In the aerospace, automotive, electronics and other industrial fields, it is required to improve the strength and rigidity of parts, toughness, corrosion resistance and fracture resistance, while reducing their weight. For this reason, thin-walled monolithic structures made of light alloy materials are widely used. They can also reduce the total number of parts and assembly work. However, the removal rate of the thin-walled monolithic structural parts is as high as 90% or more. It is necessary to strive to control the processing distortion and increase the efficiency, and high-tech requirements are imposed on machining. High-speed cutting is one of the world's advanced manufacturing technologies. Its high processing efficiency, small cutting force, low specimen surface temperature, can improve the processing accuracy, especially suitable for processing thin-walled integral structural parts, a typical foreign application example is through the high-speed milling Boeing and Airbus aircraft fuselage overall structural parts As a result, the thickness of the fins has been significantly reduced and the height has increased, effectively reducing the aircraft's own weight and reducing fuel consumption. As a result, intercontinental non-landing flights from the Far East to Western Europe have been realized. The application development of domestic high-speed cutting is still in its infancy. In this paper, the aluminum alloy triple waveguides are studied to investigate the optimization of high-speed milling process for thin-walled monolithic structures. 1 Test task, conditions and process methods
Fig. 1 Triple sided waveguide
Figure 2 C1200U high-speed milling triple waveguide front
Test task Figure 1 shows a triple waveguide product with a minimum wall thickness of 2mm and a mass of 2.35kg. It is a typical multi-bar, thin-walled monolithic structure. The blank material is a rust-proof aluminum LF21 (GB1173-86) rectangular parallelepiped plate, which is plastic. Aluminum alloy, with a mass of approximately 12.25kg, was "hollowed out" by milling, the material removal rate was 80.8%, its material hardness was 45HB, tensile strength was 180MPa, and elongation was 12%. The metal removal amount is large, the finished wall is thin, and the rigidity is low. The first problem to be solved in the processing is to control and reduce the deformation. With the ordinary speed CNC milling process, the processing time is as long as 50 hours. Intermediate heat treatment is needed to eliminate the processing stress and control the deformation. Therefore, the processing also needs to work hard to improve the processing efficiency, shorten the time and reduce the cost. According to foreign data reports, for aluminum alloys with significantly lower tensile strength than iron and steel materials, high-speed cutting can be used to complete all coarse and fine machining tasks. Starting from the principle of process concentration, only one high-speed machining center is used. Test conditions The test adopts German Hermle C1200U five-axis linkage high-speed milling machining center, as shown in Figure 2. Its main operating parameters are: spindle speed n = 20 ~ 24,000r/min, maximum output power is 23kW, torque is 79Nm; travel along x, y, z axis is respectively 1,200,800,500mm, maximum linear feedrate 30m/min, maximum acceleration 4m/s-2, positioning accuracy 0.01mm; A-axis swing range -97° to 15°, C-axis for worktable, 360° rotation; worktable diameter 800mm, load capacity 1t. The CNC (computer numerical control) system is the German Heidenhain iTNC 530, which calculates and processes the speed of a CNC command, from 4ms in the previous generation system to 0.5 ms. The machine tool has a tool magazine with 30 tool positions, and a laser type tool online detection system. And contact-type workpiece online detection, data infrared wireless transmission device. Before the test processing, the three-dimensional modeling of the test piece was performed using the UG NX CAD/CAM software system installed on a high-performance microcomputer workstation. Then, the high-speed milling process was developed as described below, then the tool path file was generated using UG NX, and the CNC programming was performed. Post processing. The obtained CNC machining program is transmitted to the CNC machine tool of the machine tool through the network, and after trial operation and necessary modification, it is used for test processing. There are many factors that affect the machining distortion caused by high-speed milling process control process distortion, including the structure of the blank, the distribution of the fiber's trend and internal stress, the various forces and heat during processing, and the physical and chemical changes caused by the test piece. To control and reduce the processing distortion, it is necessary to reasonably prepare or select blanks, reasonably select the clamping method, heat-treat the blanks or semi-finished products, and arrange the process and cutting path reasonably, select the tool and cutting quantity parameters reasonably, and reasonably cool and lubricate. Reduce cutting force and cutting temperature. The experimental study in this paper does not involve the preparation or selection of blanks, and one of the goals of omitting the intermediate heat treatment process is to focus on other process optimization measures. The size of the three-way waveguide of the clamping method is 791mm×156.8mm×32mm, and the ratio of the length-width area to the thickness dimension is not particularly large. After analysis and comparison and experiment, press the plate from the four points of action of the two sides of the blank to press it on the work table of the machine tool (as shown in Fig. 2), which can effectively prevent deformation of the test piece, and it is convenient and time-saving. Need to make a special fixture. High-speed milling aluminum alloy material has a small cutting force, and appropriately reducing the clamping force helps to prevent the deformation of the fixture. If the ratio of the length and width of the test piece to the thickness dimension is large and the rigidity is low, it may be necessary to add a large number of clamping points and manufacture special fixtures. The mounting method of the specimen is often one of the key factors in the process. The process arrangement and the route of the three-way waveguide have a cavity and a long slot on both sides of the waveguide, and there are two sets of I-shaped through slots in the middle, as shown in Figure 2. Therefore, when arranging processes, it is necessary to observe the principle of separate processing and rough finishing. Among them, priority is given to sub-surface processing, that is, machining the negative side and then processing the front side, and only repeat the clamping once; complete rough finishing operations on one side of the test piece in one clamping. The purpose is to reduce the number of times of trial and error in repeated fixtures and improve accuracy and efficiency.
Figure 3 Radial cut or incision in radial direction or obliquely
In order to avoid a partial continuous and in-depth removal of a large number of materials in thin-walled specimens, resulting in rapid changes in stress distribution and processing distortions, rough finish machining follows the principle of layered cutting; after the milling cutter reaches a certain depth, In the same type and separated by the cavity or slot in the knife in turn. Among them, arrange the path of the knife as far as possible to take into account to maintain the overall geometrical symmetry and thin-walled bilateral symmetry. There are many changes in rough finishing in terms of the cutting mode of the tool, the path of the cutting tool, and the amount of cutting parameters. High-speed cutting has high relative moving speed, so that the milling cutter cuts into the test piece radially or obliquely (as shown in Fig. 3) or feeds axially spirally (as shown in Fig. 4), which helps to keep the cutting process stable. Improve machining accuracy and surface quality and extend tool life. In this experiment, the rough machining cavity and the first cutter cut into the blank body in an axial direction, adopting an inclined feed approach with an angle of less than 5° with the upper plane. It is difficult to cut the deep cutting depth, spiral or inclined feed without cutting, so let the milling cutter drop directly in the axial direction. The finished tool path trajectory forms the final contour surface, avoiding advancing and retracting the knife on the contour. In addition, the finishing process requires removal of the rough machining residual area (also referred to as residual material milling), and the tool diameter D is relatively small. The tool test processing mainly adopts the overall cemented carbide cutting tool suitable for various workpiece materials such as cutting steel, cast iron, and plastic aluminum alloy by internationally famous cutting tool manufacturers, and has external TiCN coating. They have high precision, good dynamic balance and long service life. They have a good effect on controlling deformation and improving machining accuracy and surface quality. Because the bottom and wall of the test piece are both thin, a flat-bottomed end mill is used in the machining to avoid cornering. Milling cutters produce a downward force when cutting, causing the bottom of the specimen to warp. The cutting amount is the same as the ordinary speed milling. The high speed milling rough machining is still mainly to increase the material removal rate. Generally, the axial depth ap, the radial depth ae, and the feed per tooth fz are relatively large; The main processing accuracy and surface quality, cutting speed vc higher.
Figure 4 Axial spiral feed and layered pass
Through investigation and testing of cutting force and cutting temperature, it was confirmed that when high-speed milling of plastic aluminum alloy materials, the milling line speed vc>1,583m/min, or at least vc>804m/min should be selected so that the cutting force and cutting temperature Vc increased significantly, while reducing the processing distortion, improve processing quality and efficiency. Limited to the milling cutter diameter, tool overhang and ambient temperature and other constraints, the spindle speed in the test did not reach the maximum value, the actual vc max ≈ 1,13 lm/min. Tests and tests have further shown that reducing the axial depth of cut and appropriately increasing the radial depth of cut, especially the feed rate, is conducive to reducing the cutting force and cutting temperature and controlling the machining distortion. Therefore, the ap ≤ 1mm of each process in the test processing, and from the roughing to finishing in descending order. In principle, ae = 0.75D, ae = D when milling cavity and groove first cut into the entity, but limited by the groove width, the second pass usually ae <0.75D. When milling the cavity, the higher strength of D is higher, fz = 0.107-0.200 mm, and the fz of the inclined face of the milling and milling cavity is equal to 0.094 mm. When the slot is milled, the strength of D is small. After testing, choose fz=0.048 ~ 0.072mm. Among them, when D=3mm, fz=0.048 to 0.056mm, which can avoid the cutter from breaking. The above feed per tooth is comparable to normal speed milling. However, the spindle speed n of this test was as high as 15,600 r/min. According to the feed rate calculation formula vf = fzZn/1,000 = fzZvc/pD (Z is the number of milling cutter teeth), can be calculated vf = 3 ~ 6m/min, much higher than the ordinary speed milling. The small axial depth of cut, large feed rate, is another basic feature of high-speed machining, and it is also a basic premise to reduce machining distortion and improve machining quality and efficiency at the same time. Milling methods and cooling lubricating blanks do not have a rough, hard outer skin. Tests have shown that the cutting force is significantly reduced in the down milling mode, and theoretical analysis and literature indicate that it is conducive to the formation of cuttings, the smoothness of the cutting process and the quality of the machined surface. Machining of plastic aluminum alloys is achieved by the use of highly efficient emulsified cutting fluids for cooling and lubrication, which helps to reduce cutting forces and cutting temperatures. It also prevents the chips from sticking to the solid carbide milling cutters and cannot be separated and the tools are scrapped. Improve the processing efficiency As mentioned above, many of the process measures to control the processing deformation, including the priority of the facet processing, the rough finishing separately, and the large feed rate, can simultaneously improve the efficiency and shorten the processing time. In addition, the arrangement process should pay attention to reduce the number of tool change as much as possible. This test combines the clean horns into one process and puts it to the end of each process, saving time without affecting the quality of the process. To determine the cutting path for high-speed milling, first of all, care must be taken to avoid sudden changes in the direction and feedrate of the pass; using the layered, circumferential pass illustrated in Fig. 4, it is possible to avoid the jerk caused by the reciprocating pass. Impact, there is no trace of knife-to-knife that is required to move laterally for a short distance after a closed-loop pass. Therefore, machining efficiency and quality are high, and tool life is long. 2 Test results By adopting the above process optimization measures, the trial processing of triple-link waveguides eliminates the intermediate heat treatment process. The rough finish machining takes a total of 14.13 hours, which is lower than the predetermined optimization target time of 16 hours. With the use of a three-coordinate measuring machine and a surface roughness meter, the final shape of the finished product cavity was qualified for dimensional accuracy. Taking the front as a benchmark, the measured non-flatness is 0.16mm and the surface roughness Ra ≤ 1.6μm, which meets the requirements of the drawings. The experimental study shows that the application of high-speed milling technology to process thin-walled integral structural parts can effectively control and reduce the processing distortion, and greatly improve the efficiency and shorten the time. The key process links are blanks, workpiece clamping, process arrangement, tool path, tool and cutting parameters, milling methods and cooling lubrication. The so-called high speed, of course, is the high cutting speed and spindle speed of the machine tool, but it also needs to have high axial feed speed and acceleration, as well as the high computational processing speed of the CNC system, and high CAD/CAM hardware and software systems. Computer-aided modeling and programming speeds.
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