The most commonly used aerospace components are high-strength aluminum alloy materials (such as 2A12), which are easy-to-cut plastic materials with low strength and hardness, low melting point, good thermal conductivity and low tensile strength. Although the high-strength aluminum alloy has good machinability, the structural parts of aerospace products have the characteristics of complex shape and structure, large material removal, thin wall and easy deformation, and have higher precision in parts processing precision, quality and processing efficiency. Requirements, therefore, the aerospace manufacturing industry has continued to conduct high-efficiency machining research on high-strength aluminum alloys. Efficient machining is a new technology that combines high-speed machining technology and cutting process optimization. It is the key technology for solving complex aerospace structural components. The high-efficiency machining technology is characterized by high material removal rate and short single-piece machining time during machining, and optimization of cutting parameters to ensure machining accuracy and surface quality.
Efficient processing solution
1 Processing system optimization
High-speed and efficient machining has higher requirements for the entire machining system than ordinary machining. In the process of CNC milling of aerospace structural components, to ensure the machining quality and machining accuracy of the parts and to improve the material removal rate of the machining process, it must first be composed of the machine tool-tool-workpiece and their mutual interface (tool holder and fixture). The machining system is optimized to ensure the stability and reliability of the entire system during high-speed cutting.
1.1 Machine tools
The requirements for high-speed machining on machine tools mainly include the following:
(1) The machine tool structure must have excellent static, dynamic and thermal characteristics;
(2) The spindle unit can provide high speed, high power and high torque;
(3) The feed unit can provide a large feed speed.
If necessary, the modal analysis test of the cutting system can be used to obtain the dynamic performance parameters of the milling system, and provide basic data for solving the milling dynamics model and predicting the machining stability.
For the selection of high-speed machining tools and tool holders, the safety of the machine should be considered first, otherwise the machine tool spindle will be damaged and even a safety accident will occur. The selection of tools and tool holders for high-speed machining mainly includes the following aspects.
(1) The allowable speed of the tool must be greater than or equal to the actual machining speed.
(2) High-speed machining tools and tool holders must meet the requirements of dynamic balance and radial runout, and should generally meet the requirements of G6.3 or G2.5.
After the dynamic balance test (see Figure 1), the hydraulic tool holder and tool, the powerful dynamic balance tool holder and the tool can meet the dynamic balance accuracy requirements of G2.5, while the spring chuck type tool holder and tool can reach G6.3. Dynamic balance accuracy requirements, but not up to the dynamic balance accuracy requirements of G2.5.
When the maximum radial displacement jitter measurement (see Figure 2) is performed on the powerful dynamic balance tool holder, the spring chuck holder and the handle hydraulic tool holder, respectively, it is found that the powerful dynamic balance tool holder has the minimum radial runout (see the figure). 3).
Figure 1 Dynamic balance detection of tool holder and tool
Figure 2 Radial displacement jitter measurement site map
Figure 3 Maximum radial displacement runout for different tools and different speeds
It can be seen from the radial displacement runout measurement test that under the condition of the tool rotation speed of 20000r/min, the powerful tool and the tool holder with the dynamic balance quality accuracy of G2.5 grade are selected. After the radial displacement jump detection, when the spindle speed of the machine tool reaches 20000r/min, the maximum radial displacement of the tool is 10.95~12.05μm, which satisfies the requirements of the tool radial runout in high-speed precision machining.
(3) The tool holder is required to have a higher clamping force in high speed machining.
Figure 4 shows the maximum torque comparison that can be achieved with the hydraulic shank, the powerful shank and the heat shrinkable shank commonly used in the HSK series. It can be seen from the figure that the maximum torque that can be achieved with the ordinary spring collet ER32 can be seen. Only 196N·m, the maximum torque that can be achieved with the hydraulic shank HYDRO32 is 441 N·m, and the maximum torque that can be achieved with the heat shrink shank is 784 N·m, which is the same as the maximum torque that can be achieved with the powerful shank MAXIN20. .
Figure 4 Comparison of maximum torque in different clamping modes
In summary, the powerful dynamic balance tool holder has high dynamic balance accuracy and small radial runout, which is suitable for high speed and high precision machining. In addition, the use of heat shrinkable shank can also achieve higher dynamic balance accuracy, lower tool holder weight, and higher tool holding force.
1.3 clamping system
Efficient machining special tooling not only ensures effective clamping and positioning during high-speed cutting, but also facilitates rapid positioning and clamping. The design points mainly include the following aspects.
(1) The positioning contact area of the tooling and the workpiece is as large as possible, and the largest possible plane (or curved surface) is selected as the positioning surface on the workpiece, and the processing precision requirement of the positioning surface is improved, and the workpiece positioning surface and the fixture reference surface are ensured. Natural, close fit, increasing contact stiffness.
(2) For thin-walled parts with poor rigidity, the positioning method is often used, especially in the case of weak rigidity, to increase the process rigidity of the workpiece.
(3) Under the premise of ensuring reliable clamping, use the smallest possible and uniform clamping force, increase the number of clamping points and increase the effective clamping area. A clamping force is applied in the direction in which the part is rigid and acts on a rigid surface.
At present, the clamping fixtures commonly used in the manufacturing industry mainly include mechanical, hydraulic adjustable, vacuum adsorption, pneumatic and the like. Compared with traditional manual mechanical clamping fixtures, hydraulic, vacuum adsorption and pneumatic clamps are characterized by automation, high clamping efficiency, and controllable clamping force, which are more suitable for efficient machining.
The vacuum adsorption clamp has a small clamping force (the unit pressure does not exceed one atmosphere, that is, less than 105 Pa), but the distribution is uniform, and it is suitable for clamping any object with a smooth surface, especially a non-metallic type that is not suitable for clamping. The small-size workpiece is cleaned, polished, polished and other small amount of processing. However, the vacuum adsorption jig requires high surface roughness and contour size of the workpiece in contact with the jig.
The hydraulic adjustable clamp can realize the real-time adjustable and controllable posture and state of the clamped original, and can ensure the continuity of the cutting path of the tool. In addition, some components of the clamp can be changed or replaced to change the structure of the clamp to adapt to different parts. Processing and clamping requirements have a wider range of applicability.
The air pressure clamp can realize the function of the hydraulic clamp and achieve a sufficient pressing force. Unlike the hydraulic clamp, the air pressure clamp uses air pressure as a power source.
2 Process improvement
The main purpose of roughing is to remove the material and leave a suitable margin for finishing. Therefore, rough machining generally does not need to consider the dimensional accuracy, surface quality and deformation of the workpiece. As long as the power of the machine allows, the production efficiency can be improved in many aspects.
There are certain differences between the finishing and roughing of parts. In the finishing process, it is necessary to fully consider the influence of clamping, cutting, process parameters on the internal stress of the part, and the influence of cutting force and cutting heat on the part structure during cutting, and control deformation to avoid Deformation caused by efficiency increases, resulting in part accuracy and surface quality damage.
2.1 Cutting tools
Choosing a more reasonable tool can directly increase production efficiency. The cutting process of aluminum alloy materials does not require high tool materials. Generally, carbide milling cutters can be used. The coatings can be coated with no coating or diamond. In roughing, since it is not necessary to consider the accuracy and quality problems, the metal material can be cut off to the maximum extent, so the large-diameter tool can be selected to reduce the number of passes and shorten the pass time. In addition, in the roughing, try to choose a dense tooth cutter instead of the sparse cutter, which can increase the feed per revolution, and the cutting speed can be increased at the same speed. In the finishing process, in addition to considering the problem of efficient material removal, the problem of force deformation control of thin-walled members during cutting should also be fully considered. For the finishing of aerospace aluminum alloy thin-walled parts, K-series carbide tools should be used (equivalent to China's original tungsten-cobalt, the main component is WC+Co, codenamed YG). The rake angle of the tool should not be too small, otherwise the cutting deformation and friction force will be increased, and the rake face wear will be increased to reduce the tool life. The cutting test showed that the cutting force increased by 1% for every 1° reduction of the rake angle during high-speed milling of AlCuMgPb. For this reason, it is generally recommended to use γ0 to be about 12°. The selection of the back angle of the tool will affect the rigidity of the tool. In order to reduce the friction between the tool and the workpiece, the back angle must be selected to be larger. If necessary, the double chamfered back angle can be used to ensure the rigidity of the tool while increasing the back angle. The inclination of the blade affects the direction of the chip outflow and the amount of each component of the cutting force. A large blade inclination angle should be selected when cutting the aluminum alloy. When milling aluminum alloys at high speeds, it is generally recommended to use λs of 20° to 25°. In addition, the choice of the radius of the tool nose should be appropriate, the teeth should not be too dense, which facilitates the discharge of chips, which is beneficial to further increase the feed rate, prevent the cold hardened layer and prolong the service life of the tool .
2.2 Walking path
A more effective method for speed-increasing efficiency is to optimize the path of the tool. In the high-speed cutting, the directionality of the tool path should be guaranteed, that is, the tool path should be as simple as possible, with fewer turning points, the path should be as smooth as possible, and the rapid steering should be reduced; Knife time, increase the proportion of cutting time in the whole workpiece as much as possible; try to use circuit cutting, reduce the cutting and cutting times of the tool without interrupting the cutting process and tool path, and obtain a stable, high-efficiency and high-precision cutting process.
In the high-speed machining of large-scale complex curved surfaces of aerospace structural parts, when the curvature of the surface changes greatly, the direction of the maximum radius of curvature should be taken as the optimal direction of the cutting, as shown in Fig. 5; when the curvature of the surface changes slowly, the radius of curvature is the direction of the knife. The influence is weakened, and the direction of the longest average length of a single tool rail should be selected, as shown in Figure 6.
Figure 5: The path of the path with a small radius of curvature
Figure 6 Surface path with large radius of curvature
In the beveling process, if the horizontal horizontal pass shown in Fig. 7 is used, the distance of each pass is very short, the spindle needs frequent commutation during the cutting process, the cutting stability is poor, and the cutting is a bevel, the horizontal pass The linkage of the X or Y axis with the Z axis is required, which is not conducive to the improvement of the cutting speed. Therefore, for such bevel machining, the path of the tool is arranged as parallel as possible to the longest bevel (see Figure 8). Not only is the path of the tool the longest, the number of commutations is the least, and the single path is only cutting in the XY plane. The movement in the Z-axis direction is arranged outside the contour of the workpiece, and the tool damage can be reduced even under high-speed cutting.
Figure 7 Horizontal horizontal path
Figure 8 oblique parallel path
2.3 Cutting parameters
In roughing, it is generally possible to select a "high-power" high-efficiency cutting with a large feed rate and a moderately large cutting depth combined with a medium cutting speed, which can achieve a high material removal rate, thereby greatly improving production efficiency. For finishing, it is feasible to increase the rotational speed and increase the number of teeth. Increasing the feed per tooth may reduce the surface accuracy and cause residual stress to cause deformation. Therefore, the "light cut and fast cut" of the high cutting speed and low feed per tooth is often used to ensure the improvement of production efficiency and the accuracy and surface quality of the product.
The cutting parameters can be finally determined by cutting finite element analysis and cutting test: using the Third Wave AdvantEdge software, the machine tool spindle power and torque requirements under different process parameters are simulated and calculated, and the machine tool spindle can be well satisfied. The range of parameters such as spindle speed, feed per tooth and depth of cut required for high speed machining processes. Provides guidance recommendations for cutting test parameter selection.
In the Third Wave AdvantEdge software, through the new task, complete the workpiece material, tool material and coating, tool structure parameters, cutting amount setting, simulation, and obtain the cutting torque demand curve under a certain cutting amount. According to the graph display data, the maximum demand of cutting power and cutting torque under the current cutting amount is determined, and the compliance of the process parameters can be determined by comparing the actual performance parameters of the machine tool (electric spindle torque and power map).
For example, the machining center with a maximum speed of 24000r/min performs high-speed rough milling of 2219 aluminum alloy thin-walled structural parts, and adopts indexable cutters with uncoated fine-grain carbide inserts. The cutter diameters of φ25mm and φ32mm are selected. , simulation calculations, the results are shown in Table 1. Among them, the number 24-1 tool diameter is 25mm, the number of teeth is 3, the spindle speed is 6000r/min, the feed per tooth is 0.15mm/z, and the cutting depth is 1.5mm.
Table 1 Power and Torque Simulation Calculation Results for High Speed Milling Roughing
(1) Power demand is shown in Figure 9.
Figure 9 Cutting power requirement under No. 24-1 cutting amount
(2) Torque demand is shown in Figure 10.
Figure 10 Cutting torque requirement under No. 24-1 cutting amount
As can be seen from Figures 9 and 10, the maximum cutting power requirement is 7 kW and the cutting torque requirement is 11 N·m at the current cutting amount. The maximum power output of the gantry high-speed machining center with a maximum spindle speed of 24000r/min at 6000r/min is 42kW (rated working condition S1), and the maximum output torque is 67N·m (rated working condition S1), which can meet the current processing. demand.
By confirming the results of the eight sets of simulations, it can be found that in order to make the machining power and torque of the machine tool meet the requirements of the cutting processing amount, the tool and the cutting amount must be selected reasonably, as shown in Table 2.
Table 2 Analysis of machine power and torque requirements
The existing gantry CNC machining center has a maximum spindle speed of 24000r/min. In the roughing process of thin-walled siding, if φ25mm or φ32mm indexable tool is used, the spindle speed should be appropriately improved for the optimization of cutting amount. The range is 12000~15000r/min; the feed per tooth and cutting depth should not be too large, and the selectable range is 0.15mm/z and 2~3mm respectively.
The cutting test can be designed within the range of parameters obtained by finite element analysis. The cutting efficiency, surface roughness and surface topography are judged as criteria, and the optimal cutting parameters are finally selected.
According to the above improvement ideas, a typical folding wing has been improved in efficient processing.
According to the multi-angle bevel, arc surface transition and high-precision hole groove characteristics of the part, DMG high-precision five-axis machining center is selected as the processing equipment, and the special hydraulic adjustable tooling is designed and manufactured, and one hydraulic power unit is provided for the special hydraulic tooling. The power source, the oil enters the hydraulic cylinder inside the tool body through the pipe to control the clamping and slack movement of the clamping original.
The removal of each profile material of the wing parts accounts for 70% of the total material removal, and the time ratio in the machining is also very high. By improving the tool, optimizing the cutting parameters and improving the processing efficiency of the profile, the wing can be greatly shortened. Class part manufacturing time.
After the improvement, SECO brand R220.69-0050-10-5Aφ50mm insert end mill head with XOEX10T304FR-E05 H15 insert (see Figure 11) to complete the roughing of each profile