The concept and use of high-speed processing machine tools

High-speed processing machine tools are efficient, high-performance processing machine tools developed on the basis of CNC milling machines and machining centers to meet the development needs of aviation, aerospace, automobile and mold industries based on the development of modern tool materials. Therefore, its basic It is characterized by not only high cutting speed (5 to 10 times the conventional cutting speed), fast feed/rapid traverse speed (up to 40m/min to 180m/min), large acceleration and deceleration (currently mostly 1g to 2g), but also It also includes the short tool and/or workpiece exchange time (within a few seconds to 1 second) and often multi-axis linkage functions.
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1. Advantages of high-speed processing machine tools
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High-speed processing machine tools have many advantages, such as:
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1. High production efficiency, the material removal rate is 3 to 6 times that of conventional cutting machine tools, which can greatly shorten the processing time and manufacturing cycle of parts;
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2. The cutting force is 30% to 50% less than that at conventional speeds and about 30% more of the cutting heat will be taken away by the chips, so the temperature rise and deformation of the workpiece are less, which is beneficial to cutting thin-walled parts and improving processing accuracy;
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3. Due to the high cutting speed, the forced vibration frequency generated during the cutting process is generally far away from the natural frequency of the machine tool process system, so the cutting process is smoother, which is conducive to improving the machined surface quality and tool life, and eliminating many time-consuming and labor-intensive manual sequences. Operation;
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4. Many parts used in electromechanical products, whether single-piece or in batches, can be processed in multiple processes or even assembled in one go on corresponding high-speed processing machine tools (such as multi-axis high-speed machining centers and cnc turning and milling centers). The clamp realizes all processing.
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Therefore, high-speed machining machine tools have attracted widespread attention since their emergence in the mid-1980s. With the rapid development and increasing development of related technologies, such as high-speed electric spindles, linear motors, CNC servo systems with strong functions and good performance, etc. Perfect, the production and application of high-speed processing machine tools have now become very common; not only large internationally renowned machine tool manufacturers can produce it, but general domestic and foreign machine tool factories have also begun manufacturing, and they have become the first to develop in the world’s machine tool industry. The leading products are not only used in single-piece or small-batch production in the mold, aerospace, aviation and other industries, but are also widely used in mass production in the automotive and aviation machinery manufacturing industries, thus becoming the mainstream processing equipment in these industries. .
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However, in order to better successfully develop and fully rationally apply high-speed processing machine tools, I believe that designers, manufacturers and users of high-speed processing machine tools should still have more and deeper understanding and research on the following aspects: And good at learning, summarizing and innovating.
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2. Basic requirements and design principles for machine tool structure for high-speed machining
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Since the cutting speed, feed speed and acceleration and deceleration in high-speed cnc machining are large, the heat generated by the machine tool and the inertia of the moving parts are also large, which can easily lead to excessive temperature rise, thermal deformation and impact vibration of the machine tool structure, which will ultimately affect the to machining accuracy, quality and even the working life and reliability of machine tools and cutting tools. Therefore, the basic requirements for high-speed machining on machine tool structures are first of all three highs, namely high static stiffness, high dynamic stiffness and high thermal stiffness, that is to say, The “three rigidity” characteristics must be good; secondly, the moving parts must be lightweight, that is, the inertia of the transmission system must be reduced as much as possible. To this end, the principle measures that should be taken in the structural design of machine tools are:
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1. In order to improve the static stiffness of the structure, firstly, choose materials with large elastic modulus, such as steel, cast iron, etc. as the basic materials of structural parts; secondly, according to the nature of the force (tension, compression or torsion) and conditions (force size, direction and point of action), choose reasonable structural cross-sectional shape, size, rib wall arrangement and overall layout of the machine tool; third, the joint surface between structural parts should be flat, the area should be appropriate, and the distribution of contact points on the joint surface should be The connection should be uniform and the connection should be firm; fourth, try to use box-shaped and overall structures.
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2. In order to improve the dynamic stiffness of the structure, firstly, on the premise of ensuring static stiffness, materials with large damping coefficients, such as artificial granite, cast iron, etc., are selected as materials for basic structural parts; secondly, rational design is done through model tests or modal analysis. And adjust the mass distribution of the structure and the stiffness value of the structural joint surface to change the natural vibration frequency of the structural system itself, keeping it away from the forced vibration frequency generated during the cutting process, and avoiding the possibility of resonance; third, intentionally adopting the ability to increase Structural design with additional damping, such as double-walled castings with sandwich cores and discontinuous welded weldments; fourth, the distance between the support rail surfaces of linear motion components should be as wide as possible, and the line of action of the driving force should be centered and as far as possible Close to the center of gravity of the moving parts, there should be no backlash in the transmission chain to ensure smooth movement without impact.
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3. In order to improve the thermal stiffness of the structure, in principle, materials with large heat capacity, small thermal expansion coefficient and materials with similar thermal expansion coefficient should be used as structural materials; secondly, according to the distribution of heat source and temperature field on the machine tool, try to use Thermal symmetry and structures that facilitate heat dissipation or forced cooling, including structures that adopt thermal compensation measures, can reduce the impact of thermal deformation on the geometric accuracy and working performance of machine tools.
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4. In order to reduce the weight of moving parts and the inertia of the transmission system, firstly, materials with small specific gravity, such as aluminum alloys and composite materials, are used as structural materials for moving parts; secondly, while ensuring stiffness and load-bearing capacity, try to Remove excess material; third, use direct transmission to simplify the transmission system and shorten the transmission chain to improve the movement quality of the machine tool.
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In reality, there are no materials and structures that can meet the above conditions at the same time. They can only be selected after a comprehensive evaluation based on actual requirements.
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3. High-speed electric spindle unit
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A typical application of high-speed machining is to use small-diameter carbide milling cutters to mill molds, models and cnc machining aluminum 6082 alloy parts of various materials. The machine tool spindle speed is based on the economical and reasonable cutting speed range that modern tool materials can achieve (Figure 3) and the tool/spindle speed calculated based on this speed and different milling cutter diameters (Figure 4). It can be seen that except when cutting titanium or nickel alloy, the reasonable cutting speed that the tool can achieve is low (300m/ min), the maximum speed of the tool spindle can be below 10000r/min, the maximum speed of the tool/spindle required for cutting other materials is above 10000r/min, and even requires 50000r/min to 80000r/min. Such a high spindle It is impossible to achieve the speed using the main transmission structure (motor plus pulley and gear transmission) used in general machine tools. Generally, it is necessary to use the so-called “electric spindle” direct drive that is integrated with a variable frequency speed motor and the machine tool spindle. .

The electric spindle achieves wide speed regulation of the spindle through AC frequency conversion speed regulation and vector control. Its advantage is not only to simplify the main transmission structure and reduce the rotational inertia of the main transmission system, but also to reduce power consumption and improve the efficiency of achieving higher The ability of the spindle speed and acceleration and deceleration can also realize the fast adjustment (C-axis control) function of a fixed angle, which is very important for high-speed processing machine tools.
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Of course, the design and manufacturing of the high-speed spindle itself will involve many special issues, such as the structural type and lubrication method of the spindle (motor rotor) support, the heating and cooling measures of the motor, the connection between the spindle and the tool, and dynamic balancing issues. Fortunately, these problems Many scientific research units have conducted research and solutions. High-speed electric spindle units have been produced by specialized manufacturers at home and abroad for specialized and serialized production. Machine tool designers and manufacturers only need to determine the required components based on the requirements of the processing objects. Just choose the spindle speed and torque range reasonably.
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4. High-speed feeding system
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The high-speed feed system is an extremely important part of the high-speed machining machine tool. Its design requirements should first be to provide the high feed/rapid speed and acceleration and deceleration required for high-speed cutting; secondly, it should have all the necessary The required speed regulation width and trajectory tracking accuracy; at the same time, it should also have the ability and stiffness to withstand dynamic and static loads to ensure the efficiency and quality that high-speed machining should have.
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There are three main factors that determine the above-mentioned performance requirements of the high-speed feed system: namely, the transmission mode of the feed motion, the mutual structural relationship between the feed motion of each axis and the CNC servo control system.
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1. Transmission mode of feed motion
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There are currently two widely used transmission methods for high-speed feed motion: one is indirect transmission by a rotary servo motor through a ball screw, and the other is direct drive by a linear motor.
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The advantages of indirect transmission through ball screws are mature technology, relatively simple structure, acceleration characteristics are less affected by load changes of moving parts, and many domestic and foreign manufacturers currently carry out standardized, serialized and modularized professional production. However, the ball screw used for ordinary transmission has large inertia, small lead, and is limited by critical speed. The feed/rapid traverse speed it can provide is only 10~20m/min and the acceleration is 0.3g, which meets the requirements. Therefore, the feed ball screw for high-speed processing generally adopts the following improvement measures.
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Increasing the lead of the screw and increasing the number of threads. The former is to increase the feed amount per revolution of the screw (i.e., the feed speed), and the latter is to compensate for the axial impact caused by the increase in the screw lead. Decrease in stiffness and load-bearing capacity. Changing the solid screw to a hollow one is not only to reduce the weight and inertia of the screw, but also to facilitate the use of water-cooled internal cooling of the screw, in order to increase the screw speed and feed/ The ability to move quickly and accelerate to reduce thermal effects; improve the design and manufacturing quality of the ball return device and raceways to make the circulation of balls smoother and reduce friction loss; use ball screws to fix the nuts and connect them to the moving parts The servo motors are integrated together to complete rotation and movement, thus avoiding the critical speed limit of the screw.
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After taking these improvement measures, the feed method of ball screw drive can provide feed/rapid traverse speeds of 60m/min~90m/min and acceleration of 1~2g. However, due to the limitations of the principle structure, it is necessary to It is difficult to further increase the motion speed and acceleration of the ball screw drive, and due to the limitation of the manufacturable length of the screw, the motion stroke that the ball screw drive can provide is also limited.
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Compared with the above-mentioned indirect transmission method through ball screws, the main feature and advantage of using linear motor direct drive is that the stator and mover of the servo motor are directly combined with the machine tool bed and moving part respectively, without intermediate links. , the length of the transmission chain is shortened to zero, that is, the so-called “zero transmission” is achieved, thereby greatly improving the mechanical stiffness, reducing the inertia of the transmission system, obtaining higher speed and acceleration capabilities, and making it easy to control the damping force and damping force of the system. Dynamic characteristics: the highest feed/rapid speed of linear motors can reach 120m/min or even 240m/min, and the acceleration can reach 2~10g; the stroke length can be unlimited; it has strong adaptability, high sensitivity, good follow-up performance, and no reaction. If there is a directional gap, a linear grating ruler can be used as a measurement feedback component to achieve full closed-loop control to obtain higher positioning accuracy and tracking accuracy.
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However, direct drive of linear motors also has some shortcomings: such as low efficiency, high power consumption, relatively large structural size and weight; the temperature rises during the working process, requiring strong cooling; it is easy to attract iron filings and metal objects due to the influence of magnetic field forces. , so anti-magnetic measures need to be considered. In particular, it should be noted that its acceleration value is directly inversely proportional to the load of the moving parts (the dead weight of the workbench/slide plus the workpiece and other external loads), that is, it is sensitive to moving loads, so It is suitable to be used in situations where the load of the moving parts is constant or the change is small. When the load changes significantly, it must be considered during CNC programming, otherwise the efficiency and quality required for processing cannot be guaranteed. In addition, linear motor direct drive does not have Self-locking capability, external braking measures should be considered during design and use, especially when used in vertical axis feed systems.
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5. The mutual structural relationship between the feed motion of each axis
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Like general processing machine tools, high-speed processing machine tools generally have more than 2 and up to 5 feed motion axes. The mutual structural relationship between these motion axes currently exists in three types: series, parallel and mixed.
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The tandem structure is a commonly used type of traditional machine tools. Its characteristic is that the layout of each motion axis adopts the Cartesian rectangular coordinate system. The machine bed, column, slide plate, table/turntable and spindle box and other components pass through the corresponding guide rail support surfaces. When connected in series, the motion of each axis can be performed independently. Because of the series connection, the weight of each moving part is often larger and inconsistent, and special adjustments are required to maintain the consistency of the acceleration characteristics of each axis.