Note: Descriptions are shown in the official language in which they were submitted.
- 1335437
l The invention relates to an electrolytic,
localized surface layer loosening and removing tool for
machining electrically conductive surfaces.
There are two well known methods of machining
workpieces using electrolytic action and these are:
i) Electrochemical Machining (ECM) which is done by
using a cathode having the shape of the part to be
machined. The shape on the cathode is transferred to the
workk piece (anode) by deplating, similar to
electroplating in accordance with Faraday's Laws. An
electrically conductive solution, electrolyte, is pumped
at high pressure between the cathode and the work piece
(anode) an-d prevents the deplated material from plating
out on the cathode, see, for example, Precision
Engineering, April 1988.
ii) Electrochemical Grindinq (ECG). In this process the
cathode is a grinding wheel. As the electric current
flows between the workpiece and the wheel the material
removed by electrolysis is carried off by the abrasives
in the rotating wheel. The wheel is made of a conductive
material into which abrasive particles have been
imbedded, see, for example, Precision Engineering, April
1988.
The above processes have their respective places
in industry. ECM is used extensively in shaping parts
like turbine blades and is considered in general to be a
high volume process. For general cutting and shaping
--1--
133S437
1 more conventional lathes and milling machines are used.
The advent of electrical discharge machining (EDM) has
also resulted in the ECM process being relegated to
specialized work, mainly because of the difference in
capital cost and the time taken to produce cathodes.
Electrochemical grinding (ECG) is also used in
specialized areas, e.g. the grinding of flat surfaces or
cutting formed surfaces with preformed grinding wheels.
It has also been proposed in United States Patent
No. 3,873,512, dated March 25, 1975, R.M. Latanision, to
machine a metallic workpiece with a worktool by passing
an electrolyte in contact with the workpiece; immersing
an auxiliary electrode in said elec~rolyte; providing a
reference electrode; applying an electrical potential to
the workpiece, in relation to the reference electrode,
sufficient to control the mechanical and physical
properties of the workpiece without significantly
chemically deplating metal from the workpiece; and
removing metal from the workpiece principally by the
frictional movement of the worktool in direct contact
with the workpiece.
While the processes described above are useful,
there is a need for a process where the area of the
electrolytic action is very highly localized to the
cutting tool, where, under the control of a computer,
precise removal of material by the cutting tool from an
electrically conductive surface is achieved in a
1335437
1 discrete, particulate form, and the stream of electrolyte
floods the localized area to entrain and wash away
material as it is being removed from the electrically
conductive surface by the cutting tool.
According to the present invention there is
provided a method of electrolytically assisting the
mechanical shaping of a workpiece, comprising:
a) directing a stream of electrolyte to flood a gap
immediately ahead of a cutting tool of electrical
insulating material and between a cathode and a
localized, electrically conductive surface layer of a
workpiece,
b) applying a low voltage, direct c,urrent across the gap
using the localized surface layer of the workpiece as the
anode,
c) moving the cutting tool to remove the localized
surface layer, and wherein the improvement comprises
d) the low voltage direct current is of such a magnitude
and is applied only for a sufficient length of time for
the surface layer ahead of the cutting tool to be
structurally weakened electrolytically for easy removal
by the cutting tool in a discrete, particulate form which
is washed away by entrainment in the electrolyte.
The direct current applied across the gap may be
in the range of about 10 to about 100 amps.
The low voltage, direct current may be applied
across the gap from an electrically conductive tool shank
anode of the cutting tool.
1335437
1 The stream of electrolyte may pass along a bore
in the tool shank and be delivered to the gap by exit
ports in the tool shank.
Further, according to the present invention,
there is provided an apparatus for electrolytically
assisting the mechanical shaping of a workpiece,
comprising:
a) an electrically conductive tool shank forming a
cathode,
b) a cutting tool of electrical insulating material
mounted on an end of the tool shank for, in operation,
relative cutting movement between the cutting tool and a
conductive surface layer to be sha~ed, while a low
voltage, direct current applying gap is maintained ahead
of the cutting tool and between the tool shank and a
localized surface layer of conductive surface layer, and
c) means for directing a localized flow of electrolyte
to flood the gap, whereby, in operation,
d) when the conductive surface layer is energized as an
anode with a low voltage direct current,
e) a stream of electrolyte from the said means floods
the gap,
f) the tool shank forms a cathode,
g) relative cutting movement is caused between the
cutting tool and the energized conductive surface, and
h) localized surface portions of the conductive surface
layer ahead of the cutting tool are progressively
1335~37
1 electrolytically weakened and removed by the cutting tool
in a discrete particulate form, and the material thus
removed in particulate form is washed from the cutting
tool by the stream of electrolyte.
The tool shank may have an electrolyte conveying
bore extending therealong, and exit ports from the bore
for delivering electrolyte to the gap.
The tool shank may be mounted for rotation in a
casing, and brushes may be provided in the casing in
electrically conductive contact with the tool shank for,
in operation, conveying the low voltage, direct current
therefrom.
In the accompanying drawings which illustrate, by
way of example, embodiments of the present invention,
Figure 1 is a schematic view of an apparatus for
electrolytically assisting the mechanical shaping of a
localized surface layer of a workpiece,
Figure 2 is a partly sectioned side view of a
tool shown in figure 1, and
Figures 3 and 4 are sectional side views of two
different tools to that shown in Figures 1 and 2.
In Figures 1 and 2 there is shown an apparatus,
generally designated 1, for electrolytically assisting
the mechanical shaping of an electrically conductive
surface layer 3 of a workpiece 2, comprising:
a) an electrically conductive, in this embodiment copper
alloy, tool shank 4 forming a cathode,
1335437
1 b) a cutting tool 6 (Figure 2) of electrical insulating
material, in this embodiment a reinforced ceramic
material, and mounted on an end portion of the tool shank
4 for, in operation, relative cutting movement between
the cutting tool 6 and the conductive surface layer 3 to
be shaped while a low voltage, direct current applying
gap is maintained ahead of the cutting tool and between
the tool shank 4 and a localized surface layer of the
conductive surface layer 3, in a direction ahead of the
cutting tool and
c) means generally designated 8 for directing a stream
10 of electrolyte to flood the gap, whereby, in
operation,
d) when the conductive surface layer 3 is energized by a
source 12 as an anode with a low voltage d.c. current,
e) the stream of electrolyte 10 from the said means 8
floods the gap,
f) the tool shank 4 forms a cathode for the source 12,
g) relative cutting movement is caused, in this
embodiment by a spindle 14, between the cutting tool 6
and the surface layer 3, and
h) localized surface portions of the conductive surface
layer 3 ahead of the cutting tool 6 are progressively
electrolytically weakened and removed by the cutting tool
6 in a discrete particulate form 16 (Figure 2), and the
material thus removed in particulate form is washed from
the cutting tool 6 by the stream 10 of electrolyte.
133~37
1 The tool shank is mounted for rotation in the
direction A (Figure 1) by the spindle 14 and for
movements in the directions x, y and z.
The cutting tool 6 comprises a number of cutters,
two of which are shown and designated 18 and 20 (Figure
2). The cutters such as 18 and 20, each extend as strips
along and around the end of the tool shank 4, so that the
tool shank 4 is held spaced from the surface layer 3 to
provide a gap for the flow of electrolyte therebetween.
The means for directing a localized flow 10 of
electrolyte comprises an electrolyte tank 22, a pump 24,
a feed pipe 26, a nozzle 28 and a return pipe 30. The
nozzle 28 directs a stream of electrolyte ahead of the
cutting tool 6, in the cutting direction, and into the
gap between the tool shank 4 and the surface layer 3.
The source 12 of d.c. electrical current has the
positive lead 32 electrically connected to the workpiece
2 and the negative lead 34 electrically connected by
brush contacts 36, in a stationary casing 37, to the tool
shank 4.
In operation, with the apparatus arranged as
shown in Figures 1 and 2, electrolyte is pumped from the
tank 22 by the pump 24 and is directed as a stream by the
nozzle 28 ahead of the cutting tool 6, in the cutting
direction, into the gap between the tool shank 4 and the
surface layer 2. The spindle 14 is rotated in the
direction A and the source 12 is used to apply a positive
d.c. current along the lead 32 to the workpiece 2.
1335437
1 The electrolytic action, caused by the length of
time that the d.c. current is applied from the tool shank
4 through the electrolyte to the surface layer 2 and the
magnitude of the d.c. current are such that localized
portions of the surface layer 2 ahead of the cutting tool
6 in the cutting directions x, y and z, are progressively
electrolytically weakened so that as the cutting tool 6
is moved the weakened material is easily removed thereby
in a discrete particulate form. It will be appreciated
that the length of time that the d.c. current is applied
to the surface layer 2 through the electrolyte depends on
the rate of removal of material by the cutting tool 6.
The material removed by the,cutting tool 6 in a
discrete particulate form is swept away from the cutting
tool 6, by the flow of electrolyte, and along the return
pipe 30 to tank 22 where it settles or is filtered and is
periodically removed for disposal.
In tests to verify the present method using the
apparatus similar to that shown in Figure 1, the
workpiece was of tool steel and the material removal
rates were similar to those of ECM, i.e., 2.131 cu.
cm/1000 amp-mins. The cutting tool used was silicon
carbide and the gap between the tool shank and the
workpiece was approximately 0.284 cm with an average
depth of 0.127 cm. The electrolyte was sodium chloride
and was directed as a jet between the cutting tool and
the workpiece at a flow rate of 4.55 L/min. The dc
current used ranged between about 100 to 1,000 amps.
133~37
1 The tests showed that electrolytically weakening
the surface layer facilitated removal of the surface
layer by the cutting tool.
The tests were not intended to optimize the
process but only to show that a surface could be
electrolytically weakened and then removed by a cutting
tool.
Figures 1 and 2 also show a different method of
delivery of the electrolyte by means of a bore 38 along
the tool shank 4 to radially extending outlets 40.
In Figures 3 and 4, similar parts to those shown
in Figures 2 and 3 are designated by the same reference
numerals and the previous description is relied upon to
describe them.
In Figure 3 the electrolyte is fed along a pipe
42 to a plenum 44 having outlets 46 for flooding gaps 48
exposing portions of the tool shank 4 and extending
downwardly between the cutters of the reaming tool 6.
The tool shown in Figure 4 is for use in, for
example, the embodiment shown in Figure 1 where the
electrolyte is supplied by means of the nozzle 28.
In Figure 4, the brush contacts 36 are spring
loaded, by compression springs 50, in the stationary
casing 37 into electrically conductive contact with the
tool shank 4. The tool shank 4 is mounted for rotation
in the casing 37 by bearings 52 and 54. The bearings 52
and 54 are located against each end of a stepped portion
1 335437
1 56 of the tool shank 4 by retaining rings 58 and 60
respectively. The tool shank 4 has a spade drill 62
mounted on an end portion. A flat nose cutter could be
used instead of the spade drill 62. The stationary
casing 37 has an electrically conductive pin 64 mounted
to project therefrom, for, in operation, holding the
casing stationary and conveying the low voltage direct
current from the brushes. The pin 64 will locate in a
stationary part of the machine (not shown), which is
grounded and in which the spindle is mounted for
rotation.
It should be pointed out that with the present
invention:
1) When the invention is used in conjunction with a
multi-axis, computer numerical control (CNC) machining
centre it is particularly suited to the contouring and
sculpturing of materials which are difficult to machine
by traditional methods.
2) The entire apparatus can be fabricated to fit into
any automated tool changing system found on modern CNC
machines. In this way a conventional machine can be
readily converted from traditional to the applicant's
method of electrolytically assisted machining, if so
desired.
3) Since the electrolytic action is simply to
structurally weaken rather than remove material as in
conventional electrochemical machining processes, the
-10-
1335~37
1 system may be used to machine poorly conducting materials
such as certain ceramics.
4) With the electrolyte flowing through the centre of,
say, a tool holder and exiting close to cutting edges
arranged thereon as spade drills, the present invention
may be used to form holes in hard, but conductive
materials. This could include certain conductive
ceramics and glasses.
The following is given for a better understanding
of the present invention. Faraday's Law states that the
mass of metal removed from an electrode is proportional
to the current applied and the time for which it flows.
In the process according to the present invention,
material is not actually removed in accordance with
Faraday's Law but addresses or adjusts the material
condition just prior to reaching the point of material
transfer from the anode to the cathode. The action of
say a ceramic cutting tool is to remove the material in
the electrolytically weakened state. The weakening or
conditioning is highly localized, so the depth of "cut"
is very shallow, however, the material removal rate by
the cutting, or more properly, the wiping or scraping
tool is very high because the conditioning action is
almost instantaneous, so the machine tool can move at a
high surface speed.
