This research aimed to develop a coherence function model to describe the behaviour of coherence with tool wear. A cutting tool in turning, typically mounted as a cantilever, is used to explain the model. The dynamic forces that occur during cutting can be resolved into three mutually perpendicular components along the radial, tangential and feed directions. The theory postulated that the degree of correlation between the dynamic tangential and feed vibration components, measured as acceleration, was inversely related to the rate of tool wear. When the wear rate was high, the correlation was low. Three sets of machining tests were conducted corresponding to the roughing, semi-roughing and finishing conditions. It was observed that for roughing and semi-roughing conditions, in the frequency range of 2.5-5.5 kHz, the above-mentioned prediction of coherence turned out to be valid; for the finishing condition, there was a greater discrepancy.
