In the past 10 years, with the rapid development of high-speed processing technology and the increasingly widespread application, the demand for high-speed CNC machine tools has been increasing in various industrial sectors, especially in the aerospace, automotive, mold processing and motorcycle industries. Industrialized countries such as the United States, Japan, Germany, Italy, and Switzerland have produced a variety of commercial high-speed machine tools. The following table lists the high-speed machining centers of several famous brands that have emerged in the international machine tool market in recent years.
New high-speed machining center watch manufacturer
(Country) Machine name and model spindle maximum speed
(r/min) Maximum feed rate
(m/min) Spindle Drive Power (kW) Cincinati Milacron (US) Maxim 500 Horizontal Machining Center 20000 28 12 Ingersoll (US) HVM800 High-speed Horizontal Machining Center 20000 76.2 45 Mikron (US) HSM700 High-speed Vertical Machining Center 42000 40 14 Ex-cell-O (De) XHC241 High-speed Horizontal Machining Center 24000 120 40 RODERS (Germany) RFM1000 Machining Center 42000 30 20 to 30 Makino (Daily) A55-A128 Machining Center 40000 50 22 Niigata Ironworks (Japan) VZ40 Machining Center 50000 20 18.5 Mazak (Daily) Super-400H Machining Center 25000 15 18.5
(a) No vector control
(b) With vector control
Figure 1 Torque-power characteristics In general, high-speed machine tools are CNC machine tools and precision machine tools. The most important feature of the transmission structure is the realization of the "zero drive" of the machine tool. From the point of view of the main drive system of the machine tool, this transmission method eliminates all mechanical transmission links (such as belts, gears, clutches, etc.) between the main motor and the main shaft, and realizes the integration of the main motor and the machine tool spindle. This transmission method has the following advantages: The mechanical structure is the simplest, and the transmission inertia is small, so the rapid response is good, and it can achieve extremely high speed, plus (minus) speed, and rapid quasi-stop at a fixed angle (C-axis control). The use of AC variable frequency speed control and vector control electrical drive technology, output power, wide speed range. With an ideal torque-power characteristic (Fig. 1b), a single clamping can achieve both roughing and high-speed finishing. The realization of the unitization of the spindle components, can be made independently of the standardized functional components, and specialized factory for serial production. The machine tool main engine factory only needs to select according to the user's different request, may convene the high speed machine tool of various performances very conveniently, accords with the development direction of the modular design of the modern machine tool. Although the mechanical structure of the spindle is relatively simple, the manufacturing process requirements are very strict. This kind of structure also brings a series of new technical problems, such as the heat dissipation of the built-in motor, the balancing of the high-speed spindle, the rational design of the spindle support and its lubrication method, etc., which must be properly resolved to ensure stable and reliable high-speed spindle. Operation to achieve high-precision precision machining. This paper discusses the problems in the design and manufacture of high-speed, high-power spindles for milling and boring by combining the research and development of high-speed spindles in our school. 1 Basic Parameters and Structure of Motor Spindle The main parameters of the motor spindle are: (1) Maximum spindle speed and constant power speed range: (2) Rated power and maximum torque of the spindle: (3) Spindle front diameter and front and rear bearings Span, etc. The maximum spindle speed, front journal diameter, and rated power are the basic parameters. Electric spindles are usually equipped on high-speed machining centers. In the design of electric spindles, these parameters must be determined according to the user's process requirements using statistical analysis of typical parts. The machine tool factory will generally design two types of high-speed machine tools of the same size, namely "high-speed type" and "high-rigidity type". The former is mainly used in aviation, aerospace and other industrial processing of light alloys, composite materials and cast iron and other parts: the latter is mainly used for mold manufacturing, automotive industry, high-strength steel or heat-resistant alloys and other difficult-to-machine materials and steel processing. When designing the electric spindle, it is also necessary to pay attention to selecting a variable frequency motor and its control module with better torque—power characteristics and a sufficiently wide speed range. According to the relative position of the main motor and the main shaft bearing, there are two layout methods for the high-speed motor spindle:
1. Coding disk 2. Spindle housing 3. Cooling water jacket 4. Motor stator 5. Oil gas nozzle 6. Motor rotor 7. Stepped interference sleeve 8. Balance disk 9. Angular contact ceramic ball bearing
Figure 2 GD-2 electric spindle
1. Hydraulic cylinder 2. Pull rod 3. Spindle bearing 4. Disc spring 5. Collet 6. Spindle 7. Built-in motor Fig. 3 Electric spindle with main motor behind the rear bearing
The main motor is placed between the front and rear bearings of the main shaft. Figure 2 shows the GD-2 high-speed electric spindle developed by our university, and it is also the basic structure of the general electric spindle. Its advantage is that the axial dimension of the spindle unit is short, the spindle stiffness is high, and the output is large, which is more suitable for medium and large-scale high-speed machining centers. At present, most electric spindles adopt this structure form. After the main motor is placed behind the main shaft rear bearing, as shown in Fig. 3, the main shaft box and the main motor are axially arranged coaxially (some joints are used). This layout method is conducive to reducing the radial size of the front end of the electric spindle, and the motor has better heat dissipation conditions. However, the axial dimension of the entire spindle unit is large, and it is often used for small high-speed numerical control machine tools, and is particularly suitable for high-speed precision machining of mold cavities. Regarding the span of the front and rear bearings of the electric spindle and the extension of the end of the main shaft, calculation shall be made according to the requirements of static rigidity and dynamic stiffness.
2 Main heat source of the electric spindle and its solution The problem of heat generation in the operation of the electric spindle used in milling and boring high-speed machining centers has always been the focus of attention. There are two main sources of heat inside the spindle: one is the main motor and the other is the spindle bearing. Unlike general spindle components, one of the most prominent problems with electric spindles is the heating of internal high-speed main motors. As the main motor is next to the spindle bearing, the heat of the motor will directly reduce the bearing's working accuracy. If the main motor's heat dissipation problem is not solved well, it will also affect the reliability of the machine tool. Figure 4 shows the external circulating oil % water cooling system of our high speed electric spindle. An aluminum jacket 3 (Fig. 2) with a spiral groove is added outside the stator of the main motor. During the operation of the machine tool, the cooling oil constantly flows in the spiral groove, so that the heat of the main motor is promptly and rapidly removed. The flow of cooling oil can be calculated based on the heat generated by the main motor.
Fig. 4 Oil-water heat exchange system of GD-2 electric spindle
1. Lubricant tank 2, 6. Pressure switch 3. Quantitative dispenser
4. Spray nozzle 5. Pump 7. Compressed air
8. Solenoid valve 9. Time relay 10. Pressure gauge
Fig. 5 Oil-air lubrication system of GD-2 electric spindle
At the same time, the heating of the spindle bearings is also one of the main heat sources of the electric spindle. Due to the high operating speed of the motor spindle, the dm·n value is large (dm—the spindle front journal diameter, mm:n—the maximum spindle speed, r/min), and therefore there are very strict requirements on the dynamic and thermal performance of the spindle bearings. . In addition to individual ultra-high-speed electric spindle using magnetic bearings or hydro-static hydrostatic bearings, the current majority of high-speed spindles at home and abroad are using angular contact ceramic ball bearings, the ball made of Si3N4 material, diameter than the same size ball bearings Small 1/3. This material has the advantages of low density, high hardness, low coefficient of thermal expansion, large elastic modulus, etc. The use of this type of bearing enables the motor spindle to achieve high operating speed (dm·n value up to 2×106), small temperature rise, and high stiffness , long life and a series of excellent features. Although the price of this type of bearing is 2 to 2.5 times higher than that of a steel ball bearing of the same specification and accuracy, its service life is 3 to 6 times higher, so its performance/price ratio is not bad and is easily accepted by the machine tool designer. . In order to further reduce the temperature rise of the spindle bearing, an oil-air lubrication system is used on the GD-2 electric spindle, as shown in Figure 5. The actual measurement shows that under high-speed operation conditions, the temperature rise of the main shaft bearing can be reduced by 9 to 16° C. than the temperature rise of the oil mist lubrication, and the cooling effect is better as the dm·n value increases. 3 The dynamic balance design of the electric spindle The maximum speed of the electric spindle is generally 10000r/min or more, and some of them are as high as 60,000 to 100000r/min. A small unbalanced amount of the spindle operation will cause a huge centrifugal force, causing vibration of the machine tool and affecting the machining accuracy. And the surface quality. Therefore, the motor spindle must be strictly balanced. The components on the main shaft and the main shaft must be processed, assembled and adjusted with precision so that the precision of the dynamic balance of the main shaft assembly can reach a level of 0.4 or more.
1. Rotor inner sleeve 2, 4. End cap 3. Rotor silicon steel sheet
Fig. 6 Dynamic balance design of rotor of high speed electric spindle
When designing the motor spindle, the principle of structural symmetry must be strictly observed. The key connection and the screw connection are forbidden on the motor spindle. As shown in Figure 2, the motor rotor and the machine tool spindle use an interference fit to achieve torque transmission. The amount of interference should be calculated based on the transmitted torque, which is sometimes as high as 0.08 to 0.10 mm. The threaded sleeve for the axially fixed part on the main shaft is also replaced with a disk having an interference fit with the main shaft. When designing this kind of interference coupling device, it must also consider the convenience of its disassembly. As shown in FIG. 6 , the rotor inner sleeve 1 installed with the silicon steel sheet is made of chromium-manganese spring steel, and its wall thickness is small, so it has better elasticity. When it is necessary to remove the rotor from the main shaft (in order to replace the worn front bearing, see Fig. 1), a portable high-pressure pump can be used to press the oil from the small hole a at the left end of the inner sleeve of the rotor, and the high pressure oil enters the ring of the inner sleeve. After the inner hole e, the inner diameter of the sleeve 1 can be expanded, so that the rotor can be smoothly removed from the main shaft. In order to maintain the symmetry of the spindle structure, another small hole b is also symmetrically machined in the inner sleeve of the rotor. When pressure oil is applied from the small hole a, the small hole b can be plugged with a bolt. In order to achieve a precise dynamic balance of the rotor after assembly, in addition to the above symmetrical design, the following two process measures must be taken: First, the outer diameter D of the rotor silicon steel sheet 3 should have a certain allowance before assembly. After the rotor is loaded into the main shaft by hot pressing (the rotor is generally heated to 180-200°C), the front and rear journals are used as positioning supports to clamp the main shaft on the lathe and the silicon steel outer circle D of the rotor is performed. The last fine car. Another measure to ensure the dynamic balance of the spindle is to symmetrically machine 16 to 24 screw holes c, d (M4 or M6) with slightly different diameters on the two end caps 2, 3 of the rotor of the motor. After the motorized spindle is assembled, according to the test result of the balancing machine, in a certain position, screw in the corresponding balance screw and adjust the depth of rotation. Until the dynamic balance accuracy is fully achieved, balance the screws with epoxy resin. Curing. 4 Conclusion As the most important part of high-speed CNC machine tools, the performance of the spindle is determined to a large extent by the processing precision and production efficiency of the entire high-speed machine tool. Therefore, all industrial countries are very concerned about the research and development of high-speed spindles. With huge investments, well-equipped processing and testing equipment, and establishing a constant temperature and clean assembly environment, a number of professional production bases for electric spindles have been formed. The design and manufacturing technology of China's high-speed motorized spindles has just started. At present, the scale of mass production has not yet been formed. The performance indicators of electric spindles and foreign countries still have a large gap. In order to speed up the development and application of high-speed processing technology in China, and accelerate the upgrading of CNC machine tools, it is recommended to further organize the forces to carry out technical research on the above issues and realize the professional mass production of electric spindles as soon as possible.
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