Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02676851 2009-08-27
ELECTRO-EROSION EDGE HONING OF CUTTING TOOLS
FIELD OF THE INVENTION
[001] The invention relates to a process for honing the edge of a cutting tool
using a
series of controlled and rapid spark discharges.
BACKGROUND
[002] Industrial tools for cutting and shaping materials are fabricated from
hard
materials in order to maintain their edges and to withstand the concentrated
stresses
that are present at the cutting edge. These tools are frequently fabricated
from
materials including high speed steel (HSS), cemented carbide, ceramic,
polycrystalline diamond (PCD), polycrystalline cubic boron nitride (PCBN) or
similar
ultra-hard materials.
[003] Investigations have found that imparting a small radius on the cutting
edge of an
industrial tool on the order of several micrometers has various advantages. A
tool
with an edge appropriately prepared is typically less susceptible to
catastrophic
chipping during machining, which leads to a manyfold improvement in tool life.
In
addition, edge preparation improves the overall surface quality of the
machined parts.
The process of imparting a very small radius on the cutting edge of a cutting
tool is
known as edge honing. Due to the aforementioned advantages, edge honing has
become a critical element in the manufacture and performance of industrial
cutting
tools.
[0041A variety of different devices and methods have been developed to hone
the
edges of cutting tools. US Patent No. 5,178,645 to Nakamura, et al. discloses
a
method for honing the edge of a PCD cutting tool by applying a YAG laser to
the
cutting edge of the tool. The tool is inclined with respect to the laser beam
and moved
to hone the cutting edge. Alternatively, the laser beam is adjusted while the
tool is in a
fixed position. The laser beam processing parameters are pre-selected such
that a
radius forms along the exposed portion of the cutting edge.
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[005] The method of US Patent No. 5,709,587 to Shaffer consists of directing a
pressurized fluid stream that is comprised of an abrasive grit entrained in a
fluid. The
fluid stream is directed against the sharp cutting edge of an elongated rotary
tool for a
pre-selected time to transform the sharp cutting edge into a relatively honed
edge.
[006]The apparatus of US Patent No. 6,287,177 to Shaffer comprises a base with
a
rotating brush with abrasive bristles mounted to a variable speed motor. A
mount with
a fixture for holding the cutting tool is attached to the base. The mount has
a
translational mechanism for controlling the position of the edge of the
cutting tool
relative to the rotating brush. The edge of the cutting tool is honed by
controlling the
movement and position of the cutting tool through the volume of the abrasive
bristles,
which results in the formation of a tapered edge.
[007] The method of US Patent No. 7,063,594 to Engin, et al. consists of
immersing
the cutting edges of a cutting tool in an abrasive liquid bath. The bath
contains an
abrasive granular media, which is circulated through the bath such that the
cutting
edges are disposed within the flow path of the abrasive media. Alternatively,
the
cutting edges of a cutting tool are immersed in an abrasive liquid bath and
the cutting
tool is rotated. The abrasive media is comprised of very small abrasive
granules such
that a radius along the full length of the cutting edges forms after prolonged
exposure.
[008] It is very difficult with the foregoing approaches to achieve a
consistent and
repeatable radius along the full length of the cutting edge. Variation in the
edge radius
affects cutting performance in terms of efficient chip formation, which has
adverse
implications in terms of tool life and the quality of the machined surface.
[009] In addition to problems with generating consistent hone radii, these
methods are
also expensive, and are limited when applied to ultra-hard tool materials like
such as
PCD or PCBN, on account of their extreme hardness. The demand for such tools
is on
the rise, and hence the need for better honing methods.
[0010] Accordingly, an improved process for honing the edges of cutting tools
with
consistent and repeatable results is highly desired.
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SUMMARY OF THE INVENTION
[0011] The present invention is a process of preparing the cutting edge of a
cutting
tool having a rake face and a clearance face, with a cutting edge at least
partially
disposed therebetween. The process comprises the melting and vaporization of
material from the cutting edge by providing a series of rapidly recurring
electrical
spark discharges in a gap located between the tool edge and a counterface. The
spark
discharges vaporize and melt the tool edge to form the desired radius.
[0012] In some embodiments, the gap can range from approximately 2 m to
approximately 100 gm.
[0013] The spark discharges form a radius on the cutting edge from 2 m to a
pre-
determined threshold which includes a radius of infinity that corresponds to a
plane
surface, known as a chamfer.
[0014] In some embodiments, the process comprises flushing of the discharge
gap by
a gaseous or liquid dielectric fluid. The fluid may contain abrasive or
metallic
particle additives.
[0015] In some embodiments, the removal of discharge material is enhanced by
the
use of an electrolyte or a dielectric.
[0016] In some embodiments, the cutting tool is positioned in various
orientations
relative the tool axis for symmetric or asymmetric edge preparation.
[0017] In some embodiments, including when preparing the edges of a complex
tool,
the tool is moved relative to a fixed counterface in two or more dimensions.
In other
embodiments, the counterface is moved relative to the fixed tool in two or
more
dimensions. In still other embodiments, both the tool and counterface are
moved
relative each other in two or more dimensions.
[0018] The counterface is a metallic or electrically conducting material such
as
aluminium or graphite.
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BRIEF DESCRIPTION OF THE FIGURES
[0019] Fig. I depicts a side view of a cutting tool disposed a predetermined
distance
from a counterface which is submerged in a tank containing a dielectric fluid.
[0020] Fig. 2 is a flow diagram illustrating a process for honing the edge of
a cutting
tool according to the present invention.
[0021] Fig. 3a shows the cutting tool and counterface of Fig. 1 and a spark
that
occurred following the application of a pulse voltage applied between the
cutting tool
and the counterface.
[0022] Fig. 3b shows the cutting tool and counterface of Fig. 3a with material
removed due to the heating associated with the spark.
[0023] Figs. 3c to 3g shows the cutting tool and counterface of Fig. 3a with
additional
material removed due to the heating associated with additional sparks.
[0024] Fig. 4a shows a side view of a cutting tool obliquely orientated
relative to a
counterface with an asymmetric radius formed on the cutting edge.
[0025] Fig. 4b shows a side view of a cutting tool with a flat chamfer edge
formed on
the cutting edge.
DETAILED DESCRIPTION OF THE INVENTION
[0026] The present invention is now described with reference to the drawings.
In the
following description, for purposes of explanation, numerous specific details
are set
forth in order to provide a thorough understanding of the present invention.
It may be
evident, however that the present invention may be practiced without these
specific
details.
[0027] Referring now to Fig. 1, a cutting tool 10 is disposed a predetermined
distance
from a counterface 20. Cutting tool 10 has a rake face 11, clearance face 12
and edge
portion 15, and is fabricated from an electrically conductive material.
Counterface 20
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is comprised of a pre-selected electrically conductive material such as
copper,
aluminium, graphite or steel and has a relatively flat, planar surface 22.
Counterface
20 is submerged in a tank 25 containing a dielectric fluid 30. Dielectric
fluid 30 is re-
circulated through a filter 35 attached to tank 25.
[0028] Referring now to Fig. 2, a flow diagram 200 of a process for honing the
edges
of a cutting tool using controlled and rapid spark discharges is shown in
accordance
with the present invention. At 201, cutting tool 10 and counterface 20 are
provided.
Counterface 20 is submerged in tank 25 containing dielectric fluid 30, and
edge
portion 15 is spatially disposed at a predetermined distance and angle from
surface 22
of counterface 20 at 202. Typically, the distance between edge portion 15 and
surface
22 can range from approximately 2 m to approximately 100 m.
[0029] At 203, a pulse voltage having pre-determined electrical parameters is
applied
between cutting tool 10 and counterface 20, which function as the electrodes.
After
the electrical resistance of dielectric fluid 30 has been overwhelmed, a spark
40
occurs at the closest point between edge portion 15 and counterface 20, as
shown in
Fig. 3a. The temperature of the plasma channel associated with spark 40
results in the
removal of material from both edge portion 15 and counterface 20 as shown in
Fig.
3b. The resulting particles formed from the removed material are flushed away
by
dielectric fluid 30 and are filtered from dielectric fluid 30 by filter 35. A
person of
skill in the art will appreciate that a gaseous dielectric may substituted for
dielectric
fluid 30.
[0030] Additional pulse voltages are applied between cutting tool 10 and
counterface
20 until a pre-determined threshold of machining time has been reached at 204.
As
material is continually removed, the gap between the tool and the counterface
increases and, if a consistent gap is required in order to achieve the desired
results, the
cutting tool 10 may be fed towards the counterface 20 to compensate for the
increase
in gap or vice versa at 205. Alternatively, both cutting tool 10 and
counterface 20 are
moved simultaneously.
[0031] Since first electrical discharge or spark 40 will take the path of
least electrical
resistance between edge portion 15 and counterface 20, the heat associated
with each
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successive spark, 40b, 40c, 40d, 40e, 40f, 40g will melt and/or vaporize and
remove a
small amount of material from each of cutting tool 10 and counterface 20 as
shown in
Figs. 3b to 3g. After a pre-determined number of pulse voltages have been
applied
between cutting tool 10 and counterface 20, a uniform radius will form along
the
entire length of edge portion 15. Optionally, material removal may be aided
electrolytically by the use of an electrolyte (not shown).
[0032] In some embodiments, cutting tool 10 is oriented perpendicularly to
counterface 20 to form a symmetric uniform radius. Optionally, cutting tool 10
is
obliquely orientated relative to counterface 20 to form an asymmetric radius
such that
more material may be removed from either the rake face 11 or clearance face,
as
shown in Fig. 4a.
[0033] The process may also be applied to complex tools by moving the tool
relative
to the counterface in two or more dimensions.
[0034] The formation of a uniform radius along the entire length of edge
portion 15 is
influenced by the ratio of the amount of material removed from edge portion 15
and
counterface 20. The ratio of material removed, known as the wear ratio, is
affected by
several parameters, including but not limited to the choice of polarity, the
electrical
parameters, the spatial position of edge portion 15 relative to counterface
20, and the
choice of materials for the cutting tool 10 and counterface 20.
[0035] If the wear ratio is set too low, edge portion 15 will machine directly
into
counterface 20 with minimal material removal from edge portion 15. If the wear
ratio
is too high, a flat chamfer edge will form on edge portion 15, and minimal
material
will be removed from counterface 20, as shown in Fig. 4b. In some embodiments,
a
chamfer edge is desirable. Where the desired radius of the cutting edge is
extremely
high a chamfer edge will form.
[0036] Several experiments were conducted using a die-sink electro-discharge
machine to determine the influence of the various parameters on the wear
ratio. Other
apparatus which employ the process of the present invention are contemplated.
The
results of these experiments are summarized below.
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Experiment I
[0037] Experiment I was conducted using a copper counterface, a HSS AISI T15
cutting insert, a voltage of 180 V, current of 2.4 A, and an on-time and off-
time of 3.7
gs for 60 seconds. Copper was chosen as the counterface material since it is a
common electrode material, and copper electrodes experience minimal wear when
the
electrode polarity is set to negative.
[0038] It was observed in Experiment I that when the polarity of the copper
counterface was set to negative, this resulted in the formation of a flat or
chamfer
edge on the cutting insert, as opposed to a radius.
[0039] When the polarity of the copper counterface was set to positive, a
significant
amount of material was removed from the copper counterface and considerably
less
material was removed from the cutting insert. A proper radius did not form on
the
cutting insert as a result. Based upon the observed results, the wear ratio
for a copper
counterface and a HSS cutting insert was not within the desired range for
forming an
ideal radius on the cutting insert.
Experiment 11
[0040] Experiment II was conducted using a steel counterface, a HSS AISI T15
cutting insert, a voltage of 180 V, current of 2.4 A, and an on-time and off-
time of 3.7
s for 60 seconds. Steel was chosen as the counterface material since the
material
removal rate is higher when set to positive polarity.
[0041] It was observed in Experiment II that when the polarity of the steel
counterface was set to negative, this resulted in the formation of a flat edge
on the
cutting insert, although the flattening was less severe compared to Experiment
I.
Based upon the observed results, the wear ratio for a steel counterface and a
HSS
cutting insert was not within the desired range for forming an ideal radius on
the
cutting insert.
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Experiment III
[0042] Experiment III was conducted using an aluminium counterface, a HSS AISI
T15 cutting insert, a voltage of 180 V, current of 2.4 A, and an on-time and
off-time
of 3.7 gs for 60 seconds.
[0043] It was observed in Experiment III that an ideal radius formed on the
cutting
insert which increased linearly with increased machining time. Although the
wear
ratio was found to be in the ideal range, the surface finish on the edge of
the cutting
insert was rough.
Experiment IV
[0044] Experiment IV was conducted using an aluminium counterface, carbide and
HSS cutting inserts were machined using identical EDM machine parameters of
100
V, current of 1.8 A, and an on-time and off-time of 0.6 .is.
[0045] It was observed that the amount of material removed per spark is highly
influenced by the spark energy. As such, a larger radius was generated in the
same
period of machining time by increasing the energy. In addition, the cutting
edge
radius developed faster on the carbide cutting insert compared to the HSS
cutting
insert. It was also observed that when the discharge current was increased,
the rate of
the cutting edge radius formation did not increase linearly with the
increasing current.
[0046] Unlike the result of Experiment III, the surface finish on the edge of
the
cutting insert was considerably smoother, and the finish on the carbide
cutting insert
was smoother than the finish on the HSS insert.
Experiment V
[0047] Experiment V was conducted to determine the reproducibility of the
present
invention. Six cutting inserts fabricated from HSS and carbide were machined
using
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the parameters from Experiment IV, for a machining time of 80 seconds. From
the
results of Experiment V the radius along the cutting edge of the HSS cutting
inserts
varied by a maximum of 2.2 m and the highest standard deviation was 1.5 gm.
The
variability along the cutting edge of the carbide cutting inserts was larger
compared to
the HSS results, however the mean radius was 50.2 m as opposed to 50.0 gm for
the
HSS inserts.
[0048] Based upon the results of Experiment V, The maximum deviation along the
edge of a prepared HSS insert was 3.8 % of the desired radius compared to 35 %
for
commonly used methods such as abrasive brush edge honing methods.
[0049] The reproducibility of the process according to the present invention
is
superior to the prior art processes for honing the edge of cutting tools. The
present
invention is readily adaptable for honing cutting tools fabricated from ultra-
hard
materials, since most PCD and PCBN cutting inserts are fabricated with a
metallic
binder, typically cobalt. A worker skilled in the art will appreciate that the
process
parameters include selecting the material for counterface 20, the spatial
parameters of
edge portion 15 relative to counterface 20, the electrical parameters and the
threshold
of machining time.
[0050] Although the foregoing experiments were conducted using a die-sink
electro-
discharge machine, a person of skill in the art will appreciate that the
present
invention is readily adapted for other similar machines, including
electrochemical
discharge machines.
[0051] Given that edge honing typically improves the life expectancy of
cutting tools
several fold, the economic advantages associated with the present invention
are
significant, and will greatly benefit the tool manufacturing industry.
[0052] Although the description above contains many specific details, these
should
not be construed as limiting the scope of the embodiments but as merely
providing
illustrations of some of the presently preferred embodiments. Thus the scope
of the
embodiments should be determined by the appended claims and their legal
equivalents, rather than by the examples given.
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