Most diamond and
cBN abrasive is
used in so
called 'bonded
tools'. However
much lapping,
polishing and
fine finishing,
particularly
with
Micron+
products, is
carried out
using the
abrasive as a
loose powder or
in a slurry when
mixed with an
oil or other
carrier medium.
Diamond and cBN
abrasives are
normally used in
particle sizes
of approximately
1mm down to less
than 0.1 micron,
and their sizing
is subject to
international
standards.
In a bonded
tool, the
abrasive
particles are
held in a bond
or matrix
material,
usually
distributed
randomly but in
a controlled
concentration.
The main
functions of the
bond are:-
-
to hold the
abrasive
particles
-
to form a
moulded
profile of
the required
shape which
is usually
attached to
a hub or
carrier;
-
to release
worn
abrasive
particles as
they reach
the end of
their useful
lives, thus
exposing new
ones.
It is therefore
a general
requirement of a
bonded tool that
the abrasive and
the bond
material wear at
similar rates.
The most common
types of bond
used are:
-
Resin,
normally
thermosetting
phenolic or
polyimide
resins.
-
Vitrified;
based on
glass frits.
-
Metal;
normally
sintered
alloys.
-
Electroplated;
a single
layer or a
few layers
of abrasive
attached to
a hub by an
electro-deposited
metal layer,
normally
nickel.
In the case of
the first three
bond types, it
is very
important to
match the size,
impact strength
and fracture
mode
characteristics
of the abrasive
to the
properties of
the bond if
optimum
performance is
to be obtained.
Workpiece
material plays a
major role in
this selection
process, but the
type of
machining
operation, the
machining
conditions used
and the primary
requirement of
the operation
are also
important
considerations.
Maximum tool
life, high rates
of material
removal or high
quality of
surface finish
are common
requirements but
normally a
compromise has
to be made
between one or
more. In the
case of the
fourth bond type
- electroplated
or E.P. -
abrasive
particles are
embedded in a
metal layer to
something less
than 50% of
their total
dimension.
