Titanium milling strategies.

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.