Section 13. qxd

MACHINING AND GRINDING OF CERAMICS
The technology of machining and grinding of ceramics, as well as com-
posite materials, has advanced rapidly, resulting in good surface char-
acteristics and product integrity. Ceramics can be machined with
carbide, high-speed steel, or diamond tools, although care should be
exercised because of the brittle nature of ceramics and the resulting pos-
sible surface damage. 
Machinable ceramics
have been developed which
minimize machining problems. Grinding of ceramics is usually done
with diamond wheels.
ADVANCED MACHINING PROCESSES
In addition to the mechanical methods of material removal described
above, there are a number of other important processes which may be
preferred over conventional methods. Among the important factors to
be considered are the hardness of the workpiece material, the shape of
the part, its sensitivity to thermal damage, residual stresses, tolerances,
and economics. Some of these processes produce a heat-affected layer
on the surface; improvements in surface integrity may be obtained by
postprocessing techniques such as polishing or peening. Almost all
machines are now computer-controlled.
Electric-discharge machining (EDM)
is based on the principle of erosion
of metals by spark discharges. Figure 13.4.20 gives a schematic diagram
of this process. The spark is a transient electric discharge through the
space between two charged electrodes, which are the tool and the work-
piece. The discharge occurs when the potential difference between the
tool and the workpiece is large enough to cause a breakdown in the
medium (which is called the 
dielectric fluid
and is usually a hydrocarbon)
and to procure an electrically conductive spark channel. The breakdown
potential is usually established by connecting the two electrodes to the
terminals of a capacitor charged from a power source. The spacing
between the tool and workpiece is critical; therefore, the feed is con-
trolled by servomechanisms. The dielectric fluid has the additional func-
tions of providing a cooling medium and carrying away particles
produced by the electric discharge. The discharge is repeated rapidly, and
each time a minute amount of workpiece material is removed.
The rate of metal removal depends mostly on the average current in
the discharge circuit; it is also a function of the electrode characteris-
tics, the electrical parameters, and the nature of the dielectric fluid. In
practice, this rate is normally varied by changing the number of dis-
charges per second or the energy per discharge. Rates of metal removal
may range from 0.01 to 25 in
3
/h (0.17 to 410 cm
3
/h), depending on sur-
face finish and tolerance requirements. In general, higher rates produce
rougher surfaces. Surface finishes may range from 1,000 
m
in R
q
(25 
m
m) in roughing cuts to less than 25 
m
in (0.6 
m
m) in finishing cuts.
The response of materials to this process depends mostly on their
thermal properties. Thermal capacity and conductivity, latent heats of
melting and vaporization are important. Hardness and strength do not
necessarily have significant effect on metal removal rates. The process
is applicable to all materials which are sufficiently good conductors of
electricity. The tool has great influence on permissible removal rates. It
is usually made of graphite, copper-tungsten, or copper alloys. Tools
have been made by casting, extruding, machining, powder metallurgy,
and other techniques and are made in any desired shape. Tool wear is
an important consideration, and in order to control tolerances and min-
imize cost, the ratio of tool material removed to workpiece material
removed should be low. This ratio varies with different tool and work-
piece material combinations and with operating conditions. Therefore,
a particular tool material may not be best for all workpieces. Tolerances
as low as 0.0001 to 0.0005 in (0.0025 to 0.0127 mm) can be held with
slow metal removal rates. In machining some steels, tool wear can be
minimized by reversing the polarity and using copper tools. This is
known as “no-wear EDM.”
The electric-discharge machining process has numerous applications,
such as machining cavities and dies, cutting small-diameter holes,
blanking parts from sheets, cutting off rods of materials with poor
machinability, and flat or form grinding. It is also applied to sharpening
tools, cutters, and broaches. The process can be used to generate almost
any geometry if a suitable tool can be fabricated and brought into close
proximity to the workpiece.
Thick plates may be cut with