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Description  |
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BACKGROUND OF THE INVENTION
1 . Field of the Invention
This invention relates to a discharge apparatus and method used for
diesinking and other metal machining applications, and more particularly
to a laser guided discharge apparatus and method used for machining the
surface of a workpiece.
2. Description of the Prior Art
Discharge machining is used to cut and shape metal products. In
conventional discharge machining, the workpiece is disposed opposite the
work electrode and a discharge is induced from the electrode to the
workpiece, with the workpiece being machined by the impact of the
discharge. There is a diesinking discharge machining method and a wire
discharge machining method. In the former method, the machining is carried
out with an electrode having the required shape to which the workpiece is
to be machined pressed onto the workpiece, while in the latter method
cutting is performed using a straight electrode.
The conventional discharge machining methods therefore use a technique in
which the workpiece and electrode are positioned in opposition to each
other and a discharge is produced from the electrode to the workpiece,
whereby the heat of the discharge melts a portion of the workpiece, and
the explosive vaporization of the machining fluid that takes place at the
same time is used to blow away the melted portion. Thus, in both of the
above discharge machining methods the discharge electrode has to be
arranged opposite the part of the workpiece to be machined. Even in the
case of conventional laser-induced discharge machining, the discharge
electrode has to be more or less directly opposite to the portion to be
machined. As such, a discharge machining apparatus has only been able to
be used to machine parts which can be directly seen from the discharge
electrode, meaning that machining the inside of a pot with a small mouth,
for example, has not been possible with such an apparatus. This has made
it desirable to develop discharge machining technology that can be used to
machine parts which are not in a direct line of sight position relative to
the electrode.
An object of the present invention is to provide a discharge machining
apparatus and method that can be used to machine the rear surface of a
workpiece or other such parts that are not in a direct line from the
discharge electrode.
SUMMARY OF THE INVENTION
In accordance with this invention, the above object is attained by a laser
guided discharge machining apparatus comprising a vacuum chamber able to
accommodate a workpiece and a discharge electrode and in which it is
possible to create a reduced pressure condition of 3000 Pa or below, and a
laser irradiation apparatus able to irradiate a part of the rear surface
of the workpiece that is not facing the discharge electrode, and by a
laser guided discharge machining method comprising housing a workpiece and
a discharge electrode in a vacuum chamber, reducing the pressure in the
vacuum chamber to 3000 Pa or below, and projecting a laser beam at a part
of the workpiece that is not in a direct line of sight with the discharge
electrode.
In a reduced pressure environment of 3000 Pa or below, the laser irradiates
a part of the workpiece that is not in a direct line of sight position
relative to the discharge electrode, and when the discharge is triggered
the charged particles discharged from the electrode follow the line of
electrical power around to the rear of the workpiece, and are induced by
the laser beam to cause them to impinge on the workpiece at the point at
which the workpiece is being irradiated by the laser beam, thereby
machining the workpiece at that point. Moving the laser beam results in a
corresponding shift in the location of the machining. In this way, it is
possible to perform three-dimensional machining of parts that are not
directly viewable from the discharge electrode.
Further features of the invention, its nature and various advantages will
be more apparent from the accompanying drawings and following detailed
description of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of the discharge machining apparatus according to
one embodiment of the invention;
FIG. 2 is a block diagram of an experimental discharge machining apparatus
according to the invention;
FIG. 3 is a block diagram of another experimental discharge machining
apparatus according to the invention;
FIG. 4 is a graph showing the relationship between pressure and discharge
induction characteristics.
FIG. 5 is a graph showing the relationship between minimum discharge
voltage for laser guidance and distance between the anode and the hole in
the cathode; and
FIG. 6 is a graph showing the relationship between minimum discharge
voltage for laser guidance and the distance between the hole in the
cathode and the laser beam spot.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Details of an embodiment of the present invention will now be described
with reference to the drawings. In FIG. 1, reference numeral 1 denotes a
discharge machining apparatus which is provided with a vacuum chamber 2, a
discharge apparatus 3, a laser irradiation apparatus 4, a controller 5 and
an electrical power source 6. The pressure in the vacuum chamber 2 can be
reduced to 3000 Pa or below, and the interior of the vacuum chamber 2 can
accommodate a workpiece 7. The discharge apparatus 3 has an electrode 8
and an electrode controller 11. The electrode controller 11 pays out more
of the electrode 8 to compensate for wear to the electrode 8, and controls
the current from the power source 6 to a set value for delivery to the
electrode 8. The electrode controller 11 is controlled by the controller
5. The controller 5 may be constituted by a computer, for example.
The laser irradiation apparatus 4 is comprised of a laser-beam generator 12
forming a laser induction apparatus, which is provided with a laser
controller 13, a mirror 14 and a mirror controller 15. As the laser-beam
generator 12, there may be used a YAG laser, for example, that has a pulse
power of 0.5 J and produces a discharge at 100 to 1,000 volts. The laser
controller 13 adjusts the path of the laser to guide the laser beam 16
inside the vacuum chamber 2; a commercially available scanner may be used
as the laser controller 13. The mirror 14 is arranged inside the vacuum
chamber 2 facing a target region 17 of the workpiece 7 to be machined. The
mirror 14 reflects the laser beam 16 from the laser controller 13 onto the
region 17. The mirror controller 15 controls the angle of the mirror 14 in
accordance with commands received from the controller 5 and is used to
adjust the position of the workpiece 7 that is irradiated by the laser
beam 16.
With the discharge machining apparatus 1 having the above configuration, to
perform discharge machining of a region 17 that is on the other side of a
workpiece 7 and therefore is hidden from the electrode 8, the laser
controller 13 and mirror 14 are adjusted to cause the laser beam emitted
by the laser-beam generator 12 to be reflected by the mirror 14 onto the
region 17. When this takes place, the pressure in the vacuum chamber 2 has
been reduced to 3000 Pa or below. Such a reduced pressure makes it easier
for charged particles generated by the laser irradiation to reach the
electrode 8, so that in accordance with Paschen's law, it becomes possible
for the discharge distance to be increased, enabling the discharge to
reach a distant point such as the inside of a pot. Upon irradiating the
workpiece 7 with the laser beam 16, the charged particles 19 are generated
and spread within the vacuum chamber 2. When the charged particles have
reached the electrode 8, the discharge from the electrode 8 goes around
the edge 18 of the workpiece 7 to the interior thereof to start machining
of the region 17. Machining of a prescribed shape is carried out by moving
the laser beam 16 appropriately by using the mirror controller 15 to
adjust the angle of the mirror 14 as required.
The following experiments were performed to demonstrate that a part of a
workpiece not in a direct line from the discharge electrode can be
discharge machined.
As shown in FIG. 2, an electrode 8a and a workpiece 7a in the form of a
brass plate were arranged facing each other inside a vacuum chamber 2, and
point 17 on the rear side of the workpiece 7a was irradiated by a YAG
laser beam (0.5 J). Laser-beam irradiation and discharge were performed
with the air pressure inside the vacuum chamber at 10 Pa, a workpiece 7a
thickness of 1 mm, a distance between the edge 18a of the workpiece 7a and
point 17 of 50 mm and a distance between the electrode 8a and the surface
of the workpiece 7a of 30 mm. The discharge current was from 30 A to 50 A
and the pulse width was from 10 to 15 .mu.sec. The discharge from the
electrode 8a went around the workpiece edge 18a. reaching to the rear
surface, and was guided to point 17 by the laser beam 16, and point 17 was
thereby machined.
With reference to FIG. 3, an experiment was performed consisting of using a
YAG laser 12 (with a power of 0.45 J and a pulse width of 1 msec) to
irradiate one surface (the surface on the left side, in the drawing) of a
cathode plate 18b as the workpiece, and causing a discharge from the anode
8b disposed at the opposite (right) side to be induced through a hole in
the cathode plate to the point of laser beam irradiation. The anode 8b was
copper wire 1 mm in diameter, 1 mm of which was exposed so that discharge
occurred only at that tip. As the cathode 18b, three brass plates were
prepared, each measuring 100 mm by 150 mm by 1 mm thick and having a hole
25 mm in diameter. The distances l.sub.1 from the center axes of the holes
formed in the three plates to the point of laser-beam irradiation were 25
mm, 45 mm and 75 mm, respectively. Further, the distance between the anode
8b and the cathode plate 18b was denoted by l.sub.2 and the distance
between the anode 8b and the center axis of the hole formed in the cathode
plate 18b by l.sub.3. For the experiment, voltages ranging from 100 v to 1
kV were applied across the electrodes, and laser beam irradiation was
carried out manually. At high voltages discharge took place within a
radius of about 10 mm around the point of laser irradiation (with a beam
radius of 1 mm). In this case and when the discharge was within the laser
beam point, it was considered that the discharge was guided.
FIG. 4 is a plot of discharge guidance characteristics at each pressure
with respect to an electrode position l.sub.1 of 45 mm, l.sub.2 of 32 mm
and l.sub.3 of 0 mm. This plot can be divided into the following three
regions: "A", where spontaneous discharge takes place or guided discharge
does not take place, "B", where discharge is guided, and "C" where
discharge cannot be triggered by laser. Lowering the air pressure lowers
the minimum voltage of laser guided discharge (hereinafter also referred
to as "minimum discharge voltage"). Also, there is an increase in the
maximum voltage at which a discharge can be guided, which at 10 Pa exceeds
1 kV.
At high discharge voltages discharge takes place around the laser beam
spot, and if the voltage is raised still further, guidance becomes
impossible, as can be seen from FIG. 4. It was thought that for precision,
discharge machining should be performed using the lowest possible voltage.
The minimum discharge voltage was therefore investigated while varying the
relative position of the electrodes. FIG. 5 shows the results of the
investigation of minimum discharge voltage in which l.sub.1 was fixed at
45 mm and l.sub.3 at 0 mm, and l.sub.2 was varied from 8 mm, to 32 mm, to
67 mm. FIG. 6 shows the results of the investigation of minimum discharge
voltage in which l.sub.2 was fixed at 8 mm and l.sub.3 at 0 mm, and
l.sub.1 was varied from 25 mm, to 45 mm, to 75 mm. If it is considered
that l=l.sub.1 +l.sub.2 +l.sub.3, while l may not necessarily be the
discharge distance, it can be regarded as being a value approximately
proportional thereto.
Looking at the minimum discharge voltage, while there is a broad similarity
with respect to FIG. 5 in which l=53 mm and 77 mm and FIG. 6 in which l=33
mm and 53 mm, the voltage becomes high at longer distances of l=112 mm or
83 mm. This indicates that while at smaller distances of from around 33 mm
to 77 mm the minimum discharge voltage is within a fixed range with
respect to the product of pressure and distance (P.times.l), an increase
takes place when a certain distance is exceeded, such as at 83 mm to 112
mm.
Although there is only a small difference between l in FIG. 6 (83 mm) and l
in FIG. 5 (77 mm), under the same pressure conditions the minimum
discharge voltage is high. It is rather that with the 83 mm of FIG. 6 and
the 112 mm of FIG. 5, the minimum discharge voltage values at each
pressure are close. This shows that l.sub.1 has more of an influence than
l.sub.2 on the rise in the minimum discharge voltage.
It was confirmed that the position of a discharge on the rear of a
workpiece or other such hidden locations can be controlled using a laser
to guide the discharge. It was found that the laser guidance of a
discharge is dependent on the air pressure, the voltage and the distance
between the electrodes. Up to a certain distance the minimum discharge
voltage is determined by the product of the pressure and the distance, but
when that distance is exceeded, the distance between the hole in the
cathode and the laser beam spot has more of a bearing on the minimum
discharge voltage than the distance between the cathode hole and the
anode.
The discharge machining apparatus of this invention can be used to machine
the rear surface and other such parts of a workpiece, which is not
possible with conventional discharge machining methods. Moreover, unlike
conventional methods, this invention does not require the use of an
electrode having the same shape as the shape to which the workpiece is to
be machined. Furthermore, with this invention the machining contour is
controlled by the laser beam spot, without any need to move the electrode.
Because the discharge machining point is precisely guided by the laser
beam, precision machining is possible. Also, the machining operation is
facilitated by the fact that discharge timing can be controlled by laser.
Japanese Patent Application No. 5-213339 filed Aug. 5, 1993 is hereby
incorporated by reference.
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