In the numerical control processing, the process route arrangement, processing strategy and tool selection have a great influence on the processing efficiency, processing quality and processing cost. In the past few years, we have been devoting ourselves to research and experiments in this area. We have achieved certain results in improving processing efficiency and reducing tooling costs. The following takes the car crankshaft type forging die and PowerMILL as the programming software as an example to discuss some of its
cnc machining strategies and tool selection.
Structural analysis and processing technology Structural analysis
The structure of the car crankshaft forging model cavity (see Figure 1) is characterized by deep and narrow crank parts, small concave round corners, crank depth of about 58mm, crank width generally less than 16mm, minimum draft angle of 1°, cavity concave circle Most of the corners are R3. The connecting rod diameter of the crankshaft and the rounded corner of the crankshaft are R2. There are annular grooves on both sides of the middle spindle diameter. The dimensions of this type of die (forging die) are approximately 600mm x 330mm x 160mm, the die material is H11, the die hardness is dB = 2.9-3.1, the groove dimension accuracy is ± 0.10mm, and the surface roughness Ra1.6.
Fig.1 Schematic diagram of car crankshaft forging model cavity
2. Processing technology analysis
Die processing technology for the shape processing - hole processing - cavity NC roughing - heat treatment - contouring - cavity cavity CNC finishing - bench polishing. The current tooling technology is fully capable of machining hardened tool steels. The roughing and finishing of the cavity here are mainly considered from the aspect of saving the tool cost. Before the heat treatment, most of the margin is removed. After the heat treatment, the finishing process can reduce the tool consumption.
The rough machining, semi-finishing and finishing machining should be performed in the cavity NC machining process to obtain better surface quality and dimensional accuracy. In the semi-finishing before finishing, first remove the excess, especially the corner, leaving a margin of about 0.1 mm for finishing, and the balance is even, so that the finishing cutting is smooth and the tool life is improved. Ensure dimensional accuracy and surface roughness.
Processing strategy analysis 1. Roughing strategy and tool selection
For the structural characteristics of car crankshaft forging model cavity, two cutters can be selected to complete the roughing process of the cavity with the offset area removal strategy and the contour processing strategy.
The first knife selects the bias region removal strategy. The tool processes the contour section contours from the top contour of the trough, one cut surface and one cut surface to the cut plane of the cut surface. In the forging die machining, the first blade generally selects the bias region elimination strategy, which is not discussed here.
The second tool selects the contour machining. There is no choice for residual machining and contouring because there are so many tools available for these two machining methods. In actual machining, the number of pick-ups has a great influence on the actual machining time, which is several times the simulation time. With the same cutting parameters, there are fewer high-speed machining operations and high efficiency. The comparison of knife mention is shown in Figure 2.
Figure 2 Comparison of knife mention
The selection of roughing tools (see Figure 3) should be considered from the following aspects:
Figure 3 Roughing tool
First, the number of knives should not be large, and two knives can be used to reduce manual tool change time. It should be noted here that for most forging die cavity machining, only a maximum of 5 to 6 tools are generally required. Therefore, we did not select a tool magazine when we purchased CNC machine tools, which can reduce the investment in fixed assets while reducing the number of machine tool failures. rate. Because, according to statistics, more than 50% of CNC machine tool failures occur at the magazine location.
The second is to use indexable inserts as much as possible to reduce the cost of tools.
Third, the tool can be processed to the deepest and narrowest part of the cavity, such as the crankshaft crank parts, to remove as much of the headroom as possible, so that the less the fine processing after heat treatment, the better.
Fourth, tool selection must be considered in conjunction with processing strategies. If the second machining strategy is equal height machining, the second tool diameter should be larger than the first tool radius, so as to ensure that the tool path covers the last machining path of a tool and does not leave a local convexity. Bags make it difficult to finish.
2. Finishing strategy and tool selection
PowerMILL software (see Figure 4) provides a very rich processing strategy for finishing. Here we use the contour finishing and parallel processing (beach) strategy to complete the cavity finishing. The boundary R2 is also determined by contouring.
Figure 4 PowerMILL software interface
Contour polishing is a machining method in which the height defined by the cutting step is pressed and the tool path is horizontally projected onto the model for finishing. For most forging dies, contour finishing is the best and safest finishing strategy. However, as the cavity surface transitions to the shallows area, the tool's step distance on the curved surface will actually increase gradually, so that the shallow area will not be smooth. Therefore, it is necessary to find the boundary of the shallows and use the parallel processing strategy to supplement the processing, so that the entire The surface roughness of the mold cavity meets the drawing requirements. The best contour finishing is aimed at the above problems, but in the actual programming, for the complex mold of the crankshaft, the calculation speed is too slow, we did not use. Contours and shallows are shown in Figure 5.
Figure 5 Contour Road (left) and Shoal Knife Road (right)
Finishing tooling (see Figure 6) In the structure, we choose a solid carbide coated milling cutter, which should be considered in terms of the minimum roundness and structural characteristics of the cavity. From the above analysis of the structure of the crankshaft forging die, it can be seen that finishing with φ6R3 is the most ideal, and the entire cavity can be processed at one time, without the presence of knife marks, and the surface roughness is good. However, the tool's overhang length is 60mm, and the tool's aspect ratio reaches 10:1. As we all know, in numerical control machining, the length-to-diameter ratio of the tool is preferably within 4:1, more than 7:1 processing is more difficult, and more than 10:1 is more difficult. We have conducted machining tests on high-speed milling machines. Due to the very high accuracy of spindle rotation and reasonable cutting parameters, the φ6R3 ball nose tool has an equal aspect ratio of 10:1 and finishes at a constant height of 6 h. The tool is intact. One knife can process two car crankshaft forging dies. But the spindle is a gear-driven
CNC Milling can not, the tool wear faster, the general life of less than 2h. Therefore, to improve the rigidity of the finishing tool, it is necessary to consider the structure of the tool. According to many years of processing and observation, we chose a 1° taper ball end mill for the minimum draft angle and the minimum roundness, which satisfies the requirements of the machining strategy.
Figure 6 Finishing tool
There are two problems that need to be pointed out here. One is to increase machining efficiency when machining on ordinary
Cnc Milling, and it is not possible to simply increase the speed and feedrate. When the ratio of length to diameter of the tool exceeds a certain ratio, the rotation speed is not easy to be too high. Otherwise, the tool oscillation will increase and the wear will be faster. The second is that because of the large aspect ratio of the tool and the poor rigidity, it is easy to break the tool at the corner of the machining cavity or at a large margin. Therefore, the requirement for the whole hard alloy milling cutter material is to be able to process hard materials. The tool has a high degree of durability, but also has a certain toughness, not easy to break.
in conclusion Practice has verified that the above strategy is very effective for further improving the
Cnc Machining efficiency of car crankshaft forging dies. According to the above processing strategy and tool selection principle, we performed actual tracking test on a car crankshaft forging die (see Fig. 7), completed roughing, semi-finishing and finishing machining of the cavity with five tools and six tool paths. The results showed that the machining efficiency was increased by more than 30% compared with the previous time. The processing time of each pair of mold cavities did not exceed 32 hours, the machining accuracy was ±±0.05 mm, the surface roughness reached Ra1.6, and the tool cost was reduced by 15%.
Figure 7 Crankshaft forging die of a car
In order to better carry out production work, we should also pay attention to the following items in actual production:
1. The contour finishing machining process is stable, with little lifting and high efficiency. It is not only suitable for precision machining, but also suitable for semi-finishing and root cleaning. Therefore, in the actual programming process, various processing strategies should be applied flexibly.
2. NC programmers must not only be fluent in programming software, but also have knowledge of machining technology, cutting principles and tools, and also have rich field experience.
3. The numerical control program must not only be simulated on the computer repeatedly, the numerical control programmer must often observe and observe the actual processing situation on the spot at the same time, optimize the procedure, improve the processing precision and efficiency.
4. CNC programmers should fully communicate with the CNC machine operators and let the operators understand the programmer's ideas so as to obtain the best results.
5. In order to reduce the cost of mold manufacturing, the molds that fail in general are either refurbished or refurbished. Therefore, when we compile a new mold program, we must fully consider the processing of the refurbished mold, and the procedure is the best. We can process new molds and process refurbishment molds to facilitate the use of the production department and also save programming time.
May 06, 2022