US3027791A

US3027791A - Electrospark means for perforating webs

Info

Publication number: US3027791A
Application number: US691512A
Authority: US
Inventors: Harold C Early; Warren A Dutton
Original assignee: Meredith Publishing Co
Current assignee: Meredith Publishing Co
Priority date: 1957-10-21
Filing date: 1957-10-21
Publication date: 1962-04-03
Anticipated expiration: 1979-04-03

Description

A ril 3, 1962 H. c. EARLY ETAL 3,027,791

ELECTROSPARK MEANS FOR PERFORATING WEBS 3 Sheets-Sheet 1 Filed 001;. 21, 1957 may van/16E SUPPLY 450 g/if N TORS mm jwid wma 8 4m 1 ATTORNEYS.

i 1 g /2 VOLT sup H A ril 3, 1962 H. c. EARLY ETAL 3,027,791

ELECTROSPAP-K MEANS FOR PERFORATING WEBS 3 Sheets-Sheet 2 Filed Oct. 21, 1957 Nl/ENTORS' awflg i/M46 r By 6442: 071M ATTORNEYS 3,027,791 ELECTRQSEARK MEANS FUR PERFGRATING WEBS Harold C. Early, Ann Arbor, Mich and Warren A. Dutton, Dallas, Tex., assign'ors to Meredith Publishing Company. a corporation of Iowa Filed Get. 21, 1957, Ser. No. 691,512 18 Claims. (Cl. 83-477) This invention relates to an apparatus for printing or erforating a rapidly moving web or tape utilizing the energy generated from the discharge of an electric arc. A device of this type is' shown and described in Early et al. Patent 2,737,882. That patent is particularly concerned with a high speed printing device for delivering information at a relatively high rate of speed to computing machines. Intelligence inthe form of depressions or perforations may be embossed upon a tape fed between a pair of opposed, closely spaced surfaces which confine a space therebetween. One surface constitutes a die or printing means and the other constitutes what is referred to herein as an anvilto confine gases between the tape and the surface of the anvil. A pair of electrodes project intothe' confined space from the anvil side of the device. When a high voltage electrical arc is discharged across the electrodes, air or other gas which may be confined between the tape and the anvil is instantaneously heated and expanded. The force produced by the rapid expansion of the gas impel's the tape against the die to emboss or perforate it with the indicia in the face of the die.

The present invention consitutes an improvement over this device and is particularly adapted for punching holes in rapidly moving webs. For example, in printing newspapers and magazines it is sometimes desirable to punch holes in the margin of some of the leaves for subsequent filing in a looseleaf binder. The present invention permits perforation of such holes in the web as it advances through a rotary printing press at very high speeds, say over 1200 feet per minute. The apparatus of the invention is also useful in perforating non-breathable coated fabrics, such as those used in upholstery, to permit free circulation of air therethrough.

In applying the method described in the aforesaid Early patent to punching holes in webs a number of problems are presented. Instead of making tiny embossments or perforations constituting indicia, it is necessary to produce a hole much larger, for instance, enough to slide over a ringv in a binder. Such holes usually have a diameter of A to about inch. To explosively produce an aperture of this size by an electrical discharge approximately 20,000 volts at 1200 amperes is required. The power released isin the range of 2 to 4 watt seconds. The intense heat resultingfrom an arc having this energy may raise the temperature of the confined gas to from 10,000 to 15,000 Kelvin. The expansion of the confined gas heated by the arc instantaneously builds up to around 30 atmospheres pressure. Such conditions cause erosion of the anvil with accompanying loss in compression because the defaced. anvil is not capable of closely confining the space in which the explosion takes place. As each successive discharge occurs, a thin film of material is vaporized from the surface of the anvil and it becomes less and less effective in confining the gases. Another problem in connection with these very high voltage arcs is themechanical damage to the die. Most refractory dielectric materials will not stand the mechanical shock and tend to chip very rapidly and lose their sharp definition. Where the die has a hole therethrough for perforating, the shearing edge becomes rapidly dulled and the hole produced has a ragged edge. Another problem in connection with perforating rapidly moving webs is ire the pounding which occurs when the web becomes slightly separated from the die and is explosively impelled against the die throughthis slight distance by the arcproduced explosion. Because of the tremendous forces involved, such pounding also causes a physical breakdown of the die surface, thus accelerating its deterioration.

It is a primary object of the present inventionto obviate these disadvantages to produce a printing or perforating device which is capable of running continuously for long periods of time without breakdown.

Another object is to provide a device of thi type in which the anvil is of improved construction and capable of maintaining a confined space in cooperation with the die and the Web for extensive periods ofoperation.

Another object isto provide an improved electrode construction which produces a sheet-like arc rather than an arc column.

Another object is to provide an improved means for insuring that the tape or web is in continuous contact with the die to eliminate the pounding above referred to, said means also serving to remove pieces of material seared from the web when the web is being perforated.

Another object is to provide an improved and more rugged die construction, capable of producing sharply defined characters or perforations in the sheet material passing thereover.

A further object is to provide ananvil made of a material which is capable of volatilizing under the arc to generate gas for increasing the force exerted against the web.

These and other objects are pointed out in detail in the appended claims and will be more fully understood from the following detailed description when read in conjunction with the accompanying drawings, wherein:

FIGURE 1 is a perspective view of the apparatus of the invention shown from the anvil side;

FIGURE 2 is a perspective view of the cooperating die and its mounting assembly, which is adapted to be mounted on the opposite side of the plate 10 shown in FIGURE 1;

FIGURE 3 is a circuit diagram of the electrical circuit for producing an electric arc at predetermined tim intervals synchronized with the rate of web travel;

FIGURE 4 is a side elevational view of the rotating anvil and the cooperating die;

FIGURE 5 is a sectional view taken along the line 5- -5 of FIGURE 4;

FIGURE 6 is a front elevational view of the electrodes and their mounting plate taken along'the line 66 of FIGURE 5;

FIGURE 7 is a sectional view taken along' the line 77 of FIG. 6; and

FIGURE 8 is a sectional view of a modified form of the die and holder.

Referring particularly to FIGURES 1 and 5', the plate 10 made from aluminum or other rigid material serves as a support and guide for the moving web W. Preferably the plate is disposed vertically, although the apparatus will operate satisfactorily with the plate 10in horizontal position. The web W may be paper, coated fabric, a-plastic sheet or other material which is to be perforated or embossed by bringing the same into contact with adie of predetermined configuration. Suitable means (not shown) is provided for moving the web upwardly across the face of plate 10 opposite the die assembly, which is indicated generally by the numeral 14. Such means may comprise part of another apparatus on which the web is being processed, e.g. a rotary printing press. The anvil assembly is mounted on the other side of the plate .10 and is designated generally by the numeral 12. The anvil consists of a thick-walled cylinder 22, the end face 30 of which is in contact with the web through an arcuate opening 62 in the plate 10. A pair of spaced electrodes are mounted on the plate It) flush with the surface thereof between the anvil and the web. The die is mounted opposite the face 30 of the anvil and has some suitable depression in the face thereof which imparts an embossment or perforation in the Web. The face 30 of the anvil serves to confine gases between the web and the surface 30. The cylindrical anvil 22 is secured to an end disc or wheel 24, rotatably mounted on a shaft 20, which is secured to a block 21 mounted on the plate It It will be noted that the shaft 20 is disposed at an acute angle with respect to the plate 10. This angle may range from 75 to 85 and permits the portion of the anvil diametrically opposite the operative portion extending through plate 10 to pass over a dressing means 48 to maintain the face 36 in smooth operative condition. It has been found that the dressing means 48 can be interposed between the plate 10 and the face 30 of an 8 inch diameter cylinder if the angle between the plane of the end of cylinder 22 and plate 10 is about or more. For smaller diameter cylinders, of course, the angle will have to be a bit steeper to provide the same amount of space between the plate and the face of the cylinder; but in any event the angle should be minimized to provide maximum contact area opposite the die. The angle of face 36 with respect to shaft 20 must complement the tilt of the shaft so that the face is parallel with the web W at the point opposite the die.

Preferably the cylinder is made from polytetrafluoroethylene or polytrifluoroethylene. This material offers low frictional resistance in sliding over the electrodes, it has excellent dielectric strength, and has been found to stand up better than any other under the high temperature electrical discharge. Any similar noninflammable dielectric resinous organic material will function as an anvil and such materials are preferred because they do evaporate under the heat of the arc to provide additional quantities of gas. This is an important part of the invention. Ceramic materials are also satisfactory, but do not vaporize as the dielectric organic materials. thickness of the wall of the cylinder 22 is about inch in the apparatus shown in the drawings. This apparatus is adapted to punch inch diameter holes through the web. Obviously the thickness of the cylinder can be varied in accordance with the particular use to which the apparatus is put, and is not critical as long as the contact face at least partially confines gas adjacent the web.

In order to present a new surface at the point where the electrodes discharge, the cylinder is adapted to rotate continuously about the shaft 20 while the apparatus is in operation. A bushing 28 fits inside the hub 26 to provide a freely moving bearing. A spring guide 36 slides over the end of the shaft and bears against the face of the hub. Spring 38 is compressed by means of a collar .40 which is secured against axial movement on the shaft by a set screw 42. Thus, the spring urges the cylinder axially on the shaft into firm contact with a knife edge 48 mounted on a bracket 50 secured to the plate 10. A

.Carborundum wheel may be used to dress ceramic cylinders. Any suitable dressing means may be employed in .place of the knife 48. The knife edge 48 may be adjusted angularly by means of a pin 56 threaded in the adjusting bar 54 which is secured to the vertical member 52 of the angle 50. This arrangement is best shown in FIGURE 1. -Member 52 is pivotally secured by means of a pin 55 to the mounting member 57 which is Welded to the plate The the cylinder 22 and prevent any cocking thereof with respect to the shaft.

To provide power for continuously rotat'mg the cylinder we have provided an electric motor 34 mounted on a suspension arm 16. The upper end of arm 16 slides over the shaft 2% and the lower end terminates in a right angle portion 18 on which the electric motor 34 is mounted. The arm 16 is keyed to the shaft so that it does not rotate with respect thereto. A ring gear 32 is fixed to the wheel 24 comprising the upper end of the cylinder. A pinion (not shown) connected to the shaft of the motor 34 engages the teeth of the ring gear 32 to rotate the cylinder at constant speed.

It is essential that the members in contact with the electrodes be completely insulated from the rest of the apparatus and for this purpose we have provided an insert 58 (FIGURE 5 for plate 10 made from Bakelite or other dielectric material which has structural rigidity and can be machined. The insert 58 has an arcuate opening 62 extending completely therethrough, as best shown in FIGURE 6. The arcuate opening is slightly wider than the thickness of the cylinder wall 22. The face 30 of the end of the cylinder projects into space 62 and contacts the underide of

electrodes

68 and 70. Another opening 63 extends approximately normal to the arcuate opening 62 and serves as a guide for sliding nylon blocks 60 and 61 to which the electrodes are secured. The blocks may be made from any other good dielectric material, cg. polytetrafluoroethylene, polyesters, epoxy resins, and the like. The opening 63 has a step 64 in the face adjacent the electrodes. As best shown in FIGURE 7, the nylon blocks 69 and 61 complement the

opening

63, 64, being T-shaped in cross section, and are adapted to slide longitudinally in the opening. They are locked by means of a washer and a screw 66, the washer being wide enough to bear against the anvil side of the insert 58. The die side of the insert 58 is recessed slightly, as indicated at 73 (PlGURE 6) so that the disc-shaped

electrodes

68 and 70 may be mounted flush with the surface of the plate 10 and the Bakelite insert 58. The

electrodes

68, 70 are secured to the nylon blocks 60 and 61 respectively by means of screws 74 which screw into tapped holes in shafts '72, rotatably mounted in the blocks. The shafts 72 are prevented from sliding out of the blocks by means of snap rings 76. Electrical contact to each of the

electrodes

68 and 70 is made through the shafts 72, as explained hereinafter. By loosening the screws 66, the

blocks

66, 61 may be moved in the

groove

63, 64 to adjust the distance between the

electrodes

68 and 70.

The

electrodes

68 and 70 are adapted to rotate since the shafts 72 on which they are mounted turn freely within the mounting block. As the web moves over the surface of the electrodes they are pressed between the web and the face 30, causing rotation. The ends out of contact with the face 30 are relieved so that reverse rotation is prevented. Thus, the discharge takes place successively along different portions of the circumferences. This, of course, increases the life of the electrodes considerably over the conventional electrode design which comprises a pair of stationary pointed conductors. The disc electrodes preferably are made from a thin sheet of refractory metal, such as molybdenum or tungsten. The arcuate space between the face 30 and the moving web W is very slight, limited only by the thickness of the electrodes. The gas which is expanded by reason of the electrical discharge is closely confined by the face 30, the opposed edges of the

electrodes

68, 70 and on the other side by the web W supported by the die 80.

The die 86 mounted opposite the face 30 of the anvil may be made of any suitable heat resistant dielectric material. One best able to resist the repeated high-voltage electrical discharges is aluminum oxide, under the trade name Alsimag 491. The die has an aperture 32 extending therethrough which determines the size of the opening punched through the web. The cutting edge of the die is chamfered, as indicated at 87', to prevent chipping-. If desired, the diemay provide only a slight depression in the surface thereof for producing an embossment on the web, or it may have a much smaller hole to make pinpoint perforations in the web. Its function isto impart physical indiciain the web as the web is forced against it. Die d is force-fitted within a die holder 84 which is pivotally mounted by means of pins 86 on a ring- 85 which in turnis secured to plate 88. The holder 84 is made from a dielectric material, as is the plate 88. Holder 84 also has anopening 89 extending therethrough, concentric with: die opening 82 This opening in turn connects with a tube 91 connected-to asource of vacuum. The portion of the web-which may be punched out during. the perforating operation is sucked into the tube and carried away. The flow of air thru opening 82 tends to remove ionized particles, which are the cause of short circuits, from the space. Furthermore, the vacuum connection causes the web W to run in contact with the surface of the die. Consequently, when the discharge takes place and the air is explosively impelled against the web, the perforation is made without any movement of the web-toward the die. Without the vacuum, the web has a tendency to move away from the face of the die an infinitesimal distance. This distance, however, is sufficient to cause pounding of the die and disintegration thereof under the terrific forces imposed by the explosive gases.

An alternative form of die mounting, particularly useful in conjunction with veryhigh-voltagehigh-temperature arcs, is shownin- FIG. 8, The die holder 184 is spaced a substantial distance fromthe mounting plate 188 to permit rapid flow of large quantities ofair past the rear of the die into the projecting end of the tube 191, as indicated by the arrows. This construction provides more efficient cooling of the die than that shown in FIG. 5. At the same time the vacuum tube tunctions to remove the slug, whisk out ionized particles and keep the web in contact with the die surface.

The mounting plate 88' for the die is connected to a support by means of: a parallelogram linkage which is commonly known in the art. A pair of yokes 90 and 92 are pivotally connected to the plate 88 and these yokes in turn connect to a second pair of

yokes

96 and 98 through pairs of rods 94.

Yokes

96 and 98 are pivotally secured to the plate 100- Which in turn is fixed to the end of the rotatablearm 102 by means of a coupling member 118, asbest shownin: FIGURE 4. Plate 100 is rigidly fixed at" right anglesto' the surface of member 118 which in: turn is secured to the arm 102 by means of bolts 120 which enter tapped holes in the bottom of the arm -2. A- block 116 secured to member 118 serves as a stop for yoke 98' to limit downward movement of the die when it is moved out of contact with. the web and supporting plate 1 0; A coil spring 108 extends between the bottom of plate 88 and the member 118 to constantly urge the die into contact with the web. By means of this parallelogram linkage the face of the die is always maintained in a vertical position even though the web may move slightly away from the face of the plate 10. The arm 102- is rigidly fixed to ashaft- 1-04 which in turn is rotatably mounted in vertical supports (not shown).

Means for automatically removing the die from the face of the web consists of a pneumatic cylinder 112 having, a shaft 114 which pivotaily connects with a crank 110 secured to the shatt104. The cylinder may be actuated by a solenoid valve to automatically separate the die from the web in the event some breakdownhas occurred in-the apparatus whichsuppliesthe web.

Thus it will be seen thatthe present apparatus provides ani-mproved means for confining the gases which are rapidly heated by means of the electric discharge betweentheelectrodes. The design of the aluminum oxide die which. providesf'or a chamfered shearing edge imparts long life to this member. By utilizinga cylinder made from polytetrakuoroethylerie and dressing means therefor, we have provided a fresh smooth surface for each successive explosion. These precautions, which insure close confinement of the gases in spite of the very high voltage discharge, produces sharply cut perforations over long periods of time without having to replace the critical parts.

Power means for supplying the high voltage energy to the electrodes is provided by'means of a high voltage power supply which is shown and described in detail in the copending application of Harold" C. Early, Serial No. 669,187. The circuit diagram is illustrated in FIG- URE 3. The electrodes 68 and 70' are indicated by arrows in the circuit diagram with a spark gap therebetween. The high voltage supply connects to the electrodes through a very high resistance 152 and a high capacity condenser 155. The condenser is charged over a relatively long period of time through the high resistance 152 and the low resistance 164, which in one preferred embodiment may have the values of 200,000 ohms and 2500' ohms, respectively.

No discharge takes place until the spark gap across the discharge device 154 is bridged. This is brought about at definite timed intervals by means of a cam 156 which makes and breaks a contact 158 each time the are is to be produced across the

electrodes

68 and 70. When the contact or switch 158 is closed, the l2-volt power supply energizes the ignition coil and when switch 158 opens, a high voltage spark is produced across the spark gaps 168 and 169 The spark across 169 serves as a trigger to cause the gap in device 154 to break down. The spark gap 168 serves to minimize the duration of the trigger spark. The output of the ignition coil tends to be oscillatory in nature and has a duration of several milliseconds. When gap 168 is not present, the oscillatory trigger spark causes multiple firing of the main gap 154. When gap 168 is properly adjusted, only the first high voltage peak of the ignition coil voltage is able to bridge the gaps168 and 1 69', and thus multiple firing of 154 is prevented. The breakdown of the triggered spark gap 154' provides a low impedance discharge path for the energy stored in capacitor This results in placing substantially the entire charge voltage of condenser 155 across the

electrodes

68 and 70 The spark gap defined by' these electrodes breaks down to heat the gases as explained heretofore. The resistance 164 is necessary t'o c'ornplete' the circuit during the interval when capacitor 155 is re-charging. Since this resistance is several thousand ohms, which is high compared to the spark impeda'nce across

electrodes

68 and 70, only a small fraction of the energy stored in capacitor 155 is lost in the resistance' 1'64.

The ignition coil 162 and the points 158 are of the type used in the ignition system of an internal combustion engine; A peaking capacitance 166 is in parallel with the resistance 164' to provide a complete and consistent breakdown of gap 154- before the voltage appears across 68 and. 70; Otherwise, the performance is more erratic and there is more time jitter in the breakdown process.

The triggered spark gap 154 consists of two hemispherical electrodes made from copper and reinforced at the points nearest the gap with a refractory metal, such as molybdenum and tungsten; The trigger wire extends through an opening in the lower hemispherical member.

Some of the electrical elements mentioned are shown in physicalfor'm in FIGURE 1.

It is believe'dthat the operation ofthe device is quite apparent from the foregoing description. However, the operation will be reviewed briefly for sake of clarity. The earn 156 is synchronized with the movement of the web" W by direct-coupling to a shaft over which the web passes. By providing projections on the cam at appro-. priate places, it is possible to'regulate the distance be tween the perforations produced in the web. The initiation of the

discharge a'c'rossthe electrodes

68, 70 1's es'sentially instantaneous in response to the cam. The spark duration is in the range of l to 1 /2 microseconds. Consequently, the machine is adapted for very rapid op ration. Perforations 1% inches apart can be produced in a web traveling at a speed of 1200 feet per minute. About 7 /2 milliseconds expire between successive discharges. The spacing between perforations will remain the same regardless of the speed of the web. The power requirements vary depending upon the size of the perforation. A 4 inch diameter hole will require 2 to 2 /2 watt seconds of energy.

The arcuate flat end 30 of the cylinder 22 is adapted to coincide with the opening 62 through the dielectric plate 58. The angle of the face 30 is such that the face is substantially parallel with the web portion passing between the plate and the die 84 thus confining a space between the anvil and the web. By rotating the cylinder continuously, a new arcuate portion of the anvil surface is presented at the point where the arc is produced. In addition to heating and expanding air which is trapped between the web and the face 30, the arc volatilizes some of the polytetrafiuoroethylene from the cylinder. This material also expands to assist in generating a rapidly expanding gas which explosively impels the web against the die, thus shearing the portion of the web which overlies the opening 82 in the die. The punched out portion is sucked into the tube 91 and disposed of in thisway. As the anvil cylinder 22 is shortened due to dressing, the spring 38 urges the cylinder toward the plate 10, thus keeping the face 30 in close contact with the web on the underside of the

electrodes

68 and 70. As the movement of the web rotates the cam 156 another are is struck across the

electrodes

68, 70 to punch another hole in a precise distance from the last one.

One of the important features of the invention is the construction of the revolving electrodes. The thin flat discs provide many advantages of the conventional wire type, point electrodes which operate much less effectively in the invention. A word of theoretical explanation will provide a better understanding of this feature.

Any electrical spark tends to form an anode spot" and a cathode spot of high current density and small area. In the case of the electrospark hole puncher, the current leaves the anode and the cathode through spots having an area of the order of one square millimeter. The cross-sectional area of the spark column is also of very small cross-sectional area (of the order of one square millimeter). When the spark is confined to a thin gas film between two adjacent dielectric surfaces, the spark channel can no longer have a circular cross section but spreads out so that the cross section of the spark channel is about inch wide and .003 inch in thickness. Thus, the very small spacing is necessary in order to cause the spark channel to spread out and develop a pressure over the entire area surrounding the die hole.

A similar philosophy applies to the electrodes. When the only available electrode area is the edge of a thin metal disc of .003 inch thickness, the current does not leave the metal at a single spot, but spreads out along the thin edge of the metal for about of an inch in order to obtain suflicient area for the anode and cathode spot. Thus the spark channel resembles a sheet instead of a column.

With all other forms of electrodes which have been tested or considered, the spark channel starts out as a small diameter column (adjacent to the electrode) and spacing, the current density and gas pressure are substantially higher than elsewhere, and the tendency toward die damage is greatest at this point. In contrast, the thin disc revolving electrodes do not have any regions of high current concentration and gas pressure.

Another important feature of the invention is the chamfering of the edge of the die opening, which is indicated in the drawing at 87', FIGURE 2. We have found that where the opening is square with respect to the face of the die it chips much more easily than where a slight chamfer has been provided.

It is also important in constructing the die to make certain that it is well insulated electrically from the frame of the machine. This is done by making the supporting plates 88 and and the connecting element 118 from dielectric plastic material, such as polystyrene, polymethylmethacrylate, phenol-formaldehyde, or other suitable plastic material.

The die proper should be made from a material which is machinable or which can be cast to a predetermined shape and it should preferably have a polished surface in contact with the web. We have found that dies constructed from aluminum oxide will withstand the high energy levels of the electrical discharge which are in the range of 2 to 4 watt seconds. If the energy level is increased to, say, 6 or 8 watt seconds, the wearing of the die will be much faster. If desired, the edge of the die where the shearing action takes place can be made from inserted sapphire or diamond chips.

Although specific forms of our invention have been shown and described herein, it will be understood that various modifications can be made in the construction and the materials employed without departing from the true spirit and scope of the invention.

We claim:

1. Apparatus for making a predetermined formation in a web comprising a forming member, an opposed anvil made from an organic dielectric resin cooperating therewith to provide an at least partially confining space therebetween, a base plate supporting said forming member and anvil, said anvil being formed by a portion of the substantially fiat end of a thick-walled cylinder mounted on said plate for rotation with respect to said forming member, a pair of spaced electrodes mounted on said plate contiguous with the flat end of said anvil, means for advancing said web between said forming member and said electrodes and means for producing a controlled electrical discharge across said electrodes whereby rapidly heated gases within said confined space explosively force said web against said forming member to deform the web.

2. Apparatus of claim 1 including means for continuously rotating the cylinder to present a fresh surface to each successive electrical discharge.

3. Apparatus of claim 2 wherein said cylinder is tilted so with respect to said plate that the portion of said flat end diametrically opposite said forming member is raised at an angle of 5 to 15 above said plate, and dressing means for said flat end is disposed on said plate beneath said diametrically opposite portion.

4. Apparatus for perforating webs comprising a plate, an opening in the plate, a die having a perforation therethrough mounted on one side of said opening, an anvil mounted on the other side of said opening and cooperating with said die to provide an at least partially confined space therebetween, said anvil being formed by an arcuate segment of the substantially flat end of a thick-walled cylinder mounted on said plate for rotation with respect to said die, a pair of spaced electrodes disposed within said opening between said die and said anvil, means for advancing a web between said die and said electrodes, and means for producing a controlled electrical discharge across said electrodes whereby gases within said confined space are explosively expanded to shear out the portion of said web overlying said die perforation.

5. Apparatus of claim 4 wherein said electrodes are disc-shaped and are adapted to rotate as the web advances through the apparatus.

6. Apparatus of claim 4 including a tube communicating with the perforation in said die for carrying away ionized particles and the pieces of web material sheared from the web.

7. Apparatus of claim 4 wherein the edges of said die surrounding the perforation therein are chamfered.

8. Apparatus of claim 4 wherein said die is made from aluminum oxide.

9. Apparatus of claim 4 including a high voltage power source connected to said electrodes through a discharge device, cam means synchronized with the movement of said web for actuating said discharge device, thereby perforating said web at predetermined spaced intervals.

10. Apparatus for perforating a web comprising a fiat plate made from dielectric material over one face of which said Web is advanced, said plate having an arcuate-shaped opening therethroug h, a pair of electrodes mounted on said plate and extending into said opening from opposite sides thereof, said electrodes being essentially coplanar with the face of said plate, a die made from dielectric material having an aperture therethrough and mounted to bear with yieldable pressure against said plate opposite said opening, a relatively thick-walled hollow cylinder mounted for rotation on the other face of said plate with an arc of the end thereof disposed in said arcuate shaped opening, thus confining a space between said cylinder end and said die, means for producing an electric are between said electrodes to explosively expand gas within said confined space, thereby shearing from a web moving through said apparatus that portion of said web overlying said apertures.

11. Apparatus of claim 10 wherein said cylinder is mounted with its axis at an angle of 75 to 85 with respect to said plate and which includes dressing means mounted on said plate diametrically opposite said arcuateshaped opening for planing the end of said cylinder as the cylinder rotates.

12. The apparatus of claim 11 wherein said cylinder is made from polytetrafluoroethylene and said dressing means comprises a knife for shaving the end face thereof.

13. In an apparatus for perforating sheet material by explosively expanding a gas within a confined space against a die having an opening therein, the improvement which consists in chamfering the edge of the die surrounding the opening, said die being made from aluminum oxide.

14. An apparatus for perforating a rapidly moving web comprising a die having an opening therethrough and a cooperating anvil for providing an at least partially confined space therebetween support means for said die and anvil, a pair of electrodes mounted on said support means and disposed within said confined space opposite said die opening, means for advancing said web between said die and said electrodes, vacuum means communicating with said die opening to maintain said web in continuous contact with the face of said die, power means connected to said electrodes to produce a high voltage electrical discharge across said electrodes for explosively expanding gas within said confined space, and means for synchronizing said electrical discharge with the rate of advance of said web.

15. Apparatus for making a predetermined formation in a rapidly moving web comprising a flat plate over one face of which said web is advanced, said plate having an arcuate-shaped opening therethrough, a pair of dielectric blocks mounted in said plate on opposite sides of said arcuate-shaped opening, a disc-shaped electrode rotatably mounted in each of said blocks to provide a spark gap across said arcuate-shaped opening, said electrodes being coplanar with said one face of said plate, a smooth-faced forming member made from dielectric material and mounted to bear with yieldable pressure against said plate opposite said arcuate-shaped opening, a relatively thickwalled hollow cylinder mounted for rotation about a shaft extending outwardly from the other face of said plate with an arc of the end thereof disposed in said arcuateshaped opening, thus confining a space between said cylinder end and said die, said shaft being disposed at an angle with respect to said plate whereby the portion of said cylinder end diametrically opposite said are is raised above the plane of said plate, dressing means mounted on said plate beneath said raised portion to reface the end of said cylinder as it rotates, resilient pressure means bearing against said cylinder to urge it into contact with said dressing means, and means for producing an electric are between said electrodes to explosively expand air within said confined space to impel said web against said forming member.

16. The apparatus of claim 15 wherein said die is mounted on a parallelogram linkage to keep the webcontacting face thereof parallel with said web and simultaneously movable laterally with respect to said web.

17. Apparatus for punching holes in a rapidly moving paper web which comprises a base plate, an opening through said plate, a die mounted on one side of said opening, a circular anvil mounted for rotation on said plate and having an arcuate portion thereof extending into said opening opposite said die, which portion cooperates with said die to provide a confined space therebetween, a pair of spaced electrodes mounted on said plate and disposed within said opening between said die and said anvil, means for advancing said web over said plate between said die and said electrodes, and means for producing a controlled electrical discharge across said electrodes whereby gases within said confined space are explosively expanded to shear out the portion of the web overlying said die, said anvil being made from an organic dielectric material capable of being volatilized by said electrical discharge.

18. The apparatus of claim 17 which includes a high voltage power source connected to said electrodes through a discharge device and means synchronized with the movement of said Web for actuating said discharge device, thereby perforating the web at predetermined spaced intervals.

References Cited in the file of this patent UNITED STATES PATENTS 1,555,280 Dunham Sept. 29, 1925 2,405,714 Ryan Aug. 13, 1946 2,737,882 Early et al Mar. 13, 1956