1. Field of the invention
The present invention relates to a method of cutting a metal pipe and an apparatus for cutting a metal pipe orthogonally to a central axis line thereof and capable of effectively cutting even a metal pipe coated with a resin coating layer at an outer peripheral face thereof.
2. Description of the prior arts
FIG. 5 shows a conventional apparatus of this kind for cutting a metal pipe and a metal pipe 1 to be cut, is held from directions orthogonal to a central axis line of the metal pipe 1 such that a cut position thereof is sandwiched at portions forward and rearward from the cutting position by holding holes 8a and 9a generally in a semicircular shape respectively formed at opposed faces of a pair of holding members 2a and 3a capable of being opened and closed opposedly in the horizontal direction and other pair of holding members 2b and 3b spaced apart from the pair of holding members in the axial direction of the metal pipe 1.
Further, in respect of the metal pipe 1, there is arranged a cut-off blade 5 for cutting off a portion of a pipe wall of the metal pipe 1 at a position of cutting the metal pipe 1 orthogonally to a central axis line of the metal pipe 1 and movably in an arrow mark X direction which is a direction of a tangential line of an outer peripheral face thereof. Further, there is arranged a cut blade 6 which is movable in an arrow mark Y direction constituting a direction orthogonal to the central axis line for cutting to penetrate the metal pipe 1 at a deficient wall portion where a portion of the pipe wall has been cut off by the cut-off blade 5. Accordingly, the arrow mark X direction and the arrow mark Y direction constitute an angle of substantially 90 degree.
According to the conventional apparatus of cutting the metal pipe 1 having such a constitution, portions of the metal pipe 1 forward and rearward from the cut position are fixedly held by the holding holes 8a and 9a of the holding members 2a and 3a and the holding members 2b and 3b and successively, the cut-off blade 5 is firstly moved in the direction orthogonal to the central axis line of the metal pipe 1 indicated by the arrow mark X direction of FIG. 5 relative to the metal pipe 1 and a portion of the pipe wall of the metal pipe 1 is cut off in the tangential direction by the cut-off blade 5.
Successively, the cut blade 6 is moved from the direction orthogonal to the central axis line of the metal pipe 1 indicated by the arrow mark Y direction of FIG. 5 and at the deficient wall portion of the metal pipe 1 where a portion of the metal wall has been cut off, the metal pipe 1 is cut by the cut blade 6 from the deficient wall portion.
However, according to the conventional apparatus of cutting the metal pipe 1, in cutting of f a portion of the pipe wall in the tangential direction by the cut-off blade 5, when angular moment is applied on the metal pipe 1 in the clockwise direction by the cut-off blade 5 as shown by an arrow mark M direction in FIG. 6 and the angular moment exceeds frictional force based on static frictional coefficient between peripheral faces of the holding holes 8a and 9a and the metal pipe 1, the metal pipe 1 is rotated at insides of the holding holes 8a and 9a. 
When such a rotation by the angular moment in the arrow mark M direction is caused, not only scratch marks are produced at the outer peripheral face of the metal pipe 1 but also the metal pipe 1 is stopped at a cut-off position which is not in parallel with the arrow mark X direction but is inclined to the X direction by an angle as shown by FIGS. 7A and 7B as a result of the cut-off operation by the cut-off blade 5.
Further, when cutting operation is started by the cut blade 6 under the state, the cutting operation is started from a portion of the metal pipe 1 where a portion of the pipe wall is not cut off or a portion thereof where an extremely small amount of the pipe wall is cut off and accordingly, burrs cannot be produced symmetrically in the left and right direction relative to the cut blade 6 and an extremely irregular or deformed cut face is constituted.
Further, even when the cut-off step is finished without rotating the metal pipe, in finally cutting the metal pipe by the cut blade 6, there is brought about a state in which a lower side of the cut face of the metal pipe 1 is more roughened and deformed than an upper side thereof since the cutting operation is finished while the lower side of the cut face is being pressed by burrs from an inner face of the metal pipe.
In order to prevent various problems posed by rotating the metal pipe 1 from causing in the cut-off step, conventionally, widths of holding the metal pipe 1 by the holding members 2a and 3a and the holding members 2b and 3b, are widened, a circle formed by inner peripheral faces of the holding holes 8a and 9a is deviated from a completely round circle into an ellipse having a longer length in the vertical direction and an outer peripheral face of the metal pipe 1 is held while deforming the metal pipe 1 into the elliptical shape while exerting strong tightening force to the holding members 2a and 3a and the holding members 2b and 3b to thereby carry out the cut-off operation.
Accordingly, there poses a problem in which particularly in the case of a thin-walled one of the metal pipe 1, at a vicinity of the cutting portion of the metal pipe 1 after having been cut to penetrate, as shown by FIG. 8, the metal pipe 1 is frequently deformed in an elliptical shape deviated from a completely round circle.
Further, in the case of a resin-coated metal pipe 11 produced by providing a resin coating layer 7 on the outer peripheral face of the metal pipe 1, the resin coating layer 7 is cut off in the tangential direction along with a portion of the pipe wall of the metal pipe 1 by the cut-off blade 5 and successively, the cutting operation by the cut blade 6 is started from a position of the deficient wall portion where the portion of the pipe wall is cut off. At an instance at which the cut blade 6 exerts shear force from above the resin-coated metal pipe 11, the resin coating layer 7 having Young""s modulus far smaller than that of the metal pipe 1 is deformed to compress at insides of the holding holes 8a and 9a on the side of the deficient wall portion and the cutting operation is progressed under the state. As a result, at a final portion, that is, immediately before finishing the cutting operation, an inner peripheral end face of the metal pipe 1 is partially deformed toward an outer side and as shown by FIG. 9, a shear droop portion 10 is produced at the portion of cutting the metal pipe 1 and a highly accurate cutting operation may not be carried out.
The invention relates to a method of cutting a metal pipe for cutting a metal pipe orthogonally to a central axis line of the metal pipe and an apparatus, capable of effectively cutting even a metal pipe coated with a resin coating layer at an outer peripheral face thereof.
The invention has been carried out in view of a current situation of cutting by a conventional apparatus of cutting a metal pipe or a resin-coated metal pipe of this kind, described above, and it is an object of the invention to provide a method of cutting a metal pipe or a resin-coated metal pipe with high accuracy by preventing the metal pipe or the resin-coated metal pipe from being rotated, preventing burrs in cutting from being caused and preventing a cutting face from being deformed and preventing shear droop caused by a resin coating layer of the resin-coated metal pipe from being produced, and an apparatus to implement the same.
In order to achieve the above-described object, according to a first aspect of the invention, there is provided a method of cutting a metal pipe which is a method of cutting the metal pipe orthogonally to a central axis line of the metal pipe, the method comprising a step of sandwiching an outer peripheral face of the metal pipe from a direction orthogonal to the central axis line, a step of simultaneously cutting off portions of a pipe wall held in the sandwiching step and opposed to each other in a diameter direction of the metal pipe orthogonally to the central axis line and substantially in a same direction of tangential lines of the metal pipe and a step of cutting the metal pipe from one of deficient wall portions cut off in the cutting off step toward other of the deficient wall portions orthogonally to the central axis line.
Further, according to a second aspect of the invention, there is provided an apparatus of cutting a metal pipe, which cuts the metal pipe orthogonally to a central axis line of the metal pipe, the apparatus comprising a pair of first holding members capable of being opened and closed in a direction orthogonal to the central axis line of the metal pipe and in directions opposed to each other and having holding holes in a semicircular shape having a radius of curvature substantially equal to a radius of curvature of an outer peripheral face of the metal pipe, a pair of second holding members having holding holes substantially the same as the holding holes of the pair of first holding members and provided to be spaced apart from the pair of first holding members in an axial direction of the metal pipe, a pair of cut-off blades movable in synchronism with each other in the direction orthogonal to the central axis line for simultaneously cutting off portions of a pipe wall of the metal pipe opposed to each other in a diameter direction of the metal pipe substantially in a same tangential direction and a cut blade for cutting to penetrate the metal pipe from one of deficient wall portions where the portion of the pipe wall has been cut to other of the deficient wall portions where the portion of the pipe wall has been cut orthogonally to the central axis line by moving in an interval between the two pairs of holding members.