BACKGROUND OF THE INVENTION
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a table-top ELID processing
apparatus, according to the preamble portion of claim 1. for efficiently and accurately
grinding a small workpiece and processing it to a mirror surface.
An apparatus of the above type is known from
US-A-5 639 363
. Table-top lathes, table-top milling machines, etc. are widely being used
in homes and laboratories for machining small workpieces. These table-top machining
apparatuses process easily machinable materials such as copper, aluminum, wood,
and plastics. However, for tough materials, e.g., fine ceramics, optical glasses,
semiconductor single crystals, and other hard brittle materials and cemented carbides,
a general-purposed large machine must conventionally be used, even if the workpiece
is small. Therefore, it has been demanded to develop a table-top machining apparatus
that can be used like a table-top lathe etc. and can process even a hard brittle
material and cemented carbide with high accuracy and efficiency.
However, there were the following problems in developing
the aforementioned table-top machining apparatus.
SUMMARY OF THE INVENTION
- (1) A hard brittle material and cemented carbide may most appropriately be machined
by grinding using a grindstone. But, the resistance of a material to grinding is
greater than to normal cutting, and the resistance of the linear guide that guides
a workpiece is also normally larger. Consequently, the motor etc. required for machining
becomes larger and more powerful to drive the equipment. Normally, the above-mentioned
processing machine is driven by a three-phase power supply higher than 200V AC.
As a result, even if the equipment is made smaller for household or laboratory use,
it is difficult to modify the equipment to a compact, low-power system that can
be driven by a low voltage (single-phase 100V) and can be used on a table-top.
- (2) A grindstone used for grinding normally becomes loaded with cut chips and
debris within a short time, so it cannot be operated at a high efficiency. Although
the inventors of the present invention, et al. are developing an electrolytic in-process
dressing grinding (refer to ELID grinding), a predetermined pulse power supply is
indispensable to utilize the grinding system. However, this pulse power supply must
provide a rather high voltage (for instance, 60V to 150V) even for a small workpiece,
together with a pulse current of about 1A to 3A, therefore, the power supply becomes
large. In addition, an ELID grinding system should unavoidably use a conductive
processing fluid whose viscosity may often increase processing resistance, so power
consumption becomes still greater. In addition, if this processing fluid enter into
the linear guide, the life of the guide becomes shorter and its sliding resistance
fluctuates. If a seal etc. is installed to prevent the fluid from ingress, processing
resistance increases, and the life of the seal is reduced by the processing fluid.
The present invention has been accomplished to solve these
problems. That is, an object of the present invention is to provide a table-top
ELID processing apparatus that can process even a hard brittle material and cemented
carbide, whereby both driving and pulse power supplies can be compact, and the equipment
can be operated at a low voltage (single-phase 100V) and can be so small that it
can be used on the top of a table.
According to the present invention, the table-top ELID
processing apparatus, according to claim 1, is provided to achieve the above object.
The apparatus is provided with a conductive grindstone (2) for processing a workpiece
(1), an electrode (4) that can be installed with a predetermined spacing from a
working surface of the aforementioned conductive grindstone, a processing fluid
supplying device (6) for supplying a conductive processing fluid between the grindstone
and the electrode, and a power supply (8) for applying power between the grindstone
and the electrode, wherein the aforementioned power supply (8) is comprised with
a DC power supply and an RC circuit that generates pulse-like voltage, and can supply
power for both electrolytic dressing and discharge truing. In addition, the apparatus
is also provided with a work driving device (12) that drives a table with the workpiece
in the horizontal orthogonal directions X and Y, a tool-driving device (14) for
driving the conductive grindstone vertically of the workpiece while rotating the
conductive grindstone around vertical shaft Z, and a numeric control device (16)
which numerically controls the work driving device and the tool-driving device.
With the above-mentioned configuration of the present invention,
because the power supply (8) is comprised with a DC power supply and an RC circuit
which generate pulse-like voltages and can supply power for both electrolytic dressing
and discharge truing, the power supply can be used for both ELID processing and
discharge truing, whenever required. In addition, because of the simplicity of the
circuit, the power supply can be made considerably smaller than a conventional pulse
power supply, which generates an identical current pulse. Furthermore, a simple
numeric control system can be achieved and a workpiece can be processed to a high
accuracy by installing work driving device (12) and tool-driving device (14), and
controlling these devices using a numeric control device (16) (for instance, a personal
Hence, an electrolytic in-process dressing grinding (ELID
grinding) method can be configured using a small, simple power supply (8), whereby
the grindstone is dressed to greatly reduce cutting resistance, the necessary power
can be decreased, and a hard brittle material and cemented carbide can be processed
at a high efficiency and a high accuracy.
According to a preferred embodiment of the present invention,
the aforementioned work driving device (12) is guided by a hard porous carbon material
whose dynamic friction coefficient and static friction coefficient are substantially
the same no matter whether it is unlubricated or it is in water. Using the above-mentioned
hard porous carbon material (for example, RB ceramics) as a guide at a sliding portion,
even if processing fluid enters the linear guide, the coefficient of friction does
not alter. Therefore, positioning accuracy which is numerically controlled, can
be maintained high. Because the coefficient of friction does not vary and the carbon
material is not attacked by the processing fluid, no seal is required, and the sliding
resistance can be kept at a low value.
Other objects and advantages of the present invention are
revealed by the following paragraphs referring to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
DESCRIPTION OF PREFERRED EMBODIMENTS
- Fig. 1 is a general perspective view of a table-top ELID processing apparatus
according to the present invention.
- Fig. 2 shows a circuit of the power supply for both electrolytic dressing and
- Fig. 3 is a load characteristic diagram of the power supply in Fig. 2.
- Figs. 4A to 4C show examples of pulse-like voltages with the power supply in
- Fig. 5 is a structural view of a guide portion of the work driving device.
- Fig. 6 shows the relationship between baking temperatures and friction coefficients
of a hard porous carbon material.
Embodiments of the present invention are described below
referring to the drawings. In each figure, common portions are identically numbered,
and no duplicate description is given here.
Fig. 1 is a general perspective view of a table-top ELID
processing apparatus according to the present invention. In Fig. 1, table-top ELID
processing apparatus 10 according to the present invention is provided with a conductive
grindstone 2, an electrode 4, a processing fluid supplying device 6, and a power
supply 8 which applies a voltage between grindstone 2 and electrode 4. In addition,
table-top ELID processing apparatus 10 is also provided with a work driving device
12 which drives a table 11 with a workpiece 1 mounted thereon in horizontally orthogonal
directions X and Y, a tool-driving device 14 which drives conductive grindstone
2 in the vertical direction, while rotating the grindstone around vertical shaft
Z with respect to workpiece 1, and a numeric control device 16 which numerically
controls work driving device 12 and tool-driving device 14.
In this embodiment, workpiece 1 is fixed directly on top
of table 11, whose upper surface is to be processed. However, the present invention
is not limited only to this configuration. For example, another table for fixing
workpiece 1 on table 11 is installed, and workpiece 1 can be rotated around an axis
(X, Y, Z, etc.) using this table.
Conductive grindstone 2 is a cylindrical or cup grindstone
which rotates around the vertical Z shaft and whose lower surface processes workpiece
1. Conductive grindstone 2 may also preferably be a metal bond grindstone using
fine diamond grains. However, conductive grindstone 2 is not limited only to these
examples. Instead, various types suitable for ELID processing can also be used freely.
Electrode 4 can be installed with a predetermined spacing
from a surface of the workpiece of conductive grindstone 2. This electrode 4 is
preferably electrolytically dressed during ELID processing, as installed away from
the working surface of conductive grindstone 2. The working surface of conductive
grindstone 2 can also be electrolytically dressed or discharge trued by installing
the electrode on a table separate from the workpiece.
Processing fluid supplying device 6 supplies a conductive
processing fluid between grindstone 2 and electrode 4. In Fig. 1, numeral 13 represents
a tank of processing fluid in which processing fluid is stored and fed between the
working surface of grindstone 2 and electrode 4 using a pump (not illustrated).
The processing fluid should preferably be an electrolyte suitable for ELID processing.
The electric conductivity of the fluid may also be adjusted according to discharge
truing intended. Moreover, a surfactant etc. may preferably be added to reduce surface
tension while reducing processing resistance due to the processing fluid.
Work driving device 12, in this embodiment, is comprised
with a Y-axis table 12a installed on top of the processing fluid tank 13 and an
X-axis table 12b installed thereon. Aforementioned table 11 is fixed on the upper
surface of the X-axis table 12b. The Y-axis table 12a and the X-axis table 12b are
provided with respective linear guides, driving motors, and position detecting scales,
and can be precisely positioned by numeric control.
Numeral 15 in Fig. 1 denotes a cover installed to enclose
the Y-axis table 12b to prevent the processing fluid from spraying out during processing,
although only the upper front portion 15a is open. Open portion 15a can be closed
with a transparent cover so that one can access the equipment and observe processing
The tool driving device 14 is configured with a column
14a and a spindle unit 14b provided thereon. Spindle unit 14a houses a spindle,
which holds and rotates grindstone 2, a motor to drive the spindle, and a Z-axis
table for driving the grindstone 2 in the vertical direction Z. The Z-axis table
can also be positioned precisely by a numeric control system, because linear guide,
driving motor, and position-detecting scale are equipped in the unit, like Y-axis
table 12a and X-axis table 12b.
Numeric control device 16 is a personal computer provided
with a parallel and/or serial interface (for instance, RS-232C), and drives Y-axis
table 12a, X-axis table 12b and the Z-axis table by NC control: However, in place
of the personal computer, a custom chip and a micro-servo controller can also be
incorporated to make up a small, simple NC control device.
Fig. 2 is a circuit diagram of the power supply for both
electrolytic dressing and discharge truing. As shown in Fig. 2, power supply 8 applied
to table-top ELID processing apparatus 10 according to the present invention has
a DC power supply circuit, not illustrated, and an RC circuit connected thereto,
and can generate pulse-like voltages. In detail, as shown in Fig. 2, this circuit
is provided with a variable resistance R connected between input and output terminals
9a, 9b at the positive side, and a variable capacitor C connected between positive
and negative lines of output terminals 9b.
According to the embodiment of the present invention, the
DC power supply circuit provides output voltages at DC 30V to 150V using a low voltage
power supply of AC single-phase 100V. In addition, variable resistance R can be
varied stepwise or continuous in a range of 50 &OHgr; to 500 &OHgr;, and variable
capacitor C can be changed without or with steps in the 0.01 µF to 10 µF
Using this configuration, predetermined pulse voltages
can be applied between grindstone 2 and electrode 4 by charging capacitor C using
a simple circuit when output terminals 9b are open, and discharging capacitor C
when the resistance between terminals 9a and 9b decreases.
Fig. 3 is a load characteristic diagram of this power supply.
As shown in Fig. 3, any voltage in the range with a maximum voltage of about 150V
and a maximum current of 1.2A can be generated by adjusting variable resistance
R and variable capacitor C.
Figs. 4A, 4B, and 4C show examples of pulse-like voltages
generated by the power supply in Fig. 2. Figs. 4A, 4B, and 4C relate to R = 50 &OHgr;,
R = 100 &OHgr;, and R = 200 &OHgr;, respectively, using capacitor C with 1 µF
in all cases. From these figures, one skilled in the state of the art can understand
that pulse-like voltages can be generated by adjusting variable resistance R and
variable capacitor C, although the voltages are not complete pulses. In addition,
in any of the cases of R = 50 &OHgr;, 100 &OHgr;, and 200 &OHgr;, a capacitance
of C = 10 µF resulted in a pulse dropping at wave front and rear, however,
as the capacitance was made small, pulses approached complete ones, and with C =
0.1 and 0.01 µF, pulses were much better than those in Figs. 4A, 4B,
and 4C, according to an experiment carried out. Even when R = 500&OHgr;, a capacitor
with C = 0.01µF rendered similar pulse waves to those in Fig. 4A, 4B,
Referring to the results of Figs. 3, 4A, 4B, and 4C, it
was confirmed that the above-mentioned power supply can be applied to both electrolytic
dressing and discharge truing whenever required. In power supply 8, output voltage,
variable resistance, variable capacitor, etc. can be freely adjusted by controller
8a shown in Fig. 1.
Fig.5 shows the structure of a guide unit of the work driving
device. In Fig.5, numeral 17 represents a guide rail at the fixed side, 18 represents
a guide block at the driven side, and 19 represents sliding materials located between
both sides. These sliding materials 19 should preferably be made of a hard porous
carbon material whose dynamic and static friction coefficients without lubricating
are substantially the same as those in water. These hard porous carbon materials
include RB ceramics manufactured by mixing a phenol resin into degreasing rice bran,
forming and processing the mix, and carbonizing it in a nitrogen gas atmosphere.
Fig.6 shows the relationship between baking temperatures
and friction coefficients of the aforementioned hard porous carbon material. In
Fig.6, a baking temperature of 500°C or more drastically reduces the coefficient
of friction to an essentially constant value of 0.13 to 0.17. This hard porous carbon
material gives, even in water, a low coefficient of friction that is substantially
identical to that in air. In addition, even in a wide pressure range, static and
dynamic friction coefficients of the material are approximately the same. Consequently,
when such a hard porous carbon material as that described above (for instance, RB
ceramics) is used as a guide at the sliding unit, even if processing fluid intrudes
to the linear guide, friction coefficients do not fluctuate, therefore, positioning
accuracy can be maintained high by numeric control. Because friction coefficients
do not vary and the carbon material is not attacked by a processing fluid, no seal
is required, so sliding resistance can be kept low.
According to the aforementioned configuration of the present
invention, power supply 8 is comprised with the DC power supply and the RC circuit,
which generate pulse-like voltages, and can be applied to both electrolytic dressing
and discharge truing. Therefore, using this power supply, both ELID processing and
discharge truing can be actuated when required. In addition, because the circuit
is simple, the size thereof can be made much smaller than conventional pulse power
supply circuits, provided the same pulse current is generated. In addition, Work
driving device 12 and tool-driving device 14 are installed and controlled by numeric
control device 16 (personal computer), so a simple numeric control system can be
achieved and high-precision processing is enabled.
As a result, electrolytic in-process dressing grinding
(ELID grinding) can be established using a small, simple power supply 8, thus cutting
resistance can be greatly reduced by dressing the grindstone, while also decreasing
power consumption. Therefore, a hard brittle material and cemented carbide can be
processed highly efficiently at a high accuracy.
As described above, the table-top ELID processing apparatus
according to the present invention can process a hard brittle material and cemented
carbide at a high accuracy, and both power supplies for driving power and pulse
generation can be made compact. Therefore, the apparatus can be operated with small
power at a low voltage (single-phase 100V), and can be small enough to be put on
a desk, which are preferred advantages.