GB2066112A - Internal threading of drill string components - Google Patents

Abstract

A method and apparatus for forming an internal thread on a drill string component, such as a coupling sleeve 10 wherein an initial sleeve is formed with a bore of constant diameter, the internal and external diameters of the initial sleeve being greater than those of the intended finished sleeve, inserting at least one mandrel 12 having the required thread 16 and nose 18 form and pressure rolling the sleeve on to the mandrel. During formation the sleeve is cold worked so that the internal grain structure of the material of the sleeve is retained or even enhanced. The component may be a hollow drill bit. Apparatus for carrying out the method is described. <IMAGE>

Description

SPECIFICATION Internal threading of drill string components This invention relates to forming internal threads incomponents of drill strings including drill bits. Drill strings are composed of a plurality of rods each having both ends externally threaded and joined to each other by coupling sleeves which are internally threaded. Drill bits are connected to the free end of the rod at the distal end of the drill string, the drill bit having a sleeve formation that is internally threaded with the same profile as that of the coupling sleeves.

There are three basic methods of internal threading of structures of this kind in use today. One method involves the use of a single point form tool which is moved in a helical path inside the sleeve to form a cut. Successive cuts are made until the full thread depth is achieved. The second method involves thread milling employing a milling cutter having the full length and full form of the thread and having an outside diameter which is smaller than the bore in the sleeve in which the thread is to be formed. The cutter is mounted on a spindle which is eccentrically supported inside the sleeve by means of an arbor. In use the spindle is rotated at the required cutting speed and the arbor is rotated and moved axially. In the art this technique is known as "whirling".While theoretically faster than single point threading, a problem arises in that accurate boring of the sleeve and whirling of the thread has to be performed on the same machine with the sleeve in the same clamping position. This is because the arbor must be accurately supported in the bore in the sleeve in order to ensure an acceptable thread form and such accurate alignment, e.g. + 0.025 mm, can be obtained only by using a single clamping of the sleeve in a chuck. The third method involves using out-of-round copy-turning with a single point tool having a standard nose radius. The machine is fitted with a three-dimensional template carrying the thread form and rotated at the same speed as the spindle. A stylus picks up the form from the template and repeats the form in the coupling, where the cutting tip is carried by a boring bar.

Each of the three methods currently in use is time consuming and, therefore, expensive.

A further problem resides in the form of the finished product itself. The length of a cutting sleeve, for example, is set by standard. A central portion of the sleeve is formed with a bridge or radially inwardly projecting rib to prevent the coupling from moving axially from the joint. This bridge represents dead weight in the finished sleeve, but obviously is unavoidable. Also between the bridge and the start of the thread form there is a cylindrical recess which is provided to make it possible to cut the thread when the thread is unsymmetrical as is most often the situation. In other words the cutting tool, usually a whirler, is started off in the recess and then moved axially from the recess to the end of the coupling sleeve. The sleeve is then un-chucked, inverted, and re-chucked for threading the other half of the sleeve.

Even when the threads are symmetrical these functions must still be performed when using a whirler because of the necessity for accurately supporting the arbor. These recesses, on both sides of the bridge, represent further dead weight in the coupling sleeve. Not only do these dead weights have a disadvantageous effect on the life of the thread because of the increased inertia of the coupling sleeve which is important in percussive drilling, but also because these recesses take up portions of the lengths of the sleeve which otherwise could be usefully employed for the coupling threads to increase the useful thread life.

This invention seeks to ameliorate the abovementioned problems by providing a method of internally threading coupling sleeves of drill strings and drill bits which is quicker in practice than conventional methods now in use and which also produces a coupling sleeve or bit which has less dead weight than is usually present in conventional coupling sleeves.

According to the invention there is provided a method of internally threading a sleeve of a drill string component including the steps of machining the outside diameter of the sleeve to a suitable diameter and the internal diameter to a diameter slightly greater than the outside diameter of the final internal thread diameter, with the bore in the sleeve being of uniform cross-section throughout the length of the sleeve; into the sleeve; pressing the sleeve on to the mandrel; and unscrewing the mandrel. Preferably the sleeve is pressure rolled on to the mandrel.

When forming threads on a drill bit, the sleeve has a blind bore, with the bottom of the bore being initially formed with the required bottom striker face and with the outside diameter of the initial sleeve being over-size at the thread position and being of the final diameter size in the region of the bottom of the hole.

With coupling sleeves, the bore in the sleeve is formed straight through and is of constant diameter and two mandrels which are identical are inserted from opposite sides into the sleeve. The exterior of the sleeve may of of constant diameter or may have portions of increased or reduced diameter, as desired.

The ends of the finished sleeve are internally tapered outwardly in the usual manner to facilitate insertion of a drill rod end.

Using the method of the invention it is now possible to form coupling sleeves or sleeves on drill bits, the full length of which is used for the coupling thread and in which there are no recesses constituting dead weight. With a coupling sleeve there is still a bridge in the center of the sleeve which, while forming a dead weight, is necessary to prevent the coupling from moving axially away from the joint between adjacent or abutting drill rods.

Using the method of the invention provides the following advantages. Firstly, that of being able to form an internally threaded sleeve far quicker than can be done using convention methods. Secondly, by cold working that of forming a far stronger sleeve than previously because of the enhanced crystal structure thereby obtained. Thirdly, and very importantly, that by using cast sleeves, particularly con tinuously cast sleeves, which by their production method have a longitudinal grain structure, the method of the invention ensures that the grain structure and, therefore, strength is retained or only slightly adversely affected. On the other hand with conventional machining methods for internal threads, the machining cuts across and breaks the grain lines so as significantly to reduce the longitudinal strength of the sleeve.Moreover with this invention the grain lines are now formed such that they follow the thread profile and, in effect, increase the hardness of the material of the sleeve in the required direcion, namely normal to the abutting and non-abutting flanks of the thread.

An unexpected advantage is that during rolling considerable heat is generated in the sleeve, which heat can be increased by using heated rollers, so that the creation of undesirable stresses during working is minimized.

In one form of the invention it is proposed to use rollers having helical ridges on their working surfaces which press the sleeve on to the mandrel, with the ridges being matched to the thread so that only material required for forming the thread is pressed into the thread forming spaces of the mandrel leaving a "shadow" thread on the outside. In this form less effort will be required for forming the internal threads since less material is compressed radially inwardly. It is also expected that stresses set up by cold working the material of the sleeve in this way will also be minimized as part of the material will be deformed by bending without there being significant plastic extrusion.

In another, preferred form of the invention it is proposed to use smooth, i.e. not ribbed, rollers for rolling and pressing the sleeve on to a mandrel. This form compared with the first mentioned form of the invention has a disadvantage in that larger pressing forces must be used for forming the thread. However, it has a number of major advantages.

Firstly, setting up a sleeve for rolling is far simpler as the pressing rollers do not have to be synchronized with the mandrel or mandrels. Also where two mandrels are used these no longer have to be synchronized either, though they usually would be with a predetermined phase difference between them. Secondly, using cast sleeves, particularly continuously cast sleeves the flow lines of the grain are substantially axial near the exterior of the sleeve while being sinusoidal at the interior. This form of the grain ensures greater strength for the sleeve compared with a sleeve made according to the first form wherein the grain lines are sinusoidal at both the interior and exterior of the sleeve.

In a third form of the invention cylindrical shells are used for pressing the sleeve on to the mandrel or mandrels, the shells being arranged around the sleeve and squeezed together then rotated on the sleeve and squeezed together again and so on.

The invention also extends to apparatus for carrying out the method of the invention comprising at least one mandrel of a hard metal, such as tungsten carbide, or hardened steel having a suitable thread and nose form, means for supporting the mandrel, at least two pressing members for forcing the sleeve on to the mandrel, and means for forcing the pressing members against the sleeve. The pressing members may be formed with a thread formation matched to the thread of the mandrel. Preferably, the pressing members are rollers. Preferably there are three pressing rollers. The pressing members may also be cylindrical shells.

The invention also includes a coupling sleeve or a drill bit manufactured using the method or apparatus of the invention.

Further features and advantages of the invention will become apparent from the following description made with reference to the accompanying schematic drawings.

Brief Description of the Drawings Figure 1 shows a coupling sleeve element prior to deformation for forming internal threads therein; Figure 2 shows the sleeve of Figure 1 during deformation; Figure 3 shows a variant of coupling sleeve after deformation, the sleeve being shown in longitudinal section; Figure 4 shows an outside view of the sleeve of Figure 3; Figures 5, 6 and 7 show respectively a drill bit sleeve prior to deformation, during deformation and the finished sleeve; Figure 9 shows an apparatus suitable for forming coupling sleeves according to the invention; Figure 8 shows a variant of apparatus of the invention; Figure 10 shows yet another variant of apparatus of the invention; Figure ii illustrates a section through part of a sleeve prior to deformation and shows the grain lines of the material;; Figure 12 shows a section through part of a sleeve after deformation according to the second form of the invention, again shown the grain lines; and Figure 13 shows a section through part of a coupling sleeve after deformation according to the first form of the invention, again showing the grain lines.

Descnp tion of Preferred Embodiments Figure 1 shows a sleeve member 10 for forming a coupling sleeve of a drill string. The member 10 has a length L which is shorter than the required final length of the coupling sleeve and internal "1 D" and external "OD" diameters which are both greater than the corresponding diameters of the finished coupling sleeve.

Figure 2 shows the coupling sleeve 10 of Figure 1 at an intermediate stage during deformation by cold rolling to form a coupling sleeve. The member 10 is supported on two mandrel 12 and 14 each of which ' has a thread formation 16 and a nose formation 18, with the nose formations 18 abutting each other.

When abutting each other the nose formations form an annular recess 20 as shown. During cold rolling the sleeve is initially deformed to form an inwardly directed radial rib or bridge 22 into the recess 20.

Subsequently, and not shown in the drawings, the sleeve is completely deformed into the recess 20 and is also deformed to form internal threads matching those on the mandrels 12 and 14. When this occurs the length of the sleeve 10 increases to the required final length of the coupling sleeve as also does the internal and external diameters.

Figures 3 and 4 show a variant of coupling sleeve 22 formed as discussed above in this case the deformation is achieved by means of pressing rollers which have thread forms on their external surfaces which are arranged to press only those regions where there are to be peaks on the internal threads. For example the sleeve 24 has internal threads 26 which are not symmetrical, such as shown at 28 and 30 on the outside of the thread has been deformed such that there is a recess or groove 32 aligned with the internal bridge 22 and grooves 34 each aligned with a peak 36 of the internal thread.

Figure 5 shows a member 56 suitable for formation according to the method of the invention into a drill bit. The member 56 includes a head portion 58 (which is shown schematically) and a sleeve portion 60. The sleeve portion 60 is formed with a bore 62. A portion 64 of the sleeve 60 is formed with the required sleeve outer diameter and sleeve inner diameter. A further portion 66 tapers outwardly from the portion 64 to a further portion 68. The portions 66 and 68 have the same internal diameter as the portion 64, but such internal diameter is in excess of the internal diameter of the finished sleeve for the drill bit. Also the outer diameter of these portions 66 and 68 is greater than the required finished outside diameter of the sleeve.

Figure 6 shows the drill bit 56 of Figure 5 with a mandrel 70 inserted into the bore 62. Pressing rollers, not shown, are applied to the portions 66 and 68 of the sleeve 60.

Figure 7 shows the drill bit after rolling and pressing. In the finished drill bit the sleeve 60 has a bottom striker face 72 with a recess 74 adjacent the striker face and then an internal thread formation 76 whereby the drill bit may be coupled to the end of a drill rod at the end of a drill string or the like.

Figure 8 shows apparatus suitable for forming the coupling sleeves 24 of Figures 3 and 4. As will be appreciated the drawing indicates the apparatus schematically only as the design of a suitable apparatus is within the scope of any engineer or designor. The apparatus comprises three pressing rollers 38 each formed with a thread formation 40 which mates with the thread formation 16 on a mandrel. Each roller 38 is journalled on a shaft 42 one end of which carries a sprocket 44. The three sprockets 44 are interconnected by an endless chain 46. One of the rollers 38 in addition has a shaft extension which carries a further sprocket 48 which is connected by and endless chain 50 to a further sprocket connected to an electric motor 52. Of course, instead of sprockets and chains interconnecting the pressing rollers 38 and the motor 52 suitable gearing may be used.It is important, however, that the gearing between the rollers is constant and is positively controlled. Also as shown there is a jockey pulley arrangement 45 for maintaining the tension of the chain 46 in the normal manner.

In use the apparatus of Figure 8 is operated as follows. Firstly a circumferential mark on one of the rollers is aligned with a reference point. A sleeve to be cold rolled is fitted over two mandrels each of which is aligned with each other using suitable index markings on their exposed ends.

The mandrels and sleeve are inserted between the rollers 38 as shown and the index markers on the mandrels are aligned with a further template or reference. This alignment of the pressing rollers and mandrels ensures that the threads 40 on the rollers matches the thread forms 16 on the mandrels. The rollers 38 are then adjusted to apply pressure to the sleeve 24 and the motor 52 is then started. The rollers cause the sleeve and mandrels to rotate and at the same time the sleeve to be deformed to form the internal thread. When the required thread form has been produced the finished sleeve 24 is removed from the apparatus, the mandrels 12 and 14 are unscrewed from the sleeve, and suitable bevelling as shown at 54 in Figure 3 is formed at the ends of the coupling sleeve.

Figure 9 shows a variant of apparatus of the invention wherein there are two rollers 80 fixed to each other by a frame member 82 with each roller carrying a sprocket 84 and with the sprockets interconnected by a endless chain 86. One roller also carries a further sprocket which is connected by an endless chain 88 to a driving sprocket 90. There is also a third roller 92, which may or may not be driven and which is supported on a hydraulic ram 94 for vertical movement. The pressure applied by the ram 94 can be varied as desired according to the dimensions of the sleeve being rolled, the material of the sleeve and the speed of rolling, etc.

Figure 10 shows another form of apparatus of the invention which is similar to that shown in Figure 9, but in which the roller 92 is supported on an arm 96 for movement in an arc about an axis which is coincident with the rotational axis of the driving sprocket 90. In this way the roller 92 can be driven synchronously with the rollers 80 simply by using an enless chain 100 connected between the driving sprocket 90 and a sprocket 102 on the roller 92.

Movement of the arm 96 is controlled by a ram 98.

Figure 11 shows schematically the grain structure of a part of a continuously cast sleeve. Grain lines 104 are parallel to the longitudinal axis of the sleeve.

Figure 12 shows schematically the grain structure after rolling of a sleeve of the type shown in Figure 2, i.e. with a smooth roller. At the exterior of the sleeve the grain lines are substantially longitudinal, while at the interior they are sinusoidal and follow the thread profile. The longitudinal grain lines ensure high tensile strength and high resistance to elongation for the sleeve, whereas the internal sinusoidal grain lines promote hardness in a direction normal to the thread flanks. Figure 13 shows schematically the grain lines of a sleeve of the type shown in Figures 3 and 4.

It is important to note that in both of the structures shown in Figures 12 and 13 the grain lines are continuous. With conventional milling or cutting methods of internal thread formation there are discontinuities in the grain lines where the machining cuts across these lines. Accordingly in conven tional coupling sleeves the threads present stress raizers where fracturing and splitting of the sieeve can originate.

As will be appreciated there is less dead weight in the coupling sleeve made according to the invention compared with conventional coupling sleeves formed by single pointturning orwhirling and practically the entire length of the coupling sleeve is used for the coupling threads. After formation the coupling sleeve is suitable carburised and tempered.

It is anticipated that the method and apparatus of the invention may be used for mass production of drill string couplings and drill bits because of the simplicity, compared with conventional methods, of setting up the sleeves for manufacture and because it is anticipated that rolling the sleeve will take considerably less time than machining the sleeves in any of the conventional manners. For example it is anticipated that setting up and rolling a coupling sleeve will take approximately one minute, while cutting a coupling sleeve using a single point tool or even using a whirler including setting up time takes five or six minutes. Another advantage is that the capital cost of the apparatus for forming the sleeves will be about one half that of a suitable centre lathe.

Operating costs, e.g. replacement of cutting inserts, will be minimized.

Claims (13)

1. A method of internally threading a sleeve of a drill string component including the steps of machining the outside diameter of the sleeve to a suitable diameter and the internal diameter to a diameter slightly greater than the outside diameter of the intended final internal thread diameter, with the bore in the sleeve being of uniform cross-section through out the length of the sleeve; inserting at least one mandrel having the required thread and nose form into the sleeve; pressing the sleeve on to the mandrel; and unscrewing the mandrel.

2. A method as claimed in claim 1, wherein the step of pressing the sleeve on to the mandrel is performed by pressure rolling the sleeve.

3. A method as claimed in claim 1, wherein the step of pressing the sleeve on to the mandrel is performed by clamping shell segments which sur round the sleeve on to the sleeve and in steps rotating the shells and sleeve relatively to one another and clamping the shells on to the sleeve after each relative rotation.

4. A method as claimed in any of claims 1 to 3, in which the drill string component is a drill bit and the sleeve has a blind bore, with the bottom of the bore being initially formed with the required bottom striker face and with the outside diameter of the initial sleeve being over-size at the thread position and being of the final diameter size in the region of the striker face.

5. A method as claimed in any of claims 1 to 3, in which the drill string component is a coupling sleeve and wherein the bore in the sleeve is formed straight through and is of constant diameter and two man drels which are identical are inserted from opposite sides into the sleeve.

6. A method as claimed in claim 5, in which the exterior of the coupling sleeve is of constant diameter.

7. A method as claimed in claim Sin which the exterior of the sleeve has stepped portions of diameter varying from that of the remainder of the sleeve.

8. A method as claimed inany of claims 1 to 7, in which the sleeve is heated during pressing, so that the creation of undesirable stresses during working is minimised.

9. A method as claimed in any of claims 1 to 8, including forming a shadow thread on the outside of the sleeve with the peaks of the shadow thread being aligned with the bottoms of the grooves of the internal thread.

10. A method as claimed in any of claims 1 to 8, wherein the sleeve is formed to have a smooth external surface.

11. A method of forming an internal thread in a drill string component substantially as herein described with reference to any of Figures 1 and 2, or Figures 3 and 4, or Figures 5 to 7, or Figure 12 or Figure 13 of the drawings.

12. Apparatusforforming an internal thread in a drill string component comprising at least one mandrel of a hard metal, such as tungsten carbide, or hardened steel having a suitable thread and nose form, means for supporting the mandrel, at least two pressing members for forcing the sleeve on to the mandrel, means for forcing the pressing members against the sleeve to compress the sleeve on to the mandrel, and means for rotating the sleeve and pressing members relatively to one another.

13. An apparatusforforming an internal thread in a drill string component substantially as herein described with reference to any one of the accompanying drawings, particularly any of Figures 8, 9 or 10 of the drawings.