EP1061781A2

EP1061781A2 - Method for providing the electric arc starting point in the electrode of a plasma cutting torch, and electrode thus provided

Info

Publication number: EP1061781A2
Application number: EP00110980A
Authority: EP
Inventors: Giorgio Bassi
Original assignee: Tecmo SRL
Current assignee: Tecmo SRL
Priority date: 1999-06-14
Filing date: 2000-05-29
Publication date: 2000-12-20
Also published as: IT1309290B1; ITBO990319A0; EP1061781A3; ITBO990319A1

Abstract

A method for providing the starting point of an electric arc in an electrode (1) of a thermal plasma cutting torch, comprising the steps of: forming a substantially cylindrical axial seat (3) in the electrode (1), at the end from which the electric arc must start, the seat (3) being adapted to receive, with tolerance, a substantially cylindrical insert (4) made of hafnium or zirconium, and heading the insert (4) inside the seat (3) so as to produce a stable and uniform intimate contact of its outer surface with the inner surface of the seat (3) which is adapted to firmly retain the insert (4) in the seat (3).

Description

The present invention relates to a method for providing the electric arc starting point in an electrode of a thermal plasma cutting torch. The invention also relates to an electrode provided by means of the method.
Conventional thermal plasma cutting torches work by supplying the excitation and ionization energy to the gas whose plasma is to be generated. For this purpose, the gas is passed through an electric arc formed between a negative electrode or cathode and a part to be cut, which acts as an anode. In order to facilitate the formation of the plasma jet, coaxially to the negative electrode there is provided a sleeve which has an annular nozzle and forms, together with the electrode, a tubular channel connected to the supply of the gas whose plasma is to be produced. The sleeve can temporarily act as an anode in order to generate a pilot arc adapted to facilitate the starting of the main arc between the negative electrode and the part. During steady-state operation, the main arc extends, through the nozzle, between the negative electrode and the part, while the gas exits from the nozzle in the form of a plasma jet which strikes the part at high temperature and high speed.
In current torches, the starting point of the electric arc is constituted by an insert which is accommodated in a seat formed axially in the electrode. The insert is typically constituted by a cylindrical pellet of hafnium or zirconium which is highly resistant to spark erosion but causes considerable drawbacks due to the current method with which it is currently implanted in the electrode. The pellet is in fact usually implanted by interference fit in a seat of the electrode, with the following negative effects:
a) during implantation, the pellet is deformed and becomes narrower in the direction of implantation in the electrode, so that the contact between the pellet and the wall of the seat that is meant to receive it remains snug only toward the insertion end and instead becomes unstable at the opposite end. Accordingly, the thermal and electrical conductivity between the material of the electrode and of the pellet decreases considerably; this leads to higher electric consumption for an equal level of performance and to electrode cooling problems;
b) the pellet remains locked in its seat of the electrode only partially, and therefore retention problems due to thermal expansion phenomena can arise;
c) since the pellet is implanted by interference fit, in order to be able to withstand the mechanical implantation forces its diameter must be larger than strictly necessary to generate a clean and stable electric arc. This leads to a high cost of the pellet, in view of the particularly expensive nature of the material used (hafnium or zirconium);
d) in view of the larger dimensions of the pellet, the electric arc is less concentrated and therefore causes a wider and less clean cut.
The aim of the present invention is to provide a new method for providing the starting point of the electric arc, so as to obviate the above listed shortcomings of the prior art.
Within the scope of this aim, an object of the present invention is to provide an improved electrode according to the new method.
This aim, this object and others which will become more apparent hereinafter, are achieved by a method for providing the starting point of an electric arc in an electrode of a thermal plasma cutting torch, characterized in that it comprises the steps of: forming a substantially cylindrical axial seat in the electrode, at the end from which the electric arc must start, said seat being adapted to receive, with tolerance, a substantially cylindrical insert made of hafnium or zirconium, and heading said insert inside said seat so as to produce a stable and uniform intimate contact of its outer surface with the inner surface of said seat which is adapted to firmly retain said insert in said seat.
The electrode provided with this method is characterized in that it has, at the end from which the electric arc is to start, a cylindrical insert made of hafnium or zirconium which is headed in an axial seat formed in said end of said electrode so as to be firmly retained in said seat.
Further characteristics and advantages of the present invention will become apparent from the following detailed description of a new method for providing the starting point of an electric arc, illustrated only by way of non-limitative example in the accompanying drawings, wherein:
Figure 1 is a longitudinal sectional view of an electrode before the implantation of the pellet according to the present invention;
Figure 2 is a view of a pellet to be implanted in the electrode of Figure 1; and finally
Figure 3 is a sectional view of the electrode after implantation of the pellet.
With reference to Figure 1, the reference numeral 1 designates an electrode of a plasma cutting torch. In a conventional way, the electrode is associated with the body of the torch and forms, together with an external sleeve which is coaxial thereto, a tubular chamber which is connected to the supply of a gas whose plasma is to be generated. The sleeve and the other parts of the torch are not shown because they are outside the scope of the present invention and are in any case already known in the art, for example from EPA 97120002.7 of November 14, 1997 by this same Applicant.
The electrode 1 has an ogive-like tapered end 2 in which a seat or blind hole 3 is formed axially and centrally.
The hole 3 is cylindrical and forms a seat for a pellet 4 of erosion-resistant material, for example hafnium or zirconium.
During the provision of the electrode, the pellet 4 is constituted in practice by an insert whose cylindrical shape is complementary to the shape of the hole, so that it can be inserted therein with a minimal tolerance of preferably 0.05 to 0.10 mm.
The hole 3 is slightly shallower than the pellet 4, so that the pellet can be riveted inside the hole by forcing from outside with a striking mass on the end of the pellet that protrudes from the hole 3.
The riveting of the pellet 4 causes the pellet to be headed inside the hole, undergoing a radial deformation which causes it to anchor firmly, with all of its surface, to the electrode. In particular, it should be observed that the radial deformation undergone by the pellet 4 allows to achieve stable and uniform intimate contact between the inner wall of the hole 3 and the outer wall of the pellet 4.
Tests have shown that the riveting of the pellet 4 inside the seat or hole 3 produces a slight barrel-shaped deformation of the pellet which increases its locking in the seat 3 of the electrode 1.
It is evident that the electrode thus obtained allows to achieve the intended aim and object, i.e., higher thermal and electric conductivity, a saving of material due to the smaller dimensions of the pellet with respect to conventional ones, and provision of a more precise cut.
In its practical embodiment, the present invention can be applied to electrodes having any shape without thereby abandoning the scope of the appended claims.
The disclosures in Italian Patent Application No. BO99A000319 from which this application claims priority are incorporated herein by reference.
Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly such reference signs do not have any limiting effect on the scope of each element identified by way of example by such reference signs.

Claims

A method for providing the starting point of an electric arc in an electrode of a thermal plasma cutting torch, characterized in that it comprises the steps of: forming a substantially cylindrical axial seat (3) in the electrode (1), at the end from which an electric arc must start, said seat being adapted to receive, with tolerance, a substantially cylindrical insert (4) made of hafnium or zirconium, and heading said insert inside said seat so as to produce a stable and uniform intimate contact of its outer surface with the inner surface of said seat which is adapted to firmly retain said insert in said seat.
The method according to claim 1, characterized in that said tolerance is preferably between 0.05 and 0.10 mm.
An electrode of a thermal plasma torch, characterized in that it comprises, at the end from which an electric arc is to start, a cylindrical insert (4) made of hafnium or zirconium which is headed in an axial seat (3) formed in said end of said electrode, so as to be firmly retained in said seat.