Design of ultrasonic block horns by finite element models

Ultrasonic block horns are common tools in many high power ultrasonic applications such as welding and cutting. Ideally, block horns operate in a pure axial mode of vibration with uniform vibration amplitude at the working surface. Modal coupling between the tuned mode and close vibration modes is often responsible for serious reliability problems. Even an appropriate slotting technique, which has been previously characterised for amplitude uniformity improvements, cannot guarantee sufficient tuned mode isolation from the nearby modes, and therefore slotting strategies often require some compromise between mode separation and amplitude uniformity. This work investigates the importance of mode shape identification and characterisation for highlighting options for improvements in frequency separation. Modes are analysed and classified using experimental data from 3D laser Doppler vibrometer (LDV) measurements. The use of the 3D LDV allows identification of both in-plane and out-of-plane responses simultaneously and the resulting modal data can be used directly to investigate mode sensitivities to structural modifications. Mode sensitivities to geometric modifications are characterised by finite element (FE) models and modal analysis data, and frequency shift limits are investigated. Strategies for improving block horn performance are proposed based on frequency separation and working surface amplitude uniformity for two different block horns. In particular, the role of additional fine slots in achieving amplitude uniformity improvements is studied with reference to the increased modal activity. Good correlation achieved between FE models and the experimental results, validates the redesign strategies.