A MACHINE AND PROCESS FOR CUTTING METAL TUBES USING A PLASMA JET

Abstract: In the machine and process according to the invention a metal tube (H) to be cut is caused to translate axially along a rectilinear path. The cutting of the tube (H) into lengths is effected by means of one or more plasma torches (180) which are moved in a helical motion concentric with the axis of the tube (H) at a speed having an axial component equal to the speed of translation of the tube.

Description: A machine and process for cutting metal tubes using a plasma jet The present invention relates to a machine and a process for cutting metal tubes into lengths using a plasma torch, according to the preamble of Claims 1 and 13. A machine and a process of this general type are known from document SU-A-1 625 617. In this known machine and process a metal tube to be cut is caused to rotate about its own axis and a fixed plasma torch cuts it with a plasma jet directed towards the axis of the tube. At present continuous tubes are cut into lengths by means of mechanical flywheel cropping machines of the type having a shear or a toothed or diamond wheel . Lengths of tube cut by shears are strongly deformed at the ends by squashing and these edge portions are cut off before proceeding to convert the lengths of tube into finished products. Cutting disc systems are very slow and constitute a real "bottleneck" at the end of a tube production line, which imposes severe limitations on the speed of the line. The object of the invention is that of providing a cutting machine and process based on the general principle known from document SU-A-1 625 617, but which lends itself to the rapid cutting into lengths, on an industrial scale, and without deformations, of a tube which moves continuously at a constant high speed, for example, coming from a machine for the fabrication of welded tubes situated immediately upstream of a cutting zone . According to the invention this object is achieved by means of a machine and a process as claimed. Due to the helical movement of one, and preferably several, plasma torches around the continuously translating tube, a machine and a process according to the invention are able to effect very rapid successive cutting operations "on the fly" without imposing limitations on the speed of production of the tube. The invention has been developed in its application to the cutting of welded stainless steel tubes of circular sections with diameters lying between 10 and 800 mm, but it is applicable to tubes of any other metal and any other section. The invention also relates to lengths of tube having ends cut with a plasma jet cutting process as claimed. The characteristics and advantages of the invention will become apparent from reading the following detailed description, in which reference is made to the attached drawings which illustrate a preferred embodiment thereof, and in which: Figure 1 is a shortened schematic side view of a machine for the fabrication of welded tubes; Figure 2 is a schematic side view of a cutting machine formed as a first embodiment of the invention, which can be supplied directly from the machine of Figure 1; Figures 3 and 4 are schematic transverse sections, on a larger scale, taken on the lines indicated III-III and IV- IV in Figure 2; Figure 5 is an explanatory representation obtained by superimposing Figures 3 and 4, on an even larger scale; Figures 6 and 7 are transverse sections taken on a larger scale on the lines indicated VI-VI and VII-VII in Figure 2 ; Figure 8 is a schematic side view of a cutting machine formed as another embodiment of the invention, which can also be supplied directly from the machine of Figure 1 ; Figure 9 is a schematic partially cut away transverse section, taken on the line indicated IX- IX of Figure 8 ; Figure 10 is a schematic side view which shows the central part of Figure 8 on a larger scale; Figure 11 is a partially sectioned partial side view which shows the part indicated by the arrow XI in Figure 10 on a larger scale; Figure 12 is a schematic front view of a detail which shows a cutting head and the associated slide in two configurations; Figure 13 is a schematic transverse section similar to Figure 9, which shows a cutting unit in a different condition from that of Figure 9; and Figure 14 is a schematic transverse section taken on the line indicated XIV-XIV in Figure 10. Referring to Figure 1, a strip of steel B unwinds from a roll A and travels along the direction of the arrow FI . An exemplary machine for fabricating a welded tube starting from the strip B comprises a series of forming rolls C and D. The strip B, which leaves the first series of rolls C is shaped, in a known way, into an almost closed form, in which there is still a longitudinal slot E. The second series of rolls D completes the closure of the strip in a known manner, transforming it into a tube which passes through a station G in which longitudinal welding of the tube takes place with the so-called "TIG" welding system or with an induction welding system or with a laser welding system. The formed continuous tube leaving the machine of Figure 1 is designated H and its axis is designated X. The speed at which the tube H travels, that is to say the speed of production of the machine of Figure 1, can reach up to 180 m/min. Figures 2 to 7 illustrate a cutting machine formed as a first embodiment of the invention which has been designed to cut the tube H into lengths suitable for transport, for example 6 m, at a rate corresponding to the high speed of production of the machine of Figure 1. In Figure 2 the tube H to be cut travels axially along a horizontal rectilinear path in the direction again indicated with the arrow FI . The cutting machine illustrated comprises a pair of robust support stands, comprising an entrance stand 10 and an exit stand 12. The entrance stand 10 supports driven rolls 14 and the exit stand 12 supports driven rolls 16. The rolls 14 and 16, which serve to support the tube H and to encourage its advance along the direction of the arrow FI, are adjustable in height depending on the diameter of the tube to be cut . Moreover, in order to compensate any possible flexure of the tube H and thus to guarantee its rectilinearity, the exit rolls 16 can be supported in such a way that their height can be further adjusted. The two stands 10 and 12 are provided with frontal annular plates 18, 20. A cutting assembly, generally indicated 22, which is rotatable about the axis X of the tube H is supported between the annular plates 18, 20. The assembly 22 is preferably in the form of a squirrel cage with two annular heads 24, 26 interconnected rigidly by a ring of bars 28a, 28b. The annular heads 24, 26 are supported by the fixed annular plates 18, 20 by means of respective thrust bearings 30, 32 of the ball-bearing type or of plain type. The stands 10, 12 support respective numerically controlled electric motors 34, 36 provided with respective toothed driving sprockets 38, 40. The sprockets 38, 40 mesh with respective toothed rings 42, 44 fixedly connected to the annular heads 24, 26. In operation the two motors 34, 36 are energised in such a way as to drive the parts of the assembly 22 to rotate together in order to guarantee that the whole assembly 22 turns without torsional deformations for the purposes of obtaining cutting precision which will be discussed further below. If the torsional rigidity of the assembly 22 is sufficient it can be driven to rotate by a single motor such as 34 or 36. Referring to Figures 3 to 5, the bars 28a, 28b which are all identical with one another, can be considered as subdivided into one group of bars 28a (Figure 3) and one group of bars 28b (Figure 4) . In each group there are four bars 28a and four bars 28b respectively, equally angularly spaced from one another. In the case shown, the bars 28a, 28b of each group are disposed at 90° from one another and (Figure 5) with the groups being offset by 45° from one another. Each bar 28a, 28b is provided with a respective box- like rail 46a, 46b. Slidably mounted in each rail 46a, 46b is a respective slide 48a, 48b. Each slide 48a, 48b is coupled to a respective lead screw 50a, 50b by means of a respective lead screw nut not referenced. The interconnections between screw and nut are preferably of the recirculating ball type. Each lead screw 50a, 50b can be driven to rotate by a respective numerically controlled geared electric motor 60a, 60b situated at the end of the respective rail 46a, 46b adjacent the annular head 24. In operation of the machine the lead screws 50a of one group and the lead screws 50b of the other group are driven in unison by respective geared motors 60a, 60b in such a way, that on the one hand, all the slides 48a of one group and, on the other hand, all the slides 48b of the other group are caused to move in unison along the respective rails 46a, 46b. To ensure a greater precision the slides 48a, 48b could be interconnected rigidly in fours . Each slide 48a, 48b carries a respective cutting head 62a, 62b. Each cutting head 62a, 62b carries in turn a respective plasma type cutting torch 64a, 64b, preferably a high definition type. All the torches 64a, 64b are directed towards the tube H to perform cutting in a manner which will be described below. In Figures 2, 3 and 4 one group of cutting heads 62a, and the associated torches 64a, is generally indicated as a unit 66a for practicality, and the other group of cutting heads 62b with the associated torches 64b is generally indicated as a unit 66b for practicality. As will be seen, especially in Figures 3 and 4, with the arrangement illustrated each unit 66a, 66b has two pairs of diametrically opposed torches 64a, 64b. Since the plasma jets emitted by these diametrically opposed torches could interfere with one another during cutting with unwanted effects, it is preferred, as shown, to offset opposite torches to one side and the other of the associated diametral plane such that all the torches 64a, 64b of the same unit 66a, 66b are directed towards the tube H at the same angle so that the plasma jets intersect the tube along a chord passing radially through the internal surface of the tube . This arrangement, however, is not limiting. Preferably, in each unit 66a, 66b each of the torches 64a, 64b is carried on the associated head 62a, 62b by motorised means of known type (not shown in detail) for numerically controlled adjustment of the distance from the tube H. Four characteristic positions which each of the units 66a, 66b can assume during a cutting operation have been indicated Pl, P2 , P3 and P4 in Figure 2; the parts of the path lying between these positions have been indicated LI, L2 and L3. A cutting operation performed by the unit 66a will first be considered. At the commencement of the cutting operation the unit 66a is in position Pl . Starting from this point the geared motors 60a are put into operation to accelerate the unit 66a to translate along the first section LI of the stroke. At the same time the motors 34, 36 are put into operation and drive the whole assembly 22, and in particular the unit 66a, with a rotating motion. At the end of the section LI, that is to say at position P2 , the unit 66a has reached exactly the same linear velocity as the tube H in the direction of the arrow FI as well as a predetermined angular velocity. The four torches 64a are now activated thereby making the cut. The unit 66a is then made to follow the same constant linear velocity as the tube H along the section L2 until it reaches the position P3. During this translation along the section L2 the assembly 22 is rotated, at the same predetermined angular velocity, through slightly more than 90° so that each of the torches 64 of the unit 66a performs a cut in the tube along an arc of slightly more than 90°, which is superimposed over the cut performed by the torch 64a which preceded it . In other words, the cutting of the tube is performed by making each torch 66a follow a helical path concentric with the axis X of the tube H and having an axial component of velocity equal to the speed of the translation of the tube H. In position P3 the torches 66a are disactivated. Whilst the cut tube length continues its motion in the direction of the arrow FI the unit 66a is caused to decelerate along the section L3 to the position P4 in which its motion is reversed. At the same time the rotation of the assembly 22 is stopped. Whilst the unit 66a performed a cut the other unit 66b was made to retract in the direction of the arrow F2 from position P4 to position Pl by means of its motors 60b and lead screws 50b. As will be noted, the cutting heads 62a, 62b are angularly offset from one another such that when the unit 66a advances and the unit 66b retracts, and vice versa, the associated heads do not interfere with one another. The cutting of the next length of tube takes place by means of the cutting unit 66b with the same procedure described for the unit 66a, but with a rotation of the assembly 22 in the opposite sense through the same angle of slightly more than 90°. Preferably the return stroke of each unit 66a, 66b is faster than the helical forward or cutting stroke. This allows the unit 66a or 66b to pause in the initial position Pl . This pause of a unit 66a, 66b can be exploited, whilst the assembly 22 does not rotate, for the periodic replacement of the electrodes of the plasma torches . Referring to Figures 6 and 7, as well as to Figure 2, each cutting unit has associated with it a centring system advantageously usable for cutting tubes of circular section. As can be seen in Figures 2, 6 and 7, each unit 66a, 66b has two diametrically opposed slides 48a, 48b carrying respective blocks 68a, 68b facing respective cutting heads 62a, 62b and movable with them upon operation of the corresponding lead screws 50a, 50b. Each block 68a, 68b carries in turn a respective shaft 70a, 70b slidable radially with respect to the tube H. The block 68a, 68b and associated shaft 70a, 70b are interconnected by control means for translation of the shaft 70a, 70b towards and away from the tube H. These control means may be electrical or fluid linear actuators . Each of the shafts 70a, 70b carries at its end nearest the tube H a respective jaw 72a, 72b with a V-shape engagement surface (or rollers) . The jaws 72a and 72b are of anti-wear material resistant to heat, for example "Xantal" bronze, and are mounted in a shock-absorbing manner with respect to the associated shaft 70a, 70b. Their operation is that of ensuring centring of the tube H with respect to the assembly 22 during cutting. In operation of the machine, when the unit 66a is at position Pl the jaws 72a are spaced from the tube H; when the unit 66a reaches position P2 the jaws 72a are frictionally engaged with the tube H and then maintain this engagement for the whole of the cutting stroke L2 as far as the position P3 ; subsequently the jaws 72a are retracted from the tube H and remain disengaged from it for the whole of the final part L3 of the stroke as well as for the return stroke to the initial position Pl . In a similar manner, in operation of the machine, when the unit 66b is at position Pl the jaws 72b are spaced from the tube H; when the unit 66b reaches position P2 the jaws 72b are frictionally engaged with the tube H and then maintain this engagement for the whole of the cutting stroke L2 as far as the position P3 ; subsequently the jaws 72b are retracted from the tube H and remain disengaged from it for the whole of the final part L3 of the stroke as well as for the return stroke to the initial position Pl . According to the invention a clamping device could however include several jaws, preferably three jaws at 120° from one another. Figures 8 to 13 illustrate a cutting machine formed as another embodiment of the invention, which has also been designed to cut a tube H into lengths suitable for transport, for example 16 m, at a rate corresponding to the high speed of production of the machine of Figure 1. The machine of Figures 8 to 13 has been particularly conceived for cutting tubes of polygonal section including the square-section tubes illustrated in Figures 8 and 13. It lends itself, however, equally well to the cutting of tubes of circular section or any other section, even relatively irregular ones. The machine of Figures 8 to 13 is intended to plasma cut a tube, again indicated H, which extends horizontally, coming for example from a machine for the fabrication of welded tubes as shown in Figure 1. Referring to Figures 8, 9, 10 and 13 the machine comprises guide means in the form of a pair of horizontal rails 100 spaced from one another. The rails 100 carry a carriage generally indicated 102. The carriage 102 comprises a base plate 104 which is coupled to the rails 100 by means of precision linear guides 106 of a known rolling contact type, for example roller guides. The carriage 102 is provided with means for driving it to reciprocate along the rails 100. Preferably, these drive means comprise a pair of reversible electric motors 108 which can be energised together and are supported by the base plate 104. Beneath the base plate 104 is carried a shaft 110 which extends transversely from one rail 100 to the other and which carries a pair of toothed sprockets 112. To each of the rails 100 is fixed a respective rack 114 with which a respective sprocket 112 is in mesh. Each of the motors 108 is connected to the shaft 110 by means of a respective toothed transmission belt 115. As will be understood, the combined energisation of the two motors in one sense or the other causes translation of the whole of the carriage 102 in one direction or the other respectively, via the meshing of the sprockets 112 with the racks 114. The adoption of two motors 108 and two opposite sprocket 112 - rack 114 pairs is advantageous in that it constitutes a guarantee against snaking of the carriage 102 during its translation along the rails 100. Towards one end (to the left in Figure 8) the base plate 104 fixedly carries a robust frame 116 with a pair of opposite lateral columns 118 and an upper beam 120. Within the frame 116 is mounted a movable assembly generally indicated 122. The assembly 122 comprises a rigid structure in the form of a box-like frame 124 composed of two side walls 126, an upper wall 128, a lower wall 130 and a front wall 132 which has a central window 134. Vertical guides 136 are fixed to the two columns 118 of the frame 116, along which the assembly 122 is vertically slidably mounted by means of bushes 138 fixed to the side walls 126 of the frame 124. The base plate 104 of the carriage 102 carries a reversible electric motor 140 which, via a worm screw reduction gear 142 and a nut (not shown) controls a vertical threaded shaft 144 fixed to the lower cross beam 130 of the frame 124. As will be understood, the motor 140 allows the height of the movable assembly 122 to be adjusted for the purpose which will be explained below. The front wall 132 of the frame 124 carries a thrust bearing 146 of the ball-bearing type on which a motorised support in the form of an annular plate 148 is rotatably mounted about a horizontal axis . The support 148 can be driven to rotate about its horizontal axis by drive means which, preferably, as shown, comprises a pair of reversible electric motors 150 carried by the front wall 132 of the frame 124. A shaft of each motor 150 is provided with a respective toothed sprocket 152 meshing with a toothed ring 154 fixed concentrically to the annular support 148. The use of two motors 150 which work in unison to drive the annular support 148 to rotate is also advantageous in that it constitutes a guarantee against annular play in the coupling between the sprockets 152 and the ring 154. The annular support 148 carries a set of four motorised slides each generally indicated 156. The slides 156 are angularly spaced in pairs opposite one another . Referring specifically to Figures 10 and 11, each slide 156 comprises a robust box-like body 158. Each of the slides 156 has an associated double-acting pneumatic actuator 160 the body or cylinder 162 of which is fixed to the annular support 148. Each actuator 160 includes a pair of parallel rods 164 which extend radially with respect to the axis of rotation of the support 148. The ends of the rods 164 of each pair are interconnected by a radially outer head plate 166 and by a radially inner head plate 168. A tangential plate 170 forming part of the box-like body 158 of the associated slide 156 is fixed to the plate 168. Each slide 156 carries a cutting head generally indicated 172. Each cutting head 172 includes a pivotable generally L- shape bracket 174 with a longitudinal limb 176 and a radial limb 178. The longitudinal limb 176 has a plasma torch 180 fixed to it, orientated in a substantially radial direction and having a plasma emission nozzle 182 directed substantially towards the axis of rotation X. The box-like body 158 of the slide 156 includes a front wall 184 with a flat radial face 186. To the wall 184 is fixed a pin 188 parallel to the axis of rotation. The radial limb 178 is mounted pivotably about the pin 188 by means of a ball bearing 190 and is retained on the pin 188 adjacent the wall 184 by means of a head 192 of the pin itself. To the radial limb 178 of the bracket 174 is fixedly connected a feeler 194 of anti-wear material resistant to heat, such as, for example, the material known by the commercial name "Celeron" . The feeler 194 has a flat contact face 196 facing radially towards the interior and normal to the axis of the torch 180. The feeler 194 projects with respect to the end of the nozzle 102 by a distance K (Figure 11) for a purpose which will be explained below. The angular pivoting of the bracket 174 with respect to the body 158 of the slide 156 is advantageously frictionally resisted for a purpose which will be clarified better below. Preferably, as represented, inter alia, in Figure 11, the radial limb 178 of the bracket 174 incorporates a mushroom headed friction shoe 198, which is pressed resiliently against the flat face of the wall 184 by a series of Bauer springs 200 or the like. As will be observed, the pin 188 is situated between the contact face 196 of the feeler 194 and the shoe 198. The advantage of this arrangement will be clarified below. Advantageously there are provided means for locking the angular pivoting of the cutting head 172 about the pin 188 with respect to the associated slide 156 in an orientation in which the nozzle 182 is directed substantially towards the axis X of the tube H. As illustrated in particular in Figure 11, these angular pivot locking means comprise a pneumatic actuator 202 with a body or cylinder 204 mounted within the box- like body 158 of the slide 156. The actuator 202 is provided with a rod 206 movable back and forth parallel to the pivot axis defined by the pin 188. In the radial limb 178 of the bracket 174 there is incorporated a bush 208 which acts as a seat in which a bevelled projecting end of the rod 206 can be engaged in the manner of a bolt for locking the pivoting motion. The purpose of locking this pivotal movement will be clarified below. As can be seen in Figure 8 , the tube H extends horizontally through the window 134 of the movable assembly 124 and between the nozzles 182 of the cutting heads 172. In order to support the tube H during cutting, as illustrated especially in Figures 8 and 14, the base plate 104 of the carriage 102 carries, upstream from the support frame 116 with respect to the direction of motion of the tube H, a stand 210 at the top of which is freely rotatably mounted a roller 212 for support of the lower part of the tube H. Downstream of the cutting machine (to the right in Figure 8) is a fixed stand 214 to the top of which is freely rotatably mounted a roller 216 for supporting the successive lengths of cut tube. As can be seen, in its passage through the cutting machine the tube H is supported with a so-called "groove bottom" type of system. With such a tube support system the lower part of the tube is always substantially at the same level during its passage through the cutting machine independently of its cross-sectional dimensions. The axis X of the tube H is therefore higher the greater its cross-section and vice versa. To adapt the machine to different tube sections there are provided means, already described, for adjustment of the height of the movable assembly 122, which includes the reversible motor 140 and the threaded shaft 144. Alternatively, a tube H could be supported during cutting by a support system of the so-called "tube centre" type, such as to maintain the axis of the tube at a fixed height independently of its cross-sectional dimensions . Preferably, as shown in particular in Figure 14, the support roller 212 has associated with it a co-operating upper clamping jaw 218. The jaw 218 is supported at the middle of a cross beam 220 which is vertically movable back and forth along vertical lateral slide guides 222 fixed to the columns 118 of the support frame 116 fixed to the carriage 102. The upper cross beam 120 of the support frame 116 carries a reversible electric motor 224 which, via a worm screw reduction gear 226 and a nut (not shown) controls a vertical threaded shaft 228 fixed to the movable cross beam 220. To ensure that the axis X of the tube H coincides with the axis of rotation of the annular support 148 during cutting, the jaw 218 is caused to descend before cutting of each length of tube, by means of the motor 224, to clamp the tube H between itself and the roller 212 as illustrated in broken lines at 218a in Figure 14. Preferably the jaw 218 is supported by the movable cross beam 220 via a telescopic column 230 incorporating resilient means (not shown) for ensuring clamping elasticity. For its adaptation to tubes of any section, round or polygonal, the jaw 218 has a V-shape engagement surface with a substantially right angle central part and side parts forming an obtuse angle with one another. Since both the roller 212 and the jaw 218 are carried by the carriage 102 these two engagement members which clamp the tube H advance together with this latter at the same speed of movement during the whole of the cutting operation. At the end of the cutting operation of one length of tube the jaw 218 is caused to rise by means of the motor 224 before return of the carriage (towards the left in Figure 8) to the start of the next cutting position. During the return strokes of the carriage and during advance of the tube H before each subsequent cut the tube always remains supported by the free roller 212. The operation of the cutting machine of Figure 8 to 14 will now be described for the case of plasma cutting of a tube H of polygonal section, for example of square section as illustrated in Figure 9 and 14. The plasma cutting sequence is similar to that described for one or the other of the cutting units 66a or 66b of the machine of Figures 2 to 7 and the details will not be described again. It will be enough to say that in each cutting operation the motors 108 and 150 are made to operate in such a way as to give the torches 180 a helical motion concentric with the axis X of the tube H at a speed having an axial component equal to the speed of translation of the tube. The cutting operation can include, as for the machine of Figures 1 to 7, a first part of the stroke of the cutting heads 172 with an axial acceleration component starting from zero, a helical cutting component with a constant axial velocity, and a final part of the stroke with an axial component of deceleration reducing to zero . The cutting heads 172 can return to the start of the cutting position without rotation of the annular support 148 and, in any case, with the torches 184 radially spaced from the tube H. In order to allow a good understanding of the purposes for which the cutting heads can pivot about their respective pins 188, reference will be made to Figure 12 which shows, among other things, one of the slides 156 and the associated cutting head 172. The parts illustrated in Figure 12 have been designated with the same references as the other Figures . The reference M indicates a part of a side face of a tube of polygonal section, for example the square section tube H of Figures 8 to 13. As has already been stated the orientation of the slide 156 is always radial or substantially radial with respect to the axis of rotation of the support 148 and with respect to the axis X of the tube H. Let us suppose, as in Figure 12, that the face M starting from which the plasma cutting is to be commenced, forms with the slide direction of the slide 156 an angle different from 90 . To start cutting the actuator 160 is energised in the direction N of advancement of the slide 156 towards the face M with the arrangement illustrated in solid outline in Figure 12. At a certain point along this stroke in the direction N an angle of the flat contact face 196 of the feeler 194 comes into contact with the face M. Since the actuator 160 continues to urge the head 156 in the direction of the arrow N, a cam effect takes place between the face M and the feeler 194 which causes the cutting head 172 to rotate about the pin 188 in the direction of the arrow Q until reaching the condition illustrated in broken outline in which the flat contact face 196 of the feeler 194 is wholly applied against the face M and the axis of the torch 180 is normal to this face, as is advantageous to perform a correct plasma cut . When the condition illustrated in broken outline in Figure 12 is achieved, a predetermined air gap remains between the nozzle 182 and the face M, having the value K in Figure 11. This air gap or distance K has been recorded once and for all for the ideal value for plasma cutting. To avoid the feeler 194 from pressing excessively against the face M under the thrust of the actuator 160 the tangential wall 170 (Figure 11) of the body 158 of the slide 156 is provided with a proximity switch 226 which, when the disposition illustrated in broken outline in Figure 12 is reached, controls disactivation of the actuator 160 maintaining however a thrust pressure in the direction of the arrow N equivalent to a resilient pressure of the contact face 196 with the tube . This resilient pressure, which alternatively could be exerted by a spring, ensures contact of the face 196 of the feeler 194, that is to say the constancy of the air gap K, independently of the external form of the tube and its irregularities in section. In Figure 13 there is illustrated a typical aspect of the slides 156 and the cutting heads 172 with the axes of the torches 180 normal to the faces of a square tube on which they have just been making a cut. If, in Figure 13, the rotation of the support 148 is assumed to be in the clockwise sense, the torches 180 are on the point of passing from one adjacent face of the tube H beyond a corner. As soon as the corner is passed, thanks to the "on the flat" engagement of the feelers 194 with the successive faces, the cutting heads 172 and their torches 180 reorientate themselves perpendicular to the faces themselves . In other words, the arrangement is such that, in the case of the cutting of a polygonal section tube, the correct orientation of the torches 180 takes place following displacement of the slides 156 towards the tube at the beginning of the cut and during the cutting, by the effect of the resilient biasing of the actuators 160. In the case of a tube of regular polygonal section with a number of equal faces, upon passing the corners a temporary apparently unwanted situation occurs in which the plasma jets, in the case of pairs of opposite torches 180 are directed towards one another. However, this transitional period is very short, and in any event does not cause unwanted effects as far as the quality of the cut is concerned. With particular reference to Figure 11 there has been described a braking or frictional resistance of the pivoting of the cutting heads 156. This frictional resistance is beneficial in that, in the first place, it resists possible unwanted angular vibrations of the cutting heads 156 and their torches 180. Another advantage of the frictional resistance is the following: if during cutting an interruption of the electrical power occurs, the control system of the machine is so arranged as to move the torches 180 away from the tube concomitant with the stopping of the rotation of the support 148. The frictional resistance of the heads 156 guarantees that the orientation of the cutting heads 172 with respect to their slides 156 remains the same as before the interruption. Upon reinstatement of the electrical power, when the cutting is recommenced, the cutting heads 172 are brought up to the tube with their orientation remaining unchanged . The machine of Figure 8 to 14 is also adapted for cutting tubes of circular section. In this case it is convenient that the cutting heads 172 be locked in positions in which the axes of the torches 180 are substantially radial. This locking is obtained (Figure 11) by the engagement of the bolts 206 into the associated seats 208. As with the embodiment of Figures 2 to 7 , in plasma cutting of a circular section tube it is convenient that the plasma jets of two opposed torches 170 do not encounter one another. Thus , in the machine of Figures 8 to 13 the diametrically opposite torches of each cutting head 172 are also locked with respect to the associated slide 156 in an orientation such that the plasma jets emitted by the torches 180 lie slightly away from the truly radial direction, in such a way as not to interfere with one another . With the torches 180 thus oriented in a permanent manner, the resilient pressure exerted by the actuators 160 also has the advantage of maintaining the air gap K constant (Figure 11) . The invention is not limited to the embodiments illustrated and described, having one or two cutting units each with four equally angularly spaced cutting heads. Thus, a machine according to the invention could also comprise a single cutting head operable to perform a helical movement as described above, or any number n of cuttings heads equally angularly spaced and movable in unison both linearly and angularly, preferably on the condition that n be at least equal to 2 in order to have two diametrically opposite cutting heads 62a, 62b or 172. In the case of n cutting heads the associated means for controlling them to rotate will be operable to make the cutting head units follow an angular path at least equal to 360 /n during the advancing stroke in order to be able to perform a cut around the entire circumference of the tube H. The presence of the cutting heads, and possible jaws, which do not perform a complete revolution is advantageous since all their electrical and possible hydraulic or pneumatic connections (not shown) can be made with flexible tubes and cables rather than with rotary connectors . Thus also, a machine such as that illustrated in Figures 8 and 9 could be arranged for cutting only tubes of circular section. In this case the cutting heads such as 172 would have a substantially fixed radial orientation. To operate the cutting machine there is provided a numerical control according to a program for all the control means (motors and actuators) under the control of suitable sensors (such as encoders) for detecting the positions of the various movable elements. Reference will now be made equally to Figures 1 and 2 or Figures 1 and 8 to illustrate an advantageous characteristic of the invention which allows almost perfect cuts to be made with plasma torches without sputtering. A pipe 74 for delivering a washing fluid leads to the machine for the fabrication of welded tubes illustrated in Figure 1, which pipe penetrates into the slot E existing in the tube in the course of formation. Within the tube H in the course of formation the pipe 74 forms an elbow and then continues with a rigid rectilinear section 76 which extends (Figures 2 and 8) up to a point situated immediately upstream of the position in which the advancing stroke of the cutting unit or units commences . The outlet of the pipe section 76 is provided with a deflector 78 which causes the fluid delivered to strike the inner surface of the tube H both for cooling it and for carrying away any sputter produced by the plasma cutting. Annular centring spacers 80 of anti-wear material are fixed at intervals to the pipe section 76, with which the inner surface of the tube H is in sliding contact. The washing fluid delivered through the pipe section 76 can be simple water or a coolant liquid such as one of the water and oil emulsions utilised in machine tools for carrying away swarf . The washing fluid then discharges from the tube H in the region of the cut and is collected in a vessel 82 (Figures 5 to 7) . At the bottom of the vessel 82 is a discharge duct 84 which leads to a recirculation pump 86. The recirculation pump 86 returns the liquid to the pipe 74 in which can be fitted a suitable filter (not shown) . As an alternative to a washing liquid, in certain cases an inert gas such as nitrogen or a reducing gas can conveniently be utilised for the internal scavenging of the tube H. An inert or reducing gas can also be delivered (in a manner not shown) to the exterior of the tube H in the cutting region, for example by means of nozzles associated with the plasma torches . Although in Figure 1 a machine for the fabrication of welded tubes having a longitudinal weld has been illustrated the invention is equally applicable to machines for the continuous or intermittent fabrication of tubes starting from open elements, for example starting from a strip which is formed in the machine with a helical winding process or starting from strips which already have a TJ-shape.

Claims: CLAIMS 1. A machine for cutting metal tubes into lengths using a plasma torch (64, 64b; 180), in which a plasma jet emitted by the torch towards the tube (H) is caused to traverse a circumference of the tube following a relative angular movement about the axis (X) of the tube, characterised in that it comprises : advancement means for causing axial translation of the tube (H) to be cut along a rectilinear path, and means (34, 36, 60a, 60b; 108; 150) for causing one or more plasma torches (64a, 64b; 180) to follow a helical motion concentric with the axis of the tube (H) at a speed having an axial component equal to the speed of translation of the tube. 2. A machine according to Claim 1, characterised in the fact that it further comprises; at least one cutting head (62a, 62b; 172) movable both parallel to the path of the tube (H) and circularly about the axis of the tube; a plasma cutting torch (64a, 64b; 180) carried by the or each cutting head (62a, 62b; 172) reciprocating translation control means (60a, 60b; 108) operable to cause the or each cutting head (62a, 62b; 172) to perform a linear advancing stroke at the same speed as the tube (H) , and a return stroke; means for activation and disactivation of the plasma torch or torches (64a, 64b; 180) to emit one or more plasma jets towards the tube (H) during the advancing stroke; and rotation control means (34, 36; 150) operable to cause the or each cutting head (62a, 62b; 172) to perform, during the said advancing stroke, an angular stroke of amplitude such that the plasma jet or jets cut the tube (H) around the whole of its circumference. 3. A machine according to Claim 2, characterised in that it includes a group (66a, 66b) of n cutting heads (62a, 62b) equally angularly spaced and movable in unison both linearly and angularly, n being at least equal to 2, and in that the associated rotation control means (34, 36) are operable to cause the group (66a, 66b) of cutting heads (62a, 62b) to perform an angular stroke at least equal to 360 /n during the said advancing stroke . 4. A machine according to Claim 3 , characterised in that it comprises two groups (66a, 66b) of cutting heads (62a, 62b) one of which is arranged to perform a linear advancing stroke whilst the other performs a linear return stroke and vice versa, and in that the cutting heads (62a, 62b) of one group (66a, 66b) are angularly offset with respect to the cutting heads (62a, 62b) of the other group (66a, 66b) in such a way that the strokes of the heads of one group do not interfere with the reverse strokes of the heads of the other group. 5. A machine according to Claim 4, characterised in that it comprises a cutting assembly (22) in the form of a squirrel cage with two annular heads (24, 26) concentric with the axis of the tube and interconnected by a ring of 2n bars equally angularly spaced and parallel to the axis of the tube (H) , in that the said rotation control means (34, 36) are operable to drive the assembly to rotate by acting on at least one of its annular heads (24, 26), in that each bar (28a, 28b) is provided with a rail (46a, 46b) for an individual slide (48a, 48b) carrying a cutting head (62a, 62b) , in that the slides (48a, 48b) of the two groups (66a, 66b) alternate from one bar (28a, 28b) to the other, and in that the said translation control means comprise, for each rail (46a, 46b) , a driven lead screw (50a, 50b) which extends along the associated rail and is engaged with a nut of the associated slide (48a, 48b) , the lead screws (50a, 50b) of each group (66a, 66b) being driven to rotate in unison. 6. A machine according to Claim 5, characterised in that centering jaws (72a, 72b) are fixedly connected for translation, parallel to the axis (X) of the tube (H) , to two or more angularly equally spaced slides (48a, 48b) of each groups (66a, 66b) the jaws being controlled to frictionally engage the tube (H) during the said advancing stroke of the group (66a, 66b) . 7. A machine according to Claim 6, characterised in that each jaw (42a, 42b) is carried by a shaft (70a, 70b) slidable radially with respect to the tube (H) on a block (68a, 68b) slidable in turn along the associated rail (46a, 46b) , and in that the block (68a, 68b) and the shaft (70a, 70b) are interconnected by means for controlling the translation of the shaft towards and away from the tube (H) . 8. A machine according to Claim 6 or Claim 7, characterised in that the means (34, 36) for controlling translation of the group (66a, 66b) of cutting heads (62a, 62b) , the means for controlling translation of the jaws (72a, 72b) and the means for activation/disactivation of the torches (64a, 64b) are so arranged as to cause a group (66a, 66b) to perform, during its advance, a first, accelerating portion (LI) of the stroke, during which the torches (64a, 64b) are disactivated and the jaws (72a, 72b) approach the tube (H) until they engage it frictionally, an advancing portion (L2) of the stroke with the jaws (72a, 72b) clamped and the torches (64a, 64b) activated, and a final, decelerating, portion (L3) of the stroke during which the torches (64a, 64b) are disactivated and the jaws (72a, 72b) are withdrawn from the tube (H) . 9. A machine according to any preceding Claim, characterised in that each of the torches (64a, 64b) is carried on the associated head (62a, 62b) by motorised /15379 38 means for adjusting the distance of the torch from the tube (H) . 10. A machine according to any of Claims 3 to 9, characterised in that all the torches (64a, 64b) of the or each group (66a, 66b) are directed towards the tube (H) at the same angle such that the plasma jets intersect the tube along a chord passing radially within the inner surface of the tube . 11. A machine according to Claim 1 or Claim 2, characterised in that it comprises a support (148) translatable along and rotatable about the axis (X) of the tube (H) , and in that the support (148) is provided with at least one motorised slide (156) which carries a cutting head (172) able to perform a radial movement with respect to the axis (X) of the tube and provided with a plasma torch (180) with a plasma emission nozzle (182) directed towards the tube (H) . 12. A machine according to Claim 11, characterised in that the support (148) is annular, is provided with a unit of n angularly equally spaced motorised slides (156) each with a cutting head (172) provided with a plasma torch (180) , and in that the said rotation control means (150) are operable to cause the support (148) to perform an angular stroke at least equal to 360 /n during the said advancing stroke. 13. A machine according to Claims 11 or Claim 12, characterised in that it comprises a carriage (102) translatable on guide means (100) extending in a direction parallel to the axis (X) of the tube (H) and on which the said support (148) is mounted for rotation about the axis of the tube, in that the carriage (102) carries at least one motor (150) for controlling the rotation of the support (148), and in that carriage translation control means are interposed between the carriage (102) and its guide means (100) . 14. A machine according to Claim 13 , characterised in that the carriage translation control means (102) comprise at least one translation control motor (108) carried by the carriage itself and which drives at least one toothed sprocket (112) , and in that the guide means (100) comprise at least one fixed rack (114) with which the sprocket is in mesh. 15. A machine according to Claim 14, characterised in that the guide means comprise a pair of rails (100) spaced from one another and on which the carriage is slidably mounted, in that a shaft (110) is mechanically connected by means of a transmission (115) to the or each translation control motor (108) , which shaft extends transversely from one rail to the other and carries a pair of toothed sprockets (112) each adjacent a respective rail (100) , and in that each rail (100) is provided with a fixed rack (114) with which one of the sprockets (112) meshes. 16. A machine according to Claim 15, characterised in the carriage (102) is slidable along the rails (100) by rolling contact linear guides (106) . 17. A machine according to any of Claims 14 to 16, characterised in that the carriage (102) is provided with a movable assembly (122) on which the support (148) for the motorised slide or slides (158) is rotatably mounted, and in that the position of the movable assembly (122) on the carriage (102) is adjustable in at least one direction in a plane normal to the axis of the tube (H) for the purpose of obtaining the coincidence of the axis of rotation of the support with the axis (X) of the tube (H) . 18. A machine according to Claim 17, characterised in that it has a configuration in which the axis (X) of the tube (H) is horizontal, in that it includes rollers (212, 216) for support of the tube (H) at a fixed height upstream and downstream of the cutting heads (156) , and in that the said direction of adjustment of the position of the movable assembly (122) of the carriage (102) is vertical . 19. A machine according to Claim 18, characterised in that the support roller (212) situated upstream of the cutting head (156) is carried by the carriage (102) , and in that the carriage (102) further carries a motorised clamping jaw (218) cooperable with the said roller (212) to ensure that during cutting the axis (X) of the tube (H) coincides with the axis of rotation of the annular support (148) . 20. A machine according to any of Claims 11 to 19, characterised in that the or each cutting head (172) includes a plasma torch (180) with a plasma emission nozzle (182) , as well as a feeler (194) fixed to the torch (180) with a contact face (196) facing towards the axis of the tube (H) and projecting from the nozzle (182) by a distance equal to a predetermined air gap (K) between the nozzle (182) and the surface (M) of the tube (H) to be cut, and in that there are provided control means (160) for biasing the slide (156) yieldably in the direction of approach to the axis (X) of tube (H) and to return it in the direction of separation from the tube . 21. A machine according to Claim 20, characterised in that the or each cutting head (172) is mounted on the associated slide (156) in such a way as to be angularly pivotable about an axis parallel to the axis (X) of the tube (H) , in that the contact face (196) of the feeler (194) is flat and normal to the axis of the torch (180) , and in that the arrangement is such that, when cutting a tube (H) of polygonal section, after displacement of the slide (156) towards the tube (H) at the commencement of cutting, and by the effect of the said biasing during cutting, the flat contact face (196) of the feeler (194) is always constrained to match with a face (M) of the tube (H) and the torch (180) is always constrained to be disposed with its axis perpendicular to the face of the tube and with the said predetermined air gap (K) between the nozzle (182) and the face of the tube. 22. f A machine according to Claim 21, characterised in that the pivotal coupling between the cutting head (172) and the slide (155) is subject to frictional resistance. 23. A machine according to Claim 22, characterised in that the slide (156) includes a wall (184) with a flat radial face (186) , the head includes a support bracket (174) for the torch (180) , with a face facing towards the this flat face (186) , the said wall (184) of the slide (156) and the bracket (174) are interconnected by a pivot pin (188) , the bracket (174) incorporates a friction shoe (198) pressed resiliently against the flat face (186) of the slide (156) , and the pivot pin (188) is disposed between the friction shoe (198) and the contact face (196) of the feeler (194) . 24. A machine according to any of Claims 20 to 23, characterised in that the said control means for the or each slide (156) are constituted by pneumatic actuators (160) . 25. a machine according to any other Claims 20 to 24, characterised in that it includes means (202) for locking the pivoting of the or each cutting head (172) with respect to the associated slide (156) in an orientation in which the nozzle (182) is directed substantially towards the axis (X) of a tube (H) of circular section. 26. A machine according to Claim 25, characterised in that the said means for locking the pivoting of the cutting head (172) comprise an actuator (202) with a body (204) fixed to the slide (156) , and a rod (206) movable to and fro parallel to the pivot axis and engageable in the manner of a bolt in a corresponding seat (208) in the cutting head (172) . 27. A machine according to Claim 25 or Claim 26, characterised in that it includes at least two diametrically opposite torches (180) and in that the said means (202) for locking the pivoting of each cutting head (172) are so arranged that, in conditions in which each cutting head is locked with respect to the associated slide (156) , the plasma jets emitted by the torches (180) are diverted with respect to the radial direction in such a way as not mutually to interfere. 28. A machine according to any of Claims 1, 2 and 11 to 27, characterised in that the translation control means (108) for the support (148) , the rotation control means (150) for the support (148) , the control means for the or each slide (156) and the activation/disactivation means for the or each torch (180) are so arranged as to cause the support (148) to perform, during its advancing stroke, a first, acceleration, stroke portion during which the torch (180) is moved towards the tube until it reaches the said predetermined air gap (K) between the nozzle (182) and the surface of the tube (H) to be cut and the torch (180) is disactivated, a cutting stroke portion with the torch (180) activated, and a final deceleration stroke portion during which the torch (180) is disactivated. 29. An installation for the fabrication and cutting of metal tubes, characterised in that it comprises a cutting machine according to any preceding Claim and, upstream of the cutting machine, a machine for the fabrication of tubes starting from an open element such as a strip (B) , and in that it further comprises a washing fluid delivery pipe (76) which extends within the cutting machine, inside the tube (H) during the course of fabrication from a point (E) situated upstream of the region in which the open element is closed onto itself and welded to form the tube and up to a point situated within the cutting machine and immediately upstream of the point (P2) at which the advancing stroke of the or each cutting head (62a, 62b; 172) commences. 30. An installation according to Claim 29, characterised in that it includes an annular centering member (80) of anti-wear material fixed to the part of the delivery pipe (76) which is within the tube (H) and with which the internal surface of the tube comes into sliding contact. 31. A process for cutting metal tubes into lengths with the use of a plasma torch (64a, 64; 180), in which a plasma jet emitted by the torch towards the tube (H) is made to follow a circumference of the tube by performing a relative angular movement about the axis (X) of the tube, characterised in that the tube (H) to be cut is caused to translate axially along a rectilinear path, and during translation of the tube (H) one or more plasma torches (64a, 64b; 180) are driven with a helical motion concentric with the axis (X) of the tube and having an axial velocity component equal to the speed of translation of the tube. 32. A process according to Claim 31, characterised in that it utilises a group of n cutting heads (62a, 62b; 172) equally angularly spaced and movable in unison along the said helical path, n being at least equal to 2, and to perform the cutting the group of cutting heads (62a, 62b; 172) , with the torches (64a, 64b; 180) activated, is caused to follow a helical cutting path with an angular movement at least equal to 360 /n after which the group of cutting heads (62a, 62b; 172) is caused to perform a return stroke with the torches (64a, 64b; 180) disactivated. 33. A process according to Claim 32, characterised in that it utilises two groups (66a, 66b) of cutting heads (62a, 62b) and causes one of the groups to perform a 15379 46 helical cutting stroke whilst the other group is caused to perform a return stroke and vice versa, the cutting heads (62a 62b) of one group (66a, 66b) being angularly offset with respect to the cutting heads (62a, 62b) of the other group (66a, 66b) in such a way that the forward strokes of the heads of one group do not interfere with the reverse strokes of heads of the other group . 34. A process according to Claim 33, characterised in that the return stroke is performed in a shorter time than the cutting stroke to allow a pause of the group (66a, 66b) in the initial position (SI) of the cutting stroke . 35. a process according to Claim 32, 33 or 34, characterised in that during cutting the tube (H) is frictionally engaged by centering jaws (72a, 72b) which move helically together with the associated groups (66a, 66b) of cuttings heads (62a, 62b) . 36. A process according to Claim 35, characterised in that before cutting the cutting group (66a, 66b) is caused to perform a first stroke portion (LI) with an axial component of acceleration starting from zero, during which the jaws (72a, 72b) approach the tube (H) until they clamp it and the torches (64a, 64b) are disactivated, a helical cutting stroke (L2) with a constant axial velocity component during which the jaws (72a, 72b) are clamped on the tube (H) and the torches (64a, 64b) are activated, and a final stroke portion (L3) with an axial deceleration component down to zero, during the which the jaws (72a, 72b) retract from the tube (H) and the torches (64a, 64b) are disactivated. 37. A process according to any of Claims 31 to 36, characterised in that the or each group (66a, 66b) has torches (64a, 64b) directed towards the tube (H) with the same inclination such that the plasma jets intersect the tube along a chord passing radially within the internal surface of the tube . 38. A process according to Claim 31 or 32, characterised in that it utilises a support (148) translatable along the axis (X) of the tube (H) and rotatable about the axis of the tube along the said helical path, this support (148) being provided with at least one motorised slide (156) which carries a cutting head (172) and is operable to perform a radial movement with respect to the axis (X) of the tube (H) , in that it utilises at least one plasma torch (180) with a plasma emission nozzle (182) fixed securely to the or each cutting head (172) and in that it is provided with a feeler (194) with a contact face (196) facing towards the axis (X) of the tube (H) and projecting with respect to the nozzle by a distance equal to a predetermined air gap (K) between the nozzle (182) and the surface (F) of the tube (H) to be cut, and in that there are provided control means (160) for biasing the or each slide (156) yieldably in the direction (N) of approach towards the axis (X) of the tube (H) and means for returning it in the direction of withdrawal from the tube. 39. A process according to Claim 28, characterised in that the or each cutting head (172) is mounted on the associated slide (156) in such a way that the head can pivot about an axis parallel to the axis (X) of the tube (H) , in that a feeler (194) is utilised with a flat contact face (196) normal to the axis of the torch (180), and in that the arrangement is such that, in the case of cutting a polygonal section tube (H) , after displacement of the slide (156) towards the tube at the commencement of cutting, and by the effect of the resilient biasing during cutting, the flat contact face (196) of the feeler (194) is always constrained to match with a face of the tube (H) and the torch (180) is always constrained to be disposed with its axis perpendicular to the face of the tube and with the said predetermined air gap (K) between the nozzle (182) and the face of the tube . 40. A process according to Claim 39, characterised in that a frictionally resisted pivotable coupling between the cutting head (172) and the slide (156) is utilised. 41. A process according to Claim 39 or 40, characterised in that, when cutting a circular section tube (H) , the pivoting of the or each cutting head (172) with respect to the associated slide (156) is locked in an orientation in which the nozzle (182) is directed substantially towards the axis (X) of the tube (H) . 42. A process according to Claim 41, characterised in that cutting is performed utilising at least two diametrically opposed torches (180) and each cutting head (172) is locked with respect to the associated slide (156) with an orientation such that the plasma jets emitted by the torches (180) are offset from the radial direction in such a way as not mutually to interfere . 43. A process according to any of Claims 38 to 42, characterised in that, before cutting, the support (148) is caused to perform a first advancing stroke portion with an axial acceleration component starting from zero during which the nozzles (182) of the torches (180) approach the tube (H) and the torches are disactivated, a helical cutting stroke at a constant axial velocity during which the or each torch (180) is activated and the said predetermined air gap (K) is maintained between the or each torch and the surface of the tube (H) to be cut, and a final deceleration portion during which the or each torch (180) is disactivated. 44. A process for the fabrication and cutting of metal tubes, characterised in that it utilises a cutting process according to any of Claims 31 to 43 on a tube (H) shaped and welded from an open element (B) such as a strip, and during cutting a washing fluid is introduced /15379 into the tube (H) by a pipe (76) which extends within the tube during fabrication starting from a point (E) situated upstream of the region in which the open element (B) is closed on itself and welded to form the tube, and up to a point situated immediately upstream of the point (SI) of commencement of the cutting operation. 45. A process according to Claim 45, characterised in that a coolant liquid is used as the washing fluid. 46. A process according to Claim 44, characterised in that an inert or reducing gas is utilised as the washing fluid. 47. A length of tube having ends cut with a plasma jet cutting process according to any of Claims 41 to 46.

Inventors: MARTINA ROBERTO, PADOAN OTTAVIO, MARTINA, ROBERTO, PADOAN, OTTAVIO
Applicants: MARTINA OFFICINE METALMECCANIC, MARTINA ROBERTO, PADOAN OTTAVIO, MARTINA OFFICINE METALMECCANICHE S.N.C. DEI F.LLI, MARTINA, ROBERTO, PADOAN, OTTAVIO
Application year: 1997
Main classification: B23K10
IPC codes: B23K10, B23K37