CAD/CAM integration is an important development direction of current computer in the mechanical industry, and one of the important contents of CAD/CAM integration is the connection between NC programming and CAD. There are many ways and possibilities for the connection between CNC programming and CAD. The most common and common is to transfer the CAD data to the CNC programming system through the standard interface. According to the needs of small and medium-sized enterprises, combined with the actual production, the author developed a 3D surface milling CAD/CAM system based on Auto CAD. The system uses its specific modeling method to construct its own basic voxels, synthesize complex entities through these basic voxels, and then use these standard interface files (DXF files) to generate NC programs (NC code). This paper focuses on the description method of the geometric information of the parts used in the system. The line model method, the establishment method of basic voxels and the method of determining the line spacing in NC machining.
1 The method of automatic NC programming using Auto CAD belongs to image programming. The main feature of this method is the input of graphical elements, without the need for a numerical control language. The method also has the characteristics of visual and high efficiency. The most critical issue in image programming is the description of the geometric information of the part, ie geometric modeling. That is how to create a geometric model of the part. Under Auto CAD, you can output a variety of file formats, such as DXF, WMF, 3DS, and more. The DXF file is an Auto CAD graphics exchange file, also known as a location file, which is an ASCII text file with a special format. It has the advantages of good readability, fast processing speed and strong versatility. It is easily handled by other programs and is one of the most common methods for implementing high-level languages ​​and AutoCAD interfaces.
Since the DXF files are arranged in the order in which the entities are drawn, that is, the lines drawn first must be in front. In this way, its data arrangement may be disorganized, and all the data is mixed. To generate a NC program, you must start with a geometric model to create a suitable model. The data in the DXF file of the graph is arranged according to a certain rule, which is convenient for data extraction and processing during NC programming.
Since the machine tool is usually fed according to a certain constant, if the tool is viewed as a face, the process of machining the part is to cut the workpiece with the equidistant surface. Therefore, regardless of the complexity of the workpiece, it can be cut with the equidistant surface to obtain its wireframe model, and the smaller the spacing of the equidistant faces, the closer the resulting model is to the actual shape of the workpiece. Based on the above facts, combined with the needs of numerical control programming, this special line drawing model is adopted in modeling. That is, when drawing with Auto CAD, all voxels are intersected by a plane family perpendicular to a coordinate axis and geometry. Line composition. In this way, the generated graph is a collection of three-dimensional equidistant lines, which is similar to the trajectory of the tool during NC milling. In addition, the model simplifies the solid operation to the operation between the curves in the same plane, so that the basics can be realized relatively easily. Complex operations such as merging between voxels to construct complex 3D surfaces. More importantly, the DXF file of the part generated by the line drawing model is easy to handle, and it is easy to automate the NC programming.
2 The basic voxel modeling can be expressed according to whether the voxel can be expressed by the equation. The basic voxels can be divided into standard voxels and non-standard voxels.
2. 1 standard voxel modeling plane body, prism, cylinder, sphere, ring, quadratic curve and other voxels, they have a common feature, that is, can be expressed by standard equations, so called Standard voxel. The plane body includes a polygonal surface, a circular surface, an elliptical surface, a parabolic surface, a hyperbolic surface, etc.;
The body includes a pyramid, a prism, a prism, etc.; the cylinder includes a cylinder, a cone, a truncated cone, etc.; the sphere includes a ball and an ellipsoid; the quadratic stretch is formed by a quadratic curve, which includes a parabola, a double Stretching body such as a curve. Standard voxel modeling is relatively easy. As long as the entity is cut according to the equation of the entity (such as the equation of the ball) and the equidistant surface is cut at a given pitch, the line representation of the entity consisting of the wireframe can be obtained. model. Figure 1 shows the line model of a partial prism.
2. 2 Non-standard voxel modeling In the actual design, often encounter non-standard surfaces, that is, free-form surfaces. Due to their complex shape and no fixed equations, it is very difficult to construct. The method used in this paper is to construct an arbitrary curve by using Bezier curve or B-spline curve. Then construct the surface with the curve as the "bus bar". After generating the curve, the curve is referred to as "weft". In the same way, a "warp" can be generated. Interpolation and fitting methods can be used to obtain multiple "weft" and "warp". In the actual modeling, the interpolation point and the fitting method should be selected according to the position of the "boundary point". Finally, the surface is partially adjusted to meet the design requirements.
A line model of a free surface is given.
3 Basic voxel synthesis To complete the more complex geometry shape, it needs to be realized by voxel synthesis. That is, the "and" and "difference" operations are performed. The basic principle of the synthesis operation is to perform "and" and "difference" operations on the line elements with equal spacing, such as the section curves on the X-Z plane with equal Y coordinates. By analogy, the "and" and "difference" operations are performed on the line elements in each of the equidistant planes, and finally, the composite shape of the parts can be obtained by repeatedly calculating the line spacing. The so-called "and"
The calculation is to select the line element with a large Z coordinate among the two line elements with the same Y coordinate. The "difference" operation is the opposite, that is, a line element portion having a small Z coordinate is selected among two line elements having the same Y coordinate to form a line element difference shape. The sum of the arc and the arc is given.
Determination of 4 line spacing and step length In this system, the three-dimensional composite surface is synthesized by the beveled surface and the curved surface, and the determination of the line spacing should be considered comprehensively. That is to say, in the geometric modeling, the spacing of the equidistant faces must be calculated first, which is the line spacing of the numerical control machining.
4. The calculation method of the surface line spacing S can be seen from Fig. 4. The size of the line spacing S is directly related to the size of the residual height h (CE in the figure) on the processed surface, that is, the roughness. In general, the choice of line spacing S depends on the cutter radius r and the required or allowed peak height h and the curvature of the surface. In the calculation, the following method is used: S = 2 AD , and AD = O 1 FÏr + Ï, when the ball nose cutter radius r is larger than the curvature radius Ï on the curved surface, and the surface roughness requirement is small, An approximate value of O 1 F can be taken, that is, O 1 F = r 2 - FC 2 = r 2 - FG - CG 2≈r 2 - r - h 2 and the line spacing S is S = 2 h 2 r - hÏr ± Ï(1 Where r is the radius of the milling cutter; Ï is the radius of curvature on the surface; h is the height of the cutting edge. When the part surface is convex in the AB segment, take the positive sign, take the concave time, and (3) the negative sign.
In order to avoid the cumbersome calculation of the curvature, the following approximate formula can also be used to calculate the line spacing S : S ≈ 2 2 rh (2) where O 1 and O 2 are the tool centers, and O is the center of curvature of the E point on the surface.
4. 2 Determination of the cutting distance L of the bevel plane The row spacing is determined by the following formula: l = 0. 8 D - 2λ, 1 2 D sinα ≤ Rα2 h sinαD - 2λ- h sinα1 2 D sinα> Rα It is the diameter of the milling cutter; λ is the radius of the corner of the milling cutter; α is the angle between the beveled surface and the horizontal plane; Rα is the roughness required after the processing of the beveled surface. Take the smaller of the S and L values ​​as the Y coordinate increment h in the programming. When generating the NC program, the line spacing value is automatically h.
4. Determine the 3-step length T. If the curvature radius of the surface is Ï and the interpolation error is δ. Then the step value T is T = 2δ2Ï-δ≈2 2Ï? δ
5 Conclusions 1) The graph generation method based on the line model provides conditions for NC automatic programming under Auto CAD.
2) Based on the numerical control machining, the pitch of the equidistant lines constituting the voxels and the step size of the numerical control machining are determined.
3) The description method of the part geometry information given in this paper is suitable for image programming.
(Finish)
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