The present invention relates to the art of plasma arc torches and, more particularly, to improvements in connection with a drag cup for use with a plasma arc torch.
It is of course well known that a plasma arc torch comprises a nozzle receiving an electrode which has a nose end facing an end wall of the nozzle and which end wall has a plasma outlet opening therethrough. The electrode and nozzle may be relatively displaced between starting and operating positions in which the electrode respectively contacts the end wall and is spaced an operating distance therefrom. The end wall of the nozzle and the end face of the electrode provide a gas chamber into which a plasma gas is supplied and from which a plasma jet is emitted through the outlet orifice when an arc current is flowed between the electrode and nozzle. Upon starting the torch, by moving the electrode out of contact with the end wall to create a pilot arc, the torch operates in a non-transferred pilot arc mode and, when the nozzle is moved into proximity with a workpiece, the arc is transferred to the workpiece and the torch then operates in an arc-transferred mode. In an alternative structural arrangement, the electrode and nozzle can be fixed relative to one another and the torch started by the use of a high frequency or other known starting procedure.
Drag cups are used with hand held plasma arc torches and advantageously allow an operator to contact the torch assembly with a workpiece such as during a cutting operation, primarily to assist the operator in maintaining a correct standoff distance between the torch and workpiece. A drag cup also functions to protect against double arcing which occurs when the nozzle contacts the workpiece. Such drag cups are generally made from a ceramic material or a metal material, the latter being preferred due to easy breakage of ceramic materials. When a metal material is used, an insulating layer is provided between the torch nozzle and the drag cup so as to electrically isolate the drag cup from the nozzle. In either event, the drag cup is cooled by shielding gas flowing along the outside of the nozzle during operation of the torch. Drag cups heretofore provided have included a cylindrical skirt portion providing a relatively large diameter opening, often larger than the diameter of the nozzle and providing a large open area at the outer end of the drag cup. The shielding gas flows through the open area without disturbing the plasma jet which exits through the outlet opening in the end wall of the nozzle, and the primary disadvantage of this drag cup structure is that the nozzle is not protected from the blow-back of molten metal. Accordingly, metal builds up at the outer end of the nozzle and, ultimately, will short the nozzle to the workpiece and cause double arcing. Other drag cups heretofore available have included an end wall across the outer end of the cup and provided with a small opening axially aligned with the outlet opening from the nozzle for the plasma jet. The end wall blocks the blow-back of molten metal and the latter is blown away from the end of the drag cup by the shielding gas which flows through the central opening therein for the plasma arc and through a plurality of laterally outwardly directed bleed ports provided in the end wall about the central opening. The shielding gas flow through the central opening and through the vent passages must be smooth and, accordingly, drag cups having this structure are undesirably expensive to manufacture due to the drilling and deburring of the multiplicity of holes therethrough.