Four characteristics of the main structure of CNC machine tools

In the initial stage of the development of CNC machine tools, its mechanical structure has not changed much compared with general-purpose machine tools, but it only makes some changes in terms of automatic transmission, tool rest and table automatic indexing and handle operation. With the development of numerical control technology, taking into account its control methods and the use of features, it only puts forward higher requirements for machine tool productivity, processing accuracy and life. The main body of CNC machine tools has the following features:

1. Due to the use of a 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 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 gaps and obtain higher processing precision, more efficient transmission components are used, such as ball screw pairs and rolling guides, anti-backlash gear transmission pairs, etc.;

4. In order to improve working conditions, reduce auxiliary time, improve operability, and increase 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:

(a) Higher machine static and dynamic stiffness

CNC machine tools are automatically processed according to the instructions provided by numerically controlled programming or manually entered data. Because the positioning accuracy caused by the geometrical accuracy and deformation of the mechanical structure (such as machine tool bed, guide rails, table, tool holder, spindle box, etc.) can not 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 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 stiffness, 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 machine tools

Under the influence of internal and external heat sources, various parts of the machine tool will undergo thermal deformation of different degrees, causing the relative motion relationship between the workpiece and the tool to be broken, and the machine tool also declines quarterly. 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 distortion, the following measures are usually used in the structure of CNC machine tools.

1, reduce fever

The main heat source that generates thermal deformation when the machine tool is internally heated should separate the heat source from the main machine as much as possible.

2, control the 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

Under the same heat 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 not only deformed in the vertical direction but also deformed in other directions. 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. Simultaneously, 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 the 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.

(c) Reduce friction between exercises and eliminate drive gaps

In order to enable 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 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 and precision of the machine tool

In order to improve the life of the machine tool and maintain the accuracy, the wear resistance of the 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 tool guide rail and the feed servo spindle component. During use, it should be ensured that the components of the CNC machine tool are well lubricated.

(E) Reduced Auxiliary Time and Improved 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 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 changer 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.