Machining error refers to the degree of deviation between the actual geometric parameters (geometric size, geometric shape and mutual position) of the workpiece after processing and the ideal geometric parameters. The degree of consistency between the actual geometric parameters and the ideal geometric parameters after processing parts is the processing accuracy. The lower the processing error, the higher the degree of conformity and the higher the processing accuracy. Machining accuracy and machining error are two ways of stating the same problem. Therefore, the size of the machining error reflects the level of machining accuracy.
The main causes of machining errors
1. Machine tool manufacturing errors
Machine tool manufacturing errors 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 at each instant from its average rotation axis, which will directly affect the accuracy of the part being processed. The main reasons for spindle rotation error include spindle coaxiality error, bearing error itself, coaxiality error between bearings, spindle winding, etc. The guide rail is the reference for determining the relative position relationship of various machine tool components on the machine tool, and is also the reference for the movement of the machine tool.
The manufacturing error of the guide rail itself, uneven wear of the guide rail and installation quality are important factors causing guide rail errors. Transmission chain error refers to the relative motion error between transmission elements at both ends of the transmission chain. It is caused by manufacturing and assembly errors of various drive chain components, as well as wear and tear during use.
2. Tool geometric error
Any tool will inevitably wear out during the cutting process, resulting in changes in the size and shape of the part. The impact of tool geometric error on machining errors varies depending on the tool type: when using fixed-size tools for processing, the tool manufacturing error will directly affect the workpiece processing accuracy for general tools (such as turning tools, etc.), manufacturing error. It has no direct impact on machining errors.
3. Geometric error of the luminaire
The function of fixture is to make the workpiece have the correct position as a tool and machine tool, so the geometric error of fixture has a great impact on the machining error (especially the position error).
4. Positioning error
Positioning errors mainly include data misalignment errors and manufacturing inaccuracies of positioning pairs. When processing a workpiece on a machine tool, several geometric elements on the workpiece must be selected as positioning data during processing. If you want to learn UG programming, you can join QQ group: 304214709 to receive learning materials and courses. selected positioning reference and design reference (in If the reference used to determine the size and position of a certain surface on the part drawing does not coincide, a reference misalignment error will occur.
The workpiece positioning surface and the device positioning member together form a positioning pair. The maximum variation in part position caused by the inaccurate manufacturing of the positioning pair and the matching deviation between the positioning pairs is called the manufacturing inaccuracy error of the positioning pair. Manufacturing inaccuracies of the positioning pair will only occur when the adjustment method is used and will not occur when the trial cut method is used.
5. Errors caused by forced deformation of the process system
Workpiece rigidity: If the rigidity of the workpiece in the processing system is relatively low compared with that of the machine tool, tool and fixture, under the action of the cutting force, the deformation of the part due to insufficient rigidity will have a greater impact on the machining error. .
Tool rigidity: The rigidity of the cylindrical turning tool in the normal (y) direction of the processing surface is very large, and its deformation can be ignored. When boring a small diameter inner hole, the rigidity of the toolbar is very poor, and the force deformation of the toolbar has a great impact on the machining accuracy of the hole.
Rigidity of machine tool components: Machine tool components are made up of many parts. Until now, there is no suitable simple calculation method for the stiffness of machine tool components. The factors that affect the rigidity of machine tool components include the impact of contact deformation of the joint surface, the impact of friction, the impact of low-rigidity parts, and the impact of clearance.
6. Errors caused by thermal deformation of the processing system
Thermal deformation of the processing system has a greater impact on processing errors. Especially in precision machining and large part processing, processing errors caused by thermal deformation can sometimes account for 50% of the total part error.
7. Setting error
In every machining process, the process system must be adjusted in one way or another. Since the adjustment cannot be absolutely precise, adjustment errors occur. In the processing system, the accuracy of the mutual position of the workpiece and the tool on the machine tool is guaranteed by the adjustment of the machine tool, tool, fixture or workpiece. When the initial accuracy of machine tools, cutting tools, fixtures and workpiece blanks meets the process requirements without considering dynamic factors, setting errors play a decisive role in machining errors.
8. Measurement error
When parts are measured during or after processing, the measurement method, the accuracy of the measuring tool, the workpiece, and subjective and objective factors all directly affect the measurement accuracy.
9, inner power
The stress that exists inside the part without the action of an external force is called internal stress. Once internal stress is generated on the part, the metal of the part will be in a high-energy unstable state. It will instinctively transform into a low-energy stable state, accompanied by deformation, causing the workpiece to lose its original machining accuracy.
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