Wei Shun: Rigid Interpretation of CNC Machine Tools

Rigidity of NC machine tools refers to the vibration of machine tools when processing products. Rigidity is high and vibration is small, while rigidity is small and vibration is large. The rigidity of the whole machine tool can be directly affected by the bed and transmission system of the machine tool. Different rigid machine tools have a great influence on tool selection and wear when processing products, and also directly affect the final production efficiency and cost.

The rigidity of machine tools is embodied in two aspects: static rigidity and dynamic rigidity.

1. Static rigidity represents the deformation of the machine tool itself caused by the cutting process of the machine tool.

For example, cutting force is 1000N in X direction, 500N in Y direction and 200N in Z direction. The stiffness K of the XYZ directional screw system is 100,100,80. Then the deformation of the three axes during cutting is as follows: Delta x=1000/100=10 um=0.01mm, Delta y=0.005mm and delta z=0.0025mm.

The K value described in the example is a value to measure the rigidity of machine tools. Usually the K value of machine tool lead screw system is between 50 and 200.

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

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

Generally speaking, the dynamic rigidity of machine tools refers to the ability of machine tools to resist forced vibration, terminologically referred to as the size of the natural frequency.

Tools rotate at high speed during cutting, and vibration is caused by non-uniform or impurities in the material during rotary cutting. The cutting part is the vibration source of the whole equipment. The machine's screw system, guide rail system and the bed will be forced to vibrate under the influence of the vibration source. If the vibration of each part of the machine tool is large, it will have a very negative impact on the processing, mainly affecting the roughness of cutting and tool life, and has a fatal impact on high-precision machining.

Dynamic rigidity is affected by many aspects, among which the most important is the dynamic rigidity of the lead screw guide rail system and the vibration frequency of the bed cast iron castings. Of course, the rigidity of the spindle is the most basic and the most important link.

According to experience:

(1). The bigger the diameter of the lead screw and the bed weight, the bigger the diameter of the front bearing of the spindle, the better the dynamic rigidity.

(2).Generally, the dynamic rigidity of roller bearings or roller linear guides is better than that of ball bearings, but the rotational speed is affected (roller bearings are generally only used in lathes).

(3).3. The dynamic rigidity of sliding guideway is better than that of rolling guideway, but the sliding speed is affected (sliding guideway speed is generally less than 30 m/min, rolling guideway can reach 180 m/min). In addition, the setting of machine tool debugging parameters has a great influence on the dynamic rigidity performance. Mechanical performance is the foundation, and the fit between electrical debugging and mechanical performance is the key condition to exert mechanical performance. Assuming that the score of mechanical performance is 80, but the number of electrical debugging can only play its 50%, which is worse than 60 points of mechanical performance and 90% of electrical debugging. (The unreasonable electrical parameters will cause the feedback tremor of the servo motor).

Dynamic rigidity has no specific detection index, but the performance of dynamic rigidity of machine tools can be roughly seen by cutting samples. Mainly through the large tool cutting test, finishing sample test, dome spherical sample cutting test three aspects to detect.

In summary, dynamic rigidity is affected by mechanical design, mechanical manufacturing, electrical debugging and other factors throughout all aspects of machine tool production. It is the embodiment of the comprehensive performance of a CNC equipment. Generally speaking, the dynamic rigidity is proportional to the static rigidity.