Nozzle for a plasma arc torch having an angled inner surface to facilitate and control arc ignition

A plasma arc torch mounts an electrode and a nozzle, with a mutual spacing, symmetrically about a common longitudinal axis at one end of the torch adjacent a workpiece. The inner surface of the nozzle has an annular, inwardly-directed projection, located opposite the electrode and at a point of closest spacing between the electrode and the nozzle. The projection, or "angle kink", is characterized by a minimum radius of curvature as compared to the radii of curvature of adjacent portions of the nozzle opposite said electrode. The inner surface is adjacent a central exit passage of the nozzle and the angle kink is spaced from the intersection of the exit port and the inner nozzle. In the preferred form the interior nozzle surface is formed by two conical surfaces each having different slopes with respect to the longitudinal axis so as to form the circumferentially extending angle kink at their plane of intersection.

BACKGROUND OF THE INVENTION 
The present invention relates to plasma arc torches, and more particularly 
to an improved nozzle construction for initiating an arc in such torches 
utilizing a high voltage, high frequency signal applied to either an 
electrode or to the nozzle. 
The starting of plasma torches has for a long time been a problem area in 
plasma torch development and the focus of much engineering attention. 
There are currently three know methods to initiate a plasma arc discharge 
and start a plasma arc torch: 1) the high frequency discharge or its 
relative, the high voltage spark discharge, 2) contact starting, and 3) an 
exploding wire technique. In each method, an arc is drawn in an ionizable 
gas between a cathode (an electrode) and an anode (a nozzle or component 
of a nozzle). 
The oldest, and most widely used method is the high frequency, high voltage 
spark discharge method. The high voltage, high frequency generates charge 
carriers which create an electrical current path in the gas in the gap 
between the cathode and the electrode to establish D.C. flow of current, a 
pilot arc discharge. It is common practice to attach the high frequency 
coil to the power supply line leading to the electrode or the nozzle, but 
on smaller systems having a lower power rating, e.g., those characterized 
by a DC amperage of 200 or less, the high frequency coil is usually 
coupled to the power line for the electrode. 
An example of this high frequency, high voltage starting method is 
described in U.S. Pat. No. 3,641,308 to Couch, Jr., et al. A brief, high 
voltage pulse applied to the cathode initiates an arc across the gap to a 
nozzle which is connected through a switch and a resistor to ground. The 
workpiece is also grounded so that once the gas flow is initiated the arc 
will transfer from the nozzle to the workpiece. The switch is then opened 
so that the nozzle is electrically floating and the workpiece remains 
connected to ground. This general method of starting is also disclosed in 
U.S. Pat. Nos. 3,082,314 to Arata et al; 3,131,298 to Browning; 3,534,388 
to Ito et al; 3,619,549 to Hogan et al; 3,787,247 to Couch, Jr.; 3,833,787 
to Couch, Jr.; and 4,203,022 to Couch, Jr. et al. 
In prior art plasma arc torches the electrode has traditionally had a 
generally cylindrical configuration, whether a cylindrical disk seated in 
a solid copper tube as described in the aforementioned U.S. Pat. Nos. 
3,641,308, 4,203,022 or the electrode-nozzle arrangement shown and 
described in U.S. Pat. Nos. 4,421,970; 4,791,268; and 4,861,962. In these 
prior art arrangements, the lower end of the electrode adjacent the nozzle 
typically has a cylindrical configuration. The immediately opposite nozzle 
surface typically mirrors the outer configuration of the electrode, or is 
smooth, conical and downwardly converging. In both instances the nozzle 
includes a central exit port where the plasma arc exits the torch and 
attaches to the workpiece. It is significant that while the transition 
between the interior nozzle surface and the exit port may be a sharp 
corner, and while this corner may be closely spaced from the electrode, it 
is not located in a region where it is closer to the electrode than 
immediately adjacent portions of the nozzle. 
There are two principal problems with the prior art designs. First, when 
the arc is initiated it is sometimes difficult to induce the breakdown of 
the gas to start the plasma arc discharge. To reliably reach the breakdown 
potential to initiate the arc, the voltage level applied to the electrode 
is increased. This, however, accentuates the electromagnetic noise 
interference problems associated with all high-frequency, high-voltage 
starting arrangements. 
Another quite significant problem is the lack of reliability as to where on 
the electrode and nozzle the arc will initiate. This lack of control over 
the location of the arc causes wear problems. For example, if the high 
frequency spark begins high on the side of the electrode (away from the 
exit port), by the time it travels to the lower portion of the electrode, 
typically containing an electron emitting element, the DC current and 
voltage levels can be ramped up to a level such that double arcing occurs. 
As is well known in the trade, double arcing will quickly destroy torch 
components such as the nozzle and the electrode. 
In the prior art it is known to machine mark the electrode to facilitate 
breakdown of the gas at the mark on start up. However, to the best of 
applicant's knowledge, there has been no nozzle structure specifically 
designed to reduce the breakdown potential (and the elapsed time required 
to achieve breakdown) and to control the location of where the pilot arc 
is initiated by applying a high frequency, high voltage to either the 
nozzle or the electrode. This is particularly true where the high voltage, 
high frequency coil is attached to the power line leading to the nozzle, 
not the electrode. In this situation with prior art constructions, 
variations in the external configuration of the electrode would have 
little or no effect on the location of the arc. 
It is therefore a principal object of the present invention to provide a 
nozzle construction or a plasma arc torch which reliably initiates an arc 
within a small, well-defined annular region of the electrode and the 
nozzle. 
Another principal object is to reduce electrode wear as compared to 
comparable prior art nozzles in comparable torches operated under the same 
conditions. 
Yet another principal object of this invention is to reduce the breakdown 
potential required to initiate an arc discharge in a given plasma arc 
torch. 
A further object in the present invention is to provide a nozzle 
construction which reduces electromagnetic interference during the high 
frequency, high voltage start up with other electrical and electronic 
components in the operating area. 
A still further of the present invention is to provide a nozzle 
construction with the foregoing advantages which is simple in 
construction, has a comparatively low cost of manufacture, and can be used 
as a replacement part for conventional nozzles of existing plasma arc 
torches. 
SUMMARY OF THE INVENTION 
A plasma arc torch includes an electrode and a nozzle mounted in a mutually 
spaced relationship at one end of the torch adjacent a workpiece. They are 
also mounted symmetrically with respect to the torch and each other about 
a common longitudinal axis. A flow of an ionizable gas passes through the 
torch and exits a central exit port in the nozzle which is aligned 
opposite the lower end of the electrode. A DC power supply is attached to 
the electrode and the nozzle and a high frequency, high voltage 
coil/generator is electromagnetically coupled to either the negative or 
positive output of the DC power supply. 
The nozzle, which is typically cup-shaped, receives the electrode within 
the nozzle with a generally uniform spacing between the electrode in the 
opposite surfaces of the nozzle. The interior surface of the nozzle facing 
the electrode has a convex, inwardly projecting, circumferentially 
extending solid angle or "angle kink" portion. This angle kink is 
characterized by a minimum radius of curvature (whether a sharp corner, a 
rounded corner, or even some more complex cross-sectional shape) for a 
narrow annular region of this surface immediately opposite to the 
electrode and adjacent the central exit port of the nozzle. This annular 
region defines the location of preferential initiation of the plasma arc 
discharge. This kink angle is located at the closest approach between the 
electrode and the nozzle, and is displaced from the angle formed by the 
intersection of the central exit port in the nozzle and the interior 
surface of the nozzle. 
In a preferred form, the angle kink is formed by the intersection of two 
conical surfaces having different angles of inclination with respect to 
the longitudinal axis. Comparatively flat angle kinks, e.g. one with an 
included angle of about 165.degree. are effective. The electrode also has 
an outwardly projecting angle kink formed by the cylindrical side surface, 
or a conical transition surface of the lower end of the electrode, and a 
planar end face oriented perpendicular to the longitudinal axis. The 
electrode angle kink is also located at the point of closest approach 
between the electrode and the nozzle and it opposes the angle kink on the 
nozzle. The electrode and nozzle angle kinks are centered in 
circumferentially extending annular regions. Because of the minimum radius 
of the angle kink and the fact that the angle kink are located at local 
points of closest approach of the electrode and nozzle, 1) the electric 
field induced by the high voltage, high frequency starting signal is 
enhanced and 2) there is a reliable and rapid breakdown of the plasma at 
this region in preference to other locations on the electrode or nozzle. 
These and further features and objects of the present invention will be 
more readily understood from the following detailed disclosure of the 
preferred embodiments which should be read in light of the accompanying 
drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
With reference to FIG. 1, a plasma arc torch 10 having a nozzle 12 and an 
electrode 14 is connected to a DC power supply 16. As shown, the positive 
output of the power supply 16 is connected by lead 18 to the nozzle 12 and 
the negative side of the power supply 16 is connected by lead 20 to the 
electrode 14. The plasma arc torch 10 can be any of a wide variety of 
currently available torch designs in the prior art. Suitable constructions 
are shown, example, in the aforementioned U.S. Pat. Nos. 3,641,308; 
4,203,022; 4,791,268; and 4,861,962. A high-frequency coil 22 can be 
coupled electromagnetically to either the positive lead 18, as shown, or 
the negative lead 20. The high-frequency coil/generator produces a 
high-voltage, high-frequency signal which is transmitted over the lead to 
the nozzle or the electrode. The high-frequency coil generator is of well 
known, conventional construction, as is the DC power supply. A typical 
high voltages produced by the coil/generator 22 are in the range typically 
5,000 volts at frequencies in the range of 2 to 3 mHz. 
During operation, a flow 24 of an ionizable gas is initiated through an 
electrode-nozzle chamber 25 exiting the torch through an exit port 44 in 
the lower face of the nozzle. It assumes a swirling flow path 24a between 
the lower end of the electrode and the nozzle just before it exits port 44 
in the nozzle. When high frequency is applied to the system via the coil 
22, a high level oscillating electric field will be generated between the 
electrode 14 and the nozzle 12. 
A significant aspect of the present invention is that the interior surface 
26 of the nozzle has circumferentially extending (closed loop) projection 
or peak which is termed herein an angle kink 28. It is significant this 
angle kink 28 is located at the point of closest spacing between the 
nozzle 12 and the electrode 14, and represents a line of localized minimum 
gap separation between the electrode and the nozzle--as compared, for 
example, to prior art constructions where the corner associated with the 
nozzle exit port may have a minimum radius of curvature, but is no closer 
to the electrode than adjacent portions of the surface. It has been found 
that this construction reduces the breakdown voltage between a narrow 
annular region 30 centered on the angle kink 28 and a narrow annular 
region 32 centered on an electrode angle kink 34. The electrode 14 has a 
conical tapered end surface 36 and a planar end face 38 oriented 
perpendicular to a common longitudinal axis 40 of the torch, the nozzle 
12, and the electrode 14. The intersection of the surface 36 and the end 
face 38 defines the angle kink 34. The intersection of conical surface 26a 
(having an angle A of inclination of about 45.degree. from the axis 40, as 
shown) and conical surface 26b (having an angle B of inclination of about 
60.degree. from the axis 40, as shown) defines the angle kink 28. In the 
preferred form shown, the angle kink therefore has an included angle of 
165.degree. and has a line intersection which is not rounded. The angle 
kinks 28 and 34 and their associated regions 30 and 32 are separated by a 
gap 42 in an electrode-nozzle chamber 25. While the angle kink 28 and 34 
are shown as being directly opposite one another, and this is the 
preferred embodiment, such a precise opposite alignment is not essential 
to the operation of this invention. 
It should be noted that the gap 42 between the angle kinks 28 and 34 is the 
point of closest spacing between the nozzle and the electrode, at least in 
the region immediately adjacent the angle kinks. As a result, the 
application of the high frequency to the nozzle 12 over the positive lead 
18 concentrates the electric field strength at the nozzle angle kink 28 
and provides a reliable breakdown in the zone between the annular regions 
30 and 32. It is also significant that the angle kink 28 formed on the 
interior 36 of the nozzle has a minimum radius of curvature as compared to 
the radius of curvature of the adjoining portions of the surface so as to 
create a protrusion which produces the electric field enhancement 
discussed above. Similarly, the angle kink 34 on the electrode should have 
the smallest radius in the immediate region adjoining the point of closest 
spacing between the electrode and the nozzle at the gap 42. 
If the high frequency coil 22 is attached to the negative lead 20 of the DC 
power supply, the breakdown voltage can be reduced between the annular 
regions 32 and 30 with respect to the remainder of the electrode-nozzle 
chamber 25. Further, by providing an angle kink 34 on the electrode and an 
angle kink 28 on the nozzle that are each characterized by a minimum 
radius of curvature, then there will be a strong electric field 
enhancement that the electrode angle kink 34 which tends to concentrate 
the electric field strength between the annular regions 30 and 32. This 
enhanced field strength in turn both assures that breakdown will occur 
between the zones across the gap 42 and that the breakdown potential will 
be reduced as compared to the potential required if the nozzle 12 had a 
simple conical configuration as depicted in FIG. 3. 
FIG. 4 illustrates alternative embodiment of the present invention where 
the nozzle has a generally cylindrical interior configuration formed by a 
cylindrical side wall 12a' and a lower end wall 12b' that includes the 
exit port 44' (like parts being identified with the same reference number 
in the different embodiments). Again, the nozzle 12' is spaced 
symmetrically from the electrode 14'. The nozzle end wall 12b' is spaced 
closer to the opposite surface of the electrode than the side wall 12a'. 
An angle kink 28 is formed on the inner surface 26' of the nozzle by the 
intersection of the generally planar inner surface portion 26a' defining 
the end wall 12b' and a conical surface 26b' extending between the exit 
port 44 and the inner surface portion 26a'. The conical surface 26b' is 
typically inclined at about 10.degree. from the plane of the surface 
portion 26b'. The angle kink 28' is located at a point of closest spacing 
from the electrode 14', and preferably directly opposite an angle kink 34' 
formed on the lower end of the electrode. As shown, the angle kink 34' is 
the circumferentially extending solid angle defined by the intersection of 
a conical surface 36' and an end surface 38'. The electrode 14' has a 
cylindrical electron emitting insert 48 at the center of its lower end 
face 38', directly opposite the exit port 44. 
The angle kinks 28' and 32' are centered in narrow annular regions 30', 32' 
which define a zone in which the plasma arc discharge will preferentially 
occur. The gap 42' between the angle kinks 28' and 34' is a point of 
closest approach. 
In operation, the nozzle construction of the present invention, when used 
with a 200 ampere plasma torch of the type sold by Hypertherm, Inc. under 
the trade designation MAX 200, reduced the maximum breakdown potential by 
as much as 10% and reduced electrode wear by as much as 30%. Comparable 
reductions have been obtained with other torches under standard operating 
conditions. Moreover, the pilot arc discharge has been found to occur 
preferentially and reliably within a zone defined at its ends by the 
annular regions 30 and 32. 
While the invention has been described with respect to its preferred 
embodiments, it will be understood that various modifications and 
alterations will occur to those skilled in the art from the foregoing 
detailed description and the accompanying drawings. For example, while the 
angle kink on the nozzle has been described as formed by the intersection 
of combinations of conical and planar surfaces, it may be formed with a 
rounded cross-section, a rounded cross-section with a circumferentially 
extending ridge at its "peak", or a variety of other configurations which 
produce a protrusion with a minimum radius of curvature, in cross section, 
located at a point of closest spacing to the opposed cathode, or anode, 
and with this closest spacing being a localized point of minimum spacing. 
Also, while the angle kinks have been described and illustrated for the 
nozzle and electrode as being circumferentially extending, they could be 
formed to extend in angular displacement over less than 360.degree. about 
the axis 40. These and other modifications and alterations are intended to 
fall within the scope of the appended claims.