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Titanium milling strategies.

Turner, Sam (2009) Titanium milling strategies. PhD thesis, University of Sheffield.

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Abstract

This thesis explores the subject of titanium milling and identifies the need for development of titanium milling strategies to address the key process limitations of chatter and tool wear. These subjects are typically studied in isolation and little work has previously been undertaken on titanium milling dynamics. Titanium is often perceived as difficult to machine as the very properties such as high strength at high temperature and low thermal conductivity that make it an attractive engineering material can cause rapid tool wear and limit process parameters. Titanium alloys are increasingly popular within the aerospace industry due to the high strength to weight ratios and titanium and carbon fibre composites have replaced many steel and aluminium components within aerostructures. Titanium is still seen by many as expensive to process and there is not the same degree of understanding and process optimisation within the machining industry as there is for aluminium and steel alloys. The literature review considers both advances in titanium tool wear mechanisms and research into machining dynamics. From the literature review three research hypotheses are developed around the knowledge gaps pertaining to titanium milling stability and process optimisation. The limitations on milling performance and productivity are considered and three areas are identified where the research could be advanced to improve titanium milling productivity through manipulation of parameters and tool geometry, these areas are pocketing strategies, special tooling geometries and process damping. A method for controlling radial immersion for pocketing strategies is developed and it is proven that through control of parameters and toolpaths that tool life and productivity can be optimised and controlled. A study is then undertaken into the performance and modelling of variable helix end mills to explore the hypothesis that the tools will outperform standard and variable pitch cutters and that the performance can be modelled. As part of the validation process an analysis of the linearity of machine tool dynamics is undertaken and it is demonstrated that under speed and load, spindle and machine tool frequency responses can differ from those measured in the static condition. The final part of the research investigates process damping performance and sensitivity to cutting tool geometry and feed rates. A method for evaluating process damping performance is developed and through optimisation of tool geometry and feed per tooth increases in productivity up to 17 fold are demonstrated. A method is then presented for tuning machine tool dynamics to optimise process damping performance and stabilise sub optimum tooling and machine tools. The three core strands of the thesis are brought together and demonstrated in an aerospace case study. Through application of the techniques developed in the thesis a titanium aerostructural component is machined at the same rates as an equivalent steel component and at less than 50% of the planned titanium milling process time.

Item Type: Thesis (PhD)
Academic Units: The University of Sheffield > Faculty of Engineering (Sheffield) > Mechanical Engineering (Sheffield)
Identification Number/EthosID: uk.bl.ethos.681737
Depositing User: EThOS Import Sheffield
Date Deposited: 09 Jan 2017 10:41
Last Modified: 09 Jan 2017 10:41
URI: http://etheses.whiterose.ac.uk/id/eprint/15127

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