Method for obtaining machining accuracy of parts in machining
Key words : machining; part machining; precision
With the rapid development of science and technology and increasingly fierce market competition, the basic requirements for the manufacture of parts in modern enterprises in the pursuit of high goals and low cost are to be more, faster, better and less expensive. The meaning of “good” includes continuously improving the quality of parts, improving their performance and service life, maximizing the elimination of waste products, reducing the defective rate and improving the qualification rate of parts. Because the quality of the part directly affects the performance, life, efficiency, reliability and other indicators of the machine, it is the basis for ensuring the quality of the machine, and the manufacturing quality of the part depends on the manufacturing method of the blank, machining, heat treatment and surface treatment. To guarantee. Therefore, we must always put the quality of guarantee in the first place in all aspects of parts manufacturing.
1. Analysis of machining accuracy and machining error
Machining accuracy refers to the degree to which the actual geometric parameters after machining the part meets the ideal geometric parameters specified in the drawing. The higher the degree of conformity, the higher the machining accuracy. In the processing, due to various factors, it is virtually impossible to completely match each geometric parameter of the part with the ideal geometric parameters, and there will always be some deviation. This deviation is the machining error. In fact, as long as the error of the part does not exceed the tolerance specified by the design requirements of the part on the part drawing, it can be said that the machining accuracy requirement of the part is guaranteed. It can be seen that the two concepts of "machining accuracy" and "machining error" are the same thing that evaluates the geometric parameters of the part from two sides. The low and high machining accuracy is represented by the large and small machining errors. Therefore, the problem of ensuring and improving the machining accuracy is actually a problem of limiting and reducing the machining error.
2. How to obtain machining accuracy
Since many factors affect the machining accuracy during the machining process, the accuracy of the same machining method under different working conditions is different. If blindly pursue processing precision, it will reduce production efficiency and increase processing costs. Therefore, under the premise of ensuring the quality of processing, we should try our best to improve efficiency and reduce production costs. Machining accuracy can be divided into dimensional accuracy, shape accuracy and positional accuracy. Therefore, the high and low machining accuracy is measured by dimensional tolerance, shape tolerance and position tolerance.
2.1 accuracy method of part size
The processing method of the part size first includes the trial cutting method, that is, first try to cut out a small part of the machined surface, measure the size obtained by the trial cutting, adjust the position of the cutting edge of the tool relative to the workpiece according to the processing requirements, and then try cutting and then measuring, so After two or three trial cuts and measurements, the entire surface to be machined is cut after the required dimensions have been reached. The second step is the adjustment method, which is to adjust the exact relative position of the machine tool, fixture, tool and workpiece with the sample or standard part in advance to ensure the dimensional accuracy of the workpiece, and the size remains unchanged during the processing of a batch of parts. Adjustment method. There is also a sizing method, which is to use the corresponding size of the tool to ensure the size of the workpiece to be processed. It is processed by a standard size tool. The size of the machined surface is determined by the size of the tool, that is, with a tool with a certain dimensional accuracy. Ensure the accuracy of the workpiece being machined. Finally, the automatic control method, that is, through the automatic control system consisting of the dimension measuring device, the power feeding device and the control mechanism, during the machining process, the dimensional measurement, the tool compensation adjustment and the cutting process are processed. The work is done automatically, thus automatically obtaining a machining method of the required dimensional accuracy.
2.2 Method for obtaining shape accuracy
The method for obtaining the shape accuracy first includes a trajectory method, which uses the trajectory of the tool tip movement to form the shape of the surface to be machined, and ordinary turning, milling, planing, and grinding are all the tool tip trajectory methods. The shape accuracy obtained by this method mainly depends on the precision of the forming motion. In addition, the forming method obtains the shape of the machined surface by using the geometry of the forming tool instead of some forming motion of the machine tool. Such as forming turning, milling, grinding, etc., the shape accuracy obtained by the forming method depends mainly on the shape of the blade.
In addition, the forming method is to use the enveloping surface formed by the tool and the workpiece to obtain the shape of the machined surface. The shape accuracy obtained by this method mainly depends on the shape accuracy of the blade and the accuracy of the forming motion.
2.3 Obtaining positional accuracy method
In machining, the accuracy of the surface position of the machined surface to other surfaces depends mainly on the clamping of the workpiece.
The straightforward direct clamping method is a clamping method that uses a dial gauge, a scribing disk or a visual inspection to directly find the position of the workpiece on the machine tool. The line-finding method is to first draw the center line, the symmetry line and the processing line of each surface to be processed on the blank according to the part drawing, and then load the workpiece on the machine tool, and correct the workpiece on the machine tool according to the line. Clamping position, this clamping method has low productivity, low precision, and high technical requirements for workers. It is generally used for processing complex and cumbersome parts in single-piece small batch production, or the blank has large dimensional tolerances and cannot be directly clamped with clamps. The occasion. Finally, the fixture is clamped. The fixture is specially designed according to the requirements of the machining process. The positioning component on the fixture enables the workpiece to quickly occupy the correct position relative to the machine tool and the tool. The positioning accuracy of the workpiece can be ensured without correction. The fixture has high productivity and high positioning accuracy, but it needs to design and manufacture special fixtures, which are widely used in batch and mass production.
3. Influence of CNC technology on machining accuracy of parts
Since the birth of the world's first CNC milling machine in 1952, high precision has become the goal of the development of CNC technology. With the development of modern manufacturing technology, CNC machine tools are becoming more and more popular. Compared with ordinary machine tools, CNC machine tools have undergone major changes in control systems, servo drives, and mechanical structures. The numerical control machine tool adopts computer numerical control. Each coordinate axis adopts closed-loop or semi-closed-loop servo drive. The mechanical transmission chain is shortened, and mechanical parts have many improvements in backlash and wear reduction. Therefore, CNC machine tools have high machining precision and production efficiency. High, stable product quality, flexible processing, and high processing performance. The influence of NC programming on machining accuracy mainly comes from the determination of programming origin, data processing, trajectory fitting and processing route selection.
The first is the influence of programming origin selection on machining accuracy. The first problem encountered in NC programming is to determine the programming origin. The programming coordinate system is generally determined by the programmer according to the machining characteristics of the parts and the drawing of the parts. The choice of programming origin directly affects the machining accuracy of the part. The most fundamental principle for determining the programming coordinate system is the programming reference, design basis, and process reference system, which minimizes the error caused by the dimensional tolerance conversion. In addition, the influence of data processing on machining accuracy during programming, the data processing during NC programming has a direct impact on the machining accuracy of the contour trajectory. The more important factors are the calculation of the unknown programming node and the conversion of the programmed dimensional tolerance band. There is also the influence of the processing route on the machining accuracy. The machining route is one of the important contents of programming. The machining route has a great influence on the machining accuracy and machining efficiency. Next is the influence of interpolation operation on machining accuracy. The influence of interpolation operation on machining accuracy depends on the interpolation method of the system. The economical numerical control system mostly uses the pulse increment method, and the standard numerical control system uses the data sampling method. Two-stage interpolation method combining software and hardware, but no matter which interpolation method produces cumulative error, when the accumulated error reaches a certain value, the machine will cause movement and positioning error, which will affect the machining accuracy. Finally, the influence of the trajectory fitting error on the machining accuracy is that the CNC machine tool uses the small straight line segment or the small arc segment to generate the fitting curve of the machining trajectory when performing the non-circular curve machining, because the general numerical control system only has the straight line and the specified plane. The circular interpolation function can only be approximated by lines and arcs when the machining path is a non-circular curve. The fitting of non-circular curve trajectory is usually equal spacing, equal chord length, equal error method, and the equal error method can improve the processing efficiency while ensuring the fitting precision. The fitting of the non-circular curve trajectory must bring the fitting error. The most important thing here is that the control fitting error is smaller than the allowable error of the workpiece, and if necessary, it must undergo rigorous calculation.
In summary, this paper analyzes and discusses the method of Precision Machining of parts in machining process, and also summarizes the influence of numerical control technology on precision machining of parts. In fact, in machining, the error is unavoidable. Regardless of the processing method used in the machining process, the machining accuracy can be greatly improved by careful operation, careful adjustment, and selection of suitable cutting parameters. It is believed that with the continuous improvement of China's machining process, a database of basic engineering can be established in the near future to improve the efficiency of CNC machining, and finally obtain qualified parts with high quality and precision.
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