Definition Of Rigidity Of CNC Machine Tool

- May 17, 2019-

CNC machine rigidity refers to the machine tool in the processing of products produced by vibration, high vibration of the rigidity of small, small vibration of the rigidity of large.Machine bed, transmission system can directly affect the rigidity of the whole machine tool.Different rigid machine tools in the processing of products on the selection and loss of the tool is very large, but also directly affects the final production efficiency and production cost.


 The rigidity of machine tool is reflected in two aspects: static rigidity and dynamic rigidity.


I. Static rigidity refers to the machine tool in the cutting process caused by the deformation of the machine tool itself.

     Take a concrete example: cutting force X direction 1000N, Y direction 500N, Z direction 200N.XYZ direction screw system stiffness K=100,100,80. Cutting the three-axis deformation:δx=1000/100=10μm=0.01mm,  δy=0.005mm,  δz=0.0025mm

     The K value mentioned in the example is a value to measure the rigidity of the machine tool.Generally, the K value of the machine tool screw system is between 50 and 200.

In addition, there are guide rail deformation, deformation of the bed and so on.Static rigidity is composed of many factors, but the most important link is the three-axis screw system of static rigidity (lathe for 2 axis).

II. Dynamic rigidity is the most important index to measure the rigidity of a machine tool.

Generally speaking, the machine tool dynamic rigidity refers to the machine tool resistance to forced vibration capacity, the term is called the size of the natural frequency.   

The cutting tool rotates at high speed when cutting, and the vibration is caused by uneven cutting and impurities inside the material.The cutting part is the vibration source of the whole equipment.The screw system, guide rail system and bed of the equipment will produce forced vibration under the influence of vibration source.If the vibration of each part of the machine tool is large, it will have a very adverse impact on the processing, the main impact on the roughness of cutting and tool life, high precision machining is fatal.Dynamic rigidity is affected by many aspects, the most important of which is the dynamic rigidity of the screw guide rail system and the anti-vibration frequency of the bed cast iron. Of course, the spindle rigidity is the most fundamental and the most important link.

 According to experience:

1. The larger the diameter of the screw, the greater the weight of the bed, and the larger the internal diameter of the spindle front-end bearing, the better the dynamic rigidity;

2. General roller bearings or roller linear guide rail than ball type dynamic rigidity, but the speed is affected (roller bearings are generally only used in the lathe);

3. Generally, the sliding guide is more dynamically rigid than the rolling guide, but the sliding speed is affected (the sliding guide speed is generally < 30m/min, and the rolling guide can reach 180m/min). 

In addition, the setting of machine tool debugging parameters has a great impact on the dynamic stiffness performance.Mechanical performance is the foundation, electrical debugging and mechanical performance fit is the key condition to play the mechanical performance.Suppose that the score of mechanical performance is 80, but the number of electrical debugging can only play its 50%, which is worse than the mechanical performance score of 60, electrical debugging play its 90% situation.(electrical parameters do not pay attention to the servo motor caused by feedback vibration).

There is no specific index for dynamic rigidity detection, but the performance of dynamic rigidity of machine tool can be roughly seen through sample cutting.It is mainly detected from three aspects: cutting test of large amount of cutting tool, finishing sample test and cutting test of domed spherical sample.

To sum up, dynamic rigidity is affected by mechanical design, mechanical manufacturing, electrical debugging and other aspects throughout the production of machine tools. It is the embodiment of the comprehensive performance of a CNC equipment.In general, dynamic rigidity is proportional to static rigidity.