Tap testing, also known as modal testing, is an experimental method that is used to excite the machine-tool system in order to extract its harmonic information such as natural frequencies, modal masses, modal damping ratios and mode shapes. This is normally done in static conditions, using the impact hammer as the excitation mechanism, and an accelerometer as the sensor. In theory, the tool tip should be given a perfect impulse which excites a range of frequencies with a constant amplitude in an infinitely short duration. This allows us to obtain a clean frequency response function (FRF) over the full frequency range of interest. In real life, however, this is not possible. Sometimes, the FRF becomes noisy at higher frequencies, which is an indication that the hammer tip may be too soft, and the impulse is not strong enough to excite the higher frequency range. If the FRF is noisy at lower frequencies it means the hammer tip is too hard and that all modes are excited far beyond the frequency range of interest. Fortunately, we can alter the hammer mass and hammer stiffness by using different sized hammer and using different hammer tip stiffness respectively, to control contact time and impact force. The combination is skillfully chosen to excite the frequency range of interest.
The FRF is then used to identify the major modes of vibration, which is significant in developing the stability lobe diagram (SLD), and its accuracy depends critically on the accuracy of the FRF. The SLD chart describes the machine-tool dynamic behavior for different rotational speed and depth of cuts, whether it is cutting stably or not. Therefore, it helps determine the best rotational speed and axial depth of cut for stable cutting. This information can allow us to increase efficiency in our machining processes.
State of the Art/Maturity
A lot of work has been done over the last decade. The TAP test has been used by academic and industrial partners to determine the stability profile of the machine-tool system. This allows operators to choose the most efficient cutting parameters and it allows researchers to investigate speed dependent and independent factors that influence machining stability. Additionally, work has been done on streamlining the method to make it more feasible for industrial use. More recently, academics have been finding ways to accurately collect the machine-tool harmonic behavior at high rotational speed because the dynamics will evolve as a result of the increased speed. Therefore, the stationary harmonic information of the machine-tool system will not be reliable to predict cutting stability at higher machining speeds.
Practical applications for machining
- Main use is to ensure chatter-free machining by using cutting parameters that exhibit stable cutting. This will improve production efficiency and increase tool life.
- Development of chatter detection and avoidance system (ChatterPro and Harmonizer)
- Implementation of stability prediction for high speed machining