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Under certain controlled conditions it is now possible to machine brittle materials
such as glasses and ceramics using single or multi-point diamond tools (grinding),
so that material is removed by plastic flow, leaving crack-free surfaces. This
process is called 'shear' or 'ductile' mode grinding.
It represents a major breakthrough in modern manufacturing engineering since it
promises to enable:
- complex optical components, both transmission and reflecting to be generated
by advanced CNC machines with very little (or even zero) subsequent polishing.
- complex shaped components such as turbine blades, nozzle guide vanes, etc. to
be finish machined after near net shape forming, to high precision in advanced
ceramics such as silicon nitride, without inducing micro-cracking and thus
lowering ultimate rupture strength and fatigue life.
Ductile mode "damage free" grinding occurs when the volume of materials stressed by
each grit of the grinding wheel is small enough to yield rather than exhibit
brittle fracture, i.e. cracking. In practice, this means maintaining the
undeformed chip thickness to below the ductile-brittle transition value; this
varies from material to material but is generally in the order of 0.1 pm or 100 nm,
(hence the term "nanogrinding" is sometimes used) . Thus the critical factors for
operating successfully in the ductile regime are machine system accuracy and
dynamic stiffness between each grit and the workpiece. In detail this means:
(i) High precision 'truing' of the diamond grits, together with dressing of
the wheel bond to ensure adequate ' openness';
(ii) Design and build of the grinding wheel spindle with very high dynamic
stiffness; error motions, radial and axial, must be considerably less
than 100 nfl.
(iii) Design and build of the workpiece carriage motion system with very high
dynamic stiffness; error motions, linear or rotary, must be well within
100 nm.
(iv) Smooth, rumble-free, high-stiffness servo-drives controlling the motions
which form the chip.
In general, and as a rule of thumb, a machine "loop-stiffness" (between tool and
workpiece) not less than 300 N,4im (static) is necessary.
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