Numerical and experimental modal analysis of machine tool spindle systems

The need for air transportation has increased drastically over the last few decades, to cope with these increasing demands manufacturing companies are trying to improve and speed up their machining processes. High precision in surface finish is required in aerospace industry. To achieve higher production rates, the cycle time (time required for a part or component to be machined) should be reduced. However, chatter often poses a limiting factor on the achievable productivity. One of the major parameters contributing to chatter is the fundamental frequency of the machining system. The system consists of cutting tool, tool-holder, and machine-tool spindle. Impact test is commonly used to determine frequency response function (FRF), which in turn is utilized to acquire the natural frequencies of the system. Impact testing at each stage of machining is impractical, as it will hinder production. Therefore, the study conducted in this report introduces Finite Element Analysis (using ANSYS®) to create an accurate model, which predicts the natural frequencies of the system. A calibrated FEM model of the spindle system, where the bearings are modelled as linear spring elements, is introduced. The spring constants are then varied such that the FEM natural frequencies match the theoretical/experimental ones. This technique is extremely useful as it reduces the downtime of the machine due to impact testing. An experimental setup of the spindle system was designed and fabricated. Impact tests were conducted on the spindle-setup and the results were used to validate the model. The proposed method could be ultimately used to incorporate the bearings degradation/aging effects into the dedicated calibrated FEM model, and to predict the system frequencies in terms of spindle age, i.e., number of in-service hours.