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Title: Structural modification for chatter avoidance in high speed milling
Author: Gibbons, Tom
ISNI:       0000 0004 6500 3886
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2017
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High speed machining operations, such as milling, are widely used in many industries including the aerospace sector. Elevated manufacturing costs coupled with ever more complex geometry components have led to the need to cut deeper and faster than ever; the dynamics of the structures involved, however, greatly restrict these boundaries. As speed and depth of cut are increased, self-excited vibrations, known as chatter, can occur due to the dynamic interaction between the tool tip and the workpiece. This has undesirable consequences such as poor surface finish, rapid tool and machine wear, and high noise levels, all of which lead to a reduction in production rates and an increase in production costs. Efforts to reduce and control chatter are therefore of great importance to industrial engineers. Selection and design of appropriate cutting tools, in an attempt to minimize the occurrence of chatter, are well established methods in the manufacturing industry; however, the choice of tool (type, diameter, length) is often restricted by the required operation, and since the spindle is set by the machine itself, the only other variable component in the machining structure is the tool holder. Little research has been carried out on the dynamics of the tool holder, despite it being a much simpler structure. This thesis shows that the geometry of the tool holder has a significant effect on the dynamics at the tool tip (source of chatter). Therefore the overall focus of this work is to show how the geometry of the tool holder may be utilised to control the speed and depth at which chatter occurs. Structural modification theory allows for models of smaller, simpler structures to be combined to predict the dynamics of larger, more complex structures. One of its main advantages is that experimental models may be combined with numerical models, allowing for experimental structures to be optimised numerically. Structural modification theory is applied to the problem of tool holder dynamics and chatter control. Inverse structural modification is used to optimise the tool holder geometry in terms of tool tip dynamics and, in-turn, the onset of chatter. A prototype tuneable tool holder prototype is designed and tested for use with this structural modification model. In addition to the focus on machining, it will be shown that spatial incompleteness is, perhaps, the largest draw-back with structural modification methods. For structural modification to give accurate results, a full spatial model, including rotational degrees of freedom is needed. Since direct measurement of such information requires specialist equipment, often not available to industry, numerical methods such as the finite difference technique have been developed to synthesise rotational data from translational measurements. As with any numerical method the accuracy of the finite difference technique relies on the correct spacing, however, there is currently no method to select an optimum spacing. An error analysis of the finite difference technique with non-exact data is carried out for application to rotational degree of freedom synthesis.
Supervisor: Sims, Neil ; Ozturk, Erdem Sponsor: Not available
Qualification Name: Thesis (Eng.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available