Machining accuracy analysis
This paper briefly analyzes and summarizes some problems existing in current machining, and according to the characteristics of machining, analyzes the causes of misoperation of three machines and two machines, so as to further improve its machining spirit.
Keywords: machinery; Processing; accurate
1 overview
1. 1 machining accuracy and machining error
The accuracy of operation refers to whether the real data of the length, width and height of the parts after operation and the specific values given in the calculation are reasonable. The specific gap between them is called machining error. This paper mainly analyzes and discusses these two aspects, so as to understand the working methods and principles of machining.
1.2 original error
Because all kinds of molds and parts interact to form a whole, they are collectively called process system. These factors are uncertain, mainly including geometric error and dynamic error.
1.3 methods to explore the accuracy of homework
Element-by-element analysis and overall analysis.
2 Geometric error of process system
2. 1 machining principle error
The inaccuracy of working principle refers to the inaccuracy caused by using similar working actions or similar working tools.
2.2 Geometric error of machine tool
The inaccuracy of machine tool manufacturing, assembly and damage in practical application are all inseparable from the accuracy of operation. What is said in the middle is the operation of the spindle, the linear operation of the guide rail and the loose operation of the transmission chain.
2.2. 1 spindle rotation error. It means that the working radian and intensity of the spindle in each time period cannot be completely consistent. Specifically divided into the following categories: end-face circular jump, radial circular jump, angle swing. The concrete factors that make the spindle run inaccurately mainly lie in the inaccuracy of the spindle, the inaccuracy of the bearing, the existence of a certain gap between them, the inaccuracy of the parts interacting with the bearing and the deformation caused by various factors during the operation. Measurement methods of spindle running accuracy: dial gauge measurement and sensor measurement. The methods to improve the spindle operation accuracy include: strengthening the manufacture of spindle parts, reducing the gap between parts and using stable spiral support.
Guide rail error. The guide rail of mechanical device is an indispensable part in practical application, and its manufacturing and assembly accuracy is related to the working accuracy of mechanical device. Its inaccurate consequences are unimaginable, which will make the operation have errors in horizontal, vertical and moving directions.
Drive chain. The inaccurate operation of transmission chain refers to the error caused by the mutual operation of components associated with both ends of transmission chain. Generally speaking, the inaccuracy of the head-tail angle of the transmission chain is considered. Measures to reduce the inaccurate operation of transmission chain: reduce the transmission chain within the power range; Reduce the assembly angle of the transmission chain; Improve the manufacturing accuracy of end-to-end interconnection of transmission chain; In the transmission link, the proportion of each transmission part is considered on the basis of a specific proportion increase; Use calibration equipment.
2.3 Tool error
No matter what kind of tools are in operation, they will inevitably be damaged, which will lead to slight changes in the size and shape of the operation results. Accurate selection of raw materials for cutting tools, effective determination of specific parameters and workload of cutting tools, accurate protection and maintenance of cutting tools, timely use of frozen liquid, etc. , can minimize the actual damage of the tool. If necessary, we should also use the filling method to fill the worn part of the tool.
2.4 Clamping and fixture errors
When the parts are assembled on the fixture to carry out mechanical operation, the factors that cause the operation accuracy to be damaged in this operation procedure specifically include: inaccurate positioning, inaccurate clamping degree, inaccurate tool centering, inaccurate fixture assembly on the fixture, and inaccurate operation procedure.
3. Inaccurate operation of process system
3. 1 Mechanical deformation of process system
3. 1. 1 Basic concepts. Under the action of external forces such as cutting force, force transmission, inertia force, power and clamping force, the working process system of the instrument will inevitably change, thus changing the position between the cutting edge and the workpiece.
3. 1.2 Workpiece stiffness. In the process system, if the hardness of the workpiece is lower than that of the machine tool, cutter and fixture, under the pressure of external force, the change of the parts due to the lack of hardness has a great influence on the accuracy of operation, and the specific change can be calculated according to the relevant physical formula.
3. 1.3 tool hardness. When machining small holes or deep holes, the hardness of the tool holder is not good, and the tool holder is deformed by external force, resulting in poor operation accuracy. This specific deformation can also be calculated according to the relevant physical formula.
3. 1.4 machine tool hardness. There is no reasonable calculation method for machine tool parts calculation. It is still determined by experiments. The deformation and stress of machine tool parts are inconsistent, and the loading radian and unloading radian are different. The area between these two radians is the force consumed in loading and unloading operations, and its specific loss is under the action of friction and contact. After many unloading operations, these two angles are likely to eventually merge into one, and this deformation disappears in the process. Elements closely related to the hardness of machine tool parts: contact deformation of connecting surfaces, parts with weak hardness, gaps between parts and grinding force.
3. 1.5 Influence of process system stiffness on machining accuracy. Error caused by stiffness change of process system. Error caused by the change of cutting force. Error caused by clamping deformation. The influence of other forces
3. 1.6 Measures to reduce stress and deformation of process system. Improve contact stiffness, workpiece stiffness, machine tool component stiffness, reasonably clamp workpiece and reduce clamping deformation.
3.2 Thermal deformation of process system
3.2. 1 Basic concepts. There are internal heat sources (cutting heat and friction heat) and external heat sources that cause thermal deformation of the process system. Cutting heat is generated by the elastic-plastic deformation of the metal in the cutting layer and the friction between the cutter and the workpiece and cutting, and is transmitted through the workpiece, cutter, fixture, machine tool, chip, cutting heat and surrounding media. Friction heat is mainly an external heat source generated by moving parts and hydraulic system of machine tools, and is mainly affected by environmental temperature and thermal radiation.
3.2.2 Main ways to reduce and control thermal deformation. Reduce the heating of the heat source. Reduce thermal deformation through thermal compensation. Adopt reasonable machine tool component structure to reduce thermal deformation, adopt symmetrical structure, and reasonably select the assembly benchmark of machine tool components. Accelerate the thermal balance of the process system and control the ambient temperature.
3.3 Error caused by residual stress of workpiece
Residual stress refers to the removal of external load. There is still residual stress in the workpiece. The causes of residual stress are: residual stress in blank manufacturing, residual stress caused by cold straightening and residual stress in cutting. Measures to reduce or eliminate residual stress: reasonable design of parts structure, heat treatment and aging treatment of workpieces, and reasonable arrangement of technological processes.
3.4 Adjustment error
In each process, the process system is always adjusted in one way or another. Because the adjustment is not absolutely accurate, the adjustment error occurs. In the process system, the mutual position accuracy of the workpiece and the cutter on the machine tool is ensured by adjusting the machine tool, cutter, fixture or workpiece. Without considering dynamic factors, when the original accuracy of machine tools, tools, fixtures and workpiece blanks all meet the technological requirements, the influence of adjustment error plays a decisive role in machining accuracy.
3.5 Measurement error
When measuring parts during or after machining, the measuring accuracy is directly affected by the measuring method, measuring tool accuracy, workpiece and subjective and objective factors.
4 methods to improve and ensure machining accuracy
4. 1 directly reduces the error.
Direct error reduction method is a widely used method in production, which is a method to eliminate or reduce the main factors that produce machining errors after finding out them.
4.2 Error compensation method
Error compensation method is to create a new original error to offset the original error inherent in the original process system, thus reducing the machining error and improving the machining accuracy.
4.3 Average Original Error Method
For the surface of parts with high machining accuracy, the method of gradually balancing the original error in the process of continuous trial cutting can also be adopted. The process of this method is to reduce and compensate the original error of the machined surface through mechanical processing.
4.4 Error Transfer Method
In essence, the error transfer method is to transfer the geometric error, stress deformation and thermal deformation of the process system to a direction that does not affect the machining accuracy.
refer to
[1] Zheng Xiuben. Mechanical manufacturing technology [M]. Beijing: Machinery Industry Press, 1999.
Wu Yuhua. Metal cutting technology [M]. Beijing: Machinery Industry Press, 1998.
Wang Yao. Analysis of the influence of geometric error of process system on machining accuracy [J]. Science and Technology Information, 2004.
[4] Zhang Liangfeng. Fundamentals and practice of machining technology [M]. Beijing: Higher Education Press, 1999.
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