The present invention relates to a method and a device for cutting off elongated hollow elements, such as tubes, wherein a milling cutter provided with a plurality of cutting elements is rotated relative to the elongated element.

The invention is particularly intended to be used in connection with oil drilling, where the well is supported by casings, i.e. tubes, having diameters ranging from about 10 to 90 centimeters. The tubes have a length of 10 to 12 meters and are provided with external threads at each end. The tubes are mutually inter¬ connected by means of internally threaded coupling sleeves. The coupling sleeves are manufactured by cutting off blanks from long (in the order of 10 to 12 meters) tubes, which blanks are machined on specially designed machines.

The cut-off operation is a problem area in the manufacturing of the coupling sleeves. In prior art technique the tubes are rotated and cut off by means of a stationary cut-off tool. Due to the length and out-of roundness of the tubes, the cutting speed is limited. Further, the tools ^' are often damaged by the tube element being cut at the moment when the tools break through the wall of the tube.

This gives low machine efficiency and high tool cost.

It is also known to maintain the tube stationary and use a machine having a rotating plate. One or several slides are radially movably mounted on the rotating plate. The cut-off tools are mounted on the slides and during the rotation the slides are moved towards the centre of the tube, thereby cutting off the tube. Also in this case low machine efficiency is obtained due to big chip problems and instability. Further, the cutting speed is limited due to balance problems.

The object of the present invention is to provide a method and a device for cutting off tubes, where a comparatively high cutting speed can be used while simultaneously giving a smoother cutting and less risks for damage to the equip¬ ment and to the tube element being cut.

This and other objects of the invention are obtained by giving the invention the characterizing features stated in the claims following hereinafter. „

The invention is described in detail in the following de¬ scription with reference to the accompanying drawings. It is to be understood that the embodiment illustrated therein is only illustrative of the invention and that various modi¬ fications thereof may be made within the scope of the claims.

In the drawings, Fig. 1 shows a top view of a device according to the invention,

Fig. 2 shows a section taken on the line II-II in Fig. 1 ,

Fig. 3 shows a portion of the cut-off tool on a larger scale,

Fig. 4 shows a detail of the cutting inserts sites in the cut¬ off tool on a still larger scale,

OMP

Fig. 5 shows a similar top view as in Fig. 4 but wherein the cutting insert is located -at the opposite side of the cut-off tool, and

Fig. 6 shows a side view of the cutting insert site in Fig. 5.

Fig. 1 shows a tube 10 mounted in a cut off machine 11. The machine comprises holders 12 and 13 with rotatable chucks 1-4 for gripping the tube. The chucks 1 are driven in syn¬ chronism by motors 15.

Between the holders 12 and 14 there is a guide bed 16 on which is slidably mounted a cutter housing 17 carrying a milling cutter 18. The cutter is rotated by a motor 19 and the housing 17 is moved along the guide bed 16 by a motor 20 through a screw transmission 21. In the illustrated embodi¬ ment, the guide bed 16 is plane but the transverse motion of the cutter may have some other shape, for instance curved or circular.

Fig. 3 shows the cutter 18 with a number of the insert sites. Each site comprises a recess containing a backing up shim plate 22, a cutting insert 23 and a clamping screw 24 with a clamping wedge 25.

The cutting inserts 23 are mounted in a staggered mutual inter-relationship so that every other insert cuts about half of the thickness of the cut and every other insert cuts the other half, the halves slightly overlapping in the middle. This is illustrated in Figs. 4 and 5. The insert sites are orientated slightly obliquely together with the inserts 23 and the shims 22. The shims are fastened with screws 26. The wedges 25 are of two different types, 25a and 25b, depending on which side they are placed on.

OMPI /,, WIPO .

According to the invention the cutting of a tube is carried out as described below. First, the tube 10 is positioned as shown in Fig. 1 and clamped by the chucks 14. The milling cutter 18, then, is rotated in counter-clockwise direction in Fig. 2 as indicated by arrow A by means of the motor 19. The milling cutter 18 is moved laterally by means of the motor 20 during the rotation of the milling cutter. The tube 10 is maintained stationary till the milling cutter 18 has cut through the wall of the tube 10. Then, the motors 15 are started, thereby rotating the tube 10 slowly in clockwise direction in Fig. 2 as indicated by arrow B. The tube 10 is rotated till the cut-off around the whole tube is completed. During the rotation of the tube the milling cutter is maintain stationary in its lateral position.

According to the invention, alternatively, the tube 10 and the milling cutter 18are rotated simultaneously during the whole cut-off operation. In this case the milling cutter 18 is moved laterally during the rotation of the milling cutter and the tube.

In the above described operations, the milling cutter 18 and the tube 10 have opposite rotational directions. According to the invention, however, the milling cutter and the tube can have the same rotational direction. The rotational speeds of the milling cutter and the tube and the speed of the lateral feed mechanism 20, 21 for the milling cutter are preferably adjusted in mutual relationship for obtaining optimal cutting with as small vibrations as possible. The rotational speed of the milling cutter shall be much higher than the rotational speed of the tube. It is believed that the ratio therebetween should be larger than 50, with preference for values in the order of 60-120, in cases where the ratio between the diameter of the tube and the milling cutter is larger than 0,7. If the diameter of the tube is smaller, then the ratio between the rotational speeds can be decreased in proportion to the decrea

of the diameter of the tube. Typically, a cutting speed of 120 m/min, i.e. a rotational speed of about 340 rpm when the diameters of the tube and the milling cutter are respectively 0,5 and 0,7 meters, and a rotational speed of the tube of about 3 rpm is believed to be suitable.

OMPI