High Velocity Press

Abstract

Presses for metal or other hard material in which the driving power for the punch is derived primarily from the kinetic energy of the punch element and the supporting structure moving at high speed. Capacitive discharge through a coil produces an electromagnetic interaction with a conductive plate adjacent to the coil and secured to the punch element to achieve the high velocity and therefore the desired kinetic energy, which energy is transformed to mechanical punching force.

Parent Case Text

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation in part of application Ser. No. 869,059 dated Oct. 24, 1969, entitled "HIGH VELOCITY PUNCH, " now abandoned, which claimed the priority of Italian Application No. 53694-A/68 filed on Oct. 31, 1968, in Italy, and Italian application No. 53038-A/69 filed on Aug. 19, 1969 in Italy.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to punch presses comprising a base, a rigid frame, a die fixed on the base and a movable punch supported on the frame with respect to the die. Particularly it relates to such a press used for stamping sheet metal and plate in industrial applications.

2. Description of the Prior Art

There are known arrangements for providing energy quickly by a process of combustion for activating the movable part of a press. Presses of this type present the inconvenience of moving a considerable mass over a long distance. Moreover, they have many difficulties in guiding and stopping the movable part of the press.

There are other known arrangements for providing energy derived from the decompression of compressed fluid for activating the movable part of a press. These presses also have the inconvenience of considerable mass in motion. Also there are other difficulties with respect to the circuits of the activating fluid.

Finally there are known devices that utilize the magnetic field generated from the discharge of a bank of capacitors across an electrical coil to form or stamp pieces of conductive material. In these devices the workpiece is held in a preestablished fixed position within the magnetic field generator. This lately excites a direct pressure on the workpiece without physically touching the workpiece with parts of the device.

With these devices it is possible to form pieces of metal having relatively high conductivity such as copper, aluminum and brass. For working metal having less electrical conductivity, as stainless steel, it is necessary to make use of an intermediate conductive screen that is normally constituted by a sheet or a tube of aluminum. Thus these devices can work only metal material or other conductive materials, utilizing the fact that an electrical current is induced when they are positioned in a varying magnetic field.

The technical problem that the present invention solves is that of actuating a punch press at high velocity in which the masses in motion are greatly reduced.

SUMMARY OF THE INVENTION

This problem is resolved in the press of high velocity according to the invention which is characterized by a conductive element integral or solidly fixed with the punch and having relatively high electrical conductivity, the element being positioned in the magnetic field of a coil connected to a source of electrical energy for providing a pulse of current such as to furnish to the punch by means of the conductive element the energy necessary to achieve the high velocity toward the die required to punch the piece. One preferred embodiment of the invention is given by way of example in the following description and the attached drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 represents a cross-section of a press according to the invention.

FIG. 2 represents schematically the electrical circuit for driving the press of FIG. 1.

FIG. 3 is a front view, partially sectioned, of a variation of the press.

FIG. 4 is a section according to the line IV--IV of FIG. 3

FIG. 5 is a section according to the Line V--V of FIG. 3.

FIG. 6 is a section according to the line VI--VI of FIG. 3.

FIG. 7 is a diagram of the input and drive circuit of the press of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A punch press of high velocity according to a first embodiment of the invention, represented in FIG. 1, can cut sheet metal for producing extremely precise cleanly cut pieces that have essentially no burr on the cut edges. The press comprises massive support plate 10, on which is fixed rigid frame 11, consisting of uprights 12 and top crosspiece 13. This last is arranged parallel to plate 10 and the distance between crosspiece 13 and plate 10 is adjustable within certain limits, by means of adjusting nuts 14 threaded on uprights 12.

At the center of plate 10 is fixed a die 15 and plate 10 has a passage 16 situated under die 15 for discharge of the punched piece.

At the top of plate 10, aligned coaxially with die 15, is disposed a punch 17 for movement in a rectilinear direction to cooperate with die 15 and to operate the punching of metal sheet 18 fixed on plate 10 in correspondence with die 15. Punch 17 and die 15 are disposed so that punch 17 travels a stroke of at least 10 percent of the diameter of coil 27.

Punch 17 is fixed by means of bolts 19 to the bottom of plate 20. This latter is guided on bushings 21, each of which is provided with axial cavities 22 and is fixed by means of a nut 23 screwed on a threaded portion 24 of the respective bushing. Each bushing 21 is inserted with the axial cavity 22 on a vertical rod 25, the lower end of which is fixed to plate 10.

In conclusion, punch 17 is able to move for a stroke of at least 10 percent of the diameter of the coil 27 in a rectilinear direction guided by the rods 25 over which the bushings 21 run telescopically.

A ring 26 of elastic material, for example rubber, is encircled on each rod 25 and is disposed in contact with the upper surface of plate 10.

Travel of punch 17 is stopped when bushings 21 in passing over rods 25, strike through rings 26 against plate 10.

The rings 26 have the function of deadening the strike of the bushings 21 against the plate 10 after the action of the punch 17 for effecting punching.

The rings 26 furnish automatically an elastic reaction in the opposite direction with respect to the movement of punch 17 and is sufficient for withdrawing the punch from the hole punched in the sheet 18.

The punch 17 is actuated utilizing force based on electrodynamic action that is manifested between two circuits for conducting current.

When a varying current runs through a winding, the flux generated links it with a conductor disposed in the field and induces currents of opposite phase. Such currents produce an electrodynamic force that tends to separate the conductor from the winding. In the present invention, this force is not used directly for punching, but for accelerating the punch 17 so as to cause this latter to accumulate the maximum kinetic energy before the punching begins. In effect the electrodynamic force is practically finished when the punching begins.

In the illustrated example of the invention's application, the winding is represented by coil 27, fixed by means of screw 28, on the bottom surface of plate 29 made of an insulating material, for example a synthetic resin of adequate thickness. Plate 29 is fixed in turn on the bottom surface of crosspiece 13. The conductor is depicted as disc 30 of material with a high electric conductivity, for example copper, fixed to the top of plate 20. For purposes of this invention, high electrical conductivity is considered to be less than 10 microohm-centimeters resistivity.

The electrical circuit driving the press, depicted schematically in FIG. 2, includes transformer and rectifier unit 31. Unit 31 is connected to a capacitor 32 by a switch 33. Voltmeter 34 is connected across capacitor 32 for monitoring the capacitor voltage. Switch 35 connects capacitor 32 to coil 27. Switches 33 and 35 are reciprocally controlled so that the operation of one of them disenables the other one.

The punch 17 is kept in close proximity to coil 27 and is returned to the rest position adjacent to coil 27, as represented in FIG. 1, by elastic means such as cylindrical helical springs 36 disposed vertically between plate 10 and plate 20. The lower end of each spring 36 is inserted into a recess 37 integral with plate 10; the upper end of each spring 36 is located by pin 38 fixed on the lower surface of the plate 20.

To carry out the operation of punching, metal sheet 18 is first fixed on plate 10 in correspondence with die 15, and capacitor 32 is charged by unit 31 (FIG. 2). For this purpose switch 33 is closed connecting the transformed and rectified current from unit 31 to capacitor 32. Switch 33 is closed until voltmeter 34 shows capacitor 32 has attained the predetermined voltage. At this point switch 35 can be closed, automatically disenabling switch 33 and discharging the capacitor 32 through the circuit including coil 27 rapidly changing the current flow through coil 27. The wave form of the current through coil 27 is similar to a damped sinusoid, of which the first half cycle is used to accelerate the punch 17. The frequency of the pulse should be such that the current wave form passes through zero after the punch 17 has traveled a stroke of at least 10 percent of the diameter of coil 27.

The current going through coil 27 produces a magnetic field which links with copper disc 30 inducing a current that is substantially of opposite phase with the current through coil 27. The result is that disc 30 is accelerated away from coil 27.

The electrodynamic force on disc 30 supplies the necessary energy at punch 17 to accomplish the high speed motion towards die 15 thus acquiring the necessary kinetic energy for cutting metal sheet 18. The movement of punch 17 is guided by bushings 21 sliding on rods 25 and is controlled at the completion of cutting when bushings 21 strike rings 26 against support plate 10.

After the punching operation, switch 35 opens automatically. The springs 36 return punch 17 to the rest position, in which disc 30 is in contact with coil 27. The press is now ready for a new operation cycle.

In a variation of the punch press, die 15 (FIG. 3) comprises portion 55 that is flared toward the bottom facilitating discharge of cut pieces. To the top of plate 10 is fixed jug 39 for registering the sheet or piece for cutting 18. Punch 17 is fixed to a support constituted by plate 40 to which copper disc 30 is fixed by means of four screws 50.

On plate 40 two guide columns 41 are fixed directed downward. Each column 41 slides in a bushing 42 pressed in a hole of base plate 10. The hole for each bushing 42 is furnished with rings 43, which cooperates with column 41 to act as a seal.

The lower end of each column 41 extends into oval chamber 44 (FIG. 6) in plate 10. Chambers 44 are closed at the bottom by another plate 45 (FIG. 3) which is fixed to plate 10.

Plate 45 is furnished with two blind holes 46 in which the bottom ends of the respective columns 41 can come to rest. Two holes 46 are interconnected by crossover trunk 47 (FIG. 6) for pressure balance. In the plate 45 are fixed two cups 48 (FIG. 3) the upper rims of which seal a flexible material against a rim in plate 45 to form chamber 49. A suitable flexible material is polyethylene terephthalate. The cups 48 are each furnished with a hole 51 which communicates with the outside.

The plate 40 is, in addition, furnished with two drilled flanges 52, each of them engaging a corresponding column 53 fixed within a cylindrical recess 54 of plate 10. Two compression springs 56, arranged each in one of the recesses 54 and a hollowed recess of corresponding flange 52, normally hold plate 40 in a raised position as in FIG. 3, with each flange 52 resting against a top flange 57 of the corresponding column 53.

The plate 40 is made of a light alloy, for example aluminum, and is furnished with fins 58 (FIG. 5) separated by channels so as to make the movable parts of the press lighter. The fins also cool plate 40, dissipating the heat produced by the current induced in disc 30 (FIG. 3). The cooling can, in addition be completed by circulating water in passage 59 coming in through a conduit 61 and leaving through a conduit 62 (FIG. 5).

The electrodynamic action on disc 30 of a turn of coil 27 varies inversely with distance of disc 30 from coil 27 and directly with square of current through the turn. To obtain maximum efficiency of the press, coil 27 is constructed of a conductor of constant section spirally wrapped in a single plane. In particular coil 27 is made of conductor 63 of copper having a rectangular section with the long side inclined vertically or parallel with the direction of movement of punch 17, so as to present a section of a large number of turns with small resistance to the current. The conductor 63 is wound spirally on a core 66 from an inside point 67 to an outside point 68. The turns are isolated by means of an insulating polyethylene terephthalate ribbon likewise wound spirally. The inside point 67 of conductor 63 is connected electrically with copper ring 69 which is connected to a terminal 70, while the outside point 68 is connected to a ferrous ring 71 connected to another terminal 72.

Coil 27 has an external diameter substantially equal to that of disc 30 and is disposed in hollow 73 of plate 74. Plate 74 is made of an insulating and nonmagnetic material and is fixed to plate 13. In particular, ring 71 is fixed to plate 74 by a series of screws (not shown). The lower surface of plate 74 is furrowed with a series of substantially radial grooves 75 (FIG. 4), which communicate with core 66 through a series of fissures 76. The hollow 73 (FIG. 3) is closed from the bottom by a membrane 77 made of a plastic material, for example reinforced epoxid resin, which is a rigid material, or polyethylene terephthalate, which is a flexible one. The edge of membrane 77 is fixed to plate 74 by a ring 78. In hollow 73, oil can circulate by entering from above through the core 66 and leaving through a horizontal pipe 79 connected laterally to the plate 74.

For efficiency it is further desirable that disc 30 initially be in close proximity to coil 27 and that it travels most of the stroke over which it maintains effective electromagnetic coupling with coil 27. It has been found that with a stroke of at least 10 percent of the diameter of disc 30, acceptably high efficiencies are obtained. In the illustrated embodiment the diameter of disc 30 is about 200mm.

The useful energy of the press depends on the final velocity attained by punch 17. The speed can be increased by increasing the path of punch 17 within the limits in which disc 30 reacts to the electrodynamic action of coil 27. In practice, the velocity of punch 17 is greater as the quantity of electrodynamic force on disc 30 is greater. Since the force that is exerted on disc 30 is the sum of the forces exerted on each turn of coil 27, the diameter of disc 30 desirably approaches the most possible so as to have a maximum number of turns of conductor 63. In the device of this invention with a diameter of disc 30 of 200 mm, 200 spirals are provided in the coil 27. The thickness of disc 30 does not influence the energy provided that it be at least equal to the depth of penetration of the inductive current caused in disc 30 by coil 27. In one embodiment with a moving part mass of 5 kg., an impact velocity of about 10 meters/sec is obtained after an acceleration stroke of about 20mm. This sheared a 40mm diameter hole in hardened steel (Rockwell C.congruent.50) 2mm thick.

In order to actuate the punch to high velocity with a maximum efficiency, it is important to choose a frequency of electric current as low as possible in relation to other elements of the machine. As is known, capacitors 32 and coil 27 create a resonant circuit that generates current oscillations having dampened pseudosinusoidal waveform and a frequency depending on the capacity of the capacitors and the inductance of the coil. The current pulse that concerns the press is the one due to a first half cycle of the waveform. The waveform is not truly sinusoidal since the inductance of coil 27 changes according to the proximity of disc 30.

The principal limiting factor for the efficiency of the press is the electrical resistance of the circuit. For a given coil this resistance increases with increase of resonant frequency due to skin effects. Thus it is necessary to use a low resonant frequency circuit such as to have a current passing through zero after the moving part has been displaced at least 10 percent of the diameter of the coil. In the press of FIG. 3, the inductance of coil 27 with disc 30 elevated is approximately 2mH. For the capacitors a capacity of 500.mu.F amp such as to produce a resonant frequency of 170 Hz has been chosen so that the action of the skin effect is within acceptable limits. With such a frequency the penetration depth in the copper is 5.6 mm and the thickness of the copper plate has been chosen on 6 mm and the thickness of the coil 12 mm, that is, twice the thickness of the plate. The initial resistance of the copper plate and the coil is 0.75.OMEGA. whereas the initial derivative of inductance with respect to the displacement is 0.12 mH/mm. Table A gives the operating data for a press in which the capacitor is charged to 4200V so that the initial electrostatic energy is 4400J. With a moving part weighing 8 kilograms, a final velocity of punch 17 is attained of about 17.5 meters per second after a displacement of 30mm acquiring energy of about 1200 J. The dissipated energy due to losses is 2000 J, so that the efficiency is e=1200/200 + 1200 = 37.5 percent. This does not count as lost energy the energy stored in capacitor 32 at the end of the cycle since this energy is used in the next press cycle. Table A also shows that the maximum acceleration is obtained after a displacement of 1.8 mm and the maximum force exerted by the coil 27 on the disc 30 is about 10 tons. The average force is about 4 tons.

The kinetic energy of the punch, however, is sufficient for cutting in a 2mm thick sheet having a Rockwell hardness HRc50, a hole of 140mm in diameter. The described press of a very small dimensions and weight can be considered equivalent to a conventional press of 100,000 kilograms.

TABLE A

Kinetic Capacitor Time Stroke Velocity Accel. Energy Losses Energy .0 .0 0.0 0. 0. 0. 4400. .0004 .00006 0.604 4279. 1. 61. 3761. .0008 .00084 3.784 11210. 57. 3.83. 2331. .0010 .00183 6.197 12604. 154. 632. 1602. .0014 .00529 10.969 10440. 481. 1145. 521. .0018 .01040 14.310 6298. 819. 1540. 50. .0022 .01653 16.171 3232. 1046. 1794. 39. .0026 .02320 17.072 1454. 1166. 1942. 285. .0030 .03012 17.449 546. 1218. 2018. 618. .0034 .03713 17.574 145. 1235. 2050. 911. .0038 .04417 17.598 13. 1239. 2059. 1082. .0040 .04821 17.599 0. 1239. 2059. 1108. Where time is in seconds, strokes in meters, velocity in meters/sec, acceleration is meters/sec.su p.2, and energy in joules.

For charging of capacitors 32, transformer and rectifier unit 31 is controlled by a control unit generally indicated as 82 (FIG. 7). Charging of capacitors 32 is automatically controlled by control unit 82 after each operation of punch 17. The discharge of the capacitors is controlled by ignitron 83 also under the control of control unit 82. Another ignitron 84 controlled by circuit 85 controls the discharge of capacitors 32 in the following operation cycle, when the capacitors are charged with an opposite polarity thus recovering the energy still held by the capacitors after the first discharge. This acts to further increase the efficiency.

The control circuit of the press if provided (secured), finally, with a voltage adjustment unit 86 for controlling the charge potential, comprising a manual adjustor 87 for selecting the desired voltage. By means of manual adjustor 87 the energy to punch 17 (FIG. 3) can be manually adjusted according to the sheet to be perforated and the size of the perforation.

The function of the press of FIG. 3-7 is as follows:

At the start of the apparatus, control unit 82 automatically calls for charging of capacitors 32. After sheet 18 is placed (FIG. 3) on plate 10, by means of a control (not shown) a signal 88 is generated which conditions the control unit 82 (FIG. 7) to trigger ignitron 83 discharging capacitors 32. Consequently a current impulse at the resonant frequency of the circuit passes through coil 27. Such current produces a magnetic field the flux of which links with copper disc 30 (FIG. 3). Disc 30 is thus motivated by a downwardly directed electrodynamic force that makes disc 30 move quickly downward together with plate 40 and punch 17. This last effects then the cutting of sheet 18 at the highest speed.

The movement of the plate 40 is guided by means of the columns 41 on bushings 42 and is effected against the action of the springs 56. Toward the end of the path of plate 40, the lower ends of columns 41 engage blind holes 46 reducing the impact as the moving parts of the press stop. At the end of the path of punch 17 columns 41 reach the position shown by dashed lines in FIG. 3.

After the first half wave of current, the ignitron 83 stops the current, then springs 56 (FIG. 3) return plate 40 upward to the rest position. The punch 17 withdraws from sheet 18, which remains retained by jig 39. When the moving parts of the press return to the upper position, it generates, in a known way, an electric signal which conditions control unit 82 (FIG. 6) for recharge of capacitors 32 with a voltage opposite the previous one. A new cycle is now started in which the ignitron 84 is energized.

The cooling oil of the coil 27 and the cooling water of the plate 40 are circulated by suitable pumps (not shown), which can function continuously during the functions of the press or intermittently.

Although the illustrated embodiments of the press, according to the invention, relate to a punch press, it is clear that the method and apparatus according to the invention could equally well be used in other types of presses such as, for instance, powder metal compacting presses and forging presses.

The invention has been described with relation to a press having a circular coil. Although this is the easiest and most efficient shape, other coils such as square or oval coils, could also be used. In these cases the disc should be the same size and shape as the coil or larger.

Claims

I claim:

1. An electrodynamic press including a punch mounted for rectilinear motion therein, comprising:

a substantially planar element of highly conductive material affixed to said punch normal to the direction of motion thereof,

a planar coil fixedly mounted in said press coaxially with said planar element and adjacent thereto,

means for positioning a workpiece in the path of said punch for being struck by said punch after said punch has traveled a stroke of at least 10% of the diameter of said coil to allow transformation of energy of a low frequency energization pulse into kinetic energy of said punch, and

means for energizing said coil with a low frequency pulse of electrical energy for generating flux linking said conductive element to repel said element from said coil, the frequency of said pulse being such that said pulse continues to accelerate said punch over a stroke of at least 10 percent of the diameter of said coil.

2. A press according to claim 1 further comprising movable guide means joined to said punch; fixed guide means fixedly mounted in the press; said movable guide means and said fixed guide means cooperating for assuring rectilinear movement of the punch toward the workpiece and for limiting the travel of said punch.

3. A press according to claim 2 in which said movable guide means comprise bushings and said fixed guide means comprise columns upon which said bushings ride.

4. A press according to claim 2 further including shock absorbing means comprising resilient washers encircling said fixed guide means and disposed between said movable guide means and said base. cm 5. A press according to claim 2 further including shock absorbing means comprising recesses that are entered and closed by said movable guide means toward

the end of travel of said punch forming hydraulic cushions. 6. A press according to claim 1 in which said coil comprises a conductor spirally wound in a single stratum, said spiral having an outside diameter

substantially equal to that of said element. 7. A press according to claim 6 in which said conductor has a constant rectangular cross section with

the longer dimension parallel to the axis of punch movement. 8. A press according to claim 1 further comprising a chamber closed by a deformable membrane disposed between said coil and said element, said chamber containing a dielectric cooling liquid circulatable by motion imparted to

said membrane from said element during operation of said punch. 9. A press according to claim 1 in which said means to change the flow of electrical

current comprises a capacitive discharge system. 10. A press according to claim 9 in which said capacitive discharge system has in conjunction with

said coil and said element a resonant frequency of less than 170 Hz. 11. The press of claim 9 further including means operable on the following cycle of said press for using the energy stored in said capacitive discharge system at the end of said low frequency pulse as a portion of the input energy for the next actuation cycle of the press.