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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
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
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
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
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