Mechanical Engineering - Theses
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ItemAn investigation of some geometrical and performance aspects of twist drills( 1977)In the last few decades considerable attention has been given to the development of sophisticated machine tools capable of carrying out machining operations with improved overall manufacturing efficiency. Despite these advances in machine tools and associated computer systems, the cutting process and its performance remains a vital element of the manufacturing system. There is a need to establish the relationships between the cutting performance parameters such as forces, power and tool life and the cutting variables which include the tool and work material properties and geometry as well as the cutting conditions. These relationships are required to rationally select the cutting variables for improved, and possibly optimum, manufacturing efficiency. In attempts to relate the cutting performance parameters to the relevant cutting variables the empirical approach has commonly been used. This approach has been applied to the geometrically complex practical machining operations due to the lack of fundamental analyses of these operations. From experimental tests the effect of some cutting variables on various performance parameters have been qualitatively studied. Simple empirical equations have sometimes been found for quantitative prediction purposes. Of the many cutting performance parameters only forces and power and to a lesser extent tool life have been widely considered. In addition turning operations have been extensively studied while other popular operations such as milling and drilling have received comparatively little attention. These empirical techniques have long since been recognized to be time consuming, expensive and relevant to the specific operation studied. Hence the available data, though very useful, have often been limited and incomplete for various common machining operations such as drilling. Recently an alternative analytical approach has been attempted for machining operations such as turning and milling. This approach has followed from fundamental studies and analyses of the mechanics of cutting of the geometrically simple single edge orthogonal and oblique cutting process. The analytical approach, which relies more on the theoretical analyses and less on experimental testing, can account for many geometrical and other variables. In a series of investigations at the University of Melbourne the analytical approach has been shown to yield good qualitative and quantitative force predictions for turning, peripheral milling and machining with form tools. While this alternative approach is promising its potential has not been fully explored. In this thesis the important and popular drilling operation will be studied using the analytical approach. An analysis based on the thin shear zone cutting model will be developed to obtain the relationships between the torque and thrust in drilling, and the relevant twist drill geometrical features and other cutting variables. A method for force prediction will be proposed and experimentally tested. The possibility of adopting an analytical approach for temperature and drill life predictions will also be considered and its practical implications and limitations will be discussed.