Abstract:
Increased demands for precision components made of brittle materials such as
glasses and advanced ceramics, are such that conventional (free abrasive) grinding
and polishing techniques can no longer meet the requirements of today’s precision
manufacturing engineering. Both fast production rates and high quality surfaces of
complex shapes are required in addition to the spherical or planar surfaces produced
which are most readily produced by conventional free abrasive techniques.
The work investigates the feasibility of using ductile-mode single-point diamond
turning both as an alternative machining technique in its own right and as a model
for certain parameters involved in (rigid-wheel) grinding. Indentation and
ruling/scribing were used to study the underlying material properties, mechanical
stress fields, the ductile-brittle transition and material removal mechanisms. Several
material removal mechanisms were identified and discussed; these were ploughing,
cutting, delamination and brittle fracture. The results of indentation and scribing
experiments show that, with penetration depth of less than a critical value (the
critical cut depth) brittle materials can be machined in a ductile manner and with
chips very similar to those obtained from the classical ductile cutting of metal, save
that, in this case it is at a much smaller scale. The influence of tool shape has shown
to be important in determining the material removal mechanism.
The experiments on single-point diamond turning (facing) machine were carried
out on a highly stiff diamond facing machine. During the present project continuous
machining of a number of materials to Ra values of nanometres order has been
achieved, these include soda-lime glass, fused silica, Zerodur and single crystal
silicon. Ductile crack-free machining has been demostrated at spindle speeds up to
4500 rpm. The technical feasibility of ultra-fine single-point machining of optical,
electronic and ceramic materials has thus been established.
Investigations were undertaken into methods of measuring the nature and extent of
sub-surface damage (SSD) using scanning acoustic microscope (SAM), Rutherford
back-scattering technique (RBS) and X-ray topography. The results of SSD studies
suggested that coarse machining marks could still be detected in the sub-surface
region even though the surface has been subsequently machined to a condition of
no (optically) visible damage.
