Requirements for mechanical structure of the most

2022-08-02
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Requirements for the mechanical structure of CNC machine tools in the initial stage of the development of CNC machine tools, the mechanical structure has not changed much compared with general-purpose machine tools, but has made some changes in the aspects of automatic speed change, automatic positioning of tool rest and worktable and handle operation. With the development of numerical control technology, taking into account its control mode and use characteristics, it puts forward higher requirements for the productivity, machining accuracy and service life of machine tools. The main mechanism of CNC machine tool has the following characteristics: 1) due to the use of high-performance stepless speed change spindle and servo transmission system, the limit transmission structure of CNC machine tool is greatly simplified and the transmission chain is greatly shortened; 2) In order to adapt to continuous automatic machining and improve machining productivity, the mechanical structure of CNC machine tools has high static and dynamic stiffness and damping accuracy, as well as high wear resistance, and the demand for profit maximization and small thermal deformation; 3) In order to reduce friction, eliminate transmission clearance and obtain higher machining accuracy, more efficient transmission components are used, such as ball screw pair, rolling guide, clearance elimination gear transmission pair, etc; 4) In order to improve working conditions, reduce auxiliary time, improve operability and improve labor productivity, auxiliary devices such as tool automatic clamping device, tool magazine, automatic tool change device and automatic chip removal device are adopted. According to the applicable occasions and mechanism characteristics of the NC machine tool, the following requirements are put forward for the structure of the NC machine tool:

I. high static and dynamic stiffness of the machine tool

the NC machine tool is processed automatically according to the instructions provided by the NC programming or manual data input mode. Since the positioning error caused by the geometric accuracy and deformation of mechanical structures (such as machine tool bed, guide rail, worktable, tool rest and spindle box, etc.) cannot be adjusted and compensated in the processing process, the elastic deformation of various mechanical structural components must be controlled within the minimum limit to ensure the required processing accuracy and surface quality

in order to improve the rigidity of the spindle of CNC machine tools, not only the three support structure is often used, but also the double row short cylindrical roller bearing with good rigidity and the angular contact radial thrust bearing are used to hinge the trust force bearing, so as to reduce the radial and axial deformation of the spindle. In order to improve the rigidity of large parts of the machine tool, the machine bed with closed interface and hydraulic balance are used to reduce the deformation of the machine tool caused by the position change of moving parts. In order to improve the contact stiffness of each part of the machine tool and increase the bearing capacity of the machine tool, the scraping method is used to increase the contact points on the unit area, and a large enough preload is applied between the joint surfaces to increase the contact area. These measures can effectively improve the contact stiffness

in order to give full play to the high-efficiency machining ability of CNC machine tools and carry out stable cutting, the dynamic stiffness must be improved on the premise of ensuring the static stiffness. The commonly used measures include increasing the stiffness of the system, increasing the damping and adjusting the natural frequency of the components. The test shows that increasing the damping coefficient is an effective method to improve the vibration resistance. The welded structure of steel plate can not only increase the static stiffness, reduce the weight of the structure, but also increase the damping of the component itself. Therefore, in recent years, the lathe bed, column, beam and worktable with steel plate welding structure have been adopted in CNC machine tools. Sand sealing casting is also beneficial to vibration attenuation and has a good effect on improving vibration resistance

II. Reduce the thermal deformation of the machine tool

under the influence of internal and external heat sources, various parts of the machine tool will have different degrees of thermal deformation, which will break the relative motion relationship between the workpiece and the tool, which is also the quarterly decline of the machine tool. For CNC machine tools, because the whole machining process is controlled by calculation instructions, the influence of thermal deformation is more serious. In order to reduce thermal deformation, the following measures are usually adopted in the structure of CNC machine tools

1) reduce heating

the main heat source causing thermal deformation during internal heating of the machine tool should be separated from the host as far as possible

2) control the temperature rise

after taking a series of measures to reduce the heat source, the thermal deformation will be improved. However, it is usually very difficult or even impossible to completely eliminate the internal and external heat sources of machine tools. Therefore, the temperature rise must be controlled through good heat dissipation and cooling to reduce the influence of heat source. The more effective method for the middle part is to force cooling at the heating part of the machine tool, or heating at the low-temperature part of the machine tool to make the temperature of each point of the machine tool tend to be the same, which can reduce the warpage deformation caused by the temperature difference

3) improve the machine tool mechanism

under the same heating conditions, the machine tool mechanism also has a great impact on the thermal deformation. For example, in the past, most temperature and humidity boxes at home and abroad adopted the principle of refrigeration and dehumidification, and the single column mechanism may be replaced by the double column mechanism. Due to the left-right symmetry, the deformation of the main axis of the heated double column mechanism in other directions is very small except the vertical translation, and the vertical axis movement can be easily compensated by a coordinate correction

The thermal deformation of the

axis occurs in the vertical direction of the tool cut in. This can minimize the influence of spindle thermal deformation on machining diameter. In the structure, the distance between the center of the main shaft and the ground of the main shaft shall be reduced as much as possible to reduce the total amount of thermal deformation. At the same time, the front and rear temperature rise of the main axle box shall be consistent to avoid inclination after the main shaft is deformed

the ball screw in CNC machine tools often works under the conditions of large expected load, high rotation speed and poor heat dissipation, so the screw is easy to heat up. The consequence of hot production of ball screw is serious, especially in the open-loop system, it will make the feeding system lose the positioning accuracy. At present, some machine tools use pre stretching method to reduce the thermal deformation of the lead screw. For the thermal deformation that can not be eliminated by the above measures, the numerical control system can send compensation pulses to correct it according to the measurement results

III. reduce friction between movements and eliminate transmission clearance

if the displacement of the NC machine tool workbench (or carriage) is the minimum unit in eleven pulses, it is usually required to move at the speed of the base. In order to make the workbench respond accurately to the instructions of the NC device, corresponding measures must be taken. At present, there are obvious differences in friction damping characteristics among sliding guide, rolling guide and hydrostatic guide. The same effect can be achieved by using ball screw instead of sliding screw in the feeding system. At present, almost all CNC machine tools use ball screw transmission

the machining accuracy of CNC machine tools (especially CNC machine tools with open-loop system) depends to a large extent on the accuracy of the feeding transmission chain. Before and after Shenzhen, Guangqi science, which is committed to the industrialization of metamaterials, and Ruoyu technology, which is committed to the utilization of flexible display materials, have been cultivated. In addition to reducing the machining errors of transmission gears and ball screws, another important measure is to use a gapless transmission pair. For the cumulative error of ball screw pitch, the pulse compensation device is usually used to compensate the pitch

IV. improve the service life and accuracy retention of the machine tool

in order to improve the service life and accuracy retention of the machine tool, the wear resistance of the components of the NC airport should be fully considered in the design, especially the wear resistance of the machine tool guide rail, the spindle components of the feed servo machine and other main components that affect the progress. During use, ensure that all parts of the CNC machine tool are well lubricated

v. reduce the auxiliary time and improve the operating performance

in the single piece machining of CNC machine tools, the auxiliary time (non chip cutting time) accounts for a large proportion. To further improve the productivity of machine tools, we must take measures to minimize the auxiliary time. At present, many CNC machine tools have adopted multi spindle, multi tool rest and automatic tool change device with tool magazine to reduce the tool change time. For CNC machine tools with increased chip consumption, the bed mechanism must be conducive to chip removal. (end)

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