The structural requirements of the machine bed that cannot be ignored when designing CNC machine tools

In the initial stage of the development of CNC machine tools, their mechanical structure did not change much compared to general machine tools, only some changes were made in automatic transmission, tool holder and worktable automatic indexing, and handle operation. With the development of numerical control technology, considering its control mode and usage characteristics, higher requirements have been put forward for the productivity, machining accuracy, and service life of machine tools. The main mechanism of CNC machine tools has the following characteristics:
1. Due to the use of high-performance continuously variable spindle and servo transmission system, the limit transmission structure of CNC machine tools is greatly simplified, and the transmission chain is also greatly shortened;

2. 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 accuracy, as well as high wear resistance, and small thermal deformation;
3. To reduce friction, eliminate transmission clearance, and achieve higher machining accuracy, more efficient transmission components such as ball screw pairs, rolling guides, and clearance reducing gear transmission pairs are used;
4. In order to improve working conditions, reduce auxiliary time, enhance operability, and increase labor productivity, auxiliary devices such as automatic tool clamping device, tool magazine and automatic tool changing device, and automatic chip removal device have been adopted. Based on the applicable scenarios and structural characteristics of CNC machine tools, the following requirements are proposed for the structure of CNC machine tools:
（1） Higher static and dynamic stiffness of machine tools
 
Numerical control machine tools automatically process according to instructions provided by numerical control programming or manual data input. Due to the geometric accuracy and positioning errors caused by deformation of mechanical structures such as machine bed, guide rails, worktable, tool holder, and spindle box, they cannot be manually adjusted and compensated during the machining process. Therefore, it is necessary to control the elastic deformation of various mechanical structural components to the minimum extent possible to ensure the required machining accuracy and surface quality.
In order to improve the stiffness of the spindle of CNC machine tools, not only is a three support structure often used, but also double row short cylindrical roller bearings and angular contact radial thrust bearings with good rigidity are used to hinge the Trust Force bearings to reduce the radial and axial deformation of the spindle. In order to improve the stiffness of large parts of the machine tool, a closed interface bed is adopted, and hydraulic balance is used to reduce the deformation of the machine tool caused by the position changes of moving parts. In order to improve the contact stiffness of various components of the machine tool and increase its load-bearing capacity, the scraping method is used to increase the contact points per unit area, and a sufficiently large preload is applied between the joint surfaces to increase the contact area. These measures can effectively improve the contact stiffness.
In order to fully utilize the efficient machining capability of CNC machine tools and perform stable cutting, it is necessary to increase the dynamic stiffness while ensuring static stiffness. The commonly used measures include increasing the stiffness of the system, increasing damping, and adjusting the natural frequency of components. Experiments have shown that increasing the damping coefficient is an effective method to improve vibration resistance. The welded structure of steel plates can increase static stiffness, reduce structural weight, and increase the damping of the components themselves. Therefore, in recent years, steel plate welded structures for bed bodies, columns, beams, and workbenches have been adopted in CNC machine tools. Sealing sand castings is also beneficial for vibration attenuation and has a good effect on improving vibration resistance. The WeChat account for metal processing has good content and is worth following
（2） Reduce thermal deformation of machine tools
Under the influence of internal and external heat sources, various components of the machine tool will undergo varying degrees of thermal deformation, causing the relative motion relationship between the workpiece and the tool to be disrupted, which is also a quarterly decline in the machine tool. For CNC machine tools, the impact of thermal deformation is even more severe because the entire machining process is controlled by computational instructions. In order to reduce thermal deformation, the following measures are usually adopted in the structure of CNC machine tools.
 
1. Reduce fever
The main heat source that causes thermal deformation during internal heating of machine tools should be separated from the main engine as much as possible.
 
2. Control temperature rise
After taking a series of measures to reduce heat sources, the situation of thermal deformation will be improved. But it is often very difficult, even impossible, to completely eliminate the internal and external heat sources of the machine tool. So it is necessary to control the temperature rise through good heat dissipation and cooling to reduce the influence of heat sources. One of the more effective methods is to forcibly cool the heating parts of the machine tool, or to heat the low-temperature parts of the machine tool to make the temperature of each point tend to be consistent, which can reduce the warping deformation caused by temperature differences.
3. Improve the machine tool mechanism
Under the same heating conditions, the machine tool mechanism also has a significant impact on thermal deformation. The single column mechanism used by CNC machine tools in the past may be replaced by a double column mechanism. Due to left-right symmetry, the main axis of the double column mechanism undergoes minimal deformation in all directions except for vertical translation when heated, and the vertical axis movement can be easily compensated for with a coordinate correction.
For the spindle box of a CNC lathe, it is advisable to ensure that the thermal deformation of the spindle occurs in the vertical direction of the tool cutting. This can minimize the impact of spindle thermal deformation on the machining diameter. In terms of structure, the distance between the center of the main shaft and the ground should be minimized as much as possible to reduce the total amount of thermal deformation. At the same time, the temperature rise of the front and rear of the main shaft box should be consistent to avoid tilting after deformation of the main shaft. The WeChat account for metal processing has good content and is worth following
The ball screw in CNC machine tools often works under conditions of high expected load, high speed, and poor heat dissipation, so the screw is prone to heat generation. The consequences of hot production of ball screws are serious, especially in open-loop systems, which can cause the feed system to lose positioning accuracy. At present, some machine tools use pre tensioning methods to reduce the thermal deformation of the lead screw. For thermal deformation that cannot be eliminated despite the above measures, compensation pulses can be issued by the CNC system based on the measurement results for correction.
（3） Reduce friction between movements and eliminate transmission clearances
In order for the workbench to respond accurately to the instructions of the numerical control device, corresponding measures must be taken. There are significant differences in friction damping characteristics between commonly used sliding guides, rolling guides, and static pressure guides. Replacing sliding screws with ball screws in the feed system can also achieve the same effect. At present, almost all CNC machine tools adopt ball screw transmission without exception.
 
The machining accuracy of CNC machine tools, especially those with open-loop systems, largely depends on the precision of the feed transmission chain. In addition to reducing machining errors in transmission gears and ball screws, another important measure is to use gapless transmission pairs. For the cumulative error of ball screw pitch, a pulse compensation device is usually used for pitch compensation.
 
（4） Improve the lifespan and precision retention of machine tools
 
In order to improve the service life and accuracy retention of machine tools, the wear resistance of CNC airport components should be fully considered in the design, especially the wear resistance of the main parts that affect the progress, such as machine tool guide rails and feed servo machine spindle components. During use, it is necessary to ensure that all components of the CNC machine tool are well lubricated.
 
（5） Reduce auxiliary time and improve operational performance
In the single piece machining of CNC machine tools, auxiliary time (non chip time) accounts for a large proportion. To further improve the productivity of machine tools, it is necessary to adopt measures that maximize the compression of auxiliary time. At present, many CNC machine tools have adopted multi spindle, multi tool holder, and automatic tool changing devices with tool libraries to reduce tool changing time. For CNC machine tools with increased chip usage, the bed mechanism must be conducive to chip removal.