The key to the CNC machining of complex profiles

1. The main processing method for complex profiles The emergence of complex profiles in tool and mold manufacturing is the result of high-volume, high-efficiency production. Forging dies and dies used in the automotive industry were primarily manufactured by hand before the advent of CNC machine tools. Since the 1970s, CNC machine tools have been widely used in the manufacture of tools and molds. The basic contours of complex profiles are usually milled. The first CNC machine tools used are three-axis linkage. After entering the 1980s, the five-axis linkage milling machine has been widely used in complex profile machining. The contour of the workpiece after milling has been very close to the final shape of the workpiece, but the final finishing process is still manual. In the late 1980s, high-speed cutting technology gradually developed, and its application in industrial production has been continuously improved from machine tools, cutting tools and other related technologies. Since high-speed cutting can multiply the feed rate, it is possible to reduce the feed pitch without reducing the production efficiency, thereby providing a prerequisite for improving the shape accuracy of the workpiece and reducing the surface roughness. At present, most of the workpieces processed by high-speed milling no longer require the last manual machining process, but can be put into use directly.
The development of new tool materials such as alumina-based ceramics, silicon nitride-based ceramics, cermets, and hard alloys, especially super-hard coatings, has made hard surface milling possible. The profile of the mold can be milled after quenching, thus avoiding deformation caused by quenching after milling. This simplifies the machining process and improves the accuracy of the workpiece.
In addition, with the application of precision forging in the manufacture of molds, the die blank after forging has its basic shape, and the remaining machining allowance is negligible compared to when the whole blank is milled, in this case, except for milling. In addition, it can be processed by efficient grinding. Compared with hard surface milling, high-efficiency grinding not only improves the shape accuracy of the workpiece, but also improves the surface roughness of the workpiece. There are many methods for efficient grinding, usually with high-speed grinding of spherical grinding wheels and abrasive belt grinding with small diameter pulleys.
2. The key to CNC machining of complex profiles (1) Five-axis multi-function machining center The three-dimensional free-form surfaces commonly found in tools and molds are usually cut on a five-axis machining center. Since the material of the workpiece is mostly alloy steel or tool steel, the structure and numerical control system of the machine tool must take into account the requirements of productivity and workpiece accuracy during the machining process, and use this as a basis for proper layout and optimization. In order to ensure that the machine tool does not undergo too much deformation when cutting various mold materials, the machine stiffness should be placed in the first place when determining the machine layout. Most of the larger five-axis machining centers use gantry structures, and some small and medium-sized five-axis machining centers sometimes use column-type structures.
Since the beginning of the 21st century, complex profiles have been processed almost entirely in high-speed cutting. The purpose is to increase production efficiency, reduce product cost, and improve the shape accuracy of the workpiece and reduce the surface roughness. In order to meet the needs of high-speed cutting, the spindle of the machine tool uses the electric spindle almost without exception. The spindle speed is continuously variable according to the diameter of the tool used. The speed range is from several thousand to several tens of thousands of revolutions per minute. The drive system of the slide table is also different from the conventional machining center in high-speed cutting. The commonly used system is driven by a high-speed screw nut and a linear motor. The maximum feed speed can reach more than 100m/min.
When machining complex profiles, the CNC system of the machine 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 impossible to transfer NC programs with a floppy disk. Therefore, the CNC system must have the function of networking with other computer systems to receive the NC program directly from CAD/CAM.
In addition, CNC systems must also use advanced control technology, first requiring Look Ahead functionality. 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 of the curved surface and the connection relationship of adjacent points, so as to ensure the workpiece. Maximum productivity is achieved with precision. In order to reduce the dynamic error in the machining process, the new data system servo error correction is no longer the conventional series proportional differential integration (PID) regulator, but a state regulator that compensates by state parameters such as position and speed. The use of this regulator can completely eliminate the driving hysteresis error, compensate for the nonlinear error caused by the gap or friction, and even cancel some vibration of the machine tool, thereby achieving the requirements of improving the shape accuracy of the workpiece and reducing the surface roughness.
(2) Tool system The tool system plays a decisive role in the production efficiency and processing quality when processing complex profiles. When selecting a tool system, you must first use the geometry of the part to be machined and use the type of tool. As shown in the workpiece of Figure 1, the geometry of each part varies greatly. If only ball-end milling cutters are used for machining, it is necessary to use a ball-end milling cutter with a small diameter, which makes it difficult to improve the machining efficiency. In addition, the arc radius of some parts is very small, that is, it cannot be processed with a small ball end mill. Therefore, considering the requirements of both production efficiency and workpiece shape, other types of milling cutters, such as end mills and three-sided milling cutters, must be equipped on the five-axis linkage machining center for machining complex profiles.
Figure 2 shows some of the types of milling cutters commonly used to machine complex profiles. As long as the size allows, regardless of the shape of the tool, the cutting edge should use the machine clamp indexable milling insert. Such a tool can be produced in a variety of combinations due to the blade and the body, and the blade and the body can be produced in different companies, so that large-scale specialized production can be formed, which is beneficial to improving the quality of the tool and reducing the production of the tool. cost.
Most of the indexable inserts on the market currently use CVD coated carbide inserts. In order to achieve higher wear resistance, the indexable inserts are multi-layer coated. For example, Al2O3 can improve the chemical stability of the blade; TiN and TiCN can enhance the wear resistance of the blade. In order to enhance the sharpness of the blade, the coating can be produced by the PVD method in addition to the low temperature CVD method. Some machining requirements are very strict on the blade. The blade must have a sharp cutting edge to reduce the roughness of the finished surface, and it must have extremely high wear resistance to ensure the shape accuracy of the workpiece. In this case, a combination of multiple coatings must be used. In order to ensure the use of the blade, the number of coatings can be as many as 100 layers.
(3) Optimization of the process The life of the tool is closely related to the feed rate, cutting speed and depth of cut. The optimum amount of cutting is often a small range and is determined by the specific tool and workpiece material.
In addition, the cutting strategy is as follows: the planning of the path of the tool, the normal surface of the tool axis surface (the normal direction of the surface at the point) or the different way along the curved surface (the tangential direction of the surface at the point), etc. A key factor in the face. It not only affects the surface roughness of the workpiece being machined, but also affects the shape and dimensional accuracy of the workpiece. Figure 3 shows the different cutting strategies used to machine a cylindrical surface. Cutting in the circumferential direction, the tool path is to be interpolated by two axes. When cutting along the busbar direction, the tool only needs to be uniaxially interpolated. In addition, the different cutting methods, the tool wear is very different, the tool wear during the down milling is significantly lower than the up-cut milling, and the wear during reciprocating milling is much larger than the one-way milling.
In order to improve the stability of the machining process and optimize the cutting strategy, it is necessary to ensure the continuity of the cutting while minimizing the movement of the cutting and the idle stroke in order to shorten the cutting time. When roughing steel parts, continuous down-cutting must be ensured to minimize the peak value of the cutting edge fluctuation during the cutting process.
When machining the workpiece shown in Fig. 4, if the machining is performed by the line cutting path division, the movement of the tool is unreasonable, the cutting conditions are not ideal, the machining time is 33 min, and the surface roughness of the workpiece is 6-9 μm. If the circle cutting path is used for machining, the machining time takes about 27 minutes, and the roughness of the workpiece can be reduced to 2 to 4 μm.