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|Document Type: ||Thesis or dissertation|
|Title: ||Machining of Optical Surfaces in Brittle Materials using an Ultra-Precision Machine Tool|
|Authors: ||Shore, Paul|
|Supervisors: ||Stephenson, David J.|
|Issue Date: ||Mar-1995|
|Abstract: ||Investigations of machining optical surfaces into brittle materials using an ultra
precision machine tool are presented. The newly developed ultra precision NION
machine is evaluated to gain a good appreciation of its operating performance. The
machining accuracy capability of this machine is established by careful measurement
of its; motion accuracy, thermal and dimensional stability and loop stiffness.
Corroboration of these measurements are provided by assessment of surfaces which
were produced in soft "easily machined" metal materials. It was found that surfaces
smooth to -1 nm Ra could be produced on the NION machine and with a form error
of less than 100 nm P-V. The main source of figure error, approximately 80 nm, was
found to be caused by the synchronous axial error motion of the workhead spindle.
Other elements of the machine, including thermal effects, incurred less than 25 nm of
additional figure error.
Assessment of the diamond turning process for the producing optical surfaces made in
a number of important optical materials, which are ostensibly brittle, were undertaken.
Turning tests were carried out to establish the relative difficulty for machining optical
surfaces in these materials and to define the most important parameters which affect
the attained surface quality. Assessment of the produced surfaces was based on their
roughness quality, surface morphology and residual stress condition. It was found that
diamond tool edge quality degraded with total cut distance. Tool cut distance was
found to be a major influence on achievable material removal rate before micro-
fractures became present at the surface. Surface quality and residual stress condition
were also greatly influenced by the overall tool cut distance.
Diamond grinding trials were also carried out using the NION machine tool. These
grinding trials were carried out using a mode of grinding which permits complex
shape optical surfaces to be produced. Various grinding technologies were employed
to establish the optimum methods. Selected grinding trials were carried out to
establish the dominate parameters affecting the optical quality. Assessment of the
machined surfaces was in regard of their surface roughness, residual stress and
severity of sub-surface micro cracking. It was found that grinding wheel specification
was a major influence on surface quality and sub-surface damage. The level of
residual stress associated with "ductile" mode grinding was not found to prohibit its
application toward the direct manufacture of optical elements. Selection of grinding parameters which ensured the grain depth of cut, GDOC,
allowed glass surfaces to be parameter did not exceed the materials critical depth, d,
ground to 1-2 nm Ra. These ground glass surfaces appeared free of any surface
fractures. Sub-surface assessments
did however reveal small levels of micro-fractures
hidden below the surface.
Discussion of both machining processes is provided. Available material removal rates
for each process is given when cutting a number of important optical materials.
Conclusions regarding the production of both Infrared and visible wavelength optics
using the NION machine tool are provided. Recommendations for future work to
improve both; the understanding of the processes and the effectiveness of applying the
processes are suggested.|
|Appears in Collections:||PhD, EngD and MSc by research theses (School of Applied Sciences)|
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