Design of tool holders for ultrasonic machining using FEM

Abstract

There have been many new materials introduced recently which are hard, brittle and difficult to machine. The need for machining these materials has led to the widespread use of several unconventional machining methods, one of which is ultrasonic machining (USM) or ultrasonic impact grinding. USM complements the other non-traditional machining techniques such as electrochemical machining (ECM), electrodischarge machining (EDM), laser-beam machining, electron-beam machining and plasma-arc machining. It has made possible an appreciable simplification of the technology of fabricating profile parts from hard and brittle materials such as graphite, ceramics, quartz, semi-conductors, jewel stones and hard sintered materials. With the advent of USM, it is now possible to make cavities of any profile in such materials. One very important part of the ultrasonic machine is the horn (also known as a concentrator or tool holder). This is a waveguide focussing device with a cross-sectional area which decreases from the input (transducer) end to the output (tool) end. It amplifies the input amplitude of vibration so that at the output end the amplitude is sufficiently large for machining. The purpose of this research is to investigate the design of efficient tool holders or horns for the ultrasonic machining process by employing the finite-element method (FEM) to analyse the pertinent design characteristics of the horn, comparing the results with those measured experimentally using horns fabricated in-house as well as those supplied by an ultrasonic-machine builder. The common practice of using empirical approximations to calculate the resonant length of the horn can be wasteful and time consuming, as the designed horn may not be tuned to the machine, thus rendering the horn less efficient. By using FEM to do a modal analysis to find the natural frequencies of the various modes of the horn, the resonant frequency of the horn can be obtained easily. In this way, the horn design can be varied quite conveniently so that a more exact resonant frequency, which matches that of the machine, can be obtained. FEM also helps to verify and fine-tune the horn designs based on empirical formulae. In addition, dynamic analysis can be carried out to determine the stress characteristics of the horn under operating conditions. © 1993.