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5-axis simultaneous milling for curved parts of large sculptures

The processing of large-scale sculpture curved surface parts mostly adopts the traditional “sand casting → manual shovel grinding → three-dimensional model inspection” process. Due to the low manufacturing accuracy of the three-dimensional template and the influence of its geometric deformation, it is difficult to improve the processing accuracy of such curved surface parts.

With the development of computer technology, CAD / CAM technology has made great progress, and five-axis linkage CNC boring and milling machines and some excellent CAD / CAM software have appeared. Among them, SDRC / CAMAND software is more flexible in 5-axis linkage CNC machining programming. At present, for large-scale sculpture curved surface parts, a large-diameter face milling cutter is used to process along the surface parameter line as the best processing method. This processing method has the characteristics of high processing accuracy and processing efficiency, good surface quality of parts, and excellent cutting conditions of the tool.

I. 3D modeling technology of sculpture surface
In order to complete the NC machining programming of the curved surface, firstly a three-dimensional model of the curved surface needs to be modeled on the computer. The design data of the sculpture surface is usually described by dot matrix data, and the source of the surface dot matrix data is mainly in two ways: One is through design means, according to the design theory of the product by the designer to obtain such lattice data through calculation, which is often given regularly according to certain rules, three-dimensional modeling is relatively easy; The other is to obtain the point cloud data of the curved surface by measuring the hand-made wooden mold prototype or real object with a three-dimensional measuring instrument. There is no accurate rule for the distribution of this type of data lattice, and the three-dimensional modeling of the surface is relatively difficult. Therefore, according to the different conditions of the original data lattice of the surface, the three-dimensional modeling of the sculptural surface can be divided into a regular lattice and a irregular lattice.

1.Three-dimensional modeling of regular lattice sculpture surface
Regular dot matrix sculpture surface means that the dot matrix data of the surface is given strictly according to a certain law. Generally, the surface data lattice is divided into several parametric spline node data, and the 3D modeling uses NURBS surface modeling to complete the required sculpted surface. The following describes the modeling steps in SDRC / CAMAND software.
(1) Use the "Pointset" function to generate the point set from the original point data of the sculpture surface. Note that a separate point set should be generated for each parametric spline.
(2) Using the "B-Spline" function, select "Thru Points" and "Non-uniform" parameters, and then directly select the corresponding spline point set to generate all the parameter splines of 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 OK to generate the sculpted surface.

2, 3D modeling of point cloud data sculpture surface

Point cloud surface data means that the surface point matrix data is not accurately given according to a certain law. It is not possible to generate a spline curve first, and then shape the surface to obtain a better model of such a sculpture surface.
Therefore, only the "point cloud data modeling surface" function is used, 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 convert it into the CAMAND file format in the I-DEAS software as a model for numerical control programming.

II. Design of NC Machining Technology for Large Sculpture Surface
In actual production, to realize five-axis simultaneous CNC machining of large-scale sculpture curved surface parts, we must first solve the problem of correcting and clamping the curved surface parts. Consider how to determine the workpiece zero point, tool setting point, machining tool plan, and detailed machining sequence.

1. Principles of aligning and clamping for large-scale sculpture curved surface CNC machining

For general large-scale sculpture curved parts, there is a certain rule to follow when setting up. Therefore, after analysis, large sculpture surfaces can be divided into two categories, and the principle of correcting the clamping is as follows.

If there are characteristic reference planes, such as planes and cylindrical surfaces, on the curved surface parts of the sculpture, the equal characteristic planes of the plane are used as the reference for clamping. This simplifies the alignment process, improves the alignment efficiency of clamping, and ensures the accuracy of surface alignment.

If the sculptural surface consists entirely of sculptural surfaces, and there is no definite benchmark, then generally the plane block is cast or welded on the curved part; Pin holes or reference pins assist in correcting the reference. The use of 3D measurement technology of large curved surfaces and the use of computer software adaptation technology to obtain the distribution of the remaining blanks to guide the correction of sculpture curved surface parts.

Of course, these two methods are based on the determination of the benchmark for large sculptured surface parts, and then the CNC machining program uses the correct benchmark as the guiding idea. Nowadays, some manufacturers also have another method of processing. For large sculpture surfaces without a fixed reference, they can be placed freely on the machine table.After the clamping is reliable, the surface is measured with a certain grid distribution, and then the measurement point data is processed again. Find out the position relationship of the surface in free space to ensure the machining allowance of the surface. Adjust the processing coordinate system, convert the NC processing program, and finally return to the machine to complete the processing of this part.

The biggest advantage of this process is that it does not require a fixed reference for clamping and alignment, and the parts can be placed freely. However, on the other hand, this process method increases the processing assistance time of the machine tool and processes more data, which also brings difficulties to the production organization.

2. Selection of CNC machining tools for large sculpture curved parts

For 5-axis simultaneous CNC machining tools for large sculpture curved parts, large diameter face milling cutters are preferred. At present, the well-known tool manufacturers are Sandvik, Ingersoll, Kennametal, Seco and so on. Among them, Sandvik's CoroMill 200 series of circular blade face milling cutters are particularly suitable for semi-finishing and finishing of sculptured curved surface parts; Kennametal's 220/221 series cutters have the advantages of impact resistance and good rigidity, which are suitable for rough machining and fast cutting. The diameter of the selection tool should be determined based on the curvature of the sculpture surface part. The principle is that the tool radius should be smaller than the minimum curvature radius of the concave surface of the sculpture surface, but it should not be too small, otherwise the processing efficiency will be reduced. For sculpted curved parts with large changes in curvature radius, the whole part can also be divided into different regions, and tools with different diameters can be selected to improve machining efficiency.

III. Calculation and Simulation of Five-axis CNC Machining Tool Position

Five-axis coordinated CNC machining tool position calculation methods for sculptured surface parts include INTERP (interpolation), NORMAL (normal), TILT (tilt), and TANGTO (tangential) in the CAMAND software. But for large sculpture curved parts, the most commonly used is TILT processing. When this machining method is used, the tool axis and the cutting point surface normal form a certain rake angle (Lead / Lag Angle) or roll angle (Right / Left Angle).

1, 5-axis linkage CNC machining tool position calculation

In the CAMAND software, TILT is used to process sculptured surface parts. The tool position calculation process: First select the "Flowline Surfaces" function of "Numerical Control". The processing method of "Flowline Surfaces" is actually processing along the parameter line direction of the surface; Next, set the tool location name (available with “New” or “Rename” function), select the machining coordinate system and the starting point, and then enter the corresponding tool parameters (tool diameter, tool length, cutting edge shape size, cutting parameters, etc.). After setting the entering and exiting tool parameters, selecting the interference check surface and setting the parameters, then click the tool position calculation function, select the machining surface and define the cutting direction and the tool axis control method TILT method. General setting plus forward tilt angle, that is, an angle of 2 to 10 ° forward in the cutting direction; Finally, the calculation of the five-axis coordinated machining tool position of the surface is completed.

2, Cutting simulation and machine simulation

For large sculpture curved parts, the cost of the blank is high. And for the safety of machine tools, tools and tooling. After the calculation of the tool position of the five-axis simultaneous CNC machining is completed, cutting simulation and machine tool simulation must be performed to check the correctness of the tool position, and the resulting processing program can be used for actual production. In the CAMAND software, there is a simple set of cutting inspections and machine tool simulations that can meet common simulation requirements. If more accurate simulation tools are needed, the more specialized CGTech Vericut software should be used. The software can not only perform cutting simulation and machine tool simulation, but also can optimize the tool path. The simulation in the CAMAND software is only briefly described here.

For cutting simulation of five-axis tool position, you can check and correct the over-cut phenomenon of the tool in the previous five-axis tool position calculation. Generally, the surface to be machined 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 overcuts and collisions 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 are mostly two-axis NC milling head) and the workpiece and fixture. Therefore, you can build a 3D model of the NC milling head according to the shape and size of the actual machine tool NC milling head, and then use the "Simulation" function plus the NC milling head to simulate the machine tool. It can check the interference of NC milling head, tool, fixture and workpiece.

IV. Post-processing of 5-axis tool position
After completing the 5-axis simultaneous machining tool position calculation of the curved surface, the post-processing technology is also required to complete the conversion of the intermediate tool position file into a G code program recognized by the CNC machine tool. The tool position file in CAMAND is given by the tool tip point coordinates and tool axis vector, then the post processing must convert the tool position data into X, Y, Z, B, C coordinate values.
The post-processor is designed according to the CNC system and machine parameters of the specific machine tool (such as the travel of each coordinate axis, the maximum feed speed, speed, etc.). Then use the post-processing function in CAMAND's Main NC to convert the five-axis tool position data into a G code program executable by the machine numerical control system.

The five-axis simultaneous machining technology for large sculpture curved parts can effectively resolve the contradiction between the machining accuracy and machining efficiency of such parts. This technology involves multi-disciplinary comprehensive technologies such as computer-aided 3D surface modeling, computer-aided manufacturing, and machining technology.
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