Taking the processing of ultra-long aluminum alloy profiles for rail transit as the research object, a comparative optimization was carried out from aspects such as drawing design, process analysis, tool selection and program programming. Aiming at the problems existing in high-speed processing, such as large vibration, noise and low programming efficiency, feasible technical solutions and process measures were proposed. After on-site processing verification, the processing difficulties of ultra-long aluminum alloy profiles have been solved.
01 Preface
At present, significantly reducing the weight of the vehicle body by using a large amount of lightweight and high-strength materials has become the most important means of lightweighting urban rail transit vehicle bodies. Due to the multiple advantages of aluminum alloy profiles such as high strength, light structure, impact resistance, structural stability and good welding performance, hollow structure aluminum alloy profiles are widely used in fields such as aviation, aerospace, automobiles, mechanical manufacturing, ships and rail transit. According to statistics, the usage of aluminum alloy profiles has reached over 60% of the total mass of the metro car body structure. However, due to its unique hollow thin-walled structure and the extremely long structural feature of the product, aluminum alloy profiles encounter problems such as easy cutting deformation, vibration cracking, high processing noise and rapid tool wear during high-speed machining, which seriously affect the processing efficiency and economic benefits of hollow aluminum alloy profiles. The following takes the aluminum alloy car body top cover side beams, top cover long beams and underframe side beams that our company has been processing for many years as examples to provide some efficient and practical high-speed processing solutions. The processing equipment used in the case is a 30m five-axis bridge-type machining center.
02 Analysis of Processing Technical Requirements
In the structure of aluminum alloy car bodies, the most common and main slender aluminum alloy profiles include top cover side beams, top cover long beams and underframe side beam profiles. They are almost the mainstream design structures for high-speed rail, EMU and metro car bodies in existing rail vehicles. Their common feature is that the cross-section of the profiles is a hollow and irregular structure. Generally, the wall thickness of the profiles is 3 to 12mm, and the cross-sectional structural dimensions vary among different vehicle models. The product length is generally 15 to 25 meters and it belongs to ultra-long and slender workpieces.
The processing of various types of side beam profiles has significant similarity and concentration characteristics. Their main processing features are all concentrated at the total length end face, door installation opening, side beam suspension rib plate notch and C-shaped slide groove, etc. The side beam, as a key and important component for vehicle body assembly and welding, has extremely high requirements for its total length, door installation opening size, and the center distance between adjacent doors (tolerance ±0.5mm), with a tolerance grade reaching precision grade f.
03 Analysis of Process Difficulties
The processing difficulties of the workpiece are as follows.
1) The rough processing efficiency is low. Rough machining is the most efficient processing method adopted to achieve the purpose of quickly removing processing materials. In the processing of side beams, more than 70% of the materials that need to be removed are concentrated at the positions of the total end face, the door installation opening, the notch of the side beam suspension rib plate and the C-shaped slide groove. The rough processing of these positions takes up more than half of the processing time of the entire side beam.
2) There is significant vibration, deformation and noise during processing. Due to the hollow, thin-walled and slender special structure of aluminum alloy profiles, their inherent rigidity is insufficient. When the inner wall cavity is squeezed by cutting force, vibration will occur. The result is high noise, product deformation or even cracking, and simultaneously leads to low processing efficiency and rapid tool wear and other adverse consequences.
3) Overcutting during finishing or improper processing dimensions. During the extrusion process of aluminum alloy profiles, the straightness and twist of the profiles cannot reach an absolutely ideal state. There are also corresponding tolerance standards for profile extrusion, according to EN 755-9:2001 "Aluminium and aluminium alloys - extruded bars, tubes and profiles - Part 9: Profiles, dimensional tolerances and shape tolerances. For aluminum alloy profiles with a cross-sectional width of 200 to 300mm and a length of over 6m, the twist standard is 7mm, and the bending deviation is ≤1.5mm/m. Therefore, during fine processing, the error between the actual state and theoretical dimensions of the aluminum alloy profiles must be fully considered. For example, a machining allowance of 5mm should be reserved during rough processing. Due to the influence of the twist degree and straightness, the actual reserved finishing allowance here is not the ideal 5mm. Therefore, the direct result is overcutting or the inability to guarantee the finishing size.
4) The auxiliary time for workpiece clamping and disassembly is long. For profiles of about 20 meters in length, approximately 10 points of tooling are generally required for fixation. Manual clamping by hand is inefficient and labor-intensive. When facing the switch of product types, the cycle of tooling replacement and adjustment is long.
5) Low programming efficiency and long program debugging cycle. It is understood that currently, when major domestic rail transit manufacturing plants process vehicle underframes, aluminum alloy side beam profiles and large aluminum alloy welded structural components, manual programming is still the main method. Manual programming can better meet the requirements of on-site process changes, size adjustments, etc., but its efficiency and accuracy are not as high as those of automatic programming, and the program debugging time is long.
04 Process Solutions
4.1 Rough machining Scheme
The rough machining allowance is removed by sawing. The total end face of the side beam and the position of the door installation opening can both be processed by sawing to cut and remove the entire piece of material. For notches such as the side beam suspension rib plate, T-shaped cutters (or three-edge milling cutters) can be used to cut and remove them instead of layering the cut material into chips. The main approach is to make a notch at both ends of the material to be cut off, so that the two ends of the material to be cut off are separated from the product, and then use a saw blade or T-shaped knife to cut the middle part as a whole. The processing efficiency of this method is several times that of conventional rough machining. However, during the operation, special attention should be paid to ensuring that the waste material does not collide with the cutting tool when separating from the product, and that the waste material can land smoothly downward.
The total length of the side beam is machined by using a rotating spindle or an Angle milling head. Generally speaking, there is only about 10mm of allowance on each side of the side beam in the length direction. When some equipment does not support large-sized saw blades, a rotating spindle or an Angle milling head can be used to process the total length by milling the plane.
Sawing processing application example: In the processing of the bottom frame side beam of a certain line in Guangzhou, a 904mm long notch needs to be machined on the upper surface of the aluminum alloy profile. The processing process is shown in Figure 1.
As can be seen from Figure 1, if a end mill is used for rough machining, problems such as large processing vibration, high noise and low processing efficiency will definitely occur. Clearly, it is not the most efficient process solution for material removal.
The sawing processing plan and difficulty analysis suggest that this notch is not a standard isosceles trapezoidal structure. At the opening position of the notch, both the left and right ends are vertical upward straight edge structures. The entire notch is surrounded by five surfaces in different directions, and one or two cutting paths cannot complete the processing of the entire notch. At a position 131mm away from the notch, a 12mm thick rib plate structure is designed. It is necessary to reasonably select the diameter of the three-sided edge milling cutter. The milling cutter should not only ensure that the notch is cut through but also prevent the tool from damaging the 12mm thick rib plate on the side of the notch. This notch is located on the upper surface of the product profile. The cut waste cannot fall off the workpiece with its own weight. When the waste is disconnected from the aluminum alloy profile, there is a risk of colliding with the cutting tool, posing a significant safety hazard.
The clamping scheme is shown in Figure 2. The specific processing procedure: In the first step, through the helical milling hole processing method, a φ100mm semi-circular through hole is processed at each end of the top of the profile where the notch needs to be machined. The purpose is to prevent the handle of the three-edge milling cutter from colliding with the workpiece when cutting the inclined surface in the second step. Because the length of the three-edge milling cutter is insufficient (a too long tool will result in insufficient rigidity), holes are made to avoid the collision between the tool and the workpiece [1]. The second step is to use a φ125mm three-edge milling cutter and, while rotating the spindle, cut the inclined surfaces at both ends of the notch. Cut once from each side of the profile respectively to separate the left and right inclined surfaces of the notch from the profile. The third step is to, in the same way, cut the bottom of the notch from both sides respectively, so that the cutting seam on the inclined plane coincides with the cutting seam at the bottom. At this point, the cutting and separation in three directions have been completed. The fourth step is to switch to a φ500mm saw blade knife. The purpose is to prevent the collision and interference between the machine tool and the top of the profile. Rotate and reposition the spindle head to a 90° position. Cut the left and right end faces of the 904mm notch from top to bottom respectively. The scrap at the notch will detach from the workpiece. At this time, the tool is directly above the workpiece, thus avoiding the problem of scrap colliding with the tool.
4.2 Measures for Improving vibration, deformation and noise
The root cause of vibration, deformation and noise lies in the insufficient rigidity of the workpiece during the processing. Vibration and other problems can be controlled and improved from the following aspects. ① Reasonably arrange the distances between each clamping point of the tooling. Based on practical processing experience, the maximum span between each clamping point of the tooling should not exceed 2 meters. For some thin-walled profiles with poor strength (profile wall thickness of 4mm or less), clamping points of the tooling with smaller distances should be set. ② The areas with large cutting allowances of the workpiece should be as close as possible to the clamping points of the tooling. ③ Adopt a processing method with small cutting parameters and fast feed rates. When using the side edge to process materials with large allowances, the feed direction of the tool center should be as far away as possible from the center of the machining allowance to ensure the cutting efficiency of the tool side edge [2].
4.3 Fine processing Plan
In the processing coordinate system, Renishaw probes are used to measure the processing position in the X-axis, Y-axis and Z-axis (any one or several of the X-axis, Y-axis and Z-axis can be selected according to the fine processing situation) directions. The result of the probe measurement is the difference between the actual state of the profile and the theoretical position. This difference will be stored under the machine tool parameter # address (R parameter for Siemens system). Substitute this parameter in the processing program to achieve the fine milling processing of the product. This method is the most accurate finishing solution, but the machine tool must be equipped with a Renishaw radio probe.
In the absence of Renishaw radio probes, the manual measurement and supplementary value method is adopted, which is also a highly operational finishing solution. The main implementation process is as follows: a certain amount of finishing allowance is reserved during rough machining. After rough milling is completed, the actual allowance after rough milling is manually measured and input into the machine tool parameters. The difference between the reserved allowance for rough milling and the actual measured allowance is calculated through the program, and the difference is automatically corrected during the machining process. For example, rough machining programming: G01 Y5 F2000 (indicating a 5mm machining allowance reserved in the positive direction of the Y-axis); Finishing programming: G01 Y[5-#1]F2000 (#1 indicates the actual measured machining allowance).
4.4 Quick clamping and disassembly solution
Tooling design should fully consider universality. A set of tooling should meet the processing requirements of all similar products as much as possible. Tooling design can adopt a modular structure. One part is the basic tooling, mainly for clamping and fixing the products. Another part is the profiling positioning structure, which serves as the positioning reference for product clamping. Give priority to using pneumatic, hydraulic and other automatic or semi-automatic clamping methods. Adopt a reasonable clamping scheme to ensure that all processing procedures are completed in one clamping, avoiding the second flipping processing of the workpiece. Make full use of the rotational function of the machine tool spindle, Angle milling head, T-shaped tool and forming tool, etc., to achieve the completion of all procedures in one clamping. Adopt a reasonable layout of tooling. For the same type of side beam profile, due to the symmetrical structure of one on the left and one on the right for different vehicle models and products, it will be processed into various different products. In the layout of tooling, one layout of tooling should try to take into account all products processed with the same type of profile.
4.5 Enhance programming efficiency and accuracy
With the progress and development of industrial software, a large number of CNC programming software have emerged. Software programming features high speed, high precision and high reliability, and in many aspects, it is incomparable to manual programming. Nowadays, the commonly used software tools in the CAD/CAM industry include MASTERCAM, CIMATRON, PRO-E, UG and CATIA, etc. In the processing of aluminum alloy profiles in the rail transit industry, two-dimensional tool paths are mainly adopted. Based on this, MASTERCAM, as a leader in two-dimensional programming, It is the most suitable programming tool. The disadvantage of software programming is that on-site program debugging is not convenient when the product changes or the size varies. In some cases, the processing efficiency of CNC programming is not as good as that of manual programming. During the fine processing of side beams, software programming cannot well control the deviation caused by the deformation of profiles.
Manual programming takes a relatively long time for the initial process preparation, but it has stronger adaptability and flexibility. Through the analysis of the side beam drawings, it can be seen that the structure and processing features of various side beams are basically the same. Therefore, a large number of macro programs, fixed loop commands, and custom subroutines can be adopted to improve the efficiency and accuracy of manual programming. For instance, for the processing of C-shaped grooves on the top cover side beam, it is only necessary to define the length and center position coordinates of each C-shaped groove in the program [3]. Create modular programming structures and custom subroutine libraries, and compile program structures for the same type of side beams. Next time a new product with a similar structure is encountered, changes can be made on the existing program. Processing programs at similar structure positions can be directly called in the subroutine library. For some complex contours, surfaces and features with large amounts of programming coordinate calculations, the role of computer software programming can be fully utilized. For some simple and similar structures, manual programming is more practical.
05 Closing Remarks
This article proposes feasible solutions to the difficulties existing in the processing of ultra-long aluminum alloy profiles. Optimization was carried out in aspects such as process planning, tool selection and program compilation. Through processing verification, excellent results were achieved. For the processing of ultra-long aluminum alloy profiles, it is necessary to be good at summarizing experiences, drawing on and promoting efficient process plans, constantly optimizing processes and making technological breakthroughs, and learning and understanding the cutting-edge knowledge and processes in processing, such as new equipment, new types of cutting tools and automatic clamping solutions, etc. Learn to integrate and apply them in actual work, and do not adhere to the existing process methods and ideas. There is no best in craftsmanship, only better. In practical applications, continuous improvement and innovation are necessary.
Expert Comment
Due to its special hollow and thin-walled structure, super-long aluminum alloy profiles encounter problems such as deformation, vibration, high processing noise and rapid tool wear during high-speed cutting. Taking all the above factors into comprehensive consideration, the author formulates feasible solutions to improve the problems of vibration, deformation and noise during processing by enhancing the rigidity of the process. By designing tooling fixtures, the rapid clamping and disassembly of profiles can be achieved. Rough machining is carried out by sawing to quickly remove most of the excess material. Numerical control machining adopts methods such as macro programs and custom subroutines, which improves programming efficiency.
