1 Introduction
Gantry machining centers are an indispensable key piece of equipment for modern aerospace, automotive manufacturing, and large mold processing enterprises. Gantry machining centers mainly come in several structural forms: beam-moving type, table-moving type, and moving gantry type. Among them, the beam-moving type, also known as the bridge-type gantry machining center, especially the bridge-type high-speed gantry five-axis machining center, has been widely used in the large mold processing industry.
The mold manufacturing in the automotive industry and the machining of complex parts in the aerospace industry require high-speed five-axis machining centers. With the continuous development of technology, linear motor drive technology has been gradually applied to high-speed gantry five-axis machining centers. The crossbeam is the main support component of the gantry machine tool and has a great influence on the performance of the entire machine tool. Therefore, the structure of the crossbeam should be designed according to the load conditions and the design parameters of the machine tool, and relevant analysis and research should be carried out. The machine tool crossbeam studied in this paper is mainly designed for direct-drive high-speed gantry five-axis machining centers.
2. Load characteristics of the crossbeam of a high-speed gantry five-axis machining center
Due to the structural characteristics of high-speed gantry five-axis machining centers, the crossbeam is supported at both ends by bridges or columns, with slides, rams, milling heads, and other components suspended in the middle. This results in the crossbeam's guide surface needing to bear significant gravity and overturning torque, making it prone to deformation.
The load-bearing structure of the crossbeam in a high-speed gantry five-axis machining center can be simplified to a two-point simply supported beam. The main causes of crossbeam deformation are the crossbeam's own weight, the weight of the slide plate, ram, and milling head suspended on the crossbeam, and the cutting forces during machining. The drooping bending deformation of the crossbeam is greatest when the moving parts on the crossbeam move to the middle of the crossbeam. In addition, since the moving parts on the crossbeam are in a cantilever state, the weight of the moving parts will also cause torsional deformation of the crossbeam. During the machine tool design process, the load characteristics of the crossbeam should be analyzed according to the load-bearing mechanism and working conditions of the high-speed gantry five-axis machining center, which will serve as the theoretical basis for the crossbeam structure design of the high-speed gantry five-axis machining center.
3. Comparison of the advantages and disadvantages of welded beams and cast beams
Currently, in advanced industrial countries such as Europe and the United States, the basic large components of machine tools are gradually being replaced by welded parts or new material structures. This is to meet green and environmental protection requirements, and also because welded parts have more advantages than castings in the application of large machine tool components, especially small batches of moving parts. This article analyzes the casting structure and the welded structure of the crossbeam of a high-speed gantry five-axis machining center, and compares the advantages and disadvantages of the two structures.
The crossbeam of a high-speed gantry five-axis machining center simultaneously bears gravity, tension, and bending moment. To reduce deformation and improve vibration resistance, the crossbeam requires good rigidity. Compared to castings, welded crossbeams of high-speed gantry five-axis machining centers have the following characteristics:
(1) The elastic modulus and mechanical properties of carbon steel used in welded parts are higher than those of cast iron used in castings. According to the elastic modulus formula E=σ/ε (E is the elastic modulus, σ is the stress, and ε is the strain), when the stress σ is the same, a larger E will result in a smaller strain ε.
For structures with the same dimensions, materials with a larger E value have higher stiffness. Therefore, under the same load, steel structures have relatively high static stiffness. During forced vibration, increasing static stiffness can reduce the amplitude and increase its natural frequency, thereby preventing resonance. For self-excited vibration, increasing static stiffness can improve the limit of self-excited vibration stability.
(2) Since welded steel plate components are lighter than castings, the weight of a welded steel plate structure with the same rigidity is only about 60% of that of a casting. Therefore, according to the formula ωn=K/m (where ωn is the natural frequency, K is the stiffness, and m is the mass), reducing weight can increase the natural frequency, which is beneficial to avoiding resonance. The natural frequency of a reasonable welded structure can be 50% higher than that of a cast structure.
The large span of the gantry beam and the reduction in weight can also reduce sag and improve the geometric accuracy of the machine tool; at the same time, for direct drive gantry machine tools, weight reduction plays a decisive role in improving rapid traverse speed and acceleration.
(3) The internal damping of cast iron is on average 3.2 times higher than that of steel. However, the damping of welded parts mainly comes from the internal damping of the steel plate material and the friction between the component joint surfaces. The internal damping of the material accounts for only a small proportion, while the friction damping between the component joint surfaces accounts for about 90% of the total damping.
Therefore, its vibration resistance is not much lower than that of castings. With reasonable welding methods and welding structures, the damping of welded parts can even be higher than that of castings.
(4) Welded structures have a short production cycle and do not require the manufacture of wooden molds and casting during the production process, saving some manufacturing costs and making them suitable for small-batch and single-piece production.
The above comparative analysis clearly shows that, firstly, welded parts have better rigidity than castings of the same dimensions; secondly, welded parts of the same dimensions are lighter than castings. For large-span structures like the crossbeam of a gantry milling machine, reducing weight can reduce the degree of bending deformation, and reducing the weight of the crossbeam also reduces the mass of the moving parts; at the same time, welded parts use less material, which also saves costs; and for single-piece or small-batch production, the production cycle of welded parts is relatively shorter.
Therefore, whether from the perspective of performance, process, cost, cycle, or other factors, welded parts are more suitable for manufacturing the crossbeams of gantry machine tools; especially for direct-drive high-speed gantry five-axis machining centers, which require high speed, high acceleration, and high rigidity, welded crossbeams can better demonstrate their superior performance compared to cast parts.
4. Direct-drive high-speed gantry five-axis machining center crossbeam structure design
The crossbeam of the GMC2550u high-speed gantry five-axis machining center adopts a trapezoidal structure to minimize its weight while ensuring rigidity. Its shape is shown in Figure 1. The difference between the crossbeam and that of a typical gantry machining center is:
(1) The mounting surface of the grating ruler of the welding beam of the GMC2550u machine tool is located on the top surface of the beam, rather than on the same surface as the mounting surface of the guide rail. This avoids electromagnetic interference caused by the strong current of the linear motor and also facilitates dust and chip prevention.
(2) Since a linear motor is used for driving, the front side of the welded crossbeam only has the upper guide rail mounting surface, and there is no screw or rack mounting surface, which greatly simplifies the structure.
(3) The connection between the direct drive welded crossbeam and the bridge is achieved through the base plate on both sides of the crossbeam, eliminating the cumbersome and complicated connecting slide used in ordinary gantry machine tools.
Based on comparative analysis of welded and cast parts, the crossbeam of the GMC2550u high-speed gantry five-axis machining center was determined to be a welded structure, with the welded steel plate made of high-rigidity 16Mn material. The surrounding plates are all 25mm thick steel plates, the central stiffeners are 16mm thick steel plates, and the guide rail mounting surface and linear motor mounting surface are both 12mm thick steel plates. The crossbeam span is 6250mm, width is 950mm, and height is 1050mm.
Insufficient stiffness of the crossbeam is the main cause of deformation. Stiffeners are an important part of welded structure design. Reasonable arrangement of stiffeners can achieve good benefits such as light weight, high stiffness and low cost. Practice has proved that in modern machine tool bed systems, due to the reasonable arrangement of stiffeners, the machining error caused by elastic deformation generally does not exceed 10% to 15% of the total error.
In the design of the crossbeam structure, considering factors such as the working environment of the machine tool, a comparative analysis of the basic internal structure of the crossbeam was conducted. To improve the torsional stiffness of the crossbeam, the diagonal stiffening plate torsional theory was adopted, arranging the internal stiffening plates in an X-shape; supplemented by a horizontal stiffening plate, with the X-shaped stiffening plates arranged at the front and back, and a vertical stiffening plate added in the middle to enhance the torsional coefficient. The horizontal and vertical stiffening plates, like the side plates around the crossbeam, will play the role of torsional polar moment of inertia, enhancing the torsional stiffness of the crossbeam. The cross-section of the crossbeam and the arrangement of the internal stiffening plates are shown in Figures 2 and 3.
Based on the above structure, a 3D model of the crossbeam was created using Pro/E. The 3D model was then imported into ANSYS for analysis. The crossbeam model structure is shown in Figure 1. The crossbeam weighs approximately 8959 kg. The deformation under gravity, as analyzed by ANSYS, is shown in Figure 4, with a maximum deformation of 0.253 mm at the front end of the spindle. The deformation under cutting force in the X direction is shown in Figure 5, with a maximum deformation of 0.304 mm at the front end of the spindle. The deformation is basically within the allowable range, and the resulting deformation can be compensated for through programming to meet the machine tool's design requirements.
5 Conclusion
Through analysis of the load characteristics of the crossbeam in a high-speed gantry five-axis machining center and a comparative analysis of welded and cast structures, the structural design scheme and manufacturing method for this project were determined. Finally, ANSYS analysis showed that the crossbeam structure and manufacturing method are suitable for high-speed gantry five-axis machining centers, especially for direct-drive gantry machining centers. Based on this, a welded crossbeam for the GMC2550u machine tool was designed. Further in-depth research on the crossbeam design is needed to design and manufacture more precise gantry machining centers.
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