Electric heating – Metal heating – By arc
Patent
1995-07-28
1997-03-25
Paschall, Mark H.
Electric heating
Metal heating
By arc
21912139, 21912155, 21912159, B23K 1000
Patent
active
056141106
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a plasma cutting method and a cutting apparatus and, more particularly, to a plasma cutting method and a cutting apparatus used for cutting a workpiece.
BACKGROUND ART
Conventionally, a plasma cutting apparatus having an electrode, a nozzle disposed as a cover with a spacing for a plasma gas passage, and an electrically insulated protective cap, having an opening facing a nozzle orifice on the outside and at the extreme end of the nozzle, is employed to cut a workpiece while causing a protective gas to flow from the opening of the protective gap to protect the nozzle.
such conventional art will be described with reference to FIG. 5. An electrode 61 is provided at a center of a torch 60, and a nozzle 65 is disposed outside the electrode 61 as a cover with a spacing for a plasma gas passage 63. A high-temperature plasma gas flow 72, formed by causing a plasma gas flow and by simultaneously generating a plasma arc between the electrode 61 and a workpiece 71, is made thinner by the orifice 65a of the nozzle 65 and is jetted to the workpiece 71, thus performing cutting. In this cutting, the shape of the orifice 65a of the nozzle 65, for making the plasma gas flow 72 thinner, largely affects cutting qualities (an inclination of a cut surface, roughness of the cut surface, attachment of dross which is a molten metal, and the like). That is, during the operation of forming a hole in order to be able to start a cutting operation (referred to hereinafter as the time of piercing), if dross 73, blown upwardly from the workpiece 71, attaches to the nozzle 65 or if the nozzle 65 and the workpiece 71 contact each other due to thermal strain or the like during cutting, the orifice 65a is deformed, so that the cut quality is considerably reduced. This is a serious problem in terms of practical use. To cope with this problem, a protective cap 69, having an opening facing the orifice 65 of the nozzle 65, is provided as shown in FIG. 5. This protective cap 69 is electrically insulated from the electrode 61 and the nozzle 65 to prevent irregular discharge (double arc). Also, a protective gas is caused to flow from the cap opening from the start of a cutting to the completion of the cutting, to protect the nozzle 65 against dross 73 blown upwardly from the workpiece 71 (refer to, for example, Japanese Patent Application No. 4-72109).
In this operation of protecting the nozzle with the protective cap, a large amount of protective gas is caused to flow at the time of piercing as well as at the time of cutting. The following two problems are therefore encountered. First, because the flow velocity and the flow rate of the protective gas are large, the high-temperature plasma gas flow downstream of the nozzle is so cooled that the cutting ability is reduced and the amount of attachment of dross is increased, thereby affecting the cut quality. If only a small misalignment occurs between the axes of the orifice of the nozzle and the opening of the protective cap, it also has a bad influence; that is, the plasma gas flow is inclined and disturbed by the strong protective gas flow. Therefore, a special mounting device or a longer mounting time is required in order to achieve a high accuracy of mounting the protective cap.
The second problem is that the operating cost is high because the amount of protective gas used is large. The operating cost is allowable when a low-priced gas such as air is used as the protective gas, but it becomes high when a high-priced gas is used. As a method for externally shielding a plasma gas flow for the purpose of improving cut qualities, the following and other methods are known: a method of shielding an oxygen plasma with oxygen (see, for example, Japanese Patent Laid-Open No. 59-229282); a method of shielding a plasma gas flow with an inert gas to prevent the plasma gas flow from involving atmospheric air (see, for example, Japanese Patent Laid-Open No. 51-98652); and a method of shielding an oxygen plasma with hydrogen gas to cut stainless steel with
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Hasegawa Masahiko
Shintani Toshiya
Komatsu Ltd.
Paschall Mark H.
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