EP0312538A1 - A rethreading tool - Google Patents

Abstract

The re-threading tool (2) described is used to execute new threads, to repair damaged threads or to clean dirty or corroded threads, in particular the damaged threads of a bore with internal threads (28) of which a part of the threads are damaged (32) and of which another part of the threads is not damaged (34), the damaged threads (32) being closer to the opening of the bore (28) than the threads not being damaged (34) . Said re-threading tool comprises, according to one of its embodiments, several fingers (12) regularly arranged around a main body (4), in order to form a cylinder with variable diameter, said fingers (12) having formation threads threads (14) at their respective distal ends and being flexibly connected to the main body (4) at their respective proximal ends. Said fingers (12) can thus move between a retracted position, in which they are spaced relatively close to each other, and an operating position, in which they are spaced relatively distant from each other. others following a corresponding movement carried out by an actuating member (6). The threading threads (14) of the fingers (12) in the retracted position can then be inserted into the threaded bore (28) until they align with the undamaged threads (34), and after rotation of the tool (2) to unscrew it and take it out of the bore (28), the thread forming threads (14) of the fingers being in the operating position repair the damaged threads, the undamaged threads (34 ) contributing to the precise positioning of the tool (2) inside the bore (28), the damaged threads being repaired so as to form a single helical thread continuously with the undamaged threads (34).

Description

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A RETKREADING TOOL

The present invention relates to a tool for cutting or reclaiming screw threads, particularly cutting new threads so as to repair damaged or distorted threads, to restore damaged threads or to clean threads which are dirty, corroded or the like and to methods of using the tool. More particularly, the present invention relates to a hand tool in the form of an variable diameter tap for cutting internal threads in situations where the thread can be approached from one side only, particularly the side of the damaged portion of the thread. Even more particularly the present invention relates to an expandable/collapsible tap which is essentially hand operated for repairing the damaged or distorted thread of the spark plug hole of an internal combustion engine or similar internally threaded bore.

Although the present invention will be described with particular reference to a hand held expandable/collapsible tap for use in repairing the damaged threads of a spark plug hole, it is to be noted that the present invention is not so limited and is more extensive in scope extending so as to include other arrangements and other applications. The tool may be an expandable/collapsible tap for cutting internal threads such as those located internally within a threaded bore, such as a spark plug hole or it may be a variable diameter die-nut arrangement for cutting external threads such as those located externally on a stud or similar, such as for example an engine stud or exhaust manifold stud. The problem associated with the use of light-weight alloys in engineering applications, particularly in the automotive industry, such as the use of light-weight alloy heads and other components made from castings of light-weight aluminium-containing alloys is that the alloys are softer than conventional materials such as cast iron. Thus, the alloy components are more susceptible to damage through the mis-use or abuse of tools used in their repair. One particular problem is the damage caused to the threads of the spark plug aperture. Due to the complexity of modern engines it is quite normal that one or more of the spark plugs are usually located in a virtually inaccessible location or are surrounded by other components which hinder access to the spark plug. Due to the crowded engine compartment often it is not easy to see that the spark plug is correctly aligned within the spark plug hole before the spark plug is rotated or otherwise screwed into the aperture. If the spark plug is not correctly aligned so that the external threads of the plug mesh correctly with the internal threads of the hole, the hole threads may be damaged, distorted or otherwise destroyed because they are made from a softer material than the threaded part of the spark plug making it impossible to insert the spark plug any further into the hole and certainly not far enough into the hole to provide a gas tight seal for the combustion chamber. Alternatively, if the area around the spark plug hole is dirty, corroded or has a build-up of deposits, the internal threads of the spark plug hole may be clogged which prevents the accurate alignment of the spark plug in its hole which in turn may cause damage to the threads of the hole as the spark plug is forced into the hole. The presence of grit or other particulate matter may cause the threads of the spark plug to become cross-threaded with the threads of the hole within the hole. The hole then needs the thread redressed or reclaimed. This is usually achieved by taking the head off the engine which is a time consuming job and thus is costly for the repairer, particularly so in the case of a professional motor repairer since the cost involved in taking the head off, repairing the thread and replacing the head cannot usually be passed onto the customer. Alternatively the thread may be repaired in situ by drilling out the spark plug hole and retapping it with an oversize tap to receive an oversize insert, such as for example a helicoil or similar and thus establish a new thread. However, with this method the swarf from the drilling and cutting operations to insert the helicoil falls into the combustion chamber through the spark plug hole with obvious dire consequences for the longevity of the engine.

Therefore it is a primary object of the present invention to provide a novel tap for cutting or reclaiming a female screw thread which is especially suited for repairing damaged threads in a bore in circumstances where the threaded bore is accessible from only one side. An instance of such an application is in respect of the threaded hole in a cylinder head or engine block in which the spark plugs are mounted. In the spark plug holes the outermost threads can be damaged whilst the remaining threads are intact and undamaged and it is very difficult indeed to use a conventional screw cutting tap in order to repair the damaged threads. The damaged threads are most likely to occur at the beginning of the bore and therefore it is very difficult to introduce the tap into the bore so that it will correctly mesh therewith. If it is incorrectly meshed, the action of the tap will substantially destroy the threads which were previously undamaged in the bore.

According to the present invention there is provided a tool for cutting a thread comprising at least two movable body portions, each body portion being provided with a thread cutting portion said thread cutting portions capable of adopting a first position and a second position in accordance with corresponding movement of the body portions such that the thread cutting portions are relatively closer together in one position than when in the other position so as to be capable of cutting the thread in use and locating means for assisting in accurately locating the cutting portions to cut the thread. According to the present invention there is provided a tool for cutting a thread comprising at least two movable body portions provided with thread cutting portions at or towards respective one ends of the body portions, said body portions capable of movement between a first position and a second position and a locating means associated with at least one of the thread cutting portions for assisting in accurately locating the thread cutting portions to cut the thread, wherein when the body portions are in the first position the thread cutting portions are disengaged from the material in which the threads are to be cut and when the body portions are in the second position the thread cutting portions are in their operative position to cut threads in use. According to one aspect of the present invention there is provided a tool for cutting a thread comprising a locating means for assisting the accurate location of the tool in a bore, externally arranged thread cutting formations carried on a plurality of body portions, and means for moving the body portions between an operative position and a retracted position such that, in an operative position thereof, the thread cutting formations are located in positions for cutting a single internal helical thread of a predetermined diameter and, in a retracted position thereof, the cutting formations are located relatively close together and can pass through a threaded bore of said predetermined diameter, said thread cutting formations being located within the bore by said locating means engaging the opening of the bore.

Typically, the body portions are substantially elongate and resiliently connected to the tool such that they are biased to adopt one position and can be forced against the bias to adopt the other position and when the force is removed resiliently revert to the first position.

Typically, in one embodiment the thread cutting portions or formations and the body portions are closer together in the normal at-rest position of the tool under the influence of the bias caused by the resilience of the body portions. Alternatively, in other embodiments the thread cutting portions or formations and the body portions are spaced further apart from each other when in the normal at-rest position of the tool due to the bias applied to these portions. Thus, in some embodiments the at-rest configuration is the collapsed or retracted position and the tool is inserted into the bore in this retracted position and then expanded against the natural bias of the tool to adopt the operative position by operation of the means for moving the body portions whereas in other embodiments the at-rest configuration is the operative position and the tool is momentarily collapsed immediately prior to inserting the tool into the aperture or bore and then allowed to expand due to the natural resilience of the tool to adopt the normal at-rest position which is the operative position for cutting the threads.

Typically, the operating means or means for moving the body portion maintains the tool in its normal at-rest configuration which is the operative configuration thus preventing it from collapsing in use. Alternatively, the operating means transforms the tool from its at-rest configuration to its expanded configuration which is the operative configuration in use.

Typically, the operating means is a substantially elongate spindle or prong. Typically, the spindle or prong is tapered at one of its ends and preferably is provided with an enlarged operating knob at its other end. Typically, the enlarged operating knob is provided with means for rotating or inserting the spindle or prong such as a substantially transverse bore for receiving a handle or shaft or it may be knurled to facilitate gripping by fingers or it may be provided with flats for engaging with a complementary handle such as a spanner or a square- or hex-drive socket and shaft arrangement. Alternatively, the operating means may additionally be externally screw threaded and received within the body of the tool which is correspondingly screw threaded. Typically, the screw thread arrangement of the operating means is a left hand thread- Alternatively, the tool is provided with a resilient means located between the operating means and the body of the tool. Typically, the resilient means is a rubber '0' ring.

Typically, the operating means may be an external sleeve fitted over the outside of the tool for axial movement therealong. In one position the sleeve contacts the elongate body portions to squeeze them together and in another the body portions are free to expand due to the bias applied to them when the sleeve is withdrawn. Alternatively, the operating means may be an internal pin centrally received within the hollow body to push against the body portions to expand them and when withdrawn from the body either partially or wholly allow the body portions to slightly collapse. Typically the internal pin may be spring mounted within the hollow body to facilitate expansion of the body portions in a gradual and controlled manner. In one position the pin may be spring loaded so that when released the pin pushes the body portions apart in an amount in accordance with a predetermined tension applied by the spring. Typically, the body portions are resiliently mounted to the tool and are flexible enough to move between the collapsed configuration and the operative configuration. Typically there are 2, 3, 4, 5, 6 or more body members more or less regularly spaced around the circumference of the tool. The body members are spaced apart from each other by elongate slits or slots located between adjacent body portions. Typically the elongate slots are provided with cut-outs or apertures forming resilient hinge means permitting the movement of the body portions at the end of the tool remote from the thread cutting portions or formations which re located at the proximal ends of the body members. Typically, the body members are substantially elongate and are arranged with their lengthwise extending axes collectively aligned parallel with the lengthwise extending axis of the tool.

Typically, the lengthwise extending axes of the slots are also aligned parallel with the lengthwise extending axis of the tool.

In use of one embodiment which is a tap, of the invention, in the retracted position, the thread cutting formations can be passed through a threaded bore of said predetermined diameter until said locating means engages with the material surrounding the opening to the bore, and then said operating means or means for moving the body portions is operated so that the body portions expand and the thread cutting formations are of the correct diameter relative to the threaded bore with the crest and troughs of the thread cutting formations meshed with the troughs and crests of the threaded bore, respectively. The formations can then be carefully introduced into the undamaged threads of the bore and rotated so that the formations mesh with the threads of the bore and will cut or form the correct thread at the site of the previously damaged threads. On removal of the tap from the bore, the threads are substantially repaired so that the spark plug can be reintroduced. Typically, the locating means is a shoulder provided on the tap adjacent the inboard end of the thread cutting formations.

Typically, the thread cutting portions are arranged with a lead-in portion which is tapered, a central portion for fully cutting the new thread and a tapered following lead-out portion. Typically, the thread cutting formations are provided with their cutting edges arranged to cut the threads when being withdrawn from the aperture. Typically, the cutting formations may have a negative rake angle, zero rake or positive rake angle.

The invention will now be further described with reference to the accompanying drawings, in whichΪ

Figure 1 is a side elevation view, partly in cross-section of one form of the invention comprising an outer body portion and inner spindle;

Figure 2 is a side elevation view of the outer body portion of the embodiment shown in Figure 1;

Figure 3 is a side elevation view of the inner spindle of Figure 1;

Figure 4 shows the tap of Figure 1 in situ in its retracted position passing through the spark plug hole in a cylinder head;

Figure 5 shows the tap of Figure 4 in use in its expanded thread cutting position relative to the cylinder head;

Figure 6 is a similar view to Figure 4 showing a modified form of the tap of the present invention;

Figure 7 is a similar view to Figure 5 showing a modified form of the tap of Figure 6;

Figure 8 is a side elevation view of the outer body portion of another form of the present invention;

Figure 9 is a side elevation view of the inner spindle of another form of the present invention; and Figure 10 is a side elevation view partially in cross-section of the combined components of Figures 8 and 9. The tap 2 illustrated in Figures 1 to 7 of the drawings comprises a generally hollow cylindrical body 4 and a spindle 6 which in use is located within the body 4. Spindle 6 is rotatable in body 4 or otherwise axially adjustable in body 4 to adjust the position of the spindle between positions which correspond to the retracted position and operative position of the tap in use.

One end of the body 4 is formed with four axially extending slots 8 which terminate in circular openings 10, the slots 8 defining four resilient fingers 12 therebetween. Fingers 12 are more or less resiliently mounted to the body about circular opening 10. The ends of the fingers 12 are provided with thread cutting formations 14. Slots 8 permit relative movement of the four resilient fingers with respect to each other when the tap is adjusted between its retracted and operative position. The preferred technique of making the body 4 is to form the thread cutting formations 14 as a single thread and then to cut the slots 8 and drill the holes 10 in the body. In this embodiment the fingers when in their retracted position are in the at rest configuration. When the fingers are in the retracted position, as seen in Figure 2, the slots 8 are closed at the free ends of the fingers and the adjacent formations 14 abut one another. In this position the finger ends can be inserted into the spark plug aperture as shown in Figure 4 but would not be suitable for cutting a thread in this position. Formations 14 can be of any type or configuration such as having negative rake angle, positive rake angle, or zero rake or can be uniform over their entire length or be provided with chamfered end portions comprising lead in and/or out thread portions. It is observed that the cutting formations cut the new threads or restore the damaged threads as the tap 2 is being withdrawn from bore 28 rather than when being introduced into the bore as is the case of conventional taps. The spindle 6 has a tapered portion 16 at one end for engaging against tapered cam surfaces 26 located on the inboard side of the thread cutting formations to expand and release the thread cutting formations and a boss 18 at the other end for rotating the spindle in use. External threads 20 are formed adjacent to the boss and in use mesh with internal threads 22 provided in the interior of the body 4 at the opposite ends of the formations 14. A lock nut 24 or similar may be provided on the threads 20. In the embodiment shown in Figures 6 and 7 no lock nut is necessary since boss 18 provides a stop to limit the travel of the spindle within the hollow body 4. In use of the device, the spindle 6 passes through the central longitudinal opening in the hollow body 4 to be more or less fully received therewithin and the threads 20 thereof mesh with the threads 22 of the body. In a retracted position, as shown in Figure 4, the tapered portion 16 of the spindle is clear of tapered cam surface 26. In this case the body 4 has the configuration shown in Figure 2 where the adjacent formations 14 are in abutment (or nearly so) . On further rotation of the spindle when inside hollow body 4 the tapered portion 16 moves axially so as to engage the cam surfaces 26 and expand the fingers 12 from their normal at rest configuration. The spindle is made of such a length that when it is fully inserted and rotated home into the body 4, as seen in Figures 1 and 5 portions 14 are spaced further apart from each other so as to form the expanded configuration and are in a position in which they cooperate with each other to form a formation for cutting a single continuous helical thread. In this position the tap can be rotated to gradually withdraw the tool from the aperture and hence cut a new thread or redress or reclaim the damaged or distorted thread. Figure 4 shows the device in its retracted position which is the normal at rest position, passing through a threaded bore 28 in a cylinder head 30. In this Figure it is assumed that the threads 32 located at the top of the bore when viewed from outside the cylinder head which are typically the first one, two, three or so threads are the damaged threads whereas the threads 34 located more inboard, such as towards the underside of the head forming part of the combustion chamber are assumed to be in good condition. Typically threads 34 are all of the threads of the spark plug aperture apart from the first few threads which are damaged. It will be observed that the formations 14 are sufficiently close together for them to pass through the bore 28 and be located adjacent to the underside of the head 30. The spindle 6 is then screwed into the body 4 so that the fingers 12 expand and the thread cutting formations 14 expand therewith to a diameter which is appropriate for cutting a thread of the same diameter of the bore 28. Thus, in its operative position for cutting the threads the fingers are expanded with respect to the normal at rest position. The device may now be carefully manipulated so that is formations 14 correctly mesh with the undamaged threads 34 and the tool 2 rotated so that the formations 14 pass through the threaded bore 28 and in doing so recut or redress the damaged threads 32. It is to be noted that the formations 14 are correctly aligned by engaging with the undamaged threads 34 which are used to direct the tap to cut the damaged threads 32 in correct alignment with threads 34. Thus, the operation of the tool relies upon the undamaged threads 34 guiding the tool to correctly restore the damaged threads 32. It will be appreciated that the spindle 6 must rotate with the body 4 at this stage and accordingly, in one embodiment, the lock nut 24 is provided to ensure that the spindle is tightly locked to body 4 and rotates together with the body. During the cutting stroke, the turning torque may be applied directly to the body 2 by a spanner or the like which engages flats 36 provided on the body 4 or by means of a pin or shaft 38 which passes through a bore 40 in the boss 18 or by any suitable means. Optionally a thick adhesive like lubricate, such as grease or similar paste-like material may be applied to the cutting formation to clean the newly cut or restored thread and retain any swarf from the recutting operation thereby preventing the swarf from falling into the combustion chamber and ultimately damaging the engine. In the embodiment illustrated in Figures 6 and 7, like numerals will be used to refer to like features of the embodiment of Figures 1 to 5, there is shown a tap 2 with some modifications which include an external shoulder 50 located adjacent the formations 14. Shoulder 50 consists of four segments generally arranged circumferentially around and associated with the fingers 12. Shoulder 50 when in the retracted position of the tool has an external diameter which is greater than the diameter of threaded bore 28 and is used to prevent the tool from being inserted too far into the bore. Additionally, there is no lock nut 24 provided on the threads 20 of the spindle 16 but rather there is a left hand thread 52 of relatively large pitch which meshes with corresponding thread 54 provided in place of thread 20 so that spindle 16 can rotate to be received fully within body 4 to separate fingers 12 until boss 18 contacts the top of hollow body 4. Left hand threads 52, 54 prevent body 4 and spindle 6 separating from each other as the tool is rotated in operation restoring the damaged threads. In use, body 4 with fingers 12 in their retracted at-rest position is inserted into bore 28 clear of damaged and undamaged threads 32, 34 until shoulder 50 contacts the head 30 which prevents further inwards movement of tap 2 into bore 28. Formations 14 are generally expanded from their normal at-rest retracted position to accurately engage with threads 34 in crest-to-trough relationship by screwing spindle 6 into body 4. Tap 2 is then screwed out of bore 28 with body 4 and spindle 6 in unison using threads 34 as a guide to correctly redress damaged threads 32. A further modified tap 2 may be provided in ' combination with a guide sleeve 42 having an internal diameter which is substantially the same as the external diameter of cylindrical portion 44 on body 4. The lower end 46 of the sleeve 42 engages the head 30 in use of tap 2 and is tightly gripped by a user and tap 2 rotated therewithin. Sleeve 42 serves to ensure that the tap moves axially outwards relative to the threaded bore 28 for further facilitating the accurate alignment of the cutting formation in threads 28.

In a preferred arrangement of the invention, the threads 20 and 22 on the one hand and threads 52, 54 on the other hand are left hand threads whereby the turning torque during the cutting stroke of the tool can be applied through the pin 38 connected with the spindle. The left hand threads ensure that the spindle 6 does not withdraw from body 4 during rotation and that both body 4 and spindle 6 move in unison. In an alternative modification, the body 4 may be formed or provided with extension pieces or segments for axially extending the length of the tool. Spindle 6 can be made of different lengths or the spindle itself can be extendible. Longer devices are sometimes necessary where the threaded bore is not accessible to a short tap.

In a further embodiment, body 4 is provided with an upper set of formations and a lower set of formations so that any material cut from the damaged thread 32 by the upper thread portions is prevented from falling into the combustion chamber by the lower set which follow the upper set but are spaced below them. Alternatively a thick paste, such as molybdenum disulphide, Vaseline jelly, or the like may be used to collect and retain the cuttings. A further embodiment is illustrated in Figures 8 to

10 which comprises having modified cutting formations 14 and a hexagonally shaped collar 60 and a modified spindle 6 which is provided with a circular groove 62 into which is received a rubber '0* ring 64. Body 4 is arranged so that fingers 12 when in their expanded position at at-rest and are accordingly biased to adopt an opening which is about the same as that of the opening of the spark plug aperture such as being about equal to or slightly larger or smaller than the opening of the spark plug aperture. The opening of the at-rest position may be, in some cases, about the same or slightly different from that required when in the operative position. In any case, the at-rest position opening is larger than the retracted

position for inserting the formations 14 into bore 28. Formations 14 comprise a distal portion 68 which is chamfered to define a follow-out cutting portion, a central portion 70 of a maximum diameter corresponding to the diameter of the thread being cut or restored for performing the majority of the cutting of the new thread, and a proximal portion 72 which is chamfered to define a lead-in portion. Formation 14 is spaced from shoulder 50 by shank portion 74 which ensures formation 14, particularly portion 70, engages undamaged threads 34 in use so that the undamaged threads can correctly guide the cutting formations in use. In another embodiment central portion 70 may be oversize to cut an oversize thread in preparation for inserting an insert such as a helicoil.

In use of this form of the invention body 4 without spindle 6 inside is gripped to momentarily retract ingers 12 against the bias of the resilient mounting of the fingers. While the fingers 12 are retracted formations 14 are inserted into bore 28 and then released allowing formation 14 to expand to mesh with undamaged threads 34. If correct alignment is not instantly achieved body 4 may be wiggled to correctly align the tap so that formations 14 mesh with undamaged threads 34. Spindle 6 is inserted into hollow body 4 and pushed against the resistance of '0' ring 64 until fully home. Taper 6 of spindle 16 contacts cam surface 26 of fingers 12 to maintain the fingers in the operative position or to slightly expand fingers 12 to the operative position. Body 4 may then be unscrewed from bore 28 in unison with spindle 6 by applying torque to collar 60 by a suitable spanner or handle means if increased turning is required. As tap 2 is unscrewed formations 14 follow undamaged thread 34 until damaged^" thread 32 is contacted whereupon a new thread is cut or the damaged thread is restored. Again, undamage thread 34 correctly aligns tap 2 to cut a continuous helical thread thus repairing the damage caused to the thread. An advantage of this form of the present invention is that if the tool binds in the bore it can be readily compressed to retract the body portions thus freeing it from the bore. A further use of the tap of the present invention is to clean dirty threads in situ whilst substantially eliminating the risk that the contamination will fall down the bore of the thread since the dirt is pushed out of the bore ahead of the thread cutting formations.

The described arrangement has been advanced by way of explanation and many modifications may be made without departing from the spirit and scope of the invention which includes every novel feature and/or novel combination of features hereindisclosed.

Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is understood that the invention includes all such variations and modifications which fall within its spirit and scope.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A tool for cutting a thread comprising at least two movable body potions each provided with a thread cutting portion, said body portions capable of movement between a first position and a second position and an operating means wherein when the body portions are in the first position the thread cutting portions are disengaged from the material in which the threads are to be cut and in response to movement of the operating means the body portions are moved or maintained in the second position which is an operative position for cutting threads in the material in use.

2. A tool for cutting a thread comprising at least two movable body portions provided with thread cutting formations at or towards respective one ends of the body portions, said body portions movable between an operative position for cutting threads and a release position wherein the tool may be disengaged from the threads, an operating means for use with the tool in the operative position, and a locating means associated with at least one of the thread cutting formations for assisting in accurately locating the thread cutting formations to cut the thread.

3. A tool for cutting a thread comprising a locating means for assisting in accurately locating the tool within a bore, thread cutting formations located externally on a plurality of body portions and means for moving the body- portions such that in an operative position the thread cutting formations are located with respect to each other for cutting a single internal helical thread of a predetermined diameter, and in a retracted position thereof the cutting formations are relatively close together and can pass freely through the threaded bore, said threaded cutting formations being located within the bore by said locating means engaging the opening to the bore. . A tool according to any preceding claim, in which the thread cutting formations when in an operative position define a substantially cylindrical thread cutting arrangement in the form of a tap or a die. 5. A tool according to any preceding claim in which the thread cutting portions when in the retracted position are in the at-rest position and are resilient expandable against the bias of the body portions.

6. A tool according to any preceding claim in which the thread cutting portions are spaced apart from each other at a spacing closely corresponding to the spacing required of the operative position when in the at-rest position and are resiliently inwardly deformable against the natural bias of the body portions.

7. A tool according to any preceding claim in which the locating means if provided is a shoulder for engaging against the article in which the threads are to be cut.

8. A tool according to any preceding claim in which movement of the operating means forms the tool into its operative position.

9. A tool according to any preceding claim in which the operating means is a spindle receivable internally within the body portion which is hollow. .

10. A tool according to Claim 9 in which the spindle is tapered at one end thereof and is provided with means for turning or rotating it at the other end thereof.

11. A tool according to Claim 9 or 10 in which the spindle is provided with external threads or with a resilient resistance means, preferably an '0* ring.

12. A tool according to any preceding claim in which the operating means is a sleeve arranged externally over the substantially cylindrical body of the tool, said sleeve axially movable along the length of the body to move the body portions between the operative position and the position by contacting the external surface of the body portions.

13. A tool according to any preceding claim in which the locating means is a shoulder means arranged externally around the tool slightly in board of the thread cutting portions. 14. A tool according to any preceding claim in which the body portions are substantially elongate fingers arranged more or less regularly around the circumference of the tool and separated by corresponding elongate slots, said fingers being resiliently connected to the body.

15. A tool according to Claim 14 in which the elongate slots are provided with apertures forming hinge means allowing movement of the body portions, said apertures being located towards the end of the tool remote from the thread cutting formations which are located at the proximal ends of the body portions.

16. A tool according to any preceding claim in which the thread cutting formations are provided with a chamfered lead-in portion located relatively in board of the remaining thread cutting portions, a centrally located main thread cutting formation and a chamfered following lead-out thread cutting portion located relatively distally with respect to the remaining thread cutting portions.

17. A tool according to any preceding claim in which the thread cutting formation comprise an upper set of formations and a lower set of formations, said upper set being spaced apart from the lower set, wherein said upper set predominately cuts the new thread and said lower set follows said upper set to contain any cuttings produced by the upper set in use.

18. A tool for cutting threads substantially as hereinbefore described with reference to the accompanying drawings.