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Five-axis simultaneous milling of large sculptured curved parts

The processing of large sculpture curved surface parts is mostly done using the traditional “sand casting → manual shoveling → three-dimensional prototype inspection” process. Due to the low manufacturing accuracy of the three-dimensional prototype and the influence of its geometric deformation, the processing accuracy of this type of curved surface parts has been reduced. Difficult to improve.

With the development of computer technology, CAD/CAM technology has made great progress. Five-axis linkage CNC boring and milling machines and some excellent CAD/CAM software have emerged. Among them, SDRC/CAMAND software is more flexible in five-axis linkage CNC machining programming. . At present, the best processing method for processing large sculpture curved surface parts is to use large-diameter face milling cutters to process along the direction of the curved surface parameter line. This processing method has high processing accuracy and processing efficiency, good surface quality of the parts, and good cutting status of the tool. Excellent features.

1. Three-dimensional modeling technology of sculpture surfaces

In order to complete CNC processing programming of curved surfaces, it is first necessary to create a three-dimensional model of the curved surface on the computer. The design data of sculpture surfaces are usually described by lattice data, and there are two main sources of surface lattice data: one is through design means, and the designer obtains this lattice data through calculation according to the product design theory. It is often given regularly according to certain rules, and three-dimensional modeling is relatively easy; the other is to obtain the point cloud data of the curved surface by using a three-dimensional measuring instrument to measure the hand-made wooden prototype or actual object. This type of data is distributed in a lattice Without precise rules, three-dimensional modeling of curved surfaces is relatively difficult. Therefore, according to the different conditions of the original data lattice of the surface, the three-dimensional modeling of the sculpture surface can be divided into regular lattice surface modeling and irregular lattice surface modeling.

1. Three-dimensional modeling of regular lattice sculpture surfaces

Regular lattice sculpture surface means that the surface lattice data is given strictly according to certain rules. Usually, the surface data point matrix is divided into several parametric spline node data, and the three-dimensional modeling uses NURBS surface modeling to complete the required sculpture surface. The modeling steps in SDRC/CAMAND software are described below.

(1) Use the “Pointset” function to generate a point set (Pointset) from the original lattice data of the sculpture surface. Note that an independent point set should be generated according to each parameter spline.

(2) Use the “B-Spline” function, select the “Thru Points” and “Non-uniform” parameters, and then directly select the corresponding spline point sets to generate all parametric splines for the construction surface.

(3) Use the “Surface” function of “Modeling” to execute the “Lofted Surface” sub-function. After selecting each spline curve in a certain order, click Confirm to generate the sculpture surface.

2. Three-dimensional modeling of point cloud data sculpture surfaces

Point cloud surface data means that the surface lattice data is not given accurately according to certain rules. It is impossible to generate a spline first and then shape the surface to obtain a better sculpture surface model. Therefore, we can only use the “Point Cloud Data Modeling Surface” function, and there is no similar function in CAMAND. In practice, we use the “Fit Points to Surface” function in I-DEAS to generate a surface from the point cloud, and then use the I-DEAS software to create a surface. Convert it to CAMAND file format and use it as a model for CNC programming.

2. Design of CNC machining technology for large sculpture curved surfaces

In actual production and manufacturing, in order to realize five-axis CNC machining of large sculptural curved surface parts, we must first solve the clamping and alignment plan of the curved surface parts, and consider how to determine the workpiece zero point, tool setting point, machining tool plan and detailed processing order etc.

1. Principles of clamping and alignment for CNC machining of large sculpture curved surfaces

For general large sculpture curved surface parts, there are certain rules to follow for clamping and alignment. Therefore, after analysis, large sculpture curved surfaces can be divided into two categories. The clamping and alignment principles are as follows.

If there are characteristic datum surfaces such as planes and cylindrical surfaces on the sculpture curved surface parts, the characteristic surfaces of the plane will be used as the clamping and alignment datum. This can simplify the alignment process, improve the clamping and alignment efficiency, and ensure the accuracy of curved surface alignment.

If the sculpture surface is entirely composed of sculpture surfaces and there is no definite datum, then generally auxiliary alignment datums such as alignment plane blocks, pin holes or datum pins are cast or welded on the curved surface parts, and three-dimensional measurement techniques for large curved surfaces are used, as well as Computer software adaptation technology is used to obtain the distribution of blank margin to guide the alignment of sculptured curved surface parts.

Of course, these two methods are based on the guiding ideology of first determining the datum of large sculpture curved surface parts, and then the CNC machining program uses the alignment datum for processing. Nowadays, some manufacturers also have another process method. For large sculpture surfaces without a definite datum, place them directly and freely on the machine tool workbench. After the clamping is reliable, measure points on the surface according to a certain grid distribution, and then process the measurement point data to find out the position of the surface in the free space. Positional relationship, ensure the machining allowance of the curved surface, adjust the processing coordinate system, convert the CNC machining program, and finally return to the machine tool to complete the processing of the part. The biggest advantage of this process method is that there is no need for a determined benchmark for clamping and alignment, and parts can be placed freely. But on the other hand, this process method increases the auxiliary time of machine tool processing, processes more data, and also brings difficulties to production organization.

2. Selection of CNC machining tools for large sculptured curved surface parts

For the five-axis CNC machining of large sculptural curved parts, it is better to use large-diameter face milling cutters. Currently, the more famous tool manufacturers include Sandvik, Ingersoll, Kennametal, Seco, etc. Among them, Sandvik’s CoroMill 200 series round blade face milling cutters are particularly suitable for semi-finishing and finishing of sculptured surface parts; Kennametal’s 220/221 series tools It has the advantages of impact resistance and good rigidity, and is suitable for rough machining, rapid machining and large-tooth machining. The diameter of the tool should be determined based on the curvature of the sculptured surface part. The principle is that the tool radius should be smaller than the minimum curvature radius of the concave surface of the sculptured surface, but it should not be too small, otherwise the processing efficiency will be reduced. For sculptured curved surface parts whose curvature radius changes too much, the entire part can also be divided into different areas and tools with different diameters can be selected to improve processing efficiency.

3. Five-axis CNC machining tool position calculation and simulation

The tool position calculation methods for five-axis linkage CNC machining of sculptural curved parts include INTERP (interpolation), NORMAL (normal), TILT (tilt) and TANGTO (tangential) in the CAMAND software, but for large sculptural curved parts , the most commonly used method is TILT processing. In this processing method, the tool axis and the normal vector of the cutting point surface form a certain front and rear inclination angle (Lead/Lag Angle) or side inclination angle (Right/Left Angle).

1. Five-axis linkage CNC machining tool position calculation

In CAMAND software, the TILT method is used to process sculpture curved surface parts. The tool position calculation process: first select the “Flowline Surfaces” function of “Numerical Control”. The “Flowline Surfaces” processing method is actually processing along the parameter line direction of the surface. ; Secondly, set the tool location name (available with the “New” or “Rename” function), select the processing coordinate system and starting point, and then enter the corresponding tool parameters (tool diameter, tool length, cutting edge shape and size, cutting parameters, etc.) , set the parameters for entering and exiting the tool, select the interference check surface and set the parameters, then click the tool position calculation function, select the processing surface and define the cutting direction and the tool axis control mode TILT mode. Generally, set the forward inclination angle, that is, forward along the cutting direction. Tilt an angle of 2 to 10°; finally complete the calculation of the five-axis linkage machining tool position of the curved surface.

2. Cutting simulation and machine tool simulation

For large sculptural curved surface parts, due to the high cost of blanks and for the safety of machine tools, tools and tooling, after the five-axis linkage CNC machining tool position calculation is completed, cutting simulation and machine tool simulation must be performed to check the correctness of the tool position, and finally get The processing procedures can be used in actual production and manufacturing. In the CAMAND software, there is a simple set of cutting inspection and machine tool simulation that can meet general simulation requirements. If you need a more accurate simulation tool, you should use the more professional Vericut software from CGTech. The software can not only perform cutting simulation and machine tool simulation, but also has features such as optimizing tool path. Here we only briefly describe the simulation in CAMAND software.

For the cutting simulation of the five-axis tool position, the overcutting phenomenon of the tool in the previous five-axis tool position calculation can be checked and corrected. Generally, the surface to be processed is first rendered and colored, and then the “Simulation” function is used to gradually simulate the tool position and check the interference of the tool. If overcutting and collision occur, the tool position must be recalculated or modified. For machine tool simulation, we mainly check the interference between the NC milling head (large five-axis gantry boring and milling machines mostly use two-axis NC milling heads) and the workpiece and fixture. Therefore, the NC milling head can be established based on the shape and size of the actual machine tool NC milling head. The three-dimensional model is then used to simulate the machine tool using the “Simulation” function plus the NC milling head. The interference between the NC milling head, the tool, the fixture, and the workpiece can be checked.

4. Post-processing of five-axis tool positions

After completing the calculation of tool positions for five-axis simultaneous machining of curved surfaces, post-processing technology is needed to convert the intermediate tool position files into G code programs that can be recognized by CNC machine tools. The tool location file in CAMAND is given in terms of tool tip point coordinates and tool axis vector, so post-processing must convert the tool location data into X, Y, Z, B, and C coordinate values. Design the post-processor according to the CNC system and machine tool parameters of the specific machine tool (such as the stroke of each coordinate axis, maximum feed speed, rotation speed, etc.), and then use the post-processing function in CAMAND’s Main NC to convert the five-axis tool position data It is a G code program executable by the machine tool CNC system.

The five-axis linkage processing technology of large sculptural curved parts can effectively solve the contradiction between the processing accuracy and processing efficiency of such parts. This technology involves multi-disciplinary comprehensive technologies such as computer-aided three-dimensional surface modeling, computer-aided manufacturing and mechanical processing technology.

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