Design and Development of Sonotrode for Ultrasonic Drilling

The design of tooling for ultrasonic machine is critical for every application. For the proper concentration of energy across the workpiece during ultrasonic machining, geometry of sonotrode plays important role. The entire ultrasonic assembly has to be tuned to the same frequency for transfer of maximum energy. If the natural or resonant frequency of the components in the ultrasonic assembly are different, it would set up vibrations of different frequencies leading to energy loss and even damage to certain components by fatigue at the interface surfaces. The issues involved in sonotrode design are material, shape, frequency, resonant length and gain. Proper design of sonotrode is essential to bridge the gap between the booster and the work surface and amplify the vibration amplitude to a value required for successful machining. The separate design of tool which can be assembled at the end of the sonotrode leads to acoustic coupling problems at that interface and separate design of sonotrode and tool and individual testing for successful tuning on the machine. It increases the possibility of failure also. For these reasons, the integrated design of sonotrode with tool being formed at the bottom end of sonotrode is preferred. The tool edge is so designed that only the periphery of the hole is to be cut to obtain through drilling. This reduces the amount of material to be removed and the machining time. The manual design procedure would require solving a second order partial differential equation with the boundary conditions applicable to the machine and the gain required. This paper discusses design of conical, exponential and step-cylindrical horn for ultrasonic machining. First the dimensions are obtained through commercial sonotrode design software CARD (Computer Aided Resonator Design). The software uses the finite difference theory for discretising the sonotrode along the length and iteratively finds the length for resonance condition with gain close to the required amplification provided by the user. It also gives the nodal point, anti-nodal point, stress variation and amplitude variation along the axis of the sonotrode over the entire length. End diameter and resonant length calculated from CARD are supplied to ANSYS to perform modal analysis. A good agreement is observed between the results obtained using CARD software and ANSYS. Based on this design, actual horn of the conical design is manufactured and tested on the machine for resonant frequency and the results are found to be close to those observed from CARD and ANSYS both.