EDM milling automatic programming system based on graphic exchange file

Traditional EDM is divided into two major categories: EDM and WEDM. One of the key points of molding processing is also the production of shaped electrodes. The design and manufacture of tools account for almost half of the total processing time and the cost is high. For many years, electric processing researchers have been looking for ways to replace molded electrodes. EDM-Mill is the use of a simple shape electrode to make a certain trajectory for the forming movement, through the discharge between the tool electrode and the workpiece for processing. This avoids the production of molded electrodes and increases productivity. Correspondingly, it also put forward higher requirements for EDM machines, and it needs to develop a special CNC system. AutoCAD is currently the most widely used CAD software. It not only has rich 2D graphics, editing commands and strong 3D modeling capabilities. It also provides programming methods such as wire-shaped files, menu files, and command files. Its flexibility and openness have determined that many applications choose it as a support platform for graphic design, editing, and pre- and post-processing. AutoCAD and its graphics format have become a de facto international industry standard. In addition, AutoCAD can exchange data with other CAD systems or CAM systems through standard data formats. This is "DrawingeXchangeFile", or DXF file for short. To realize the integration of CAD/CAM for EDM milling, it is necessary to extract useful parts information from this file and convert this information into CNC machining programs for EDM machines. 1 Interface Program Design The DXF file is an ASCII text file. A typical DXF file consists of six sections: header section, class section, table section, block section, entity section, and object section. The DXF file contains a great deal of information, but the entity section is useful for NC programming. Therefore, we only care about the content of the entity section. According to the data format of the entity section, a corresponding interface program is prepared to extract the geometric information of the graphic. However, the starting point coordinate obtained from the DXF file is the starting point of the first input pattern. The setting of the starting point of EDM machining needs to consider the stress state of the workpiece and the influence on the machining accuracy and surface roughness of the workpiece. It is often inconsistent with the starting point of the drawing. To solve this problem, we added the character "O" next to the starting point of the machining when drawing the figure. To facilitate the modification and reordering of entities in the DXF file. We use a double linked list structure to store the entity coordinates.

Figure 1 DXF file interface program flow diagram

(a)

(b)

Figure 2 side outline interface program flow chart shown in Figure 1. 2 Automatic identification of the machining direction After obtaining the part geometry information, the tool trajectory can be generated by compensating the tool radius (cylindrical tool). When tool radius compensation is performed, the tool center offset direction is determined by the inside and outside characteristics of the contour and the machining direction (rotational direction of the machining closed loop). The inside and outside characteristics of the contour are set during machining, and the machining direction must be determined based on the contour map and the tool feed direction. Therefore, the discrimination of the machining direction is the basis for solving the tool compensation. The outline of parts is generally composed of straight lines and arcs, so we discuss the calculation of the trajectories of the plane profile tools consisting of these two types of figures. The sides of the polygon are regarded as vectors, the vector direction is the direction of the cutting knife, and each vector is connected in order of the cutting knife. M1M2,..., M7M8, M8M1 in FIG. The arc contour takes the line between the start and end points. The arrow in the figure indicates the tool travel direction. For the graph shown in Figure 2(a), each point is a convex point and the method of discrimination is relatively simple. By determining the cross product of two adjacent vectors of any one vertex, the rotation of the machining closed loop can be determined. It is specified that the closed loop be counterclockwise rotation is positive and clockwise rotation is negative. When the two vector cross product is positive, the machining direction is counterclockwise: otherwise, it is clockwise. But for the outline shown in Figure 2(b), this method will not work. We designed the following two methods of discrimination. The Vertex Rotation Accumulation Judgment method defines that the vertex rotation is the cross product direction of two adjacent vertices. For any plane figure, the machining direction is related to the rotation direction of each vertex. If the number of vertices with positive rotation is greater than the number of negative vertices, the machining direction is positive: otherwise it is negative. It can be verified that this method is valid for any plane graphics. However, when the number of vertices is large, the amount of operations is large. The pole-discriminating method defines that the pole of a polygon is the one with the smallest x-coordinate. By determining the rotation of the pole, the closed loop direction of machining can be obtained. Specifically, first select the pole and adjacent two vectors in the doubly-linked list (if there are more than one pole, select one) and calculate its cross product. The cross product is positive and the machining direction is positive (counter clockwise): otherwise the machining direction is negative (clockwise). The pole discrimination method has nothing to do with the number of polygonal vertices, so the amount of calculation is small. 3 Calculation of tool trajectory The tool trajectory can be calculated based on the previously determined machining direction. Let the tool radius be dr and the unilateral discharge gap be d. Then the tool center offset b = d + r. Table 1 Symbol selection for straight line contours Partial machining direction a Symbol outer contour negative 0 ≤,

Table 2 Compensating parts of circular arc contour Tool Arc direction Tool path Arc radius Inner contour Circumference rb Inverse r+b Outer contour Cylindrical r+b Inverse rb Linear contour For linear contours The direction of the direction of the normal translation distance b, the known line ends A (xa, ya), B (xb, yb), available linear equation y = kx + c where k = yb-ya, c = xbya-xayb Xb-xa xb-xa The linear equation after translation is y=kx+c', c' can be calculated by the following formula: c'=cf(1+k2)? (1) The sign is based on the inside and outside of the machining direction and contour. Features to choose. Let the angle between the vector and the X axis be a, and Table 1 is the value under different conditions. Arc contour contour compensation is simpler than a straight line and only involves the increase or decrease of the arc radius r. The first thing to do is to determine the arc direction. From the DXF file we can obtain the starting and ending coordinates of the arc, the radius of the arc, and the starting and ending angles, so that the tangents at the starting and ending points of the arc can be obtained. The cross-product of the starting and ending tangent vectors is determined The direction of the arc. The judgment rule is: the cross product is positive, and the reverse circle: the cross product is negative and smooth. After determining the arc direction, use Table 2 to perform tool path compensation. After processing, the tool trajectory is automatically compensated. However, when the two entities intersect to form a sharp corner, it is necessary to perform transition processing on the sharp corner to avoid the interference generated by the tool at the corner or the discontinuity of the tool path. We used the arc transition method to pretreat the sharp corners and solved this problem. According to the final tool trajectory, after the post-processing can generate NC code. 4 Conclusion DXF files exported by AutoCAD provide the foundation for CAD/CAM integration of EDM milling. Based on the DXF file structure, this paper develops the interface software for extracting entity information, and proposes two methods to determine the processing direction. Both of them can effectively judge the rotation of the plane figure. According to the machining direction and the inside and outside characteristics of the contour, the tool trajectory can be calculated. This lays the foundation for the generation of the final numerical control code. Using the method described in this article, tool trajectories can be quickly and reliably obtained. However, the tool trajectory obtained in accordance with this method is a theoretical trajectory and does not consider the loss of the tool during machining. To realize the integration of CAD/CAM for EDM milling and achieve better process results, the dynamic compensation of tool wear in the machining process must also be resolved.

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