Reasonably selecting the structural form of CNC machine tool steel components

Because of the complexity of the transient and changeable processing state of CNC machine tools, it is usually difficult to calculate the structural stiffness theoretically. The designer can only calculate the stiffness of some components (such as shaft, lead screw, etc.) with calculation methods, while the bending and torsional deformation of parts such as machine bed, column, workbench and box, contact deformation of joint surface, etc., can only be calculated approximately after simplification. The calculation results always differ greatly from the actual situation, so they can only be used as a reference for qualitative analysis. In recent years, although the finite element method is used for analysis and calculation in the structural design of machine tools, generally speaking, it is still necessary to test, analyze and compare models, objects or similar prototypes to determine a reasonable structural scheme during design. However, following the following principles and measures can still reasonably improve the structural stiffness of machine tools.

(1) Correctly select the shape and size of the section

After a member is subjected to bending and torsional loads, its deformation depends on the bending and torsional moment of inertia of the section. If the bending and torsional moment of inertia is large, its stiffness is high. For sections with the same shape, when maintaining the same cross-sectional area, the wall thickness should be reduced and the profile size of the section should be increased. The torsional stiffness of the circular section is greater than that of the square section, while the flexural stiffness is smaller than that of the square section; The stiffness of closed section is much greater than that of non closed section; The opening on the wall will reduce the stiffness, and the bending stiffness can be restored by adding a flange around the hole.

(2) Reasonable selection and arrangement of diaphragms and ribs

The static and dynamic stiffness of the member can be improved by reasonably arranging the diaphragms and ribs of the supporting members. Several column structures are internally arranged with longitudinal, transverse and diagonal stiffeners. The results of static and dynamic stiffness tests are obtained for them. For some thin-walled members, in order to reduce the wall warpage and the distortion of the member section, honeycomb stiffeners are better. It can not only improve the rigidity of components, but also reduce the shrinkage stress during casting.

(3) Improve the local stiffness of components

The connecting parts of the guide rail and the supporting parts of cnc machine tools are often the parts with weak local stiffness zui, but the connection mode has a great impact on the local stiffness. Several types of connection between the guide rail and the bed are given. If the guide rail is wide, the double wall connection type shall be used. If the guide rail is narrow, the single wall connection or thickened single wall connection can be used, or the vertical ribs can be added on the single wall to improve the local stiffness.

(4) Components of welded structure

The machine bed, column and other supporting parts of the CNC machine tool are welded by steel plate and section steel, which has the remarkable advantage of reducing the quality and improving the stiffness. The elastic modulus of steel is about twice that of cast iron. On the premise of the same shape and overall dimension, if the rigidity of weldment and casting is required to be the same, the wall thickness of weldment only needs half of the casting; If the local stiffness is required to be the same, the local stiffness is proportional to the cubic of the wall thickness, so the wall thickness of the weldment only needs about 80% of the wall thickness of the casting. In addition, whether the stiffness is the same to reduce the mass, or the mass is the same to improve the stiffness, it can increase the resonant frequency of the component, so that resonance is not easy to occur. It is possible to make the component into a fully enclosed box structure by welding steel plates, which is conducive to improving the stiffness of the component.