This invention relates to a method and an apparatus for cutting off the beads of seam-welded pipes and especially of hot-shaped, seam-welded pipes More particularly, it relates to a method and apparatus for cutting off the beads of seam-welded pipes using a freely-rotatable circular cutting tool.
In the manufacture of seam-welded pipes and particularly hot-shaped seam-welded pipes, an endless strip, referred to as a skelp, is continuously passed through a heating furnace and heated to a temperature of at least 850.degree. C., after which it is passed through a series of forming rolls which bend the outer edges of the skelp and form it into the shape of a tube. The resulting tubular object is referred to as an open pipe. The confronting outer edges of the open pipe are as of yet unwelded. The open pipe is then passed through a tubular work coil which is somewhat larger than the outer diameter of the open pipe, and the confronting edges of the open pipe are selectively heated to a welding temperature, and the confronting edges of the open pipe are then pressure welded by squeeze rolls which are located downstream of the work coil, thereby forming a seam-welded pipe. Due to the heating and pressure welding, the inner and outer surfaces of the welded seam swells to form beads on the inside and outside of the pipe. The pipe is then sent to a finishing apparatus which cuts off the beads. It is important to cut off the beads smoothly so that there is no difference in thickness between the portion of the pipe which is cut and the adjoining portions. However, as the removal of the inner bead must be performed inside the pipe, it is generally difficult to cut the beads off smoothly. Normally, a slender mandrel is inserted from an open space upstream of where the forming rolls form the skelp into a tube to the downstream side of the squeeze rolls, and a circular cutting tool is mounted on the tip of the mandrel. Accordingly, the mandrel passes through the entire length of the open pipe and the cutting tool is secured to the mandrel downstream of the squeeze rolls.
In order to selectively heat the edge portions of the open pipe which are to be welded, a high-frequency current is passed therethrough by the work coil. Due to the skin effect, currents are induced on the circumferential surface of the open pipe, and heating is accomplished by the concentration of the current in the portions to be welded. However, if the induced current leaks to outside the portions to be welded, the welding efficiency decreases. Therefore, the leak current should be as small as possible, and in actual practice, an impeder having a ferrite core is disposed inside the open pipe so as to confront the work coil. Generally, since a mandrel for cutting off the beads on the inside of a seam-welded pipe passes through the center of the open pipe, the impeder is mounted on the mandrel opposite the work coil.
Seam welding can be formed in a hot or cold state. When cutting off the beads on the inside of a cold-shaped, seam-welded pipe, in order to prevent excess or inadequate cutting, a full-type cutting tool having the same curvature as the inside of the pipe is used. Even when the seam-welded pipe is itself in a cold state, as cutting is performed immediately after welding when the bead is still hot, the cutting tool normally has to be replaced after 1.5-2.5 hours of use. The production line must be stopped for a long time when the cutting tool is replaced. The time lost for replacement of the cutting tool has an adverse effect on productivity, since the forming and welding processes for seam-welded pipes and the bead cutting operation are all performed continuously, so that when the cutting tool is replaced, all three of these operations must be halted.
When manufacturing hot-shaped seam-welded pipes, if the same cutting method is used as for cold-shaped seam-welded pipes, decreases in productivity due to cutting tool replacement are even greater. When replacing the cutting tool, it is necessary to stop the production line just as when manufacturing cold-shaped seam-welded pipes. While the production line is stopped, it is also necessary to lower the temperature of the furnace in which the skelp is heated to the rolling temperature so that the skelp will not melt through and sever. After the cutting tool is replaced and the line is restarted, the furnace temperature is again raised, but skelp must be passed through the furnace for some length of time before the skelp temperature stabilizes, and the length of skelp which passes through the furnace until the skelp temperature stabilizes must be discarded. Furthermore, as the cutting tool operates in a high-temperature environment, the life span of the cutting tool and the length of time between replacements of the cutting tool are much shorter than in the manufacture of cold-shaped seam-welded pipes. Accordingly, if the cutting tool must be frequently replaced, not only are there large time losses, but the energy consumption rate rises and there is a large amount of material loss in the form of scrap.
Moreover, when the inside beads of hot-shaped seam-welded pipes are cut off on a production line, just as with cold-shaped seam-welded pipes, a mandrel is inserted into a pipe from a position upstream of where the skelp is formed into an open pipe and the inside bead is cut off by a cutting tool mounted on the end of the mandrel. In hot shaping, the temperature of the skelp is at least 850.degree. C., so even without the heat generated by welding, the mandrel would be heated to around 700.degree. C. by the heat from the open pipe. Therefore, it is generally necessary to provide some form of heat protection for a mandrel used to cut off the inside beads of hot-shaped seam-welded pipes.
For these reasons, in a production line for seam-welded pipes and particularly hot-shaped seam-welded pipes, there is a great need to increase the life span of the cutting tool used for cutting off inside beads and to cool the mandrel on which the cutting tool is mounted.
There have been various proposals for using a circular cutting tool for removing the inside beads of seam-welded pipes. Japanese Published Unexamined patent application No. 48-56547 (1973) discloses the use of a circular cutting tool for removing the inner beads of cold-shaped seam-welded pipes. The cutting tool has a skew angle of 22.5.degree., although the diameter and the tilt angle of the cutting tool are not specified. No mention is made of the life span of the cutting tool.
Japanese Published Unexamined patent applications Nos. 58-143908 (1983) and 59-144586 (1984) also disclose the use of circular cutting tools for removing inside beads of hot-shaped seam-welded pipes. By using a cutting tool with a diameter of 0.6-0.8 times the diameter of the pipe, a skew angle of 10.degree.-45.degree., and a tilt angle of 5.degree.-15.degree., a long life span of 2.5 hours (application No. 58-143908) or 5 hours (application No. 59-144586) can be achieved. For cutting in a high-temperature environment, these life spans are extremely long.
In above-mentioned Japanese Published Unexamined patent applications Nos. 58-143908 and 59-144586, in order to protect the mandrel from heat, the mandrel is formed with a hollow center, and cooling water is passed through the entire length of the mandrel from the base end and then continuing through the holder, the impeder, and the outer end, i.e., head portion on which the cutting tool is mounted. The cooling water is discharged into the seam-welded pipe through gaps between the outer end, i.e., head portion of the mandrel and a plurality of rollers which guide the mandrel along the inside beads of a pipe.
As for the cutting tool itself, in above-mentioned Japanese Published Unexamined patent application No. 48-56547, if the tilt angle is 5.degree.-15.degree., at a relatively slow line speed of at most 40 m/min, for example, satisfactory cutting can be carried out. However, when the line speed exceeds 100 m/min, the rotation of the cutting tool becomes unstable, and the cutting tool tends to remove too much or too little of the inside surface of a pipe being processed.
In Japanese Published Unexamined patent applications Nos. 58-143908 and 59-144586, the same tendency can be seen when the skew angle is small, but when the skew angle is large, the shape of the cutting tool as seen from the direction of advance of the pipe becomes elliptical, the difference between the curvature of the inside of the pipe and the cutting tool increases, and the cut surface takes on a U-shaped cross section, so that it becomes difficult to perform smooth cutting.
As the life span of cutting tools increases, the length of time which the mandrel must be inserted into an open pipe necessarily increases, and problems occur which were not experienced when cutting tools had a shorter life span.
The holder of the mandrel must support the mandrel so that the opposite end of the mandrel is located downstream of the squeeze rolls, and it must support an impeder so that the impeder confronts a work coil. Therefore, the holder is generally long, and the length of time for which cooling water passes therethrough is also long. As it passes through the holder, the cooling water undergoes a great increase in temperature, and even if the cooling water does not boil, gases which are dissolved in the cooling water come out of solution and remain inside the holder. The longer the mandrel is inserted into an open pipe, the larger the amount of gas which will accumulate inside the holder, and as the gas does not conduct heat as well as the cooling water, a difference in the rate of cooling will develop between the top side of the holder, where the gas accumulates, and the bottom side of the holder, which is in contact only with cooling water. This difference in cooling rates causes the holder to bend. Even if the amount of curvature is minute, as the holder itself is long, the misalignment between both of its ends and the center of the holder is large. Not only does this curvature affect the depth of cutting of the cutting tool, in the worst case, it becomes impossible to detach the rest of the mandrel from the pipe.
The impeder of the mandrel contains a ferrite core which is self-heated by magnetic induction caused by the action of the work coil. Accordingly, the greater the ability to cool the impeder the better. However, the gas which is generated in the holder is carried downstream to the impeder, and due to the gas, the impeder may be inadequately cooled. If the thermal load on the impeder is too large, not only will the ferrite core lose its magnetism, but the FRP cover which surrounds the ferrite core can be damaged, possibly resulting in the breakage of the impeder.