Machining error refers to the degree of deviation between the actual geometric parameters (geometric dimensions, geometric shape, and relative positions) of a machined part and its ideal geometric parameters. The degree of conformity between the actual and ideal geometric parameters after machining is called machining accuracy. The smaller the machining error and the higher the degree of conformity, the higher the machining accuracy. Machining accuracy and machining error are two ways of stating the same thing. Therefore, the magnitude of machining error reflects the level of machining accuracy.
The main causes of machining errors:
1. Manufacturing errors of machine tools
The main manufacturing errors of machine tools 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 any given moment, directly affecting the accuracy of the machined workpiece. The main causes of spindle rotation error include spindle coaxiality error, bearing inherent errors, coaxiality errors between bearings, and spindle deflection. Guideways are the reference for determining the relative positional relationships of various machine tool components and also serve as the reference for machine tool movement. Manufacturing errors in the guideways themselves, uneven wear, and installation quality are significant factors contributing to guideway errors. Transmission chain error refers to the error in the relative motion between the transmission elements at both ends of the transmission chain. It is caused by manufacturing and assembly errors in each component of the transmission chain, as well as wear during use.
2. Geometric errors of the cutting tool
All cutting tools inevitably experience wear during the cutting process, which in turn causes changes in the size and shape of the workpiece. The impact of tool geometry errors on machining errors varies depending on the type of tool: when machining with fixed-size tools, the tool's manufacturing errors directly affect the machining accuracy of the workpiece; while for general tools (such as lathe tools), their manufacturing errors have no direct impact on machining errors.
3. Geometric errors of the fixture
The function of a fixture is to ensure that the workpiece is in the correct position, which is equivalent to the cutting tool and the machine tool. Therefore, the geometric error of the fixture has a great influence on the machining error (especially the positional error).
4. Positioning error
Positioning errors mainly include datum misalignment error and inaccurate manufacturing error of positioning pairs. When machining a workpiece on a machine tool, several geometric elements on the workpiece must be selected as positioning datums for machining. If the selected positioning datum does not coincide with the design datum (the datum used to determine the size and position of a certain surface on the part drawing), a datum misalignment error will occur.
The workpiece locating surface and the fixture locating elements together constitute a locating pair. The maximum positional variation of the workpiece caused by inaccurate manufacturing of the locating pair and the clearance between the locating pairs is called the locating pair manufacturing inaccuracy error. This locating pair manufacturing inaccuracy error only occurs when machining using the adjustment method; it does not occur during trial cutting.
5. Errors caused by stress deformation of the process system
Workpiece stiffness: If the workpiece stiffness is relatively low compared to the machine tool, cutting tool, and fixture in the machining system, the deformation caused by insufficient stiffness under the action of cutting force will have a greater impact on machining errors.
Tool stiffness: External turning tools have high stiffness in the direction normal (y) to the machined surface, and their deformation is negligible. When boring small-diameter internal holes, the tool shank stiffness is very poor, and the deformation of the tool shank under stress has a significant impact on the hole machining accuracy.
Machine tool component stiffness: Machine tool components consist of many parts, and there is currently no suitable and simple method for calculating their stiffness. Experimental methods are still the primary means of determining machine tool component stiffness. Factors affecting the stiffness of machine tool components include the influence of contact deformation at mating surfaces, the influence of friction, the influence of low-stiffness parts, and the influence of clearances.
6. Errors caused by thermal deformation of the process system
Thermal deformation of the process system has a significant impact on machining errors, especially in precision machining and large-part machining. Machining errors caused by thermal deformation can sometimes account for 50% of the total error of the workpiece.
7. Adjusting the error
In every step of machining, adjustments to the process system are always necessary. Since these adjustments cannot be absolutely accurate, adjustments result in errors. Within the process system, the positional accuracy of the workpiece and cutting tool on the machine tool is ensured by adjusting the machine tool, cutting tool, fixture, or workpiece. When the original accuracy of the machine tool, cutting tool, fixture, and workpiece blank all meet the process requirements, and dynamic factors are not considered, adjustments play a decisive role in machining errors.
8. Measurement error
When measuring parts during or after processing, the measurement accuracy is directly affected by the measurement method, the accuracy of the measuring tools, the workpiece, and subjective and objective factors.
9. Internal stress
Stress existing inside a part without external force is called internal stress. Once internal stress is generated on a workpiece, the metal of the workpiece will be in an unstable state with a high energy potential. It will instinctively transform into a stable state with a low energy potential, accompanied by deformation, thereby causing the workpiece to lose its original machining accuracy.
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