Title:
IMPACT HYDRAULIC FORMING DEVICE
United States Patent 3721091


Abstract:
An impact hydraulic forming device has a hydraulic pressure chamber with a bore filled with water, and a plunger is arranged to be thrust into the bore by a hammer which is unitary with the plunger and which is reciprocable in a cylinder. Means are provided to direct a jet of water across the open end of the bore in such a way as to seal the water in the bore from flowing out of the bore, to provide a water membrance. The face of the hammer joined to the plunger has an annular recess of curvilinear cross section whose surface merges smoothly with the exterior surface of the plunger, and air jet means are arranged to be directed against this annular groove to retract the hammer. The hammer is advanced under air pressure for a pair of its stroke after which the pressure air is shut off and the advance of the hammer is carried out by expansion of the air in the cylinder. The air jets directed against the recessed end face of the hammer are also shut off after the hammer has completed part of its return stroke, with the return stroke being completed due to the expansion of the air jetted into the cylinder. Valve means control reciprocation of the hammer, and the valve means may be flow pressure operated valves of may be solenoid valve controlled by an electrical control device.



Inventors:
Tominaga, Hiroshi (Kanagawa-ku, Yokohama-shi, Kanagawa-ken, JA)
Takamatsu, Masanobu (Totsuka-ku, Yokohama-shi, Kanagawa-ken, JA)
Application Number:
05/136685
Publication Date:
03/20/1973
Filing Date:
04/23/1971
Assignee:
TOKYU SHARYO SEIZO K K,JA
Primary Class:
Other Classes:
137/455, 417/403, 417/489
International Classes:
B05B12/06; F01B17/00; F01B17/02; F01L25/06; F04B7/04; F04B9/125; F15B11/072; (IPC1-7): F15B7/00
Field of Search:
60/54
View Patent Images:
US Patent References:
3601988DEVICE FOR BUILDING-UP FLUID PRESSURE PULSES1971-08-31Chermensky et al.
3601987DEVICE FOR BUILDING-UP FLUID PRESSURE PULSES1971-08-31Chermensky et al.
3593524DEVICE FOR PRODUCING HIGH-PRESSURE PULSE-TYPE JETS OF LIQUID1971-07-20Chermensky et al.
3500641PRESSURE INTENSIFYING DEVICES1970-03-17Meekings
3412554Device for building up high pulse liquid pressures1968-11-26Voitsckhovsky
3207442High pressure fluid gun1965-09-21Kessler et al.
2955749Reciprocable piston gas compressor1960-10-11Tomkowiak
2944530Pneumatic control means for reciprocable pistons1960-07-12Severinsen
2726642Air control for immersion apparatus1955-12-13Zinty et al.



Foreign References:
GB939951A
Primary Examiner:
Schwadron, Martin P.
Assistant Examiner:
Zupcic A. M.
Claims:
What is claimed is

1. An impact hydraulic forming device comprising, in combination, a hydraulic pressure chamber having a bore filled with impact hydraulic pressure producing water; a plunger arranged to be thrust into said bore; a hammer operable to thrust said plunger into said bore and to retract said plunger completely out of said bore; means operable to seal the water in said bore against discharge therefrom when said plunger is retracted completely out of said bore; means operable to retract said hammer after thrusting said plunger into said bore, to retract said plunger; and means operable to reciprocate said hammer cyclically to thrust said plunger into said bore cyclically to produce impact hydraulic pressure repetitively and cyclically.

2. An impact hydraulic forming device, as claimed in claim 1, including a cylinder in which said hammer is reciprocated; said hammer having a diameter substantially greater than the diameter of said plunger and being integral with said plunger; the surface of said hammer facing said bore and integral with said plunger being formed with an annular curvilinear cross-section recess merging smoothly with the periphery of said plunger; said cylinder having a wall at its end adjacent said bore and formed with an enlarged aperture through which said plunger extends with clearance; said wall being formed with a series of cylinder-returning air ports connected to a source of air under pressure and arranged to direct air supplied thereto as jets directed against said recess substantially parallel to the axis of said plunger and said hammer; whereby said hammer may be rapidly returned to the retracted position, upon supply of air under pressure to said ports, by utilizing the jet effect produced by the air jets striking against said recess.

3. An impact hydraulic forming device, as claimed in claim 2, in which said means operable to reciprocate said hammer cyclically includes a feed port, for air under pressure, in the opposite end of said cylinder; a main feed valve connected between said feed port and a source of air under pressure to control supply of compressed air to said cylinder to advance said hammer and plunger; said main feed valve including a spring biased valve body normally closing said feed port and having one end subjected to the pressure of said source of air under pressure; an auxiliary feed valve controlling connection to atmosphere of the opposite end of said valve body; said auxiliary feed valve being arranged to connect said opposite end to atmosphere, to provide for opening of said main feed valve by pressure air from said source, during the initial stage of the advancing stroke of said hammer, and to supply pressure air to said opposite end to close said main feed valve when a preselected range of the air expansion coefficient has been attained during the later part of the advancing stroke of said hammer and during the return stroke of said hammer; a return valve connected between said source of air under pressure, said cylinder-returning air ports of said hammer returning means and a port for discharging compressed air during the return stroke of said hammer; said return valve being operable selectively to open or to close both said returning air ports and said discharge port simultaneously; and a cushion valve connected to said return valve and operable to apply the compressed air in the cylinder, upon completion of the advancing stroke of said hammer, to said return valve to open the latter, and to close said return valve before completion of the return stroke of said hammer to trap the remaining air in said cylinder to provide a cushioning effect for the return stroke of said hammer.

4. An impact hydraulic forming device, as claimed in claim 3, wherein all of said valves are operated by working compressed air.

5. An impact hydraulic forming device, as claimed in claim 3, wherein all of said valves are operated by the air pressure in said cylinder.

6. An impact hydraulic forming device, as claimed in claim 3, in which said feed valve is a solenoid valve; an exhaust solenoid valve controlling discharge of air from said cylinder; said return valve comprising a solenoid valve; a hammer-return confirming electric switch operable by said hammer to confirm the return stroke of said hammer; and a controller connecting said switch and said valves through a holding circuit controlled by said switch, in association with timers; all of said valves being selectively opened and closed by electric signals provided thereto by said controller, to effect reciprocation of said hammer in said cylinder.

7. An impact hydraulic forming device comprising, in combination, a hydraulic pressure chamber having a bore filled with impact hydraulic pressure producing water; a plunger arranged to be thrust into said bore; a hammer operable to thrust said plunger into said bore; means operable to seal the water in said bore against discharge therefrom when said plunger is retracted out of said bore; means operable to retract said hammer after thrusting said plunger into said bore, to retract said plunger; means operable to reciprocate said hammer cyclically to thrust said plunger into said bore cyclically to produce impact hydraulic pressure repetitively and cyclically; feed valve means connected between said bore and a source of water to supply water to said bore; and a cylinder in which said hammer is reciprocated; said means operable to seal the water in said bore comprising a water membrane chamber interposed between the adjacent ends of said bore and said cylinder, a water inlet port in said chamber connected to a source of water under pressure and operable to direct a water jet against the opening of said bore perpendicular to the axis of said bore and inclined at a small angle to a diametric plane of said bore to form a water membrane, and an outlet opposite said water inlet to discharge the water jetted into said water membrane chamber after formation of the water membrance; said water membrane chamber being formed with an aperture communicating with atmosphere.

Description:
SUMMARY OF THE INVENTION

This invention relates to an impact hydraulic forming device in which a hammer in a cylinder is accelerated by compressed air and a plunger integral with the hammer is thrust into a hydraulic pressure chamber to highly pressurize the water in the hydraulic pressure chamber instantenously, thus performing metal forming or other working.

In conventional impact hydraulic forming devices of the type described above, if the device is to be horizontally installed, the plunger is inserted into the hydraulic pressure chamber in advance to prevent water from flowing out of the chamber and a separate hammer imparts blows onto this plunger indirectly. Therefore, in comparison with the devices of the type wherein a plunger is directly thrust, the conventional devices incur loss in energy transmission and the efficiency of producing impact hydraulic pressure is lower. In reciprocating the hammer, the advancing motion of the hammer is accomplished only by means of the pressure of compressed air, and therefore the consumption rate of compressed air is increased and the energy is not utilized effectively. The return motion of the hammer is accomplished by the suction of the hammer at its rear end portion or by means of the pushing force produced by compressed air, supplied from the hydraulic pressure chamber located in front of the hammer. In the former case, a large-scale device is required. Furthermore, the valve mechanism thereof is subject to severe working conditions where both high hydraulic pressure and air pressure are encountered, and therefore the valve mechanism is liable to malfunction. In the latter case, there is a drawback that the consumption rate of compressed air is increased. Additionally, an effective means of producing impact hydraulic pressure continuously by successive blows of the hammer has not been proposed so far. In view of the above, the conventional devices have many drawbacks.

An object of this invention is to thrust a plunger directly by forming a water membrane, at the opening of a hydraulic pressure chamber, through which the plunger is thrust and filling the chamber with water, regardless of the installing location of the device.

Another object of this invention is to utilize the energy of compressed air economically by accomplishing the advancing motion of a hammer not only by the pressure of compressed air but also by the energy produced from the expansion of compressed air.

Still another object of this invention is to accomplish the motion of the hammer in a short period of time by a small amount of low pressure compressed air through the utilization of the dynamic pressure of the jet effect obtained by injecting compressed air.

A further object of this invention is to accomplish the reciprocating motion of the hammer by utilizing the pressure of working compressed air, or by electrical control, thus making it possible to produce impact hydraulic pressure continuously.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a sectional view, partly schematic, of the impact hydraulic forming device according to this invention in which the reciprocating motion of a hammer is accomplished by air pressure;

FIG. 2 is an enlarged, detailed, sectional view of the principal portion of a means for sealing the hydraulic pressure producing water which is employed in the device shown in FIG. 1;

FIG. 3 is a sectional view, partly schematic, of another impact hydraulic forming device in which a means for opening and closing the inside of a cylinder is added to the embodiment shown in FIG. 1; and

FIG. 4 is a sectional view, partly schematic, of still another impact hydraulic forming device embodying this invention in which the reciprocating motion of the hammer is electrically accomplished.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, a cylinder 1 has a hammer 3, integral with a plunger 2, movable therein. Formed at the front end of the cylinder 1 is a water membrane chamber 6 of a hydraulic pressure producing water sealing means which is separated from the cylinder 1 by a stepped portion 5 having an opening 4. The water membrane chamber 6 communicates with a plunger bore 8 of a hydraulic pressure chamber 7. A water channel 11, having a feed valve 9 and leading to a water source 10, and a water channel 12, leading to a forming die (not shown), are in communication with bore 8.

As illustrated in detail in FIG. 2, the water membrane chamber 6 comprises a wall surface 13 which is at right angles to the center line of the plunger bore 8, a water outlet 14, having a valve 59, which is inclined from wall surface 13 by a small angle φ to guide a high speed water jet, an outlet 15 for discharging water from the water membrane chamber 6, and an air vent 16 in communication with atmosphere. By virtue of this construction, a high speed water jet is injected from the water outlet 14 against the opening between the wall surface 13 and the plunger bore 8, thus forming a water membrane of comparatively small thickness to cover the entire area of the opening. When the water membrane is to be formed, a hydraulic pressure having a somewhat different direction is applied in opposition to the water which tends to flow out of the opening of the wall surface 13, thereby to prevent the water in the plunger bore 8 from flowing out. In this way, the water supplied into the plunger bore 8 from the water source 10 through the feed valve 9 is sealed within the bore 8, thus making it possible to thrust the plunger 2 directly. Since the head of the water in the plunger bore 8 is practically 1 meter or so at most, the inclination angle φ can be selected within the range of 2 to 5° by increasing the flow rate for forming the water membrane appropriately. As soon as the feed of water to the plunger bore 8 is started, the formation of the water membrane is started. When the water is fully charged, the plunger 2 is immediately thrust. Therefore, the time required for the formation of water membrane is 1 to 2 seconds at most in practice. Consequently, the amount of water required is small. The important point to note here is that when the plunger 2 is to be thrust into the plunger bore 8, the tip of the plunger 2 crashes into the water membrane and is thrust after breaking through the water membrane. Therefore, if the water membrane is inclined in either direction, the plunger 2 is given a thrust in the horizontal direction under the influence of the inclined water membrane and is pressed against one direction of the inner surface of the plunger hole 8. This is not desirable, because frictional resistance loss is caused. In accordance with this invention, however, it is possible to maintain the water membrane so as to form a centered curved surface with respect to the vertical and horizontal directions by appropriately selecting the position and the angle of the water inlet 14 with respect to the wall surface 13 and controlling the flow rate within an appropriate range. Thus, the above-described drawback can be eliminated.

The hammer 3 integral with the plunger 2 has recessed portions 17 which are formed at the junction of the hammer and plunger, so that they merge smoothly with the outer circumference of the plunger 2. The stepped portion 5 is provided with a plurality of returning air ports 18 opposite the recessed portions 17 of the hammer 3. In returning the hammer after the plunger 2 is thrust into the plunger bore 8 and impact hydraulic pressure is produced, compressed air is injected from returning air ports 18 to the recessed portions 17 of the hammer 3. At this time, the injected compressed air impinges against the recessed portions 17, changes its course by nearly 180° and is reflected. Due to the changes in the momentum of this jet effect, a thrust twice as large as that obtained by injecting compressed air against a flat surface is provided, and the hammer 3 is returned to its initial position in a quite short time interval of about 0.5 second. Thus, the hammer 3 is returned by a small amount of low pressure compressed air. In injecting compressed air from the returning air ports 18, a strong thrust is applied from a plurality of air ports 18 for the period of time of 0.1 to 0.2 second during the early part of the hammer return. After the hammer 3 is given a sufficient speed, the air jet thrust is terminated. In this way, the dynamic pressure of the air is more effectively utilized and strong shock by the hammer due to its return can be avoided. The important points to note here are that a large impact force is applied to the plunger 2 when the plunger 2 compresses the water in the hydraulic pressure chamber 7 momentarily, and that there is a possibility of incurring damage due to the stress concentration at the junction between the plunger 2 and the hammer 3. According to this invention, however, the recessed portions 17 merge smoothly with the plunger 2 and their cross-sectional areas are successively increased, and therefore excessive stress concentration is not caused. Consequently, the durability of the plunger 2 is quite high.

The cylinder 1 has a compressed air feed port 19 at the rear end thereof. A main feed valve 25, having a valve body 24 provided with a spring 23 at the back side thereof, is installed on an air pipe 22 between the supply port 19 and an air source 21. The air pipe 22 is provided with an operating valve 20. An auxiliary feed valve 26, having a spring 37 at the back side thereof, is provided adjacent to the main feed valve 25. The auxiliary feed valve 26 is also provided with a small piston 28, a valve body 29 and a large piston 30 which are all made integral and concentric with a trigger rod 27 protruding inside of the cylinder 1. The auxiliary feed valve 26 communicates with the rear end of the cylinder 1 through an air pipe 31, and communicates with valve 25 through an air pipe 32 branched from the air pipe 22. Valve 26 is also in communication with the rear end of the valve body 24 of the main feed valve 25 through a passage 33. Furthermore, the auxiliary feed valve 26 is provided with a discharge port 34. Moreover, the rear end of the larger piston 30 communicates through an air pipe 35 with a cut-off air port 36 which is determined so as to provide a desired air expansion coefficient with respect to the total working stroke of the hammer 3. Before the hammer 3 makes an advancing stroke, the hammer is positioned at the rear end of the cylinder 1, as a result of which the trigger rod 27 is pushed to the right. Therefore, in the auxiliary feed valve 26, the small piston 28 closes the air pipe 32 and the valve body 29 opens the exhaust port 34, thus opening the passage 33 to atmosphere. The main feed valve 25 is opened by moving the valve body 24 against the bias of the spring 23 by means of the compressed air supplied from the air source 21. Thus, compressed air is directed into the cylinder 1 to move the hammer 3. When the advancing motion of the hammer 3 is started, the air pressure in the cylinder 1 is directed through the air pipe 31 into the auxiliary feed valve 26 and is applied onto the small piston 28 to hold the trigger rod 27 in its retracted position. Consequently, the main feed valve 25 is opened and the feed of compressed air is continued. Thus, the hammer 3 advances within the cylinder 1 and passes by the cut-off air port 36. At this time, the air pressure in the cylinder 1 is applied from the cut-off air port 36 through the air pipe 35 onto the large piston 30 of the auxiliary feed valve 26. As a result, the trigger rod 27 is projected and the small piston 28 opens the air pipe 32. Compressed air is applied through the passage 33 onto the back of the valve body 24 of the main feed valve 25, and the valve body 29 closes the discharge port 34. Consequently, the main feed valve 25 is closed and the delivery of compressed air is stopped. From this time on, the hammer 3 is accelerated due to the expansion of compressed air in the cylinder 1 and helps the plunger 2 in producing the impact hydraulic pressure.

The cylinder 1 is provided with an exhaust port 37' at the rear end thereof. An air pipe 39 is provided to connect exhaust port 37' and a discharge port 38. A return valve 41 is provided in an air pipe 40 which is branched from the air pipe 22 and is connected to returning air ports 18. The return valve 41 comprises a first land 44, which is integral and concentric with a valve spool 43, having a spring 42 at the back end thereof and which opens or closes the air pipe 39, and a second land 45 which opens or closes the air pipe 40. A motion stop valve 46 is provided on the air pipe 40. Moreover, the cylinder 1 is provided with an air inlet 47 at a position ahead of the cut-off air port 36 but within the total working stroke of the hammer 3. A cushion valve 49 is provided in an air pipe 48 connected to air inlet 47. The cushion valve 49 has a piston 50 which is provided integral with the spool 43 of the return valve 41 on the opposite end from the spring 42. The air pipe 48 communicates with the piston 50 through a throttle 51 and a ball valve 53 biased by a spring 52. During the above-described advancing motion of the hammer 3, the pressure at the air inlet 47 becomes equal to atmospheric pressure and the return valve 41 is held in the illustrated position by the bias of spring 42, and is therefore preventing return motion of the hammer. After the advancing stroke of the hammer 3 is completed, the hammer 3 passes by the position of the air inlet 47. At this time, the air pressure in the cylinder 1 is applied from the air inlet 47 through the air pipe 48 onto the piston 50 of the cushion valve 49. Consequently, the return valve 41 is moved to the right and the cylinder 1 is exhausted by means of the first land 44 opening pipe 39. At the same time, compressed air is injected from the returning air ports 18 through the air pipe 40, which is opened by second land 45. Thus, the hammer 3 starts to return. In this way, the hammer 3 is moved back in the cylinder 1 and passes by the air inlet 47 again. At this time, the air pressure at the air inlet 47 drops and the cushion valve 49 tends to return the return valve 41 to its initial position. However, the operation is delayed due to the flow-resistance of the throttle 51. Consequently, the position of the return valve is changed when the hammer 3 is in an appropriate position on its way back, thus closing the air pipes 39 and 40. From this time on, the hammer 3 moves back due to its own inertia, while compressing the air remaining in the cylinder 1. This compression of air decelerates the hammer 3 to provide cushioning action to the hammer 3 in the last stage of the hammer return stroke.

The overall operation of the impact hydraulic forming device of the above-described construction will be described hereinbelow. Now, the hammer 3 has completed its cycle and remains stopped in the position where impacting is completed. First, the operating valve 20 and the motion stop valve 46 are opened. When compressed air is fed into the returning air ports 18 by the manual change-over of the return valve 41, the hammer 3 is returned to the reciprocating motion starting point. At this time, trigger rod 27 of the auxiliary feed valve 26 is pushed to the right to open the main feed valve 25. Then, compressed air is fed into the cylinder 1. During the early part of advancing motion, the hammer 3 is advanced by the pressure of this compressed air. In the latter stage, after the hammer 3 has passed by the cut-off air port 36, the advancing motion of the hammer 3 is accomplished by the expansion of the compressed air. During this advancing motion of the hammer 3, the water supplied from the water source 10 into the plunger hole 8 is sealed therein by the water flow injected from the water outlet 14 into the water membrane chamber 6 at a high speed. The plunger 2 is directly thrust to produce the impact hydraulic pressure, by means of which a forming operation is accomplished. At this time, the position of the return valve 41 is changed by means of the cushion valve 49. The hammer 3 is returned by the dynamic pressure of the air injected from the returning air ports 18. While the hammer 3 is returning, the return valve 41 makes a return motion. Thus, the hammer 3 completes the return stroke, while being cushioned, and then the advancing stroke is started again. In this way, the hammer 3 repeats the reciprocating motions continuously and various kinds of working are continuously performed by means of the impact force. The cycle of the hammer is completed when the motion stop valve 46 is closed. The hammer 3 is not returned but stops its motion at the position where the impacting completed. The operating valve 20 is closed. Thus, the operation is completed.

As described above, according to this invention, the direct thrust of the plunger 2, in the case where the device is installed horizontally, becomes possible by sealing the water filled in the then horizontal plunger bore 8 so as not to allow it to flow out. In the vertical type device, wherein the plunger is adapted to move upward from the bottom, the plunger bore opens downwardly. In this case, the dropping of water can be prevented by forming a water membrane in the same manner as described hereinbefore. In this construction, it is preferable to provide a means for blowing air upward from a point near the hydraulic pressure chamber in order to prevent the interior of the cylinder from rusting due to water splashed from the plunger bore disposed above into the cylinder disposed below.

Furthermore, in accordance with this invention, the plunger 2 and the hammer 3 are specifically constructed so that the jet effect can be utilized, that the air to be used can be small in amount and low in pressure and that the fatigue strength of the hammer is not decreased by reason of this construction.

Moreover, in accordance with this invention, compressed air is expanded up to a certain predetermined expansion coefficient, and therefore the energy possessed by compressed air can be effectively and economically utilized.

Additionally, in accordance with this invention, the mechanism for reciprocating the hammer comprises the main feed valve 25 for feeding or shutting off the working compressed air, the auxiliary feed valve 26 for controlling the main feed valve 25 in conformity with the position of the hammer during its advancing motion, the return valve 41 for accomplishing the reciprocating motion of the hammer, and the cushion valve 49 for controlling the operation of the return valve 41. Thus, the continuous reciprocating motion of the hammer can be repeated smoothly and successive blows can be effected continuously many times, and consequently a great deal of work can be done in a short period of time.

In another embodiment illustrated in FIG. 3, an air port 54 is provided instead of the opening 4 at the front end of the cylinder 1. A valve 57, comprising a valve body 56 biased by a spring 55, is provided at air port 54. An air pipe 58 branched from the air pipe 40 is connected to the operating side of the valve body 56. Other component parts are constructed in the same manner as the embodiment described hereinbefore. Thus, when the hammer 3 is advancing, the valve body 56 is pushed by the spring 55 to open the air port 54, thus bringing the cylinder 1 into communication with atmosphere. When the hammer 3 starts to return after the completion of the advancing motion, compressed air is fed into the returning air ports 18 and is also applied through the air pipe 58 onto the valve body 56 of the valve 57 to close the air port 54. Consequently, in comparison with the first embodiment, in which the front end of the cylinder 1 is always held open, wasteful leakage of part of the air injected from the returning air ports 18 at the time of the return motion of the hammer 3 is prevented, thus accomplishing the return of the hammer 3 effectively.

In still another embodiment illustrated in FIG. 4, a solenoid valve is used as the valve 59' of the water inlet 14. The feed valve 9' for the plunger bore 8 also is a solenoid valve. Furthermore, the return valve 41', for the returning air ports 18, and the feed valve 25', for the feed port 19 also are solenoid valves. An exhaust valve 61, in the form of a solenoid valve, is provided on an air pipe 60 which is branched from the air pipe 22 for the feed port 19. An elastic seat 62 is installed at the rear end of the cylinder 1 to provide a cushioning effect for the return of the hammer 3. Provided at the seat 62 are a return detecting rod 64 biased outwardly by a spring 63 and protruding into the cylinder 1 and a return confirming switch 66 comprising a switch operator 65 operable in conformity with the displacement of the detecting rod 64. The above-described valves are all electrically connected by way of a controller 67 comprising switch 66 and a self-maintaining or holding circuit. The advancing motion of the hammer 3 is started by operating switch 66. From that time on, the advancing motion is accomplished after the feed valve 25' is opened and the return valve 41' and the exhaust valve 61 are closed by the self-maintaining action of the controller 67. At this time, in the hydraulic pressure chamber 7, the feed valve 9' is closed and the valve 59' is opened, and therefore the water is filled in the plunger bore 8 without flowing out. When the advancing motion of the hammer 3 is completed and the return motion is to be started, the feed valve 25' is closed and the return valve 41' and the exhaust valve 61 are opened by means of a timer or the like of the controller 67. Thus, the hammer 3 is returned. When the switch 66 is operated again by the hammer 3, the return motion is completed and the next advancing motion is started. In this way, the reciprocating motion of the hammer 3 is continuously accomplished and the same effect as that obtained in the first embodiment can be achieved. However, the operation is accomplished electrically in the above-described embodiment, the construction can be greatly simplified and the reliability of operation can be improved.