Abstract: A process and apparatus for cutting a material using a plasma are jet provides improved uniformity along the length of cut, despite variations in cutting speed. This is achieved by pulsing the arc current and dynamically varying the pulsing. By this means the momentum of the plasma arc jet can be maintained substantially constant whilst the amount of energy delivered by the plasma arc jet is controllably varied. The pulsing can be dynamically varied in dependence on one or more of the cutting speed, the angle of ejection of a stream of molten material from the cut, the size of the droplets of the ejected material, the intensity of spectral pattern of light emitted from the plasma arc jet and material interface, and the arc voltage. The pulses can be varied by varying one or more of the pulsing frequency, the pulse duty, upper value of the pulse current, and depth of the pulses.

Abstract: A method and apparatus for detecting non-axisymmetric wear (i.e. grooving 22) of the orifice (12) of the nozzle (13) of a plasma arc torch (1) involves placement of a probe adjacent a plasma jet (21) that emerges from the nozzle (13) such that a number of electrically isolated elements (23) of the probe surround the jet (21) and measuring a voltage drop across an electrode (11) of the torch (1) and each probe element (23) to detect whether there is any deflection of the plasma jet (21). The presence of a groove (22) causes the jet (21) to deflect and is indicated by an increased voltage at the probe elements (23) towards which the jet is deflected and a decreased voltage at the opposite elements. The probe may be formed by segmenting a shield (17) of the torch.

Abstract: A method and apparatus for monitoring the condition of a plasma arc torch determines whether the nozzle (13) of the torch and an electrode (11) of the torch have suffered any erosion and distinguishes the two. The pressure of a plasma forming gas that is supplied for the torch (p.sub.1 or p.sub.n) is monitored while the torch is operating to detect erosion of the orifice (12) of the nozzle (13), and the voltage U.sub.ne between the electrode (11) and nozzle (13) is monitored, also while the torch is operating, to detect erosion of the electrode (11). A pressure, p.sub.1 or p.sub.n below a reference pressure indicative of a good (un-eroded) nozzle indicates erosion of the orifice (12), and a voltage U.sub.ne above a reference voltage indicative of a good (un-eroded) electrode indicates erosion of the electrode. The pressure measurement and U.sub.ne are compared with appropriate reference values to logically discriminate between wear of the nozzle and wear of the electrode (given that an increase in U.sub.