Spindle design of machining center and high-speed precision CNC milling machine
High-speed machining can not only double the production efficiency, but also further improve the machining accuracy and surface quality of parts, and solve some of the high-efficiency machining problems of some special materials that are difficult to solve in conventional machining. Therefore, high-speed machining has caused a high degree in the world. Pay attention.
1 High-speed machining requirements for machine tool spindles
High-speed machining not only requires high speed, high torque and power output, but also high spindle rotation accuracy and good rigidity, shock resistance and thermal stability during high-speed operation. At present, the highest speed of the spindles of high-speed machining centers produced by industrially developed countries in the world is as high as 20,000-100,000r/min, and the highest speeds of the spindles of domestic small and medium-sized machining centers and CNC milling machines are also 4000-6000r/min. There are mainly two types of high-speed spindles in practical applications: one type is a high-speed electric spindle with zero transmission. This type of spindle adopts the integrated structure of the motor and the machine tool spindle, and has undergone precise dynamic balance correction, so it has good rotation accuracy and Stability, but the output torque and power are limited; the other type is a spindle that combines a variable frequency spindle motor and a mechanical speed change mechanism. The output torque and power of this type of spindle are much larger, but the rotation accuracy is relatively speaking It is a little bit close to stability, so for this type of spindle, how to correctly design the machine tool spindle and its components on the machining accuracy of the machine tool is very important.
2 Structural design of the spindle assembly
1)Spindle unit structure. Most high-speed machining centers and CNC milling machines adopt a unit spindle structure. The front and rear bearings of the spindle are matched and ground under a constant temperature environment. After the grinding is completed, they are placed in a round sleeve, and then assembled in a complete The unit is installed in the spindle box of the machine tool, which not only guarantees the assembly accuracy of the machine tool spindle assembly, but also is easy to install, maintain and adjust, as shown in Figure 1.
2) Spindle bearing
Spindle bearing selection: In view of the large load, high speed and high precision requirements of machining centers and high-speed CNC milling machines, ordinary spindle double bearing structures can no longer meet the requirements. Now for high-speed machining centers and CNC milling machines, most of the angular contact bearing combinations are used design. Because angular contact bearings can bear radial and axial loads in one direction at the same time, the allowable limit speed is higher.
As shown in Figure 2, the use of two angular contact ball bearings back-to-back assembly makes the supporting points A and B expand outward, shortens the overhang of the spindle head, and greatly reduces the flexural deformation of the spindle end. Improved spindle stiffness.
Figure 2 Angular contact bearings are assembled in groups
Pre-tightening of the spindle bearing: using ordinary nuts as the axial limit of the spindle bearing, it is usually difficult to ensure that the end face of the nut has a high verticality with the axis line (as shown in Figure 3a). After locking, the bearing will be skewed, even possible Bend the shaft (as shown in Figure 3b), which will affect the rotation accuracy of the shaft
Figure 3 The effect of thread deflection on the bearing when ordinary nuts are locked
As shown in Figure 4a, the lock nut 3 can not guarantee the verticality of the end face and the hole when locking. In order to improve the adjustment accuracy of the bearing, it should be changed to use the retaining ring 5 and 6 as shown in Figure 4b. Accuracy. Use two separable nuts 8 and 9 for adjustment, or use double-groove lock nuts (see Figure 5), cross washers, and interference sleeves.
Figure 4 Schematic diagram of the locking and sealing structure of the spindle bearing
1. Compressed air pipe 2. Piston 3. Double groove lock nut 4. Disc spring 5. Pull rod 6. Spindle 7. Spindle sleeve 8. Spindle inner cooling ring 9. Tool pull nail 10. Oil retaining flange Figure 5 KX714 spindle structure
Sealing and lubrication of the spindle bearing: Due to the high spindle speed of high-speed machine tools, grease lubrication is difficult to meet the requirements when the speed is above 5000r/min, and the amount of lubricating oil in the high-speed movement of thin oil lubrication obviously affects the smooth operation of the spindle Sex. Therefore, most of the current designs adopt centralized quantitative timing oil mist or drip lubrication. In high-speed machining, in order to improve the life of the spindle bearing and ensure the rotation accuracy of the bearing, strict sealing measures must be taken. However, the contact seal with better sealing effect will inevitably affect the increase of the spindle speed. Therefore, the current common use is spindle blowing , Labyrinth seal and other non-contact sealing methods, gap sealing can be used for less demanding, but the size of the gap must be accurately controlled, generally between 0.02 and 0.04mm.
3) Spindle rod automatic tool installation system In the machining center and high-speed CNC milling machine, the tool installation is bound to adopt the automatic tool installation mechanism. The axial tension is controlled by the pre-tensioned spring, and then the pneumatic, hydraulic or mechanical screw and other actuators realize the pull rod mechanism for loosening and clamping the knife (see Figure 5). The actuator has a follow-up unit that rotates with the main shaft, and a separate structure that does not rotate with the main shaft. The former has a compact structure and high complexity, and the latter has a simple structure and low cost, but takes up more space. In addition, in order to improve the accuracy of repeated tool installation and reduce the abnormal engagement between the tool taper shank and the spindle taper hole, the automatic tool installation system must be equipped with a spindle stop mechanism and air blowing or spraying to clean the tool taper shank and spindle taper surface. Institutions.
4) Spindle water cooling system. Bearing temperature rise in high-speed rotation is one of the important factors that cause spindle deformation and decrease processing accuracy. A large amount of heat energy will cause serious thermal deformation of the spindle, which not only affects the rotation accuracy of the spindle, but also inhibits the increase of the spindle speed. The higher the speed, the more obvious this phenomenon. Therefore, how to improve the thermal characteristics of the machine tool spindle system, reduce the heat source intensity of the system, reduce the temperature rise and reduce the thermal displacement of the system, is particularly important in the design of high-speed spindles. In the main shaft design, in addition to the correct selection of main shaft components to reduce the intensity of the system heat source, the main shaft should be cooled by water to reduce the temperature rise and reduce the thermal displacement of the system. A continuous water cooling ring groove is designed outside the spindle sleeve to allow the machine tool coolant to flow through the cooling ring groove on the spindle sleeve first, and then spray to the tool tip. In this way, most of the bearing produced is taken away Heat, reduce system heat transfer to the machine tool spindle. For some machine tools with higher requirements, the spindle water cooling and cutting fluid can be divided into two different systems. Of course, this structure can only be applied to the unit spindle.
3 Design example
Figure 5 is the spindle structure design of XK714 CNC bed milling machine (the spindle speed is 50～4200r/min). Because the main shaft is longer, a structure of two main supports and one auxiliary support is adopted. The front and middle supports adopt two sets of high-speed precision angular contact ball bearings assembled back-to-back to improve the load-carrying capacity. The pre-tightening force of the bearings has been paired and ground in the bearing factory, which is a general assembly type. The rear support adopts two light series deep groove ball bearings, the outer ring of the bearing is floating, and the clearance is retained to prevent axial interference. The end of the main shaft adopts the sealing and dustproof technology that combines the gap seal and the oil throwing groove. The pull rod of the machine tool spindle is controlled by the disc spring to control the axial tension of the tool installation, and the pneumatic mechanism performs the tool loosening and clamping actions. The pull rod and the piston rod have a through hole with a diameter of f4, which is used to change the spindle taper hole and the tool taper. The handle blows clean. The whole spindle is designed as a unit structure, and a water cooling ring is arranged outside the spindle sleeve to reduce the temperature rise of the spindle.