In the initial stage of the development of CNC machine tools, the mechanical structure did not change much compared with the general machine tool, but only in the automatic shifting, tool holder and table automatic indexing and handle operation. With the development of numerical control technology, considering its control mode and use characteristics, it puts higher requirements on the productivity, processing precision and life of the machine tool. The main mechanism of CNC machine tools has the following characteristics:
1) Due to the high-performance stepless variable speed spindle and servo drive system, the limit transmission structure of the CNC machine tool is greatly simplified, and the transmission chain is also greatly shortened;
2) In order to adapt to continuous automated processing and improve processing productivity, the mechanical structure of CNC machine tools has high static and dynamic stiffness and damping precision, as well as high wear resistance, and small thermal deformation;
3) In order to reduce friction, eliminate transmission gap and obtain higher machining accuracy, more efficient transmission components, such as ball screw pair and rolling guide, anti-backlash gear transmission pair, etc.;
4) In order to improve working conditions, reduce auxiliary time, improve operability, and improve labor productivity, auxiliary devices such as automatic tool clamping device, tool magazine, automatic tool changer, and automatic chip removal device are used. According to the applicable occasions and mechanism characteristics of CNC machine tools, the following requirements are imposed on the structure of CNC machine tools:
First, the high static and dynamic stiffness of the machine tool
The CNC machine tool is automatically processed according to the instructions provided by numerical control programming or manual input data. Due to the geometrical accuracy and deformation of the mechanical structure (such as machine bed, guide rail, table, tool holder and headstock), the positioning error cannot be artificially adjusted and compensated during the machining process. Therefore, mechanical structural components must be used. The resulting elastic deformation is controlled to a minimum to ensure the required machining accuracy and surface quality.
In order to improve the rigidity of the CNC machine tool spindle, not only the three-support structure is often used, but also the double-row short cylindrical roller bearing and the angular contact radial thrust bearing with good rigidity are hinged out to believe that the force bearing is used to reduce the radial direction of the main shaft. And axial deformation. In order to improve the rigidity of the large parts of the machine tool, the bed of the closed interface is adopted, and the hydraulic balance is used to reduce the deformation of the machine tool caused by the positional change of the moving parts. In order to improve the contact stiffness of various parts of the machine tool and increase the load-carrying capacity of the machine tool, the contact point per unit area is increased by scraping, and a sufficient pre-load 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 capability of CNC machine tools and to enable stable cutting, dynamic stiffness must be improved while maintaining static stiffness. Commonly used measures include increasing the stiffness of the system, increasing the damping, and adjusting the natural frequency of the member. Tests have shown that increasing the damping coefficient is an effective method to improve vibration resistance. The welded structure of the steel plate can increase the static stiffness, reduce the structural weight, and increase the damping of the component itself. Therefore, in recent years, the bed, the column, the beam and the table of the steel plate welded structure have been used on the numerically controlled machine tool. Sealing sand castings are also beneficial for vibration attenuation and have a good effect on improving vibration resistance.
Second, reduce the thermal deformation of the machine tool
Under the influence of internal and external heat sources, different parts of the machine tool will undergo different degrees of thermal deformation, so that the relative motion relationship between the workpiece and the tool is broken, which is also a quarterly decline of the machine tool. For CNC machine tools, the effect of thermal deformation is even more serious because all machining processes are controlled by calculations. In order to reduce thermal deformation, the following measures are usually adopted in the numerical control machine tool structure.
1. Reduce fever
The main heat source that generates thermal deformation when the machine is inside generates heat, and the heat source should be separated from the host as much as possible.
2. Control temperature rise
After a series of measures to reduce heat sources, the thermal deformation will be improved. However, it is often difficult or even impossible to completely eliminate the internal and external heat sources of the machine tool. Therefore, temperature rise must be controlled by good heat dissipation and cooling to reduce the effects of heat sources. The more effective method is to forcibly cool the hot part of the machine tool, or to heat the machine at a low temperature part to make the temperature of the machine point uniform, which can reduce the warpage caused by the temperature difference.
3. Improve machine mechanism
Under the same heat conditions, the machine tool mechanism also has a great influence on the thermal deformation. For example, the single-column mechanism used in CNC machine tools in the past may be replaced by a double-column mechanism. Due to the bilateral symmetry, the main axis of the double-column mechanism is slightly deformed in the other direction except for the vertical direction translation, and the vertical axis movement can be conveniently compensated by a coordinate correction amount.
For the headstock of a CNC lathe, the thermal deformation of the spindle should be as far as possible in the vertical direction of the cutting of the tool. This minimizes the effect of thermal deformation of the spindle on the machining diameter. In the structure, the distance between the center of the main shaft and the main axial ground should be reduced as much as possible to reduce the total amount of thermal deformation. At the same time, the front and rear temperature rises of the main shaft box should be consistent to avoid tilting after the deformation of the main shaft.
Ball screws in CNC machine tools often work under conditions of high expected load, high speed and poor heat dissipation, so the lead screw is prone to heat. The consequences of thermal production of the ball screw are severe, especially in open loop systems, which can cause the feed system to lose positioning accuracy. At present, some machine tools use a pre-tensioning method to reduce the thermal deformation of the lead screw. For the thermal deformation that can not be eliminated by the above measures, the compensation pulse can be corrected by the numerical control system according to the measurement result.
Third, reduce the friction between the movements and eliminate the transmission gap
The displacement of the CNC machine table (or the carriage) is the minimum unit of the eleven pulse, and usually requires movement at the base speed. In order for the workbench to respond accurately to the instructions of the numerical control device, corresponding measures must be taken. At present, there are obvious differences in friction damping characteristics between the sliding guide rails, the rolling guide rails and the hydrostatic guide rails. The same effect can be obtained by replacing the sliding screw with a ball screw in the feed system. At present, CNC machine tools use ball screw drives almost without exception.
The machining accuracy of CNC machine tools (especially CNC machines with open-loop systems) depends to a large extent on the accuracy of the feed chain. In addition to reducing machining errors in the drive and ball screws, another important measure is the use of a gapless drive pair. For the cumulative error of the ball screw pitch, the pulse compensation device is usually used for pitch compensation.
Fourth, improve the life and accuracy of the machine tool
In order to improve the life and accuracy of the machine tool, the wear resistance of CNC airport parts should be fully considered in the design, especially the wear resistance of the main parts that affect the progress, such as the machine guide rails and the feed spindle parts. During use, it should be ensured that all parts of the CNC machine tool are well lubricated.
Fifth, reduce auxiliary time and improve operational performance
In the single-piece machining of CNC machine tools, the auxiliary time (non-chip time) occupies a large proportion. To further increase the productivity of the machine tool, it is necessary to take the initiative to maximize the auxiliary time. At present, many CNC machine tools use multi-spindle, multi-tool holder, and automatic tool changer with tool magazine to reduce tool change time. For CNC machine tools with increased chip usage, the bed mechanism must be good for chip removal.
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