In the initial stage of the development of numerically-controlled machine tools, its mechanical structure has not changed much compared with general-purpose machine tools, but only in the aspects of automatic transmission, tool holder and table automatic indexing and handle operation. With the development of numerical control technology, taking into account its control method and use characteristics, it has put forward higher requirements for the productivity, processing accuracy and life of the machine tool. The main body of CNC machine tools has the following features:
1. Due to the use of high-performance continuously variable spindles and servo transmission systems, the limit transmission structure of CNC machine tools is greatly simplified, and the transmission chain is greatly shortened;
2. In order to adapt to the continuous automatic processing and improve the processing productivity, the mechanical structure of CNC machine tools has high static and dynamic stiffness and damping accuracy, as well as high wear resistance, and small thermal deformation;
3, In order to reduce friction, eliminate transmission gap and obtain higher processing precision, more efficient transmission components are used, such as ball screw pair and rolling guide, anti-backlash gear transmission, etc.;
4. In order to improve working conditions, reduce auxiliary time, improve operability, and improve labor productivity, auxiliary devices such as automatic tool clamping devices, tool magazines and automatic tool change devices, and automatic chip removal devices are used. According to the applicable occasions and institutional characteristics of CNC machine tools, the following requirements are imposed on the structure of CNC machine tools:
(I) Higher static and dynamic stiffness of the machine tool The CNC machine tool is automatically processed according to the instructions provided by numerically controlled programming or manual input of data. Because the positioning error caused by the geometrical accuracy and deformation of the mechanical structure (such as the machine bed, guide rails, table, tool holder, spindle box, etc.) cannot be artificially adjusted and compensated in the machining process, the mechanical structural parts must be placed everywhere. 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 dual-row short cylindrical roller bearing and the angular contact centripetal thrust bearing with good rigidity are used to articulate the bearing of the bearing to reduce the radial of the spindle. And axial deformation. In order to improve the rigidity of large machine tools, a closed interface bed is used, and hydraulic balance is used to reduce the deformation of the machine due to position changes. In order to improve the contact stiffness of various parts of the machine tool and increase the bearing capacity of the machine tool, the method of scratching is used to increase the contact points per unit area, and a sufficient 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 capability of CNC machine tools and to perform stable cutting, under the premise of ensuring the static rigidity, it is also necessary to increase the dynamic stiffness. The commonly used measures include improving the stiffness of the system, increasing the damping, and adjusting the natural vibration frequency of the components. Tests have shown that increasing the damping coefficient is an effective way to improve vibration resistance. The welded structure of the steel plate can not only increase the static stiffness, reduce the structure weight, but also increase the damping of the component itself. Therefore, in recent years, the bed, columns, beams, and worktables of steel plate welded structures have been used on CNC machine tools. Sealing sand castings are also beneficial to vibration attenuation, and also have a good effect on improving vibration resistance.
(B) reduce the thermal deformation of the machine under the influence of internal and external heat sources, the various parts of the machine tool will have different degrees of thermal deformation, the relative motion between the workpiece and the tool has been broken, but also the quarterly decline in machine tools. For CNC machine tools, the effect of thermal deformation is even more severe because all the machining processes are controlled by calculation instructions. In order to reduce thermal deformation, the following measures are usually used in the structure of numerically-controlled machine tools.
1. Reduce the main heat source that generates heat distortion when the internal heat of the heat generating machine heats up. The heat source should be separated from the host as much as possible.
2. Controlling temperature rise After adopting a series of measures to reduce heat sources, the situation of thermal deformation will be improved. However, it is usually very difficult or even impossible to completely eliminate the internal and external heat sources of the machine tool. Therefore, the temperature rise must be controlled through good heat dissipation and cooling to reduce the impact of heat sources. One of the more effective methods is to force cooling in the heating part of the machine tool, or to heat the machine tool in the low-temperature part of the machine tool so that the temperature of each point of the machine tool tends to be consistent. This can reduce the warpage caused by the temperature difference.
3. Improve the machine tool mechanism Under the same heating conditions, the machine tool mechanism also has a great influence on thermal deformation. Single-column mechanisms such as those used in CNC machines may have been replaced by two-column mechanisms. Due to the left-right symmetry, the main axis of the double column mechanism is heated except for the translation in the vertical direction, the deformation in other directions is small, and the vertical axis movement can be easily compensated by a coordinate correction amount.
For the headstock of CNC lathes, the thermal deformation of the spindle should be as far as possible in the vertical direction of the tool. This can minimize the effect of hot deformation of the spindle on the machining diameter. The structure should also reduce the distance between the center of the spindle and the main axle as much as possible to reduce the total amount of thermal deformation. At the same time, the front and rear temperature rise of the spindle box should be consistent to avoid tilting after the spindle is deformed.
Ball screws in CNC machine tools often work under conditions of high expected load, high rotation speed, and poor heat dissipation, so the screw is prone to heat. The consequences of hot ball screw production 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-pull method to reduce the thermal deformation of the screw. For the thermal deformation that cannot be eliminated by taking the above measures, it can be corrected by the numerical control system to issue compensation pulses according to the measurement results.
(3) Reducing friction between movements and eliminating transmission gaps In order to enable the workbench to accurately respond to instructions of numerical control devices, corresponding measures must be taken. At present, there are obvious differences in the frictional damping characteristics between the commonly used slide rails, rolling rails, and static pressure 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 almost exclusively use ball screw drives.
The machining accuracy of CNC machine tools (especially open-loop CNC machines) depends to a large extent on the accuracy of the feed chain. In addition to reducing the machining error of the transmission gear and ball screw, another important measure is the use of a gearless transmission pair. For the cumulative error of ball screw pitch, pulse compensation device is usually used for pitch compensation.
(D) Improve the life of the machine tool and the accuracy of the maintenance In order to improve the life of the machine tool and maintain accuracy, the wear resistance of the CNC airport parts should be fully considered in the design, especially the impact of the machine tool guides, feed servo spindle components, etc. The wear resistance of the main parts of the schedule. During use, it should be ensured that the components of the CNC machine tool are well lubricated.
(5) Reducing auxiliary time and improving operation performance In the single-piece processing of CNC machine tools, the auxiliary time (non-chip time) occupies a large proportion. To further increase the productivity of the machine tool, it must be taken to maximise the compression of the auxiliary time. At present, many CNC machine tools have used multi-spindles, multi-turrets, and automatic tool changers with tool magazines to reduce tool change time. For CNC machine tools with increased chip usage, the bed mechanism must be conducive to chip removal.
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