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Five-axis machining of complex shapes

The emergence of complex profiles in tool and mold manufacturing is a product of high-volume suburban production. Forging dies and dies used in the automotive industry were mainly made by hand before the advent of CNC machine tools. After the 1970s, CNC machine tools were widely used in tool and mold manufacturing. The basic contours of complex shapes were usually processed by milling. Initially, the surrounding CNC machine tools were three-axis linkage.

After entering the 1980s, five-axis linkage milling machines have been widely used in complex surface processing. The milled workpiece profile is very close to the final shape of the workpiece, but the last finishing process is still a manual operation. In the late 1980s, high-speed cutting technology gradually developed and matured, and its application in industrial production has been continuously improved in terms of machine tools, cutting tools and other related technologies. Since high-speed cutting can increase the feed speed exponentially, it is possible to reduce the feed spacing without reducing production efficiency, thus providing prerequisites for improving the shape accuracy of the workpiece and reducing the surface roughness. At present, most workpieces processed by high-speed milling no longer require the last manual processing step and can be put into use directly.

The continuous development of new tool materials such as alumina-based ceramics, silicon nitride-based ceramics, cermets, cemented carbide, especially superhard coatings, makes hard surface milling possible. The mold surface can be milled and formed after quenching, thereby avoiding deformation caused by quenching after milling. This not only simplifies the processing process, but also improves the accuracy of the workpiece.

In addition, with the application of precision forging in mold manufacturing, the forged mold blank already has its basic shape, and the remaining processing margin is negligible compared with milling the entire blank. In this case, in addition to milling In addition, it can also be processed by efficient grinding. Compared with hard surface milling, high-efficiency grinding can not only improve the shape accuracy of the workpiece, but also improve the surface roughness of the workpiece. There are many methods for high-efficiency grinding. The commonly used methods include high-speed grinding with spherical grinding wheels and abrasive belt grinding with small-diameter pulleys.

Three-dimensional free-form surfaces commonly found in tools and molds are usually cut on five-axis machining centers. Since most of the workpiece materials are alloy steel or tool steel, the structure of the machine tool and the CNC system must consider the requirements for productivity and workpiece accuracy during the machining process, and carry out appropriate layout and optimization based on this. In order to ensure that the machine tool does not deform too much when cutting various mold materials, the machine tool stiffness should be given top priority when determining the machine tool layout. Most of the larger five-axis machining centers adopt a gantry structure, and some small and medium-sized five-axis machining centers sometimes adopt a column structure.

Since the 1990s, almost all complex profiles have been processed by high-speed cutting in production. The purpose is to improve production efficiency, reduce product costs, while improving the shape accuracy of the workpiece and reducing surface roughness. In order to meet the needs of high-speed cutting, the spindle of machine tools almost without exception uses electric spindle. The spindle speed is continuously variable according to the diameter of the tool used, and the speed range ranges from several thousand to tens of thousands of revolutions per minute. The drive system of the slide table is also different from conventional machining centers during high-speed cutting. Commonly used systems include high-speed screw nut drive and linear motor drive. The maximum feed speed can reach more than 100m/min.

When processing complex profiles, the CNC system of the machine tool must also meet some special requirements. For example, CNC machining programs for complex profiles are generally generated on CAD/CAM software. A profile program often requires several megabytes of storage space. It is no longer possible to transfer CNC programs using floppy disks. Therefore, the CNC system must have the function of networking with other computer systems in order to receive CNC programs directly from CAD/CAM. In addition, the CNC system must also adopt advanced control technology, which first requires a look-ahead function. That is to say, before the machine tool processes a certain trajectory, the data system pre-analyzes the surface to be processed, and appropriately adjusts the feed speed of the machine tool according to the curvature of each point on the surface and the connection relationship between adjacent points, so as to ensure that the workpiece is Achieve the highest productivity without sacrificing precision. In order to reduce dynamic errors during the machining process, the new data system servo error correction no longer uses the previous series proportional differential integral (PID) regulator, but uses a state regulator that compensates according to state parameters such as position and speed. Using this kind of regulator can completely eliminate the drive lag error, compensate for the nonlinear error caused by clearance or friction, and even offset certain vibrations of the machine tool, thereby achieving the requirements of improving the shape accuracy of the workpiece and reducing the surface roughness.

The tool system plays a decisive role in production efficiency and processing quality when processing complex profiles. When selecting a tool system, you must first consider the geometry of the parts to be processed and choose the type of tool reasonably. As shown in Figure 1, the geometry of each part of the workpiece varies greatly. If only a ball-end milling cutter is used for processing, a ball-end milling cutter with a very small diameter must be used, which makes it difficult to improve the processing efficiency. In addition, the arc radius of some parts is so small that it cannot be processed even with a small ball end mill. Therefore, considering the requirements of production efficiency and workpiece shape, other types of milling cutters, such as end mills and three-sided milling cutters, must be equipped on five-axis linkage machining centers that process complex profiles.

Figure 2 shows some types of milling cutters. As long as the size allows, no matter what shape the tool is, the cutting edge should be a machine-clamped indexable milling insert. Since the blades and cutter bodies of such tools can be combined in various ways, and the blades and cutter bodies can be produced by different companies, they can form large-scale specialized production, which is conducive to improving the quality of the cutters and reducing the production cost of the cutters. cost.

Currently, most of the indexable inserts on the market use CVD-coated carbide inserts. In order to achieve higher pseudo-wear resistance, indexable inserts are multi-layer coated. Bizhi uses Al2O3 to improve the chemical stability of the blade. Using TiN and TiCN can enhance the wear resistance of the blade. In order to enhance the sharpness of the blade, in addition to the low-temperature CVD method, the coating can also be produced by the PVD method. Some processes have very strict requirements on the blade. The blade must have a sharp cutting edge to reduce the roughness of the finished surface and extremely high wear resistance to ensure the shape accuracy of the workpiece. In this case, a combination of multiple coatings must be used. Some blades have as many as 100 coating layers to ensure foolproof use.

The life of the tool is closely related to the feed rate, cutting speed and cutting depth. The optimal cutting amount is often a small range and must be determined based on the specific tool and workpiece material conditions.

In addition, cutting strategies such as: planning of tool path, different methods of tool axis surface normal vector (normal direction of the surface at this point) or tangent vector along the surface (tangential direction of the surface at this point), etc., are also complex machining methods. a key factor. It not only affects the surface roughness of the workpiece being processed, but also affects the shape and dimensional accuracy of the workpiece. Figure 3 shows different cutting strategies used when machining a cylindrical surface. When cutting in the circumferential direction, the tool path needs to be interpolated with two axes. When cutting along the bus line, the tool only needs to perform single-axis interpolation. In addition, different cutting methods have great differences in tool wear. The tool wear in down milling is significantly lower than that in reverse milling, and the wear in reciprocating milling is much greater than in one-way milling.

In order to improve the stability of the machining process, when optimizing the cutting strategy, it is necessary to ensure the continuity of cutting, and at the same time reduce the tool movement and idle stroke as much as possible to shorten the cutting time. When rough milling steel parts, continuous down milling must be ensured to minimize the peak value of the cutting amount fluctuation of the blade during the cutting process.

When processing the workpiece shown in Figure 4, if the cutting path partitioned processing shown in Figure 5a is used; the movement of the tool is very unreasonable, the cutting conditions are very unsatisfactory, the processing time takes 33 minutes, and the surface roughness of the workpiece is 6 to 9 μm. If the circle cutting track partition shown in Figure 5b is used for processing, the processing time will be about 27 minutes, and the roughness of the workpiece can also be reduced to 2 to 4 μm.

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