An investigation into the design for vibration damping of extended length tool holders
This thesis presents a theoretical and experimental investigation into the design of extended length tool holders, with specific emphasis on vibration damping and the attenuation of chatter in boring bars. The theoretical strategy was to evaluate the general mechanics of vibration characteristics, as applied to metal cutting operations. This was used to provide an insight into possible control parameters, and demonstrate a practical approach to the design and optimization of the boring bar structure. Consideration of vibration control parameters and its interaction with functional specifications of the tool resulted in a modified design of the tool holder. The design aspects were confined to passive damping, to enable its application for practical use in industry. Passive damping can be separated into two areas: Material specification and system configuration. Both have been exploited here through the development of a new material. The theoretical design approaches were further examined through metallurgical consideration. From this the practical aspects of material development were confined to improving equivalent stiffness through alloying elements and processing techniques. Research into developing a Titanium Carbide (TiC) composite is detailed, involving powder metallurgy under controlled processing conditions. The experimented results indicate a 47.39% reduction in density, combined with 27.14% improvement on its modulus of elasticity leading to an increase in equivalent stiffness up to 84.59% compared to steel. Although the results demonstrated considerable improvements of mechanical properties and substantiate the suitability of such material as a candidate for the bar material, even better properties were obtained through Hot Isostatic Pressing (HIPing) process. A further 13.48% increase in elastic modulus lead to an improvement of 109.58 % on the value of equivalent stiffness. Experimental examination of tools, was confined to simple internal turning operations (boring). This required the design of fixtures for setting up the test rig. The experimental verification of the combination boring bars was undertaken through comparative stability performance, assessed from the attained machining quality under varying machining conditions. A computational verification of the combination boring bars was performed using Finite Element Method. The dynamic compliance of the tool was evaluated in the frequency range relevant to machine tool and cutting processes for the fundamental mode with appropriate boundary conditions. The computational and practical analyses, support the conclusions implicit in the theoretical model, that the combination approach to the design through material development and system configuration offers high performance, practical devices.