PLASMA TORCH

Abstract: A plasma torch capable of cutting in a dross free state, made possible by increased energy density of the arc jet, and whose operation efficiency is not reduced even with a low operating gas flow rate since the arc jet can be stably maintained in the plasma torch, and which has a high double arc resistance and excellent durability. This is realized by forming a velocity reduction space N from near a lower end (3b) of the electrode to a nozzle (9) at the front end of the plasma torch (1), the velocity reduction space being used for reducing the axial velocity component of the operating gas which flows along the outer periphery of an electrode (3). The velocity reduction space (N) is cylindrically shaped, and diameter (Dd) of the cylindrical shape is larger than diameter (da) of a lower end (3b) of the electrode. The velocity reduction space may be formed such that the diameter (Dd) of the cylindrical shape is larger than the diameter (da) of the lower end (3b) of the electrode and larger than the height (Ha) of the cylindrical space. The energy density of the arc jet is greater than 4 x 10 5 A ·S/kg.

Description: TECHNICAL FIELD The present invention relates to a plasma torch, and, more particularly, to a plasma torch in which a transferred arc jet is produced to cut a material. A comparable plasma torch is known from the EP-A-452 494. BACKGROUND ART Hitherto, there has been demanded a plasma torch capable of cutting material such as steel, stainless steel, etc. with high precision and without adherence of molten metal (hereinafter referred to as dross), and having a narrow cutting width, capable even of cutting thick plates, and having a long life. With regard to such prior art, the present applicant has proposed a transferred plasma torch, for example, in Japanese Utility Model Application No. 1-72919. For example, Figs. 7 and 8 are each cross sectional views of a nozzle and electrode section of a conventionally proposed transferred plasma torch, wherein swirling air currents are produced in the operating gas. In the transferred plasma torch 50 of Fig. 7, a switch 53 is switched to transfer the arc, formed between an electrode member 51a of an electrode 51 and a nozzle 52, to a cutting material 54. In this plasma torch 50, swirler member 55 is inserted near the electrode 51, disposed within the nozzle 52, and a plurality of holes 55a are obliquely formed downward therein. The operating gas, which has passed through the plurality of holes 55a, becomes swirling currents and is successively accelerated in an acceleration section 52a, formed into a V shape with a gentle inclination at the front end of the nozzle 52, and reaches a nozzle restriction section 52b for restricting the arc jet 56 such that it moves in a straight line. In plasma torch 60 of Fig. 8, a swirler member 63 is inserted near an electrode 62, disposed in nozzle 61, and a plurality of holes 63a are formed in the swirler member 63 perpendicular to axial center Z of the plasma torch 60 and tangential with respect to the inner peripheral face of the swirler member 63. At the front end of the nozzle 61 below the electrode 62, there is disposed a velocity reduction space 61a below and apart from the lower end of an electrode member 62a of the electrode 62. The operating gas, which has passed through the plurality of holes 63a, becomes swirling air currents, and in the velocity reduction space 61a, these swirling air currents allow arc jet 56 to be held in a low-pressure space formed in the center axis and therearound. Since the nozzle 61 has the velocity reduction space 61a at the upstream side, it is capable of preventing deflection of the arc jet 56 which is ejected from the nozzle restriction section 61b, so that they are generated with a high degree of straightness, which results in excellent cutting of a cutting material 54. However, in such above-described conventional transferred plasma torches, when current in conventional use is made to flow through an electrode and when a conventional operating gas flow rate is supplied, it is extremely difficult to achieve cutting of a material in a dross free state, which is thought to be very difficult to achieve even when the conditions are changed. Another different prior art is known, in which cutting in a dross free state is achieved by a method which consists in cutting a material by an arc jet having the operating oxygen gas further enveloped by oxygen curtain during cutting (refer, for example, to Japanese Patent Laid-Open No. 59-229282). Since oxygen is used for the curtain, however, increased gas consumption as well as reduced precision in the dimensions of the cut face or the like, due to burning, result. The present invention hash been achieved to overcome the above-described problems of the prior art and relates to a plasma torch and, more particularly, to a plasma torch in which transferred arc jet is generated, wherein dross adhesion does not occur, the arc jet is stable, and the nozzle, etc., has a long life. DISCLOSURE OF THE INVENTION According to the present invention, there is provided a plasma torch according to claim 1. In the above-described construction, the energy density of the arc jet is greater than 4 x 10 5 [(ampere x second)/kg). In this case, the energy density I/m of the arch jet is defined as [arc current value I (ampere)/operating gas flow rate m (kg/sec)], where m represents the operating gas flow rate (in kg) per unit time (in sec.). With such a construction, a velocity reduction space is formed near the lower end of an electrode, making it possible to maintain most of the arc jet in the plasma torch in the velocity reduction space. This increases the stability of the arc jet in the plasma torch interior. In addition, since the diameter of the velocity reduction space is larger than the diameter of the lower end of the electrode, there is less fluctuation of the arc jet in the plasma torch in the radial dimension, that is the arc jet becomes more stable, with less wandering. This means that the gas insulation layer is made thicker in the radial dimension, thus making it possible to prevent the occurrence of improper discharge such as double arcs, etc. In addition, since the diameter of the cylindrical space is larger than its height, the length of the arc jet held in the velocity reduction space in the axial dimension becomes relatively small, making it possible to prevent kinking instability, etc., when the arc jet is extended. Accordingly, forming it with the predetermined dimensional form makes possible cutting in a dross free state, and to design as desired. Further, since material such as steel is performed using an arc jet having a high energy density, it is possible to perform cutting in a dross free state. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1a is a cross sectional view of the front end of a nozzle of the plasma torch in accordance with the present, invention; Fig. 1b illustrates reference numerals denoting the dimensions, etc., of Fig. 1a; Fig. 2 illustrates swirling currents of operating gas flowing from the swirler member of Fig. 1a; Fig. 3 illustrates reference numerals designating the dimensions, etc., of the nozzle front end of a conventional plasma torch of Fig. 8; Fig. 4 shows experimental results of the dross adhesion height when changes are made in the operating gas flow rate and the cutting velocity; Fig. 5 illustrates experimental results of the number of double arc cumulative occurrences; Fig. 6 shows experimental results of the dross adhesion height when various changes are made in the diameter of the nozzle in the present invention; Fig. 7 is a cross sectional view of the nozzle front end of a conventional plasma torch; Fig. 8 is a cross sectional view of the nozzle front end of another conventional plasma torch; Fig. 9 shows experimental results of the relationship between parallel section length/nozzle diameter and static pressure in the present invention; Fig. 10 shows experimental results of the relationship between velocity reduction space height/nozzle diameter and static pressure in the present invention; and Fig. 11 illustrates experimental results of the relationship between the nozzle diameter length/nozzle diameter and the double arc occurrence limiting current in the present invention. BEST MODE FOR CARRYING OUT THE INVENTION A description will be given of a preferred embodiment of the plasma torch of the present invention with reference to the attached drawings. Fig. 1a is a cross sectional view of the nozzle front end of a plasma torch, while Fig. 1b shows reference numerals designating the dimensions, etc., of Fig. 1a. At the axial center of a plasma torch 1, there is provided an electrode 3, and outwardly of the electrode 3 is concentrically provided an insulation member 5, and outwardly of the insulation member is provided a swirler member 7 and a nozzle 9 concentrically with the electrode 3. The electrode has a conductive member of, for example, copper, and electrode member 3a made of hafnium, tungsten, silver, or the like, which is embedded at virtually the central part of the front end of the electrode. The lower end 3b of the electrode is a plane section of diameter da, which is an outer diameter from the electrode member 3a. A taper section E (taper angle α) is disposed extending toward an electrode outer diameter db above the lower end of the electrode. The insulation member 5 is made of insulation material such as ceramic and electrically insulate the electrode 3 from the nozzle 9. The inner peripheral face of the insulation member 5 has the electrode 3 of outer diameter db, while the outer peripheral face of the lower portion of the insulation member 5 has a swirler member 7 of inner diameter Da fitted tightly thereto. A supply gas passage 11 is formed between the outer periphery of the insulation member 5 of outer diameter dc and inner periphery of the nozzle 9 of inner diameter Db. A gas passage 13 is formed from the swirler member 7 and below a lower end 5a of the insulation member 5. The swirler member 7 is formed of material such as free-cutting steel and copper having excellent high-temperature resistance and processability. The inner peripheral face has the insulation member, while the outer peripheral face has the inner peripheral face of nozzle 9 of inner diameter Db tightly fitted thereto. The outer periphery of the swirler member 7 has formed therein gas path slits 7b at two or more places along the axial center at equal distances apart. In addition, holes 7b serving as ejection holes are formed therein at equal distances apart tangential with respect to the diameter of the supply gas path 11 and almost vertical to the axial center (X- or Y-axis in Fig. 2) toward the inner peripheral dimension, as shown in Fig. 2, from these slits 7a. It is to be noted that although in the embodiment the outer periphery of the swirler member 7 may be slightly cut to form a path. The axial center of the holes 7b is not more than ± 5°, and preferably not more than ± 3° in the vertical dimension (vertical dimension in Fig. 1a). The holes 7b are formed below the lower end 5a of the insulation member 5. The nozzle 9 is formed of conductive material such as iron-containing material, copper-containing material, and stainless steel. The inner peripheral face of inner diameter Db has the outer peripheral face of each swirler member 7 tightly fitted thereto, with one end face 7c of each swirler member 7 being in contact thereto. The upper portion of the nozzle 9 is connected to a plate (not illustrated), and is removably stopped with screws, etc., to the torch body (not illustrated). The inner diameter Dc face of the nozzle which is virtually equal to inner diameter Da of the swirler member 7 is nearly parallel to the electrode 3 face of outer diameter db, and the parallel section length is Hd. A pipe-shaped cylindrical space, formed by inner diameter dc face of the nozzle 9, and the inner diameter Dc face at the outer peripheral face of the electrode 3, is called entrance section L. It is to be noted that the outer peripheral face of the electrode 3 at the entrance section L may have a tapered lower outer diameter section. For example, it may have a tapered section E. The nozzle 9 has a tapered section M tapering from the inner diameter Dc downward (to the nozzle front end), which forms an angle , which may be either nearly equal to or greater than the taper angle α of the electrode 3. Even below this tapered section M and near the electrode lower end 3b (distance in the axial center dimension), there is formed a cylindrical section (hereinafter referred to as velocity reduction space N). The velocity reduction space N is concentric with the electrode axial center and is cylindrical in shape, with a diameter Dd greater than diameter da of the lower end 3b of the electrode and a height Ha smaller than the diameter Dd. It is to be noted that with regard to the distance Hb in the axial center dimension between the upper end of cylindrical shape of the aforementioned velocity reduction space N and the electrode lower end face 3b, in Fig. 1b, the lower end 3b of the electrode is illustrated above the velocity reduction space N, the lower end 3b of the electrode may be illustrated in the velocity reduction space N. In this case, the velocity reduction space N has its upper end formed into a recessed cylindrical shape. A tapered section (hereinafter referred to as acceleration space P) tapers downward from the velocity reduction space N from the diameter Dd at an angle , and the tapered section merges into a nozzle having diameter De formed at the end of the nozzle 7. The nozzle diameter De is set to a predetermined size in accordance with the cutting material, material thickness, cutting width precision, etc. The length Hc of the nozzle diameter De is also set in the same way. Hereunder, the nozzle restriction 9a will include both the nozzle diameter De and nozzle length Hc. With each of the components arranged in the above-described manner, the operating gas takes the path summarized below. It flows from a pipe-shaped and almost parallel cylindrical entrance section L, formed between the outer periphery of the electrode 3 and the inner diameter of the swirler member 7 and the nozzle 9, and then down through the thin conical acceleration section (hereinafter referred to as acceleration section M) with tapered inner and outer faces, formed between tapered section E of the electrode 3 and the tapered section M of the nozzle 9, and connected to the entrance section L at a gentle angle. The operating gas then reaches the cylindrically-shaped velocity reduction space N formed at the end of the acceleration section M and near the lower end 3b of the electrode. After having flowed into the velocity reduction space N, the operating gas passes down through the acceleration space P, located below the velocity reduction space N, then through the nozzle restriction section 9a formed into a cylindrical shape at the front end of the nozzle 9, and is ejected to a cutting material (not illustrated) in the form of arc jets. Although, in the above-described construction, examples of materials for each of the component members were given, they are not to be construed as limitative. A description will be given of the operation of the plasma torch 1 having the above-described construction. The operating gas flows from the supply gas path 11, formed between the outer diameter dc of the insulation member 5 and the inner diameter Db of the nozzle 7, and then through the slits 7a of the swirler member 7, through the holes 7b, formed in the swirler member 7 at equal distances apart, and through the gas path 13, located inwardly of the gas path 11. As shown in Fig. 2, the gas fluid, flowing out from the plurality of equivalent holes 7b, flow as jets, having only a tangential velocity component V, in the form of tangential swirlers. The tangential swirlers, which pass from the gas path 13 to the entrance section L, become uniform swirling currents of operating gas, and flow down into the acceleration section M connected to the entrance section L at a gentle angle. The swirling currents, accelerated in the acceleration section M, flows into the velocity reduction space N, formed near the lower end 3b of the electrode. In the velocity reduction space N, arc jet (hereinafter referred to as arc column) is stably held with respect to the electrode axis, using a gradient of low pressure of the swirling central portion symmetrical to the axis, generated by the swirling current produced by the tangential swirler, that is a gradient of the pressure symmetrical to the axis produced by the centrifugal force of the current swirling velocity component (becomes minimum on the center axial line). Here, in the velocity reduction space N, as the path area increases, the axial velocity component decreases, while the swirling velocity component, which does not decrease, remains at an appropriate value, so that it is possible to create the necessary steep pressure gradient symmetrical to the axis to stably maintain the arc column. Since the velocity reduction space N has a large diameter Dd, the distance between the outer edge of the arc column (current boundary) and the velocity reduction space N wall is large, which results in increased gas insulation layer thickness, so as to increase double arc resistance and restrict the generation of double arcs. This increases the durability of the plasma torch. From the velocity reduction space N to the next acceleration space P, the operating gas is gradually accelerated within a short distance and narrowed down, so that the arc column, maintained with respect to the electrode axis in the velocity reduction space N, is narrowed down and flows into the nozzle restriction section 9a. In the nozzle restriction section 9a, the operating gas becomes a predetermined arc jet and travels a short distance from the electrode 3 to the cutting material. Accordingly, a shorter distance from the lower end 3b of the electrode to the entrance of the nozzle restriction section 9a causes the arc column to be maintained at a shorter length, thus reducing the occurrence of various instabilities of the arc column formed in the current, such as arc column wandering. A description will be given of the experiments performed on the plasma torch 1 in accordance with the present invention described in detail above and the conventional plasma torch 60 corresponding to EP-A-452 494 proposed by the present inventor. Experimental Example 1: Dross Adhesion Height In this experiment, swirling currents were generated and the conventional plasma torch 60 having the velocity reduction space 61a (see Fig. 8) was used to examine the dross adhesion height when changes were made in the operating gas flow rate and the cutting velocity. This experiment was conducted to show that in the case of the conventional plasma torch with a nozzle and an electrode, since the double arc generation limiting current is small, it is difficult to increase the energy density I/m of the arc jet; when cutting steel plates using a plasma torch utilizing transferred arc jets, it is particularly necessary to increase the energy density I/m of the arc jet, so that it is even more difficult to perform cutting in the free dross state; and to make clear the state of dross adhesion, etc., in the energy density I/m regions of the arc jet at which cutting is not conventionally performed. Fig. 3 shows reference numerals designating dimensions, etc., in the plasma torch 60. The same component parts are given the same reference numerals, and will not be described below. (1) Principal dimensions in the plasma torch 60 used in the experiment Outer diameter db x of electrode 62 = 5.5 mm Diameter da x of lower end of electrode 62 = 2.7 mm Taper angle α x of electrode 62 = 90° Inner diameter Da x of swirler member 63 = 8.5 mm Length corresponding to parallel section length Hd of plasma torch 1 = 0 mm Diameter Dd x of velocity reduction space 61a = 2.0 mm Height Ha x of velocity reduction space 61a = 1.5 mm Nozzle 61 angle  χ nozzle 61 below velocity reduction space 61a = 120° Nozzle 61 angle  x = 90° Nozzle 61 diameter De = 0.8 mm Distance Hb x between lower end of electrode 62 and velocity reduction space 61a = 1.3 mm Length Hc x of nozzle restriction section 61a = 2.6 mm (2) Cutting conditions Arc current value I = 37 A Type of operating gas = oxygen Operating gas flow rate m (following four values) = 11.5 x 10 -5 kg/s (Line L1 of Fig. 4) = 9.5 x 10 -5 kg/s (Line L2 of Fig. 4) = 7.5 x 10 -5 kg/s (Line L3 of Fig. 4) = 6.0 x 10 -5 kg/s (Line L4 of Fig. 4) Stand-off = 2 mm Cutting material = Soft steel plate Plate thickness = 6 mm Note: according to EP-A-452 494, the following relationships are satisfied : 1.5 ≤ Dd x /De ≤ 10 and 0.75 ≤ Ha x /Dd x ≤ 10 (3) Experimental results The results of this experiment are shown in Fig. 4. In this experiment dross adhesion was observed in the L1 and L2 regions, that is the regions having a small energy density I/m, where a large amount of a conventional operating gas was used. It was found that in the line L4 [ energy density I/m = 6.2 x 10 5 (A ·s/kg) ] and the line L3 [ energy density I/m = 4.9 x 10 5 (A ·s/kg) ] regions where a small amount of operating gas was used, that is where energy density I/m was large, it is possible to perform cutting in a dross free state. However, although only small amounts of dross adhesion occurred at a cutting velocity of 60 ∼ 100 cm/min, this depends on the plate thickness, current value, etc. The inventor has found out from many experimental results that when the energy density I/m is larger than approximately 4 x 10 5 (A ·s/kg), it is possible to achieve cutting in a free dross state. However, the inventor has also found out that when cutting is performed successively for a large number of times, double arc occurs and that, as will be described below, durability of the plasma arc is decreased. Experimental Example 2: Number of cumulative occurrences of double arcs The double arc occurrence conditions and dross adhesion were checked using the plasma torch 1 of Fib. 1b, which is a plasma torch of the present invention. Cutting (described later) was performed on three nozzles 9 having the same shape. The conventional plasma torch 60 having the same dimensions as those of the plasma torch used in the aforementioned first experimental example was used, except that the nozzle diameter was De = 0.6 mm. (1) Principal dimensions in the plasma torch 1 used in the experiment Diameter da of lower end 3b of electrode = 2.7 mm Outer diameter db of electrode 3 = 5.5 mm Taper angle α = 40° Inner diameter Dc of nozzle 9 = 8.5 mm Parallel section length Hd of entrance section L = 2.7 mm Diameter Dd of velocity reduction space N = 4 mm Height Ha of velocity reduction space N = 0.6 mm Angle  of acceleration space P = 120° Angle  of acceleration section M = 60° Nozzle diameter De = 0.6 mm Length Hc of nozzle restriction section 9a = 2.0 mm (2) Cutting conditions (same for both plasma torch 1 and plasma torch 60) Arc current value I = 27A Energy density I/m = 6.5 x 10 5 A ·s/kg Stand-off = 2 mm Type of operating gas = oxygen Cutting material = Soft steel plate Plate thickness = 1.6 mm (3) Experimental results Piercing was started to perform 10-cm straight cutting repeatedly for 1000 times, and the number of cumulative occurrences of double arcs were examined. The double arc occurrences were measured from changes in the input voltage values, while dross adhesion was visually measured. Fig. 5 shows the relationship between the number of piercings and the number of cumulative occurrences of double arcs. Experimental results showed that when the conventional plasma torch 60 was used, initially, no dross adhesion occurred. However, when the number of cutting operations approaches 600 times, double arcs cumulatively occur 50 times, so that slight dross adhesion was observed. When the number of cutting operations exceeds 800 times, the occurrences of double arcs increase rapidly, so that a large amount of dross adhesion was observed. From the many experimental results, the present inventor confirmed that when the energy density I/m is greater than approximately 4 x 10 5 A ·s/kg, cutting in a dross free state is achieved. However, the inventor also found out that when cutting is repeated for a large number of times, double arcs as well as large amounts of dross adhesion were observed, with reduced durability of the plasma torch. The experimental results showed that when the plasma torch 1 of the present invention is used, double arcs occur cumulatively about 50 times at most, even when cutting operations are repeated for 1000 times, as shown by lines L8, L9, and L10. In this case, no dross adhesion was observed on the cut section. Compared to the conventionally-constructed plasma torch, even when the same energy density I/m is applied, it has more power to stably maintain the arc column with respect to the electrode axis, so that even when the operating gas flow rate is small at approximately 4.2 x 10 -5 kg/s, there is less instability of the arc column, and cutting can be stably performed for a long period of time without dross adhesion, that is in a dross free state. Experimental Example 3: Dross adhesion height with various nozzle diameters Fig. 6 illustrates the experimental results. Fig. 6 is a graph showing the relationship between gas flow rate and current allowing cutting where no dross adhesion height is visually measured or allowing cutting in a dross free state, when changes are made in the cutting current using various nozzle diameters De in the plasma torch of the present invention. The figure shows that, for example, when the arc current value I is 40 A, the operating gas flow rate m limit allowing cutting in a dross free state is approximately 10 x 10 -5 kg/s (represented by O in the figure), while in regions where the flow rate is less than this value, it is possible to perform cutting in a dross free state. From this experiment, the limit value of energy density I/m = 4 x 10 5 A ·s/kg . This means that the dross free region is located where the energy density I/m is greater than this limit value. Experimental Example 4: Cutting velocity measurement In the experiment, the plasma torch 1 of the present invention and the conventional plasma torch 60 were used to examine the cutting velocities allowing cutting in a dross free state. The main conditions are a cutting material plate thickness of 1.6 mm, a nozzle diameter size De of 0.6 mm, arc current value I of 27A, oxygen as operating gas, and operating gas flow rate at which the energy density I/m is greater than 4 x 10 5 A ·s/kg . Cutting at various velocities revealed that the dross free region of the plasma torch 1 is approximately 100 ∼ 190 cm/min, while the dross free region of the plasma torch 60 is approximately 100 ∼ 155 cm/min. This means that at the region where I/m ≥ 4 x 10 5 A ·s/kg , it is possible to perform cutting in a dross free state, while, at the same time, the cutting velocity is a practical velocity, with the plasma torch 1 of the present invention being about 1.23 times faster than the conventional ones. Experimental Example 5: Measurement by enlarged plasma torch model This experiment was conducted to find out preferable dimensions and shapes for the plasma torch 1 of the present invention. Accordingly, to find out the relationship of plasma torch shape, and the swirling current strength and uniformity, plasma torches of a model having five times the dimensions of the plasma torch 1 were manufactured for various standards to measure the static pressure at each of the points in the torch interior where operating gas flows. The reference numerals, etc., of the present plasma torch is the same as those of the plasma torch 1, so that they will not be described here. (1) Common dimensional forms of plasma torches and gas flow rate Nozzle diameter De = 3.0 mm Length Hc of nozzle diameter De = 3 De Operating gas (oxygen) flow rate = 9.5 x 10 -4 kg/s (2) Measurement position of static pressure in plasma torch interior Center of lower end 3b of electrode (static pressure at this position called Pe) Wall face of lower portion of velocity reduction space N (static pressure at this position called Pvr) (3) Experimental results The experimental results are as follows. (a) Fig. 9 shows the relationship between (parallel section length Hd of entrance section L/nozzle diameter De) and static pressure Pe, when the height Ha of velocity reduction space N = nozzle diameter De , the distance Hb between the lower end 3b of the electrode and the velocity reduction space N is 0, and the diameter Dd of the velocity reduction space N = 7 De . Since centrifugal force acts upon the operating gas, which is a fluid, swirling currents with a larger swirling velocity component V (see Fig. 2) causes a lower static pressure Pe at the lower end 3b of the electrode. From the many experimental results described above, it is preferable that the static pressure Pe is not more than about 0.7 kg/cm 2 , so that the preferable range of the parallel section length Hd of entrance section L/nozzle diameter De is 0 ≤ Hd/De ≤ 7 . b) The relationship between angle  of acceleration section M and static pressure Pe, when, for example, Ha = De , Hb = 0, and Dd = 7 De as in the aforementioned a). The results showed that the angle  at which the static pressure Pe equals the same desirable value as in the aforementioned a) of not more than about 0.7 kg/cm 2 falls in the range of 30° ≤  ≤ 100°. c) A desirable angle  for acceleration space P was selected to maintain the stability of the arc jet. More specifically, when  < 90°, the length from the bottom face of the velocity reduction space N to the nozzle restriction section 9a becomes too long, so that the arc jet becomes more unstable. On the other hand, when  > 150°, the operating gas is rapidly accelerated to the nozzle restriction section 9a, so that the flow often becomes unstable. Therefore the angle  is preferably in the range of 90° ≤  ≤ 150°. d) Fig. 10 shows the relationship between (height Ha of velocity reduction space N/nozzle diameter De) to static pressure Pvr of the wall at the lower portion of the velocity reduction space N. The graph shows the result when the distance Hb = 0 and diameter Dd = 7 De . A higher static pressure Pvr value forms a more effective pressure distribution at the lower face of the velocity reduction space N. The static pressure Pvr is preferably in the range of, for example, about Pvr ≥ 1.2 kg/cm 2 for it to exist in an extremely stable state. Therefore, although an appropriate Ha/De value would be Ha/De ≤ 2.5 , since when Ha/De < 0.5 a proper discharge gap cannot be obtained, it is preferably in the range of 0.5 ≤ Ha/De ≤ 2.5 . e) Examination of the relationship between (diameter Dd/nozzle diameter De) and static pressure Pe showed that a desirable static pressure Pe value can be obtained, that is the center of the arc jet in the plasma torch enters an effective low pressure space, when Dd/De lies within the preferable range of 4 ≤ Dd/De ≤ 10 . f) Experiments were carried out, under the condition that height Ha = nozzle diameter De and diameter Dd = 7 De , to obtain a preferable distance Hb between the lower end 3b of the electrode and the velocity reduction space N. Examination of the relationship between (distance Hb/nozzle diameter De) and static pressure Pe revealed that the preferable static pressure is obtained when it lies within the preferable range of - 0.4 ≤ Hb/De ≤ 0.6 . Experimental Example 6: Measurement by plasma torch 1 The experiment was conducted to obtain preferable dimensions as regards the length Hc of nozzle aperture De of the plasma torch 1 of the present invention. Fig. 11 shows the relationship between (length Hc of nozzle diameter De/nozzle diameter De) and double arc occurrence limiting current Ic. In this case, the nozzle diameter De = 0.6 mm and the operating gas used is oxygen. From various experiments, it can be thought that (length Hc/nozzle diameter De) value of not more than 4 is appropriate to obtain the required double arc occurrence limiting current Ic of, for example, about 30 A or more. However, when Hc/De < 2.5, the arc jet cannot be sufficiently contracted by the thermal pinch effect, which means that good cutting quality cannot be obtained. Therefore, the preferable range is 2.5 ≤ Hc/De ≤ 4 . With the constructions in Examples 5 and 6, the plasma torch 1 allows cutting in a dross free state, and, at the same time, it can be designed based on a wide range of dimensional forms, when necessary. INDUSTRIAL APPLICABILITY The present invention is effective in that it provides a plasma torch capable of cutting in a dross free state made possible by increased energy density of the arc jet, and whose operation efficiency is not reduced even with a low operating gas flow rate since it can stably maintain the arc jet in the plasma torch, and which has high double arc resistance, and high durability.

Claims: A plasma torch having an electrode (3) at its central axis, a nozzle (9) concentric with the electrode (3), a swirler member (7) with a plurality of ejection holes (7b) formed therein on a plane substantially perpendicular to the central axis of said plasma torch (1), said swirler member (7) causing generation of jets with only a swirling velocity component V  in the tangential direction and formation of operating gas into swirling currents, wherein said plasma torch (1) has a substantially cylindrically-shaped velocity reduction space (N) and has the following dimensions: 0 ≤ Hd ≤ 7 De , 30° ≤  ≤ 100° , 90° ≤  ≤ 150°, 0.5 De ≤ Ha ≤ 2.5 De , 4 De ≤ Dd ≤ 10 De , -0.4 De ≤ Hb ≤ 0.6 De , and 2.5 De ≤ Hc ≤ 4 De , where De represents the diameter of the nozzle restriction section (9a) at the nozzle output end, Hd represents the length of the cylindrical section of the nozzle (9) between the swirler member (7) and a tapering section (M),  represents the angle formed by said tapering section (M), Ha represents the length of said velocity reduction space (N) which is formed after said tapering section (M), Dd represents the diameter of said velocity reduction space (N), Hb the distance along the central axis between the upper end of said velocity reduction space (N) and the electrode lower end face (3b), Hc the length of said nozzle restriction section (9a), and  the tapering angle of the acceleration space (P) provided between said velocity reduction space (N) and said nozzle restriction section (9a). A plasma torch according to Claim 1 wherein an arc jet energy density is greater than 4 x 10 5 ampere x second/kg.

Inventors: HASHIZUME YOSHIHIKO, SAKURAGI SHUNICHI, TSURUMAKI NAOYA, HASHIZUME,YOSHIHIKO, SAKURAGI, SHUNICHI, TSURUMAKI, NAOYA
Applicants: KOMATSU MFG CO LTD, KABUSHIKI KAISHA KOMATSU SEISAKUSHO
Application year: 1993
Main classification: B23K10
IPC codes: H05H1