In CNC machining, process routing, machining strategy and tool selection have a great impact on machining efficiency, machining quality and machining costs. In recent years, we have been committed to research and testing in this area, and have achieved certain results in improving processing efficiency and reducing tool costs. Taking the car crankshaft forging die and PowerMILL as the programming software as an example, the NC machining strategy and tool selection are discussed.
Structural analysis and processing technology Structural analysis
The structural characteristics of the crankshaft forging model cavity of the car (see Figure 1) are that the crank part is deep and narrow, the concave fillet is small, the crank depth is about 58mm, the crank width is generally less than 16mm, the minimum draft angle is 1°, and the cavity is concave. Most of the angles are R3, and the connecting rod diameter of the crankshaft and the transition portion of the crank are rounded to R2, and the intermediate main shaft has annular grooves on both sides. This type of (machine forging die) has an outer dimension of about 600 mm × 330 mm × 160 mm, a mold material of H11, a die hardness of dB = 2.9 to 3.1, a groove size accuracy of ± 0.10 mm, and a surface roughness of Ra 1.6.
Fig.1 Schematic diagram of car crankshaft forging model cavity structure
2. Processing technology analysis
Mold processing technology for shape processing - hole processing - cavity CNC roughing - heat treatment - shape finishing - cavity CNC finishing - bench work polishing. The current tooling technology is fully capable of machining hardened tool steels. Here, the cavity is divided into coarse and fine machining, which is mainly considered from the viewpoint of saving the tool cost. Most of the remaining amount is removed before the heat treatment, and the finishing is performed after the heat treatment, thereby reducing the tool consumption.
In the NC machining process of the cavity, roughing, semi-finishing and finishing should be divided to obtain better surface quality and dimensional accuracy. Before finishing, semi-finishing, first remove the remaining amount, especially the margin, and leave a margin of about 0.1mm for finishing, and the balance is even, which makes the finishing cutting smooth and improves the tool life. Guarantee dimensional accuracy and surface roughness.
Processing strategy analysis 1. Roughing strategy and tool selection
According to the structural characteristics of the crankshaft forging model cavity of the car, the two cutters can be used to complete the rough machining of the cavity by the offset region clearing strategy and the contour processing strategy.
The first knife selects the offset area clearing strategy, and the tool cuts the contoured section down from the uppermost contour of the groove, one cut surface to the specified lower cut step. In the forging die processing, the first knife generally selects the offset region clearing strategy, which is not discussed here.
The second tool selects the same contour machining. There is no choice between residual machining and contour machining because there are so many knives for these two machining methods. In the actual machining, the number of times of lifting the knife has a great influence on the actual machining time, which is several times of the simulation time. Under the same cutting parameters, the contour processing tool is less and the efficiency is high. The comparison of the knife is shown in Figure 2.
Figure 2 Comparison of the knife
The selection of roughing tools (see Figure 3) should be considered from the following aspects:
Figure 3 roughing tool
First, the number of tools should not be too much, two knives can be used, which can reduce the time for manual tool change. It should be noted that for most forging model cavity machining, generally only 5 to 6 tools are required. Therefore, we have not selected the tool magazine when purchasing CNC machine tools, which can reduce the investment cost of fixed assets and reduce the machine tool failure. rate. Because, according to relevant statistics, more than 50% of the faults of CNC machine tools occur in the tool magazine.
Second, try to use indexable insert knives to reduce tooling costs.
Third, the tool should be able to be machined to the deepest and narrowest part of the cavity, such as the crank of the crankshaft. As much as possible, remove most of the cavity, so that the amount of finishing after heat treatment is as small as possible.
Fourth, tool selection should be considered in conjunction with the processing strategy. If the second machining strategy is contour machining, the second tool diameter is larger than the first tool radius. This ensures that the tool path covers the last machining path of a tool and does not leave a convex portion. The package creates difficulties for finishing.
2. Finishing strategy and tool selection
The PowerMILL software (see Figure 4) provides a very rich processing strategy for finishing. Here we use the contour finishing and parallel machining (shoal) strategy to complete the cavity finishing. After the boundary is found at the local R2, contour processing is also used to complete.
Figure 4 PowerMILL software interface
Contour finishing is a method of projecting each tool path horizontally onto a model for finishing by pressing the height defined by the cutting step. For most forging dies, contour finishing is the best and safest finishing strategy. However, as the cavity surface transitions to the shoal area, the step size of the tool on the curved surface will gradually increase, so that the shoal area will not be smooth, so it is necessary to find the shoal boundary and use parallel processing strategy to supplement the processing so that the whole The surface roughness of the mold cavity meets the drawing requirements. The best contour finishing is aimed at the above problems, but in actual programming, for a mold with a complex cavity such as a crankshaft, the calculation speed is too slow, and we have not used it. Contour and shoal cutters are shown in Figure 5.
Figure 5 Contouring (left) and Shoal (right)
Finishing tools (see Figure 6) In terms of structure, we choose the solid carbide coated milling cutter, which should be considered in combination with the minimum fillet and structural characteristics of the cavity. From the above analysis of the structural analysis of the crankshaft forging die, it can be seen that the finishing with φ6R3 is ideal, and the entire cavity can be finished at one time without any knife marks and good surface roughness. However, the tool has a suspension elongation of 60 mm and a tool length to diameter ratio of 10:1. It is well known that in CNC machining, the tool length to diameter ratio is preferably within 4:1. It is more difficult to process more than 7:1, and it is more difficult to exceed 10:1. We have carried out the machining test on the high-speed milling machine. Due to the high precision of the spindle rotation and the reasonable cutting parameters, the φ6R3 ball-nosed knife has a length-to-diameter ratio of 10:1, the contour finishing is 6h, and the tool is intact. A knife can process two car crankshaft forging dies. However, the spindle is gear-driven CNC milling, but the tool wears faster, and the service life is less than 2h. Therefore, to improve the rigidity of the finishing tool, it is necessary to consider the tool structure. Based on years of machining and observation, we chose a 1° taper ball end mill for the minimum draft angle and minimum fillet, which satisfies the requirements of the machining strategy.
Figure 6 Finishing tool
Two problems need to be pointed out here. One is to improve the machining efficiency when machining on ordinary CNC milling, and it is not possible to simply increase the speed and feed. After the tool length to diameter ratio exceeds a certain ratio, the rotation speed is not too high, otherwise the tool swing will be intensified and the wear will be faster. The second one is due to the large length-to-diameter ratio of the tool and the poor rigidity. It is easy to break the knife at the corner of the machining cavity or the larger margin. Therefore, the requirement for the solid carbide milling cutter material is to be able to process harder materials. The tool has high durability and must have certain toughness and is not easy to break.
in conclusion Practice has proved that the above strategy is very effective in further improving the numerical control processing efficiency of the car crankshaft forging die. According to the above processing strategy and the principle of tool selection, we carried out the actual tracking test of a car crankshaft forging die (see Figure 7), and completed the rough machining, semi-finishing and finishing of the cavity with 5 knives and 6 knives. The results show that the processing efficiency is increased by more than 30% compared with the previous one. The processing time of each mold cavity is not more than 32h, the processing precision is ≤±0.05mm, the surface roughness is Ra1.6, and the tool cost is reduced by 15%.
Figure 7 A car crankshaft forging die
In order to carry out the production work better, we should pay attention to the following items in actual production:
1. Contour finishing strategy The processing process is stable, the lifting tool is small, and the efficiency is high. It is not only suitable for finishing, but also suitable for semi-finishing and root cleaning. Therefore, in the actual programming process, it is necessary to flexibly apply various processing strategies.
2. CNC programmers must not only be proficient in programming software, but also have knowledge of machining processes, cutting principles and tools, as well as extensive field experience.
3. The NC program not only needs to be simulated repeatedly on the computer, but also the NC programmers often follow the scene to observe the actual machining situation, optimize the program, and improve the machining accuracy and efficiency.
4. CNC programmers should fully communicate with the CNC machine operator, let the operator understand the programmer's ideas, in order to get the best results.
5. In order to reduce the cost of mold manufacturing, the molds that are generally ineffective are subjected to fall refurbishment or burn-in renovation. Therefore, when we prepare the new mold program, we must fully consider the processing of the refurbished mold. The procedure is best. It can process new molds and renovate molds, which is convenient for the production department and saves programming time.
