Patent Application
20090166334
The present invention relates to a method and an apparatus for forming a microshaft used for electric discharge machining and more particularly, to a microshaft formation using an electric discharge phenomenon to a scanning and rotating electrode shaft with a plate member serving as a counter electrode.
The electric discharge machining method representative of the non-contact machining technique, characterized in its small reactive force during machining, is effective in the field of microfabrication which requires fine tools. The methods for forming a microshaft to be used for the micro electric discharge machining include: (1) the inverse electric discharge method; (2) the wire electro discharge grinding method (hereinafter referred to as WEDG method); (3) the electric discharge microshaft forming method using a hole; (4) the repeated transcriptional micro electric discharge machining method; (5) the fine fabrication by means of a zinc electrode; and (6) the microshaft instantaneous forming method of a fine electrode using a single-shot of electric discharge, respectively, as shown in
The microshafts formed in the above methods are useful for a measuring probe for measurement of a fine configuration and/or a surface roughness, a tool for micro manipulation, a fine hole forming tool for forming a fine hole, such as a nozzle hole, a two- or three-dimensional fine configuration creating tool for a fine mold. Especially, the inverse electric discharge machining method representing a non-contact machining technique is characterized in its small reactive force and thus allows a microshaft to be produced easily. In addition, since the inverse electric discharge machining method allows a forming shaft that has been formed in a machining apparatus to be used as a tool for carrying out a subsequent step of processing in the same apparatus without any re-chucking operation, therefore the method has become a standard machining process.
[Patent document 1]
Japanese Laid-open Publication No. 2004-142087
The WEDG method is one representative method as the microshaft forming method that takes advantage of the electric discharge machining which uses a running wire made of brass as a tool, as shown in FIG. 21(3). Although this method, owing to its capability for producing a microshaft of high precision easily, has become a standard of technique, the method has been yet suffered from defects that it requires a long forming time and that asymmetrical electric discharge occurs with respect to a side surface of the shaft and consequently the shaft with such a shaft diameter as minute as some microns or smaller is inescapable from the effect of vibration.
Besides, there is one approach in order to achieve a symmetrical electric discharge, in which a microshaft is formed in a certain type of machining process, such as a reciprocal abrasion, while using the shaft as a tool for making a hole in a plate (see FIG. 21(3) as well as the document 1). In this case, since a primary scanning direction of a shaft is normal to a top surface of the plate, disadvantageously this approach could not control a microshaft diameter and a microshaft length to be produced, independently from each other. Therefore, the approach requires a large set of experimental data in order to achieve a target shaft diameter or a target shaft configuration. Although there may be another approach contemplating the swinging of the shaft relative to a previously formed hole, it must encounter a problem of making an initial hole or another problem, such as the processing time and the shaft vibration, as is the case with the WEDG method.
From the reasons above, the machining process according to the prior art technique requires a high level of practice and skill but is not favorable for mass production, consequently turning out not to be as popular as it has been initially expected. This is a remote cause that the microfabrication has sunk in the death valley.
In the light of the above situation, an object of the present invention is to provide a microshaft forming method and a microshaft forming apparatus by way of an electrode scanning method, which is capable of forming a microshaft efficiently without the need for acquiring a high level of practice and skill as is the case with the conventional methods.
The present invention has been made to solve the problems as pointed above and a first invention provides a method for forming a microshaft, characterized in comprising: a step of providing an electrode to be processed into the microshaft; a step of providing a forming member for shaping the electrode; an electrode rotation step for rotating the electrode around a rotation center extending longitudinally through the electrode; a power supplying step for supplying a power to the electrode and to the forming member by using a discharge machining power supply in order to induce electric discharge between the electrode and the forming member; an electrode moving step for moving the electrode, as it is in a rotational motion by the electrode rotation step, from a lateral end side of the forming member transversely across the forming member; and a microshaft formation step for shaping the electrode to be formed into the microshaft, while forming a groove in the forming member during the electrode moving step by using the electric discharge induced between the electrode and the forming member by the power supplying step.
A second invention provides a method for forming a microshaft as defined by the first invention, characterized in further comprising, before the electrode moving step, a slit formation step for forming a slit previously in the forming member along a direction for the electrode to be moved. A third invention provides a method for forming a microshaft as defined by the second invention, characterized in that the forming member comprises two forming members and the slit is formed as a gap between the two forming members.
A fourth invention provides a method for forming a microshaft as defined by the third invention, characterized in that the two forming members are electrically isolated from each other. A fifth invention provides a method for forming a microshaft as defined by the third invention, characterized in that the two forming members are electrically connected with each other.
A sixth invention provides a method for forming a microshaft as defined by any one of the first to the fifth inventions, characterized in that, during the electrode moving step, a secondary motion is applied to the electrode in a direction different from the direction of the electrode moving from the lateral end side of the forming member transversely across the forming member.
A seventh invention provides a method for forming a microshaft as defined by the sixth invention, characterized in that the secondary motion is a reciprocating motion of the electrode in a direction vertical to a top surface of the forming member. An eighth invention provides a method for forming a microshaft as defined by the sixth invention, characterized in that the secondary motion is a reciprocating motion of the electrode in an angled direction relative to a top surface of the forming member.
A ninth invention provides a method for forming a microshaft as defined by any one of the first to the fifth inventions, characterized in that, during the electrode moving step, the electrode is driven to make a swing motion relative to the forming member in a direction parallel to a top surface of the forming member.
A tenth invention provides a method for forming a microshaft as defined by any one of the third to the fifth inventions, characterized in further comprising: a discharge frequency measurement step for measuring discharge frequencies between the electrode and each of the two forming members; and a discharge frequency control step for controlling the discharge frequencies measured in the discharge frequency measurement step so that the discharge frequencies are equal to each other.
An eleventh invention provides a method for forming a microshaft as defined by the tenth invention, characterized in that the discharge frequency control step comprises a distance control means for controlling distances between each of the two forming members and the electrode to be equal to each other. A twelfth invention provides a method for forming a microshaft as defined by the tenth or the eleventh invention, characterized in that the discharge frequency measurement step is a current detection step for detecting current flowing to each of the two forming members.
A thirteenth invention provides a method for forming a microshaft as defined by any one of the third to the fifth inventions or the tenth to the twelfth inventions, characterized in further comprising, before said electrode moving step, a slit discharge machining step for performing previously the electric discharge machining between the two forming members so as to shape an inner surface of the slit.
A fourteenth invention provides a method for forming a microshaft as defined by any one of the first to the thirteenth inventions, characterized in further comprising: after the microshaft formation step, a groove width adjustment step for narrowing the groove of the forming member; and after the groove width adjustment step, a reprocessing step for performing a series of operations comprising the electrode rotation step, the power supplying step, the electrode moving step and the microshaft formation step, in a sequential manner. A fifteenth invention provides a method for forming a microshaft as defined by the fourteenth invention, characterized in that the reprocessing step is repeated by multiple times.
A sixteenth invention provides a microshaft characterized in that the microshaft is formed from the electrode by using a method for forming a microshaft as defined by any one of the first to the fifteenth inventions.
A seventeenth invention provides an apparatus for forming a microshaft, characterized in comprising: an electrode to be processed into said microshaft; a forming member for shaping the electrode; an electrode rotation means for rotating the electrode around a rotation center extending longitudinally through the electrode; a discharge machining power supply for supplying power to the electrode and to the forming member in order to induce electric discharge between the electrode and the forming member; and an electrode moving means for moving the electrode, as it is in a rotational motion driven by the electrode rotation means, from a lateral end side of the forming member transversely across the forming member, wherein during operation of the electrode moving means, the electrode is shaped to form the microshaft while forming a groove in the forming member by using the electric discharge induced between the electrode and the forming member by the discharge machining power supply.
An eighteenth invention provides an apparatus for forming a microshaft as defined by the seventeenth invention, characterized in that the forming member comprises a slit that is previously formed in the forming member along a direction for the electrode to be moved. A nineteenth invention provides an apparatus for forming a microshaft as defined by the eighteenth invention, characterized in that the forming member comprises two forming members and the slit is formed as a gap between the two forming members.
A twentieth invention provides an apparatus for forming a microshaft as defined by the nineteenth invention, characterized in further comprising a frequency measurement means for measuring discharge frequencies between the electrode and each of the two forming members; and a discharge frequency control means for controlling the discharge frequencies measured by the discharge frequency measurement means so that the discharge frequencies are equal to each other.
A twenty-first invention provides a method for forming a microshaft as defined by any one of the first to the fifteenth invention, characterized in that the forming member employs a forming member made of silicon.
A twenty-second invention provides a method for forming a microshaft as defined by the twenty-first invention, characterized in that electric discharge is induced between the electrode and the forming member made of silicon, so that a silicon contained layer is formed over a surface of the microshaft while the microshaft is being formed.
A twenty-third invention provides a microshaft characterized in comprising the silicon contained layer formed over the surface of the microshaft by a method for forming a microshaft as defined by the twenty-first or the twenty-second invention.
Components in the attached drawings are designated as follows:
1 Electrode
1
a Rotation axis
1
b Microshaft
3 Forming plate
3
a End surface
3
b Slit
3
c Groove
5 Discharge machining power supply
7 Scanning direction (Electrode moving direction)
9 Electric discharge machining apparatus
11 Carrier stage
12
a,
12
b,
12
c Stepping motor
15 Drive clock controller
16 Working fluid level
17 Video microscope
18 Personal computer
19 Driver
21 Averaging circuit
23 Comparator
27 Discharge deviation detecting circuit
27
a Current detector
33 Forming plate
33
a End surface
33
b Slit
33
c Groove
Respective embodiments according a microshaft forming method and a microshaft forming apparatus by way of an electrode scanning method of the present invention will now be described with reference to the attached drawings. It is to be noted that in the drawings, like elements are designated by using the same reference numerals and any descriptions on the like elements are omitted.
A first embodiment provides a microshaft forming method according to a “direct scanning method”. In the conceptual diagram as shown in
As shown in
An exemplary processing condition of the electrode 1 by the electric discharge machining apparatus 9 is presented in Table 1. The electrode 1 employed a cemented carbide of Ø 500 micrometer and the forming plate 3 employed a zinc alloy (ZAPREC), a brass and the S50C and a cemented carbide, which are expected to bring a stability in processing.
The process of forming the microshaft could be observed from a transition of shaft diameter of the electrode 1 as a function of a processing time as shown in
Research was then made on an effect of the specific material of the forming plate 3 to a forming characteristic of the microshaft formed from the electrode 1.
To investigate the reason for the results, discharge voltage waveforms during the processing by the zinc alloy and the cemented carbide were observed, results from which observation are shown in
A second embodiment provides a method and an apparatus for forming a microshaft using a “plate with a slit”. Even if the processing is executed under the predetermined condition and the relationship between the processing time and the shaft diameter or between the scanning distance and the shaft diameter is previously established on the database, as in the first embodiment, there will still be a case that makes it difficult to obtain a desired electrode diameter with good reproducibility, because in the electric discharge machining, the condition of the electric discharge varies depending on the condition of the working fluid and the specific electrode material or forming plate material. Possibly the case that makes it difficult to apply the method of the first embodiment is more likely to occur especially with a narrower electrode diameter.
To address the problem above, in the processing method of the second embodiment, a forming plate 3 having a thickness around some mm or thinner is processed for slitting by way of the wire electric discharge machining so as to form a slit 3c previously in the forming plate 3, as shown in
A third embodiment provides a method and an apparatus for forming a microshaft according to a “slit forming plate comprising a set of two plates”. The “slit forming plate comprising a set of two plates” refers to a set of two forming plates 33, each having a thickness of some mm or thinner, which are fixedly positioned with side surfaces of the two plates 33 close to each other in a parallel relationship so that a slit (a gap) 33c is defined therebetween, as shown in
Then, a forming electrode 1 is driven to scan from a groove end surface 33a side of the forming plate 33 toward the inner side of the slit 33c to thereby form a groove 33b wider than the slit 33c for defining a configuration of the electrode 1.
In this case, advantageously there is no need for forming the slit previously by way of the electric discharge machining and the width of the slit can be desirably adjusted, as compared to the second embodiment.
A fourth embodiment provides a method and an apparatus for forming a microshaft according to “insulated coupling plates”. The “insulating coupling plates” refers to a set of two forming plates used for forming a slit 33c, which is basically similar to that in the third embodiment. However, in the fourth embodiment, one 33d of the forming plates is electrically connected with a discharge machining power supply 5, while the other 33e of the forming plates is electrically insulated from the discharge machining power supply 5, as shown in
Then, a forming electrode 1 is driven, as in the rotating motion, to scan from a groove end surface 33a side of each of the forming plates 33d, 33e toward an inner side of the slit 33c. During this step, the electric discharge is induced between the forming plate 33d and the electrode 1, so that the forming plate 33d and the electrode 1 are abraded, while on the other hand, no electric discharge is induced between the forming plate 33e and the electrode 1, so that the forming plate 33e and the electrode 1 would be abraded little.
In this case, the side surface of the slit 33c defined in the insulated forming plate 33e would not be subject to the electric discharge machining, and so the electrode 1 can be driven to scan along this side surface.
Further, in the fourth embodiment (
A fifth embodiment provides a method and an apparatus for forming a microshaft according to “up and down scanning”, “orthogonally upward scanning” and “intermittent scanning”. As described with reference to the first embodiment, an electrode 1 can be moved in any desired directions relative to a forming plate 33 with the aid of the stepping motors 12a, 12b and 12c. A secondary motion 50, as will be described below, is a motion applied to the electrode 1 in a direction different from the direction of a primary motion defined by a movement along the scanning direction 7 of the electrode 1.
For example, during the horizontal scanning of the electrode 1 as shown in
Although the fifth embodiment (
A sixth embodiment provides a method and an apparatus for forming a microshaft according to a “swing motion”. As shown in
Although the sixth embodiment (
A seventh embodiment provides a method and an apparatus for forming a microshaft according to “electric discharge frequency difference proportional control”. The “electric discharge frequency difference proportional control” provides the control of the scanning direction 7 of an electrode 1 so that the current flowing to or the electric discharge frequency of the two forming plates 33, which are insulated from each other, may be equal therebetween.
As shown in
It is to be noted that the current flowing to each of the forming plates 33 is proportional to the electric discharge frequency, and the electric discharge frequency is in turn proportional to a distance between a surface of the electrode 1 and a side surface of the forming plate 33 facing to the slit 33c. Accordingly, if the control is provided so that the electric discharge frequency is equal between the two forming plates 33, then the electrode 1 can be driven to scan with the distance between each of the two forming plates 33 and the electrode 1 held equal.
In a first additional embodiment, before the scanning of an electrode 1, two insulated plates are previously applied with a two- or three-dimensional motion to control a groove sectional contour, as shown in
Further, in the first additional embodiment (
Further, a second additional embodiment provides what is called “slit width control in a repeated scanning” or “repeated scanning” in any one of the first to the seventh embodiments for performing the processing of microshaft formation repeatedly while controlling the groove width. In the second additional embodiment, a servo voltage providing an indication of a discharge electricity condition and a distance between electrodes in each process of the repeatedly performed forming processes may be shifted sequentially to meet the condition for forming a finer shaft.
The second additional embodiment will now be described with reference to
It is to be noted that respective embodiments described above may use as a material for the forming plate 3, 33, a less consumable plate, such as a copper-tungsten plate; a consumable plate, such as a silicon plate or a green compact and a semi-sintered compact; and a small work function plate, such as a zinc alloy plate.
Further in any one of the second to the seventh embodiments, instead of the electrode 1, non-conductive shaft may be driven to scan for forming the microshaft. Specifically, when one of a set of two forming plates is used as a conductive coating electrode while the other of the forming plates is used as a shaping electrode and the non-conductive shaft is driven to scan between the two plates, then the non-conductive shaft may be shaped by the electric discharge between the two plates.
According to a method and an apparatus for forming a microshaft of the present invention, since a basic motion (scanning direction 7) is defined to be parallel with a top surface of the forming plate, the electric discharge frequency (a number of electric discharge occurrences as per a unit time) becomes greater, achieving an approximately ideal value. As for a direction orthogonal to the direction for the electrode to be moved, the electric discharge frequency can be made equal between the left and the right with respect to the electrode, so that the effect from the vibration during processing can be reduced and thus a stable electrode shaping can be achieved. In addition, since a rear side with respect to the moving direction of the electrode is cleared, so that an efficiency of removing a processing chips can be improved and consequently a processing time can be shortened.
Further, in the embodiments where the slit (groove) is previously formed in or between the forming plate(s), since a distance determined by subtracting an electric discharge gap (a space between the electrode and the groove inner surfaces) from a slit width defines a final shaft diameter, the electric discharge stops automatically and so there would be no such chance that the shaft is annihilated in the course of processing, and accordingly the control of configuration of the shaft would be extremely easy. In addition, as the shaft diameter for processing becomes finer, inaccuracy between the direction of the slit in the forming plate(s) and the scanning direction 7 of the rotating shaft of the electrode becomes relatively greater, whereas if the two forming plates are disposed, as they are insulated, and a moving control is provided so that the electric discharge frequency is relatively equal with respect to the two forming plates, then the scanning direction and the groove orientation can be held in a parallel relationship. In this case, if the electric discharge machining is performed previously between the two forming plates before the scanning of the electrode, the surfaces of the forming plates opposing to each other within the slit can be completely matched.
Further, if the material for the forming plate employs a material having a lower work function, then the electric discharge may be induced more easily, and with such arrangement, the electric discharge machining operation can be carried out stably even in the case of processing in the air. If a material, such as silicon, is selected as the material for the forming plate, then the surface of the shaft to be formed can be modified to be the one having extremely high corrosion resistance.
It is to be noted that in addition to the respective embodiments as well as the additional embodiments as described above, such a method and an apparatus for carrying out the electric discharge machining operation can be provided that may use a microshaft produced from said electrode 1 as a tool for a subsequent step without any re-chucking operation of the microshaft. Specifically, the forming plate(s) 3, 33 may be removed from the carrier stage 11 without removing the microshaft that has been formed by any one of the respective embodiments as well as the additional embodiments as described above from the electric discharge machining apparatus 9. Subsequently, a new workpiece to be processed is mounted on the carrier stage 11 of the same electric discharge machining apparatus 9, and the microshaft is now used as the tool with respect to the workpiece for forming a fine hole or creating a scanning configuration in the workpiece.
This example is directed to modify a surface of a formed microshaft to have extremely high corrosion resistance by selecting silicon as a material for a forming plate.
It has been known that if the finishing electric discharge machining is applied to a steel material by using the silicon as an electrode (forming plate), then a higher processing rate and improved roughness of a processed surface are obtained and no deterioration in the surface roughness is observed even with a larger electrode area, as compared to a copper electrode. In addition, the inventors of the present invention have reported that if the electric discharge machining is applied to the stainless steel by using the silicon as an electrode, a robust layer having corrosion resistance and wear resistance is formed over a surface of a workpiece.
As explained in the embodiments as described above, it has become possible to form a fine linear shaft having a high aspect ratio by carrying out a microshaft formation by means of the scanning electric discharge machining. In this connection, using an apparatus and a method of the above embodiments, the scanning electric discharge machining for microshaft formation was performed with a silicon plate as an electrode (a forming plate) and the research was made with the resultant microshafts on the corrosion resistance, the surface roughness and so on.
In this example, applying the forming method as illustrated in the foregoing embodiments, the scanning electric discharge machining was carried out on the stainless steel (SUS) round bar 1 by using a silicon wafer (thickness of 0.5 mm, specific resistance of 0.02 ohm cm) as an electrode 3 (forming plate).
It is expected that a silicon film should have been formed over the fine linear surface, and if so, it means that the formation of the fine linear shaft having excellent corrosion resistance will become possible. In this viewpoint, a corrosion test by using a hydrochloric acid was performed against the round bar that has been applied with the silicon electrode processing.
It is believed that in the electric discharge machining by way of the silicon electrode, the surface should be covered with a silicon film in the processing time around 5 minutes. In order to confirm that, the condition of a workpiece (round bar made of SUS (stainless steel)) was observed periodically (every one minute). As a result, it was confirmed that even if the eccentricity of the round bar is taken into account, a smooth trace of the electric discharge considered as the silicon film was observed over the outer surface in about 4 minutes, and the surface modification was successfully achieved with the electric discharging for about 5 minutes. The diameter at this point of time was about 400 micrometer. Further, the brass electrode was processed under the same processing condition to achieve the diameter around 400 micrometer, and then those round bars produced from respective electrodes were dipped in a solution of hydrochloric acid (26% concentration) for comparison of the degree of corrosion.
a) shows a processed site of each round bar after having been processed to the diameter around 400 micrometer by using the BS electrode material and the Si electrode material along with an unprocessed round bar.
Turning now to
In the observation with a microscope, the surface processed with the Si electrode is seen much smoother than that processed with the BS electrode. In this regard, the surface roughness measurement was carried out by using a confocal three-dimensional microscope and a surface roughness meter.
The surface roughness profile for each of the two is shown in
Observations by means of a SEM (Scanning Electron Microscope) and an EDS (Energy Dispersive X-ray Spectroscopy) were carried out on the round bar having its surface modified through the electric discharge machining carried out by using the Si electrode.
It can be seen from the EDS analysis that the Si (or a compound containing Si) is placed over the workpiece surface. In addition, it can be confirmed from the SEM image that the thickness of the film was some micrometer.
To summarize the foregoing description, in the illustrated embodiment, the electric discharge machining was carried out by using the silicon as the electrode (the forming plate) and the microshaft was successfully formed. Thus the formed microshaft had obtained the corrosion resistance and the improved surface roughness, actually much smoother than a typical processed surface by the electric discharge machining. Since the silicon film formed over the workpiece surface was not eroded even by the hydrochloric acid, therefore it is believed that this can be possibly useful for an application, such as a scanning probe or a handling tool in a corrosive environment.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2005-331656 | Nov 2005 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2006/322351 | 11/9/2006 | WO | 00 | 5/14/2008 |
