Plasma piercing with non-oxidative plasma gas and plasma cutting

Details Plasma piercing with non-oxidative plasma gas and plasma cutting Plasma piercing with non-oxidative plasma gas and plasma cutting

1996-11-25

1998-09-01

Paschall, Mark H.

Electric heating

Metal heating

By arc

21912139, 21912155, B23K 1000

Patent

active

058013555

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

The present invention relates to a method and an apparatus for plasma cutting which are employed to cut a workpiece with a plasma gas.


BACKGROUND ART

In a method and an apparatus for plasma cutting heretofore known, a workpiece is cut by using a non-oxidative gas as the plasma gas. When a workpiece is, e.g., stainless steel or an aluminum alloy, a non-oxidative gas, e.g., nitrogen gas, argon gas, hydrogen gas, or methane gas, is used as the plasma gas. By using a non-oxidative gas, a cut product can have less dross, affixed thereto, and a clear cut surface.
Also, when an electrode is made of a metal, e.g., tungsten, that lowers its melting point and is quickly consumed when oxidized, a non-oxidative gas is used as the plasma gas, as mentioned above. By using a non-oxidative gas, it is possible to prevent oxidation of the electrode and early consumption of the electrode.
The piercing start in the prior art will be described. When cutting a workpiece 6 to produce a cut product 62, circumscribed by b-c-d-e-b, for example, as shown in FIG. 4, if the cutting operation is started from the point b, a hole bored at the point k has a larger diameter and blown-up dross affixes thereto in an increased amount, resulting in an inability to maintain the cutting quality. For this reason, the piercing start is practiced as a method of first forming a through-hole (piercing hole) 61 at a point a near the point b by boring (piercing), and then cutting the workpiece 6 from the through-hole 61 along b-c-d-e-b-f in the order named, thereby producing the cut product 62 with high cutting quality. Generally, when the workpiece 6 is a thin plate, the piercing is made by a plasma arc, but when the workpiece 6 is a thick plate, it is pierced beforehand by drilling.
In the prior art plasma cutting method of cutting a workpiece with a non-oxidative gas, it is sure, as stated above, that the cutting operation is performed with a smaller amount of dross affixed, a clean cut surface, and less consumption of the electrode. At the piercing start, however, because the plasma gas contains no oxygen gas, O.sub.2, the heat of oxidation reaction cannot be utilized, giving rise to a problem in that the piercing time is prolonged. This problem is significant, particularly when the workpiece is a thick plate or is made of stainless steel. Further, the prolonged piercing time increases the chance that blown-up dross affixes to the plasma torch. Thus, the plasma torch generates a double arc, due to the dross affixed thereto, and the life time of the plasma torch is shortened.


BRIEF SUMMARY OF THE INVENTION

The present invention has been accomplished with a view of solving the above-stated problems in the prior art, and its object is to provide a method and an apparatus for plasma cutting which can produce a cut product having less dross, affixed thereto, and a clear cut surface, which can reduce consumption of an electrode, and which are capable of high-speed performance even at the time of the piercing operation.
The present invention provides a method for plasma cutting with which a workpiece is cut by ejecting a plasma arc from a nozzle while plasma gas is supplied around an electrode, wherein an oxidative gas is supplied as the plasma gas at the time of a piercing operation, and the composition of the plasma gas is changed to a non-oxidative gas when shifting from the piercing operation to a cutting operation, thereby cutting the workpiece by the non-oxidative gas. The oxidative gas can be oxygen gas or a gas containing oxygen gas, and the non-oxidative gas can be a gas containing at least one of nitrogen gas, argon gas, hydrogen gas, and methane gas. Further, when shifting from the piercing operation to the cutting operation, the supplying of the non-oxidative gas can be started before the supplying of the oxidative gas is stopped.
With the arrangements set forth above, the composition of the plasma gas can be selected to be optimum for the piercing operation and the cutting operation in consideration of material properti

REFERENCES:
patent: 5214263 (1993-05-01), Sakuragi
patent: 5591357 (1997-01-01), Couch, Jr. et al.
patent: 5614110 (1997-03-01), Shintani et al.

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