size of the machining error reflects the level of machining accuracy. The main causes of machining errors are as follows:


1. Manufacturing errors of machine tools


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 processed workpiece. The main causes of spindle rotation errors are spindle coaxiality error, bearing error, coaxiality error between bearings, spindle winding, etc. The guide rail is the benchmark for determining the relative positional relationship of various machine tool components on the machine tool, and also the benchmark for machine tool movement.


The manufacturing error of the guide rail itself, the uneven wear of the guide rail, and the installation quality are important factors that cause the guide rail error. Transmission chain error refers to the error of relative movement between the transmission elements at both ends of the transmission chain. It is caused by the manufacturing and assembly errors of each component of the transmission chain, as well as the wear during use.


2. The geometric error of the tool


Any tool will inevitably wear out during the cutting process, which will cause changes in the size and shape of the workpiece. The influence of tool geometry error on machining error varies with the type of tool: when using a fixed-size tool for machining, the tool manufacturing error will directly affect the machining accuracy of the workpiece; while for general tools (such as turning tools, etc.), the manufacturing error There is no direct influence on machining errors.


3. The geometric error of the fixture


The function of the fixture is to make the workpiece equivalent to the tool 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).


4. Positioning error


Positioning errors mainly include fiducial misalignment errors and positioning pair manufacturing inaccuracy errors. When processing a workpiece on a machine tool, several geometric elements on the workpiece must be selected as the positioning datum during processing. If the selected positioning datum and design datum (used to determine a certain surface size and position on the part drawing are based on Benchmark) do not coincide, and benchmark misalignment error will occur.


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


5. Errors caused by force and deformation of the processing system


Workpiece rigidity: In the processing system, if the rigidity of the workpiece is relatively low relative to the machine tool, cutting tool, and fixture, under the action of cutting force, the deformation of the workpiece due to insufficient rigidity will have a greater impact on machining errors.


Tool rigidity: The rigidity of the external turning tool in the normal (y) direction of the machined surface is very large, and its deformation can be ignored. Boring inner holes with a small diameter, the rigidity of the toolbar is very poor, and the force and deformation of the toolbar have a great influence on the machining accuracy of the hole.


Machine tool component stiffness: The machine tool component consists of many parts. So far, there is no suitable simple calculation method for the stiffness of machine tool components. At present, experimental methods are mainly used to determine the stiffness of machine tool components. The factors that affect the stiffness of machine tool components include the influence of the contact deformation of the joint surface, the influence of friction, the influence of low stiffness parts, and the influence of clearance.


6. Errors caused by thermal deformation of the processing system


The thermal deformation of the processing system has a relatively large impact on the machining error, especially in precision machining and large parts processing. The machining error caused by the thermal deformation can sometimes account for 50% of the total error of the workpiece.


7. Adjustment error


In each process of machining, the processing system must be adjusted in one way or another. Since the adjustment cannot be absolutely accurate, an adjustment error occurs. In the processing system, the mutual position accuracy of the workpiece and the tool on the machine tool is ensured by adjusting the machine tool, tool, fixture or workpiece. When the original accuracy of machine tools, tools, fixtures, and workpiece blanks all meet the technical requirements without considering dynamic factors, adjustment errors play a decisive role in machining errors.


8. Measurement error


When measuring parts during processing or after processing, the measurement accuracy is directly affected by the measurement method, the accuracy of the measuring tool, the workpiece, and the subjective and objective factors.


9. Internal stress


The stress that exists inside the part without external force is called internal stress. Once internal stress is generated on the workpiece, the workpiece metal will be in a high-energy unstable state, which instinctively transforms to a low-energy stable state, and accompanied by deformation, so that the workpiece loses its original machining accuracy.


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