Plasma arc cutting process and apparatus using an oxygen-rich gas shield

A method of operating a plasma arc cutting torch including operation in a pilot arc mode and then transferring to a transferred arc mode for piercing a metal workpiece and then cutting the workpiece by translating the torch where the torch has a plasma gas flow and a secondary gas flow therethrough, the plasma flow forming a pilot and transferred arc, and said gas flows having a preflow associated with said piercing and an operating flow associated with said cutting comprising

• directing said secondary flow onto said plasma jet, and
• increasing said secondary flow rate during piercing as compared to an operating flow rate for cutting when said plasma jet is in the cutting mode.

A method of improving the cutting speed and cut quality of a plasma arc torch operating on a metal workpiece, where the torch has a body, an electrode and a nozzle mounted in the body in a mutually spaced relationship that defines a plasma chamber, a plasma gas flow in said body that forms a plasma jet in said plasma chamber, the nozzle having a central passage and a nozzle exit orifice through which said plasma jet passes, the method comprising:

• forming a secondary gas flow of a mixture of an oxidizing gas and a non-oxidizing gas, wherein the oxidizing gas comprises at least 40% of the flow as measured by flow rates;
• directing the secondary gas flow onto the plasma jet as said plasma jet leaves the nozzle exit orifice, wherein said secondary gas flow is (i) not substantially ionized and (ii) highly uniform.

In a plasma arc torch having a primary gas flow therethrough that is ionized in a plasma chamber to produce a plasma jet that exits the torch through a nozzle exit orifice to pierce and cut a workpiece, the torch comprising,

• a secondary gas flow path extending through the torch to a secondary gas flow exit orifice located adjacent the nozzle exit orifice,
• a swirl ring disposed in said secondary gas flow path adjacent said exit orifice and configured to direct the secondary gas flow therethrough in a highly uniform flow,
• said secondary gas flow path having a secondary gas flow passing therethrough, said a secondary gas flow leaving through said secondary gas flow exit orifice and being directed onto the plasma jet as it leaves said nozzle exit orifice without being substantially ionized.

A nozzle for a plasma arc cutting torch formed of a conductive material and having an exit orifice at one end for a plasma jet, comprising

• a hollow body portion formed having a generally conical, thin-walled configuration sloping toward the exit port, and
• an enlarged head portion formed integrally with said body portion,
• said head portion (i) being solid except for a central passage aligned with said exit orifice, (ii) having a generally conical outer surface that also slopes toward said exit orifice, and (iii) having a diameter adjacent said body portion that exceeds the diameter of said body portion to define a cut back recess.

A replaceable shield for an annular secondary gas cap mounted at one end to a plasma arc torch and having a body portion extending toward an exit port of a nozzle that directs a plasma jet toward a workpiece for piercing and cutting of the workpiece, said shield comprising

• a metallic member having (i) a central circular opening aligned with the exit port of the nozzle and sized to closely surround the plasma jet, (ii) a set of ports that surround the central opening that are sized to divert a significant portion of the secondary gas flow during the cutting, but not so divert a flow of the secondary gas at a higher rate associated with the piercing, said ports venting the high rate secondary gas flow upon its termination in favor of a flow for cutting, and means located at the outer edge of said member for replaceably sealing said shield to the body portion of the secondary gas cap, said shield intercepting molten metal sprayed from the workpiece toward the torch during cutting.