Types and characteristics of cutting vibration There are two main types of vibration in machining: forced vibration and self-excited vibration (chatter). (1) Forced vibration: vibration excited by external periodic disturbing force (internal or external vibration source of process system). Its main characteristics are: 1) The frequency of forced vibration is the same as that of external periodic interference force, or an integer multiple of it. 2) In addition to the forced vibration caused by the inhomogeneity of the cutting process itself, the interference force is generally related to the cutting process. The disturbing force is eliminated and the vibration is forced to stop. 3) The amplitude of forced vibration is related to the amplitude of interference force, the stiffness and damping of the process system. When the frequency of interference force is constant, the greater the amplitude of interference force, the smaller the stiffness and damping of process system, and the greater the amplitude of forced vibration. 4) When the ratio of the frequency of the interference force to a certain natural frequency of the process system is equal to or close to 1. The system will produce * * * vibration with the largest amplitude. (2) In the process of self-excited vibration (chatter), in the absence of periodic external force (relative to the cutting process), the excitation feedback inside the system produces periodic vibration. Its main characteristics: 1) The frequency of self-excited vibration is equal to or close to the natural frequency of the system. 2) Whether the self-excited vibration can be generated and its amplitude depends on the comparison between the energy obtained by the system in each vibration period and the energy consumed by the system damping. 3) Because the disturbing force for maintaining self-excited vibration is excited by the vibration (cutting) process itself, once the vibration (cutting) stops, the disturbing force and the energy supplement process disappear immediately. Vibration Source of Forced Vibration and Diagnosis (1) Vibration Source of Forced Vibration 1) The periodically changing centrifugal force is caused by the imbalance of rotating parts on the machine tool, such as motor, grinding wheel, pulley, chuck and workpiece. 2) The inhomogeneity of the cutting process itself, such as the periodically changing cutting force caused by milling, drawing and turning the surface of the workpiece with keyway. 3) Periodically changing transmission force caused by defects of machine tool transmission components, such as inaccurate gear manufacturing or poor installation, uneven thickness of V-belt, flat belt joint, size and shape errors of bearing rollers, pressure pulsation of working fluid of hydraulic pump 4, etc. 4) Inertia impact generated when the moving direction of reciprocating parts changes. 5) Interference force from other external vibration sources. (2) Diagnosis of forced vibration 1) Pick up vibration on site and do spectrum analysis. Under the condition of on-site machining, the vibration signals in the machining process are picked up by sensors (accelerometers, force sensors, etc.). ) along the vibration sensitive direction near the processing site, carry out frequency spectrum analysis and draw frequency spectrum diagram. There are many obvious peaks in the frequency spectrum, so there are many vibration frequency components in the machining system. 2) Do environmental tests, find external vibration sources, pick up vibration signals in shutdown state and analyze the frequency spectrum. At this time, the obtained vibration frequency components are all frequency components of external interference force sources. Comparing these frequency components with the vibration frequency components of field machining, if they are completely the same, it can be judged that the vibration generated in mechanical machining belongs to forced vibration and the vibration source is outside the machine. If the main vibration frequency component of field processing is not consistent with the frequency of external interference force, it is necessary to continue the idle running experiment. 3) Run the test for a short time to find the vibration source in the machine. The machine tool idles according to the motion parameters used in the machining site, picks up vibration signals and carries out frequency spectrum analysis. Comparing the spectrum obtained by no-load test with that obtained by field processing, it can be concluded that the vibration generated by field processing belongs to pre-vibration, and the interference force source also exists in the machine, but the frequency component of the external interference force source has been identified. If there is a frequency component (integer multiple) different from the spectral line component of machine tool idling in field processing, it can be judged that there is self-excited vibration besides forced vibration. If the interference force source is inside the machine tool, also find the specific location. The specific position of the vibration source can be found by driving the moving parts of the machine tool independently and carrying out the idle running test, or for all the moving parts that may become the vibration source, the interference frequency of each moving part can be calculated according to the motion parameters, and the specific position of the vibration source in the machine can be determined by comparing it with the frequency spectrum of the idle running test of the machine tool.