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Cad/cam technology for WEDM based on AutoCAD

1 introduction

in recent years, the single machine numerical control system has been gradually replaced by the group control system (DNC) with microcomputer as the core. Manual programming is transitioning to graphical automatic programming. Cad/cam technology has been widely used in manufacturing technology. Due to the powerful drawing and function of AutoCAD software and the open DXF data interface, it has become an important part of the graphics input platform and automatic programming. However, at present, many automatic programming systems based on AutoCAD draw according to the machining sequence, without considering the automatic compensation of tool path, and the machining direction needs to be set manually. For complex parts, it is often necessary to determine the drawing parameters manually, which has many inconveniences. In this paper, ARX development technology and visual c++6.0 are used to carry out the secondary development of AutoCAD, so that the graphical input, automatic tracing, NC programming and master-slave communication processes are completed under the AtuoCAD platform, and the cad/cam integration is realized with the transformation of WEDM system as the goal

2 system structure

the system adopts master-slave control mode, in which the host adopts medium-grade microcomputer and the slave adopts single-chip microcomputer system to directly control the wire cutting machine tool. A host computer controls several slaves through RS-485 interface, and makes full use of the powerful functions of the microcomputer to input and output graphics, and establish a cutting process parameter database. The automatic programming module of the host machine converts the graphic data information into NC machining instructions, and transmits the machining instructions to the slave machine, which executes in turn to complete the machining process. At the same time, the slave machine feeds back data according to the requirements of the host machine, which provides a basis for real-time control, automatic tracking and the establishment of process parameter database. The host software system includes six functional modules: graphic input, process parameter processing, automatic programming, simulation, tracking and communication (Figure 1)

Figure 1 host software system structure

3 automatic programming technology

the automatic programming module mainly solves the functions of automatic tracking, machining direction discrimination, tool path automatic compensation, etc., and realizes the integration of cad/cam. The process is shown in Figure 2

Figure 2 flow chart of automatic programming software

3.1 obtain graphic data and generate basic machining closed loop

DXF file generated after AutoCAD drawing input, and use feature-based information modeling method to obtain machining entity feature information. What the system needs is the geometric information of the entity, so it only needs to read and process the entities section of the DXF file and store the relevant feature information. Based on this information, the outline of the part and the topological relationship between its constituent elements are preliminarily formed. CAD drawing entities are stored in DXF files according to their drawing order, which has nothing to do with the processing order; When deleting or adding an entity in the processing closed loop, the entities in the DXF file are not stored in the order of the processing closed loop, so it is necessary to reorder according to the end connection to form the processing closed loop. In this way, drawing and machining are separated. When AutoCAD is used for drawing input, the actual machining sequence does not need to be concerned, and the data input process is simplified. The system uses the double linked list data structure to store entity information. See the literature for the double linked list data structure

3.2 judging the machining direction

this paper uses the vector method to judge the machining direction. Next, take any processing closed loop shown in Fig. 3 as an example to illustrate the discrimination algorithm of processing direction

first traverse the double linked list, and find the endpoint closest to the X or Y coordinate axis as the processing starting point, that is, d=min (| x-x0 |, | y-y0 |). Let this point be point B, and the front and back end points in the linked list are point a and point C respectively. It can be inferred that the connection of the two vectors AB and BC connecting point B will uniquely determine the direction of the original figure

any closed-loop diagram in Figure 3

uses the vector method to identify the direction of triangle ABC. For the convenience of programming, the quadrants to which the vector direction belongs are classified according to the following principles. Set points a (XA, ya), B (XB, Yb), C (XC, YC), then ab=ob oa= (xbi+ybj) - (xai+yaj) = (XB XA) i- (Yb ya) j

when XB XA> 0 and Yb ya ≥ 0, ab ∈ I quadrant

when XB Xa ≤ 0 and Yb Ya> 0, ab ∈ Ⅱ quadrant

when XB Xa < 0 and Yb ya ≤ 0, ab ∈ Ⅲ quadrant

when XB XA> 0, Yb Ya <0, ab ∈ Ⅳ quadrant

according to the selection principle of machining starting point, it can be deduced that the vectors AB and BC cannot be in the same quadrant, thus simplifying the judgment process. See Figure 4 for the closed-loop direction judgment process. In Figure 4, K Ⅰ, K Ⅱ, K Ⅲ and K Ⅳ respectively represent the slope of each quadrant vector. This method has the advantages of simple programming, fast running speed, short program execution time and accurate automatic tracing

Figure 4 flow chart of judging machining direction by vector method

3.3 forming actual machining path

when forming actual machining path, it is necessary to consider the influence of concave and convex die characteristics of machining objects, molybdenum wire diameter, discharge gap and machining direction, determine the tool path compensation in the automatic programming system, and adjust the basic machining route to ensure machining accuracy. Let the radius of molybdenum wire be r molybdenum, and the unilateral discharge gap be δ Electric, then the offset compensation of machining path is

f=r Mo+ δ Electric (1)

the contour of the target workpiece is generally composed of straight lines and arcs (other curves can be fitted by straight lines and arcs). Therefore, adjusting the actual machining path of the tool actually increases or decreases f for the arc radius r, and shifts f for the straight line in the normal direction

set the two ends of the straight line as PS (XS, YS), PE (Xe, ye),

the general equation of the original straight line is l:ax+b to be confirmed y+c=0 (2)

the straight line equation after translation f is l ': ax+by+c (2) if the machine is not used for a long time after the experiment' =0 (3)

where (4)

a=ye ys, b=xs Xe, c=xeys yexs (5)

for circular arc, R ′=r ± f (6)

the selection of the sign in formula (4) and (6) depends on the convex Concave model and processing direction. See reference [2] for the adjustment under various conditions. The system is accurate to 1 when performing relevant calculations μ m。 After the corresponding adjustment of each entity, the actual motion path of the tool is obtained, which eliminates the errors caused by the process parameters such as the tool and discharge gap that may cause deflection and torsion, and improves the precision of the process

in actual machining, the workpiece contour may not be smooth, and corners, sharp corners, etc. may easily cause wire breakage, short circuit, angle collapse, empty travel and other problems. A transition arc (R ≥ f) can be added to make the machining path smooth transition, so as to protect the molybdenum wire and improve the machining efficiency. The system interpolates each entity according to the machining sequence, stores the machining command in the file, and sends it to the slave system for machining, thus realizing the integration of cad/cam

4 conclusion

the cad/cam technology based on AutoCAD introduced in this paper has been applied in the precision instrument factory of our institute, and the technical transformation and group control management of ckx-1 and dmk6732 numerical control WEDM machines have been realized. In the process of use, users only need to be familiar with the basic drawing operations and input drawings according to the design requirements, without caring about the processing process. The complex calculation and trajectory compensation are completed by the automatic programming system, so as to realize the separation of graphic input and machining. After drawing, the master directly controls the slave machining, realizing the integration of cad/cam. The system can also realize the precise machining of complex workpiece contour. The system has friendly interface and simple operation. It can accurately carry out automatic tracking and tool path compensation. The operation is stable and reliable, which reduces the burden of operators and improves the machining efficiency. (end)

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