ArticleImpact Testing and Its Applications in Machine Tool Spindles
by Ming Xu and Frank A. Findley
This excellent paper lays down the foundation for performing impact testing properly. A large number of illustrations and graphs are used to ensure that the subject material is clearly described. This is a very practical paper that walks the analyst through the steps and information needed to be successful in performing impact testing. While explaining the underlying theory, this remains a practical paper and does not explore the depth of the mathematics needed to develop the tooling. Two case studies are presented at the end of the paper that illustrate impact testing and this is where the paper focuses on machine tool spindles. Even though machine tool spindle testing is the focus of the case studies, the material in this paper would be applicable for all impact testing applications.
The impact testing is an effective way to troubleshoot resonance related problems. Resonance is the condition where the frequency of the disturbing force coincides with the natural frequency of a structure or mechanical system. In rotating machinery, the disturbing force can be induced by unbalance, misalignment or other mechanical faults. Impact testing aids in understanding how forces are transmitted throughout a structure or mechanical system. The natural frequencies of a structure, machine or component are determined by its mass, stiffness and damping characteristics. Thus, the natural frequencies are inherent properties of a mechanical system. Sometimes, the natural frequency is also called resonance frequency. The system response is determined by both the input and the characteristics of the system, as illustrated in Figure 1.
There are basically two types of vibration measurement:
1. Only response output is measured
2. Both input and response output are measured
It is obvious that the system characteristics, such as mistral frequencies of the system, can be determined completely when both the input and response are measured. In the traditional vibration measurement using a single channel FFT analyzer, the resonant frequencies were examined by striking the structure or machine component and measuring system response only. At times, it is difficult to say whether a particular large response level is due to a strong excitation or to a resonance of the system. In the impact testing, both input and response signals are measured simultaneously. The system natural frequencies and other properties can be determined directly from the measurement data.
In the machining industry, the importance of dynamic stiffness is emphasized in recent years because it reflects the machine component’s vibration characteristics under the cutting condition. Dynamic stiffness is the ratio of the force amplitude to the displacement amplitude at a certain vibration frequency. In general, dynamic stiffness is a function of frequency. The vibration problems in a machining system, which includes machine tool structure, workpiece, tooling and fixture, are among the major obstacles to greater productivity and quality of the machined parts. The vibratory motion between a cutting tool and a workpiece is recorded on the workpiece surface. Depending upon the severity of the oscillatory motion and the surface finish specifications, the surface quality of the parts may be unsatisfactory. The high frequency vibrational condition known as chatter may seriously decrease tool life. In addition, chatter also affects the life of bearings and other machine components.  It is necessary to solve a vibration problem if unsatisfactory tool life, surface finish, or noise result.”
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