The rotary table commonly used in CNC machine tools has an indexing table and a CNC rotary table. When machining certain parts of CNC machine tools, in addition to the linear feed motion of the three coordinate axes X, Y and Z, sometimes there is a need to have circular motion around the three coordinate axes X, Y and Z, respectively A, B, C axis.
The CNC rotary table can be used to achieve the circumferential feed motion, in addition to the indexing motion. The function of the indexing table is only to change the surface of the workpiece, and it can be used together with the automatic tool changing device to realize the multiple installations of several surfaces in one installation of the workpiece, thus greatly improving the working efficiency. The shape of the CNC turntable and the indexing table are not much different, but there are a series of features in the structure. Since the CNC rotary table can realize the feed motion, it has much in common with the feed drive mechanism of the CNC machine. The difference is that the drive mechanism realizes the linear feed motion, and the CNC rotary table realizes the circular feed motion.
Rotary tables are widely used in various CNC milling machines, boring machines, various vertical vehicles, and vertical milling machines. In addition to requiring the rotary table to withstand the weight of the workpiece well, it is also necessary to ensure its rotation accuracy under load. The rotary table bearing, as the core component of the turntable, not only has a high load carrying capacity during the operation of the turntable, but also needs to have high rotation precision, high anti-overturning capability, and high rotational speed capability.
1. Thrust ball bearing + cylindrical roller bearing
Thrust ball bearing can withstand a certain axial force, so the bearing is mainly used to bear the weight of the workpiece; and cylindrical roller Bearings are primarily used for radial positioning and to withstand external radial forces (eg cutting forces, milling forces, etc.). This type of design is widely used and relatively inexpensive. Since the thrust ball is a point contact bearing, its axial bearing capacity is relatively limited, and it is mainly used for small or medium-sized machine tool rotary table. In addition, the lubrication of the thrust ball is also difficult.
2.Static bearing + precision cylindrical roller bearing
The hydrostatic bearing is a kind of externally supplied pressure oil, which establishes a static pressure bearing oil film in the bearing to achieve liquid lubrication. Sliding bearing. Hydrostatic bearing always works under liquid lubrication from start to stop, so there is no wear, long service life and low starting power. In addition, this bearing also has the advantages of high rotation precision, large oil film stiffness, and can suppress oil film oscillation. Precision cylindrical roller bearings have good radial bearing capacity, and the precision of the rotary table can be guaranteed due to the use of precision bearings. The rotary table designed with this type can withstand high axial forces. Some workpieces weigh more than 200t and the turntable has a diameter of more than 10m. However, this type of design also has some shortcomings. Because the hydrostatic bearing must be equipped with a special oil supply system to supply the pressure oil, the maintenance is complicated and the cost is relatively high.
3. Crossed Roller Bearings
Cross roller bearings are commonly used on turntables. Crossed roller bearings are characterized by two raceways in the bearing and two rows of rollers arranged in a row. Compared with the traditional thrust bearing + radial centering bearing combination, the cross roller bearing is compact and compact, and simplifies the workbench design, thus reducing the cost of the turntable.
In addition, due to the use of optimized preload, this type of bearing has a high stiffness, so the rigidity and accuracy of the turntable are also guaranteed. Thanks to the design of the two-row cross-roller, the effective span of the bearing can be significantly improved, so this type of bearing has a high anti-overturning moment. In the cross-roller bearing, it is divided into two types: the first is a cylindrical cross-roller bearing, and the second is a tapered cross-roller bearing. In general, cylindrical cross-roller bearings are less expensive than conical crossed roller bearings and are suitable for use in turntable applications where the rotational speed is relatively low. Conical crossed roller bearings use a purely rolling design of tapered rollers for high running accuracy and speed capability. Strong, reducing the advantages of shaft length and processing cost. Crossed roller bearings are suitable for all types of vertical or horizontal boring machines, as well as vertical grinding, vertical turning and large gear milling machines.
In summary, the bearing, as the core component of the machine tool spindle and turntable, plays a decisive role in the performance of the machine tool. In order to be able to choose the right size and type of bearing, we need to consider a variety of working conditions, such as operating speed, lubrication, installation type, spindle stiffness, accuracy and other requirements. As far as the bearing itself is concerned, only by fully understanding its design features and the advantages and disadvantages brought about by it, can the best performance of the bearing be achieved.