Plasma Arc Metal Cutting Apparatus With Actuation Spring

1. Field of Invention

This invention relates to a plasma arc cutting apparatus comprising an over-center spring mechanism which very rapidly and precisely separates a cathode from an anode, moving the cathode from one operating position to a second operating position for contact arc starting.

2. Description of The Prior Art

It is known in the art generally to use a snap mechanism to move a member into or out of an operating position.

In Armstrong U.S. Pat. No. 3,367,463 an arcuate clutch shoe and a restraining pin are mounted at a drive plate intermediate a leaf spring and a brake shoe. The leaf spring co-acts with the pin to maintain the shoe to be engaged or disengaged.

O'Brien et al. in U.S. Pat. No. 2,561,405 shows a snap disc spring with a throw rod using a compression coil spring to increase the effective length of throw of the snap disc spring in moving the rod.

Rasmussen et al. in U.S. Pat. No. 2,410,680 shows a snap action mechanism with a quick snap action from an inactive to an active position embodying the use of a diaphragm.

In Sanders U.S. Pat. No. 4,791,268, a movable cathode is disclosed in a plasma torch, said cathode being automatically separated from an anode or nozzle contact by a build-up of a gas pressure opposing and overcoming a normal spring pressure whereby the torch is useable in a non-transferred mode as well as in a transferred mode.

The above are indicative of the present state of the art.

It is an object of this invention to provide in a plasma torch structure means to start a contact arc in which the cathode and anode have relative movement for separation whereby the act of separation creates a pilot arc across the gap formed by this separation.

It is another object herein to provide a structure embodying a movable cathode and a stationary anode or the reverse structure in which both the cathode and anode are capable of movement.

It is a further object herein to provide a plasma torch structure embodying an over-center spring mechanism to separate the cathode and anode by a switch lever action, the switch being in circuit with said cathode or anode or by a remote operating means such as energizing a solenoid to actuate the spring mechanism.

It is also an object of this invention to provide a plasma torch structure wherein the electrode and nozzle are normally in engagement and in circuit. An over-center spring mechanism is in engagement with the electrode. The over-center spring mechanism is actuated upon the circuit becoming energized whereby the electrode is withdrawn from the nozzle and the pilot arc created by the circuit becoming energized initiates a plasma jet in a non-transferrable mode, the jet projecting from the nozzle through an orifice therein is brought into contact with a work piece and the plasma arc is transferred from the nozzle to the work piece and in this transferred mode current flows through the plasma arc back to ground through the work piece.

Finally it is also an object herein that when current is shut off from the torch, the over-center spring mechanism snaps back to its normal position of pressing the electrode against the nozzle for engagement therewith and the resulting impact between the electrode and nozzle provides a cleaning action by breaking up the non-conductive oxides which may have developed and gathered on the surfaces of either the electrode or the nozzle.

These and other objects and advantages of the invention will be set forth in the following description made in connection with the accompanying drawings in which like reference characters refer to similar parts throughout the several views.

FIG. 1 is a side elevational view of the invention in operating position in connection with a work piece shown in vertical section; and

FIG. 2 is a view of the invention in a longitudinal section; and

FIG. 3 is an enlarge view of the head portion of the invention in vertical longitudinal section; and

FIG. 4 is a view similar to that of FIG. 3 in another operating position; and

FIG. 5 is a view in horizontal section taken on line 5--5 of FIG. 3 as indicated; and

FIG. 6 is a partially schematic wiring diagram.

Shown in the drawings is a plasma torch 10 comprising a handle 12 with the near side thereof being removed, as shown in FIG. 2, said torch comprising a grip portion 12a and a forward end or head portion 12b having much of the working parts of the torch disposed therein.

Received within the rear end portion 12c of said handle is a cable 13 which in a regular manner runs to it from an associated power supply now shown, the power supply of said plasma torch comprising a pressurized air supply and an adequate current source which are conventionally transmitted through said cable 13 to operate said torch. The pressurized air supply and its path are indicated by the arrows 14.

Pivotally mounted within the upper portion of said handle is a switch 15 which is a push lever type of switch, said switch being horizontally elongated and being pivoted at its rear end portion 15a by a pivot 17 for vertical movement, said pivot being a pin which extends axially to a side wall of said handle to be suitably secured.

Extending forwardly and upwardly from said rear end portion of said switch integral therewith and extending forwardly thereof is an operating switch arm or lever 15b operable through a top opening 12e in said handle 12. Positioned under said switch lever arm 15b spaced rearwardly of the forward end thereof is a coiled compression spring 21 seated upon a support member 22 which is suitably secured to an adjacent side wall portion of said handle. Said spring extends upward into a recess 15d of said switch having its upper coil suitably secured therein about a projection 15e. The forward end portion of said switch is indicated at 15c.

Spaced rearwardly of said spring 21 and said support member 22 is a microswitch 24 suitably secured to an adjacent wall portion of said switch and extending upwardly to be engaged and operated by an adjacent portion 15f of said switch lever 15b.

Said switch lever is adapted to be depressed to two levels of operating positions as will be further described.

Pivotally depending from the forward end portion 15c of said switch 15 and secured by a pin or pivot 18 is a guide member 40. Said guide member has a central upstanding portion 40a forming a recess 40c therein and extending laterally from the bottom of said upstanding portion are arms 41 and 42 having downwardly and inwardly turned end portions form small recesses or notches A and D to be further alluded to.

Depending from the leading or forward end of said handle 12 is a relatively short brass contact tube 33 having threaded onto the outer end thereof a nozzle cap 34 which has about the sides thereof and secured thereto a cylindrical insulator 35.

Disposed within said nozzle cap 34 and extending outwardly thereof is a nozzle 36 having at its tip portion a restrictive orifice 36a and having an inner mating surface 36c for electrical connection with said nozzle as will be described.

Disposed within said contact tube and extending upwardly thereof are a pair of concentric cylinders or tubes 38 and 38a of which 38a is the inner and shorter one and therebetween is a passage 38c.

Within said tube 38a is a cylindrical plunger 37 having an electrode 43 suitably attached as by threading to its lower end portion as an extension thereof and the same is preferably formed of copper. Said plunger and said electrode extend substantially from a contact position with the surface 36c at the inner top of said nozzle to a point having its upper end 37c spaced from the top of said recess 40c. This will be further described. An annular passage 36b extends around the lower portion of said electrode as an extension of the passage 38c.

Inserted into the tip of said electrode is a small cylinder 43a of a material serving as a thermionic emitter such as the metallic element hafnium which when energized emits electrons to the ionizable gas passing thereby.

As a word of explanation, thermionic emission which is electron or ion emission, results from and increases rapidly with the increasing temperature of the emitter. The gas used here is pressurized air and is charged with current in passing by the emitter. The air has a certain resistance to current passing therethrough and the resistance generates heat.

The orifice in the nozzle constricts the ionized air carrying the current. All of the cutting current passes through an area with a small cross section resulting in high resistance and high heat.

In the heated state indicated the pressurized air turns into a plasma state and this extends only between the metallic insert 43a and the nozzle 36 in a non-transferred mode or between the metallic insert and the work piece 70, to be described, in a transferred mode.

Mounted upon said electrode 43 and bearing against the adjacent end of the cylinder 38a and retained by the nozzle 36 is a swirl tube 47 having angled grooves 47a therein which spins the plasma passing through the passage 36b and through said swirl tube.

Said tubes 38 and 38a have a non-conductive top cover 38d formed as a plate member having a close fitting opening 38e allowing the vertical movement of said plunger 37. With the nozzle cap 34 positioned and therein, the nozzle 36, the electrode 43, the swirl tube 47 and the plunger 37 are assembled in order.

With the plunger 37 in position engaging the nozzle 36, an annular groove or notch 37b is cut thereabout adjacent the top of said plate member 38d and providing a reduced diametrical portion 37c of said plunger, said portion having an upper shoulder 37e and a lower shoulder 37f which serve as upper and lower limit or stop members as will be described.

Engaged by said upper and lower shoulders 37e and 37f is a wafer plate-like member 38g suitably secured to the top surface of said member 38d and having an opening 38h therein to permit the vertical movement of the reduced diametrical portion 37c of said plunger 37.

The lower stop member permits a solid contact between said electrode and the adjacent surface 36c of said nozzle. The upper stop member provides an exact proper gap between the electrode and nozzle or between said cathode and said anode, as has been described.

Spaced downwardly from the top of said plunger is an annular groove or recess 37a thereabout. With the electrode 43 engaging the nozzle 36, said annular recess will be in a plane below the plane of the recesses A and D. Opposed points B and C are indicated in said recess.

Now, mounted upon said plunger 37 and said guide member 40 is an over-center spring mechanism 45. Referring to FIGS. 3 and 5, said spring mechanism is shown formed of a pair of leaf springs 46 and 47 configured as illustrated with said springs having their respective upper tip portions 46a and 47a engaging the notches A and D and their respective lower tip portions 46b and 47b engaging the opposed annular recess 37a points B and C at either side thereof. Said spring mechanism in its normal at rest position holds the plunger in its downward position with the electrode 43 engaging the nozzle 36.

Briefly described will be the mechanical effect of operating the circuitry. When the switch lever 15b is depressed, it deflects until the surface 15g comes into contact with surface 15h. As the switch lever 15b is further deflected, region 15f of the lever arm depresses the plunger on the microswitch 24 and closes its contacts 60a and 60b shown in FIG. 6. See also dotted line depressed position of 15b in FIG. 3.

As the switch 15b is depressed, the effect is to lower the forward end switch portion 15c pivoting it at 17 and which in being thus moved, moves the guide member 40 downwardly and this in turn moves the leaf spring end portions 46a and 47a downwardly to come into the plane of the spring ends 46b and 47b. Thus the respective ends of said springs 46 and 47 are brought into a common plane at a toggle point. When said switch lever 15c of said switch 15 is further moved into its further depressed position, the effect is to further lower the projected end 15c of said switch 15 and thus further lower the guide member 40 moving the spring ends 46a and 47a downwardly below the plane of the ends 46b and 47b which moves said ends below or beyond said toggle point. Immediately that said spring points 46a and 47a are below their toggle point with respect to said spring points 46b and 47b, the ends 46b and 47b snap upwardly causing upward movement of said plunger 37 and thus the consequent sudden withdrawal of said electrode 43 from its physical contact with the nozzle 36. See FIGS. 3 and 4. Thus as will be further described, in being energized and suddenly withdrawn from its physical engagement with said nozzle, an arc 40 is formed and this arc is essential to the following briefly described plasma cutting process. The rapid separation of said electrode and nozzle increases the useful life of said electrode and improves its otherwise performance.

The circuitry as such is not a part of the invention as claimed and is conventional for achieving the operation herein. The circuitry comprises two circuits, C1 and C2.

The cable 13 carries the wiring and the pressurized air gas supply which is conducted through a hose 50 secured to a nipple 51 which provides direct access to the annular passage 38c. From the annular passage 38c, the gas has direct passage to the annular passage 36b and through the angled slots 47a in the swirl tube 47 to the annular passage 36d about the electrode 43. From the annular passage 36d about the electrode 43. From the annular passage, the gas flows either through the orifice 36a or is allowed to escape back between the plunger 37 and the tube 38a and out of the opening 38c.

One wire 53 carried by said cable is connected to the micro-switch 24 and the return wire is 54.

A wire 56 referred to as a plasma arc wire is connected to the plunger 37 at the electrical connection 58. The ground wire 59 is connected to the contact tube 33 and contact 33a thereon.

Referring to FIG. 6, a description of the operation of the circuitry is now given. To activate the apparatus, the micro-switch 24 is depressed and the initial depression of said micro-switch thus close the first circuit C1 whereupon the relay coil 60 is energized pulling in its contacts 60a and 60b to close what is a second circuit.

The depression of the micro-switch is continued as the second circuit C2 is energized. The electrode and nozzle are in electrical or touching engagement. The relay has closed its contacts 60a and 60b energizing the transformer 62 whose output is rectified by a rectifier 63 and current flows through a n inductor 64 and by line 56 through the electrode 43, the nozzle 36 and back through line 59 and resistor 65. The purpose of the inductor is to maintain a steady, smooth current flow. The resistor limits the arc starting current and also puts the grounded or work piece 70 at a somewhat lower potential than otherwise. The resistance is relatively small. A two ohm resistor, for example, has been found to be practical.

With said further depression of the micro-switch, the plunger guide 40 is moved downwardly to place the spring ends 46a and 47a below their toggle point with respect to the spring ends 46b and 47b and upon this occurrence, the spring ends 46b and 47b with the engagement of the plunger 37 therebetween, snaps said plunger upwardly into the recess of the guide 40c.

As said plunger and its attached electrode is withdrawn or pulled away from engagement with said nozzle 36, an arc is generated therebetween. The high temperature of the arc starts the thermionic emission of the insert 43a, resulting in a plasma jet or arc. With the gas passing through the swirl tube, the same having angled vertical slots therein to encourage the swirling action thereof, the plasma arc extends through the orifice 36a in said nozzle 36 as indicated in FIG. 6 at 75. With current flow taking a path of least resistance, the current flows through the work piece 70 and back through a ground clamp not shown, rather than through the arc starting path.

When the current is shut off from the torch, the over center spring mechanism is snapped back to its normal position pressing the electrode against the adjacent surface of the nozzle. Thus is completed the operation.

It will of course be understood that various changes may be made in form, details, arrangement and proportions of the product without departing from the scope of the invention which, generally stated, consists in a product capable of carrying out the objects above set forth, such as disclosed and defined in the appended claims in the parts and combination of parts disclosed and defined in the appended claims.