IL33084A - Power tools - Google Patents

Description

Power Toole ma »MF o' yD Dn rrnay GKN Screws & Fasteners Limited and ¾ The Plessey Company Limited 9 This invention relates to power tools. It is particularly concerned with power hand tools, i.e. tools arranged to he hand held but having a power driven working element. The invention has been developed in connection with tools for the insertion of self-tapping and piercing screws into a work-piece but may also be used for running a nut onto a threaded shank and then locking the nut by giving it an impact.

Such power hand tools have already been proposed and manufactured to operate off compressed air which is normally available in a factory. However, due to the normally available pressures the application of such tools is limited, so far as they are applied to self-tapping and piercing screws, to inserting these into comparatively thin sheet metal. The pressures normally available in factory compressed air systems are insufficient to enable screws to be driven into comparatively thick sections, e.g. steel sections of a thickness of •jjr" or more.

It is an object of the present invention to provide a power tool which will be capable of inserting self-tapping and piercing screws into comparatively thick sections although not limited to such use.

According to the invention we provide a power tool comprising a body; a working element mounted on the body for reciprocation and rotation relative thereto, an impact member arranged in the body and operable to deliver impacts to the working element; a rotary drive member arranged in the body and operable to rotate the working element; receiving means in the body to receive an explosive charge which, when exploded, produces high pressure gas; and directing means to direct said gas to operate said members. "fcke normally available compressed air. The energy available will thus be capable of effecting work which cannot be effected with known guns using compressed air normally available in factories.

The explosive charge may be of any type and may be provided by cartridges or by caseless charges although, as described above, it is preferred to use a liquid mono-fuel such as isopropyl nitrate.

In one arrangement, the directing means is so arranged that the gas produced by the explosion first operates one of said members and then the other of said members. Thus the impact member may be operated before the rotary drive member where a screw is to be driven, the screw being given an impact to cause it to pierce the workpiece and then rotate it to drive it into the workpiece. Alternatively, the rotary drive member may be operated before the impact member and this can be used when the tool is employed for driving a nut, the nut being driven on by rotation and then impacted to lock it.

In a second arrangement, the directing means may be arranged so that the gas operates both of said members simultaneously.

In either arrangement, the directing means may be so arranged than the explosion of a single charge causes the impact member to deliver a series of impacts to the working element. Thus where the directing means is arranged to produce simultaneous rotation and impact, the working element may be rotated while it is being given a series of impacts.

Manual control means may be provided associated with the directing means and operable to control the flow of said gas produced by the explosion. Thus where the members are to prevent, until operated, the gas operating the second of the members until after the gas has operated the first member.

In an alternative arrangement, instead of having manual control means to control the flow of the gas, the gas flow may be controlled automatically where the members are moved sequentially. Thus the first member to be operated may cause or permit the gas to flow along the directing means to operate the other member after the first member has completed or substantially completed its operation. Thus said one member may uncover passageways forming part of the directing means along which the gas flows to operate the other member.

The impact member may be arranged to be held against the action of a spring in a cocked position by detent means which, when rendered inoperative, allow movement of the impact member under the influence of the spring to initiate the explosion of a charge in said receiving means, the resulting gas causing the impact member to deliver an impact to the working element, compress the spring and return to its cocked position. The detent means can be rendered inoperative by an end load applied to the impact member by the working element. This end load can be applied to the working element by the operator of the tool when he applies, in the case of the tool for inserting screws, the screw to the working element.

The detent means may be spring loaded and the spring loading thereof may be sufficient to hold the impact member in its cocked position against the action of the spring acting thereon but insufficient to hold the impact member against any further end load.

Preferably, the tool includes manually operable safety means including a safety element engageable with the working since the tool operation can be initiated by an end load on the working element it is necessary to provide some sort of safety means to prevent the tool operating if it should inadvertently be dropped.

The tool may include disabling means selectively operable to control the tool so that an explosion causes the working element to be either only impacted or only rotated. Thus a tool having this feature will have at least two modes of operation. The normal mode of operation will be that impact and rotation of the working element will take place either sequentially or simultaneously. However in some cases it may be required that the working element execute only an impact or only a rotation and the disabling means may be operated to this end. Preferably such a tool will have three modes of operation that is to say a first mode which will involve both rotation and, impact of the working element either sequentially or simultaneously, a second mode which will involve only rotation of the working element and a third mode which will involve only impact of the working element.

The explosion may be prevented from causing an impact on the working element by physically preventing the impact member from transmitting an impact to the working element. This may be effected by inserting a shutter or the like between the impact member and the working element and the shutter may be spring loaded. If, as will normally be the case, a clutch is provided between the rotary drive member and the working element, the disabling means may be operable to dis-engage the clutch.

In one convenient form of construction, the tool may comprise two co-axial cylinders in the body, one cylinder containing the impact member which is in the form of a first forms one part of a screw jack which forms the rotary drive member, the other part of the screw jack "being connected to the working element. Thus the gas can move the first piston to deliver an impact to the working element and can move the second piston to operate the screw jack and thus to rotate the working element. In this arrangement the second piston may he prevented from rotating relative to the body and the other part of the screw jack may be connected to the working element through a torque-limiting clutch. In one arrange-ment, the one cylinder, i.e. the cylinder containing the impact piston may lie within the other cylinder which contains the second piston.

As has been mentioned above, the charge for operating the tool may be in the form of a cartridge, a caseless charge or a liquid monofuel. Preferably, the receiving means is arranged to receive a charge of a liquid monofuel and the tool includes means for initiating decomposition of said fuel to produce said explosion. The preferred monofuel is isopropyl nitrate.

The receiving means may comprise a breech and said means for initiating decomposition may comprise a firing pin capable of entering the breech. The monofuel is fired by compressing the air or gas in the breech by the rapid movement of the firing pin which will increase both the temperature and pressure in the breech to ensure initiation of decomposition of the monofuel.

Where the impact member itself is arranged to initiate the explosion, the firing pin may be carried on the impact member and arranged to enter the breech when the detent means holding the impact member in its cocked position are rendered 29.9.1969 In another arrangement, the firing pin may "be manually operable and held in a cocked position against a spring "by catch means, release of the catch means allowing the firing pin to enter the breech and the subsequent explosion returning 29.9.5969 the firing pin to its cocked position. 8.2.1969 The tool may include adjustable means for delivering a variable quantity of the monofuel to the receiving means.

By adjusting the quantity of the monofuel the amount to be exploded is varied and the pressure of the resulting gases is 10 also varied. Thus for arduous work a greater quantity of monofuel would be delivered to the receiving means than for 18.2.1969 less arduous work. ¾1.3.1969 The tool may be used for inserting a self-tapping and piercing screw in a workpiece and in this instance the working 15 element comprising a screw-driver bit and the working element is given an impact either before rotation or simultaneously with rotation. If the screw-driver bit is impacted simultaneously with rotation then it is preferred that a series of impacts be delivered to the screw-driver bit.

In an alternative arrangement, the tool can be arranged for running up a nut and in this case the working element will be a nut-engaging member and will be caused to rotate prior to being impacted. A nut may be thus run up on a threaded shank and when it has been tightened it may be impacted and thus 21.3t1969. . . . locked m position.

The invention will now be described in detail by way of example with reference to the accompanying drawings in which: - 29.9.1969 FIGURE 1 is a side elevation, partly in section, of a first embodiment of the invention arranged for inserting a 50 self-tapping and piercing screw in a workpiece; certain parts 9 FIGURE 2 is a diagrammatic axial section of the pump for charging the receiving means with the monofuel; FIGURE 2A is explanatory of the symbol employed for illustrating the check valves in Figure 2 ; and .1969 FIGURE 3 is a section through a tool constituting a second embodiment of the invention for inserting a self-tapping and piercing screw in a workpiece.

Referring now to Figures 1 , 2 and 2A, these show a power tool arranged to be hand held for inserting a self-tapping and piercing screw in the workpiece. The tool is in the form of a gun and comprises a barrel portion 1 , a handle portion 2 , a monofuel reservoir 3 and a pump for the monofuel. The barrel portion 1 has a first cylindrical bore or cylinder 5 formed therein, and this contains an impact member 6 in the form of a piston which is slidable in the cylinder 5 · T e impact member is urged to the left by a means of a spring 7 which surrounds a stem 8 forming part of the impact member. The spring 7 is interposed between the impact member and an anvil 9» the latter being arranged to be engaged by the stem 8 when the latter is moved to the right as will be described.

The anvil 9 is connected to a working element 10 arranged to receive a screw driver bit, not shown, and an impact delivered to the anvil 9 from the impact member 6 is transmitted to the working element 10.

The impact member is provided with an annular flange 6a which is arranged to enter a dashpot 11 to absorb the momentum of the impact member 6 should it inadvertently be caused to move to the right while the anvil 9 is in a position in which it would not be engaged by the stem 8.

The working element 10 is connected via a torque-limiting part 13 being rotatable in bearings one of which is indicated at 13a adjacent to the dash pot 11. The left hand portion of the part 13 is surrounded by a member 40 which is secured to the barrel portion 1.

The other part of the screw jack device comprises a piston 14 having a piston head 14a which is a sliding fit in a bore 15 in the barrel 1 , the bores 15 and 5 being co-axial. The piston head 14a is annular and slides between the bore 15 and the fixed member 40. A splined connection 16 is provided between the piston 14 and the barrel 1 to prevent rotation of the piston 14 while allowing the latter to move longitudinally. It will be apparent that as the piston 14 moves longitudinally from the position shown it will cause rotation of the part 13 and thus rotation of the working element 10 through the torque limiting clutch 12.

At the left hand end of the barrel portion 1 there is receiving means for a liquid monofuel comprising a decomposition chamber 17 which is connected to the cylindrical bore 5 through a breech 17a . A firing pin 18 projects into the chamber 17 and is held in a cocked position shown against the action of a spring 19 "by means of a self-resetting catch means 21 which can be released by a first trigger 20. When the firing pin 18 is released it is arranged to enter the breech 17a .

When the impact member 6 is at the left hand end of its stroke, a projection 6b thereon closes the right hand end of the breech. As will be described below, when the breech is thus closed, release of the firing pin 18 will cause decomposition of the monofuel in the breech 17a and in the chamber 17 thus causing an explosion and at the same time resetting the firin in 18 in its cocked osition as shown. .1969 A plate valve 22 controls the flow of gas from the cylinder 5 to the cylinder 15 such flow being able to take place by means of a groove 5a in the wall of the cylinder 5 · The reservoir 3 can be filled with the liquid monofuel through a filler plug 1 and the pump 4 is so constructed that each stroke . thereof will deliver a charge of the monofuel to the chamber 17 and the breech 17a .

The pump 4 comprises a cylinder 2 in which is slidable a piston 26 secured to a graduated piston rod 23 » the latter having an operating knob 2 at its free end. A spring 2 tends to move the piston 26 to its fully withdrawn position as shown in Figure 2 up to a stop 42. The cylinder 25 has three longitudinally spaced ports, 28, 29 and 30 and the piston 26 has a longitudinal passage 31 having three transverse branches 32 , 33 and 34. In the fully withdrawn position shown the branch 3 communicates with the port 28 via the annular space between the piston rod 3 and the piston 25 - The port 28 communicates through a non-return valve 35 with the reservoir 3. The convention used for the non-return valve is shown in Figure 2A.

The transverse branch 33 in the piston 26 communicates with a longitudinal groove 36 in the piston and through this groove with the port 29 · The port 29 communicates via a nonreturn valve 37 with the breech 17a and the decomposition chamber 17 · Means, not shown, is provided for proportioning the amount of fuel delivered between the breech 17a and the chamber 17 · The transverse branch 34 communicates with the port 30 which is connected to atmosphere.

A set screw 38 as shown in Figure 1 is adjustably screwed the piston rod 23 and thus the piston 26 by engaging the operating knob 24. The pump is operated by depressing the knob 24- and when this occurs the piston will move to the right in Figure 2 against the spring 27 to the extent determined by the setting of the screw When this movement begins, the transverse branch 3 of the passage 31 moves out of alignment with the atmospheric vent port 30 so that continued movement of the piston to the right will draw monofuel from the reservoir 3 past the non-return valve 35 and through the port 28 into the annular space between the piston rod 23 and the cylinder 25. At the end of the inward movement of the piston the knob 24 is released and the spring 27 will return the piston 26 to its illustrated position. Liquid will thus be forced from the annular chamber surrounding the piston rod 23 , through the branch 32 , the passage 31 > the branch 33 » the groove 36 , the port 29 and the non-return valve 37 into the breech 17a and the decomposition chamber 17 , the apportionment being obtained as described above. At the end of the return stroke of the piston 26, the branch 3 again becomes aligned with the atmospheric vent port 3 thus preventing any build up in the monofuel reservoir 3 via the port 28 in the case of any leakage from the decomposition chamber 1 - If desired, a nonreturn valve may be incorporated into the passage 31 to act as an additional safe guard in case the port 30 should become partly blocked or should not be fully uncovered when the piston 26 reaches its fully withdrawn position.

The operation of the tool will now be described.

Assumin that the impact member 6 is at the left hand end of its stroke, the projection 6b will close the breech 17a. The pump 4· will now be operated as described above to deliver mono-fuel from the reservoir 3 to the chamber 17 and the breech 17a. The amount of fuel delivered b the um will de end on the setting of the set screw 38 and the heavier the work to he done and the greater the pressure required the more fuel will he delivered. Some of the fuel will be delivered to the "breech 17a and some to the decomposition chamber 17, the apportionment being obtained by means not shown. The first trigger 20 may now be operated to release the catch means 21 and thus to release the firing pin 18 so that the latter enters the breech 17a. The gas trapped in the breech will be subjected to high compression so that its temperature will rise and the monofuel in the breech will begin to decompose and will explode and the decomposition of the initial amount of monofuel in the breech 17a will initiate decomposition of the monofuel in the decomposition chamber 17. The high pressure gas produced by this decomposition will have two effects. Firstly, it will move the firing pin to the left to the position shov/n so that the firing pin will again return to its cocked position where it will be held by the catch means 21. The high pressure gas will also move the impact member to the right thus allowing the gas to flow from the decomposi-tion chamber and the breech through the breech 17a into the first cylinder · As result, the impact member will be forced at increasing speed to the right and will compress the spring 7 so that the stem 8 delivers an impact to the anvil 9 the impact being transferred by the anvil to the working element 10. Assuming that the working element has been furnished with a self-tapping and piercing screw the impact will cause the screw to pi^ce the workpiece. The gas will now be in the first cylinder 5 and can then be caused to rotate the work element 10. This is effected by opening the valve 22 by means of the second trigger 22a. The gas flows along the groove by-passing the impact member 6 and flows result, the piston 1 is moved to the right and is prevented from rotating relative to the barrel portion 1 by means of the splined connection 16. As a result, movement of the piston 1 to the right will cause rotation of the part 13 of the screw jack and thus rotation of the working element 10 through the torque-limiting clutch 12. A vent, not shown, is provided for allowing the gas to leave the cylinder 15 after having moved the piston 1 . The piston 14 then returns to its initial position shown and the tool is ready for a further operation which is initiated by delivering a charge of the monofuel from the reservoir 3 to the decomposition chamber 1 and the breech 17a by the pump 4. 969 Referring now to Figure 3 this shows a modified tool comprising a body 50 having a hollow handle portion 51 within which is provided a reservoir 2 for a liquid monofuel, e.g. isopropyl nitrate. A manually operable pump 53 is provided on the body to pump the monofuel from the reservoir 52 to a decomposition chamber 54- provided in the body 50· Slidably and rotatably mounted at one end of the body 50 is a working element in the form of a screwdriver bit 55· An impact member 56 is slidably mounted in the body and is arranged to be driven into contact with the rear end of an anvil 57 as a result of explosion of the monofuel in the chamber 59 so as to deliver an impact to the screwdriver bit 55. In addition, the gases produced by the explosion are fed to act on a piston 58 connected to a ball nut 59 which causes, when the piston is driven, the anvil 57 to rotate, thus to rotate the bit 55· The reservoir 5 is provided with a filler aperture closed by a filler plug 60 which enables the reservoir to be filled with the monofuel. A tube 61 extends from the bottom cylinder 62. A piston 63 is slidably mounted in the cylinder 62 and has a manually operable piston rod 6 connected thereto at its upper end and is acted upon, at its lower end, by a coil compression spring 65 which urges the piston 63 upwardly.

The piston 63 is provided with an axially extending passage 66 and two transversely extending passages 67 and 68 which communicate with the passage 66 and with the periphery of the piston 65 · At the outer end of the passage 68 the piston 65 is formed with an axially extending slot 69 - The body 50 is formed with a passage 70 which extends from the exterior of the body to the wall of the cylinder 62 at a position intermediate its ends. The body 50 is also formed with a further passage 71 extending from the exterior of the body to the bottom of the cylinder 62. A flow control valve 72 is provided in the passage 71· A passage 73 extends from one end of the decomposition chamber 5 to the cylinder 62. Sealing rings 7 are provided between the cylinder 62 and the piston 63.

The body is formed with a cylindrical bore 75 at its front end 76. A nose piece 77 is mounted on the front end of the body and is provided with a bearing 78 in which a cylindrical portion 79 of the anvil 57 is slidably and rotatably mounted. The anvil 57 is formed with a circular shoulder 80 and an externally splined spigot 81 which is in non-rotatable engagement with an internally splined clutch member 82 so that the anvil 57 and the clutch member 82 can slide axially relative to one another but are constrained to rotate together.

The clutch member 82 has an operative face formed with teeth 83 which engage teeth 84 on a second clutch member 85 · The teeth 83 , 84 are so arranged that the clutch acts as a torque-limiting clutch so that if rotation of the bit 55 and hence of the clutch member 82 is prevented the clutch member 85 85 is carried in a holder 86 which is rotatably mounted by a bearing 87 on a boss part 88 of the body.

The external surface of the holder 86 is provided with a helical groove 89 of half-circular cross-section. Balls 90 are engaged in the groove 89 and also engage in a similar groove 91 provided on the nut 59 which is formed with splines 92 which engage with splines 93 provided in the bore 75 so that the nut 59 is permitted to move axially in the body but is prevented from rotating relative thereto. The piston 8 is annular and . slides between the bore 75 and the holder 86 , being provided with seals 94·· The piston 8 , nut 59 and holder 6 together constitute a rotary drive member for the working element or bit 55· Passageways 95 are provided between the chamber 5 · and the left hand end of the bore 75 · The boss 88 is formed with a cylindrical bore 96 having end walls 97 and 98. The end wall 97 is formed with a passage 99 of the same diameter as the diameter of the decomposition chamber 5 · and which constitutes an extension of such chamber. The wall 98 is provided with a bush 100 having a bore 101 of the same diameter as the passage 99 · The impact member 56 comprises a piston 102 having a tapered circumferential surface 103 of concave form and slidable in the bore 96 , a cylindrical spigot 104 slidable in the passage 99 and the chamber 57 » sealing means 105 being provided between the passage 99 and the spigot member 104 , and a spigot 106 slidable in the bore 101 of the bush 100. A coil compression spring 107 is located in the bore 96 between the end wall 98 and the piston 102. Spring-loaded detents 108 are provided in the wall of the bore 96 to engage with the larger diameter part of the shouldered piston 102 as hereinafter described in detail.

A manually-operable safety trigger 109 is provided on to swing up and down. The lever 110 is engaged within a slot 111 provided in a finger 112 slidably mounted in the body for movement into and out of engagement with the left-hand face of the shoulder 80 of the anvil 57· A compression spring 113 is provided to urge the finger 112 towards the anvil 57· A compression spring 114· is provided to act between a part 115 provided on the holder 86 and the shoulder 80 to urge the anvil 57 "to the right. A further compression spring 116 is provided between the clutch member 82 and said part 115 to urge the clutch member 82 into operative engagement with the second clutch member 85 · The operation of the tool will now be described and assuming that the tool has previously been operated., the parts thereof will be in the position shown in Figure 3· After the previous operation the piston 63 of the pump 53 will have been biased to its upper position by the spring 65 and the monofuel will have been dravm up the tube 61 and into the cylinder 62 below the piston 63· The amount of monofuel dravm into the cylinder 62 will depend upon how much air is bled into the cylinder 62 through the valve and passage 71 and thus the charge of monofuel can be adjusted by adjusting the valve 72 .

The piston rod 64- is moved down manually thereby causing the monofuel beneath the piston 63 to enter the passage 66 5 past a ball check-valve 117. As the piston 63 moves downwardly the passage 67 is moved away from the end of the passage 70 but because the passage 68 is in communication with the slot 69 the passage 68 remains in communication with the passage 73 and so that a charge of monofuel is pumped through the passages 68 and 73 to the chamber 54- via a ball check valve 118 in the assa e 3· When the iston 6 has been normal position under the influence of the spring 65 and hence a further charge of monofuel is drawn through the tube 61 past the ball check valve 119 and into the cylinder 62 ready for the next operation of the gun.

A screw, not shown, is engaged with the bit 55 and then the screw is engaged with a workpiece, also not shown, into which it is desired to insert the screw. The safety trigger 109 is then pressed thus swinging the lever 110 downwardly and withdrawing the trigger 112 from engagement with the shoulder 80 on the anvil 57 · The tool is then pressed against the workpiece so that the anvil 57 slides inwardly, thereby compressing the spring 114 and so moving the end 120 of the anvil into engagement with the end 121 of the impact member 56.

Continued urging of the tool towards the workpiece causes the anvil 57 to move the impact member 56 to the left thereby disabling the detents 103 and so suddenly allowing the impact member 56 to shoot to the left under the influence of the spring 107. The sudden movement of the impact member causes the spigot 104· to project into the chamber to cause a com-pression of the gas within the chamber and thus to cause explosion of the monofuel therein.

The hot gases created by the explosion drive the impact member 6 back to the right against the action of the spring 107 and thus bring the ends 120 and 121 into engagement to give the anvil 57 an impact and hence the bit so that the screw engaged by the bit is driven into the workpiece and is caused to pierce its own pilot hole in the workpiece. As the spigot 105 moves to the right it uncovers the inner ends of the passages 95 which form directing means for the gases which act on the piston 58 causing it to move the nut 59 to the right.

Since the nut cannot rotate because of the s lines 2 ¾6 to rotate, thereby rotating the clutch member 85 , the clutch member 8J, the anvil 57 through the splined connection between the clutch member 83 and the anvil, the bit 55 and hence the screw engaged therewith which is caused to tap the pilot hole it previously formed and is driven into the workpiece.

When the screw is fully driven, and assuming that this occurs before rotation of the anvil 57 has ceased, the torque limiting clutch 82' 85 operates to enable continued rotation of the holder 86 whilst rotation of the anvil 57 is prevented.

The tool is then removed from the driven screw so that the anvil 57 is moved to the right under the influence of the spring 114 and as the shoulder 80 moves to the right its curved peripheral surface 122 engages with a correspondingly shaped surface 123 of the finger 112 and so that the latter is moved downwardly and then is urged upwardly by the spring 113 to engage behind the shoulder 80 as shown in the drawing to prevent movement of the anvil to the left until the trigger 109 has again operated.

Although spring loaded balls have been illustrated in the drawing as the detent members 108, any other suitable means for preventing movement of the impact member 56 into the chamber 5 until desired may be provided. These means could be manually operable or they could be a mechanical linkage connected to a means which senses when the tool is in driving engagement with a screw.

The preferred monofuel is isopropyl nitrate but if desired other explosive media such as cartridges or caseless charges could be provided and initiation of the explosion could be started by means other than the detonation method described hereinbefore, for example the explosion could be 1 69 Various modifications could "be made to the embodiments described in which impact of the working element is effected before rotation. If desired the gases from the explosion could be fed to cause rotation of the working element, first, followed by impact. This arrangement could be used for running nuts and the impact element would be in the form to engage a nut.

If desired a single explosion could cause simultaneous impact and rotation and the impact could be in the form of a series of discrete impacts caused during rotation of the working element.

Instead of using a monofuel to provide the explosion ases, a cartridge or a caseless charge could be used.

Claims (7)

1. 33084/2 CLAIMS; .691· A power tool compriaing a body, a working element counted on the body for reciprocation and rotation relative thereto* an Impact member alidable in the body to deliver impacts to the working element, and a rotary drive member to rotate the working element, characterised by the provision of a receiver in the body to receive a charge of an explosive liquid fuel; a pump for drawing a charge of such fuel from a reservoir and feeding the charge to the receiver, adjustable metering cieans operable to vary the else of the charge fed to the receiver, means to explode the charge in the receiver, a drive piston alidable in the body to rotate the rotary drive member, and directing % means to direct gas produced by explosion of said ohargeto elide the impact member to give an impact to the working element and to slide the drive piston to drive the rotary drive member*

2. A power tool according to Claim 1 characterised in that it comprises two co-axial cylinders in the body, one cylinder containing the Impact member which is in the form of a first piston and the other cylinder containing said drive piston (a second piston) to rotate the rotary drive member and which is connected to one part of a screw jack the other part of .69 the screw jack being connected to the working element. 33084/2

3. - A power tool according to Claim 2 characterised in that said second piston is prevented from rotating relative to the body and the other part of the screw jack is connectedt to the working element through a torque limiting clutch.

4. A power tool according to either one of Claiias 2 and 3 characterised in that said other cylinder and the second piston which it contains are annular in cross section with the impact member located within the annulus of said other cylinder.

5. · A power tool according to any preceding claim characterised in that the gas first slides the impact member which uncovers passageways forming part of the directing means and along which the gas flows to operate the drive piston to rotate the working element.

6. A power tool according to any preceding claim characterised in that the means to initiate the explosion of the charge comprises the impact member which is arranged to be held against the action of a spring in a cooked position by detent means which, when rendered inoperative, allow movement of the impact member under the influence of the spring to initiate the explosion of a charge in the receiver the resulting gas causing the impact member to deliver an impact to the working element, compress the spring -½o?-) and return to its cooked positio . 33084/2

7. A power tool according to Claim 6 characterised in that the detent means is spring loaded and arranged so that an end load applied to the impact member by the working element causes the impact member to render the detent means inoperative. 3. A power tool according to Claim 7 characterised by the provision of manually operable safety means, including a safety element engageable with the working element to prevent the latter applying said end load to the impact member unless the safety element is out of engagement with the working element. 9. A power tool according to any of Claims 1 to 5 characterised in that said receiver comprises a breech and said means to, explode the charge comprises a spring loaded firing pin capable of entering the breech and releasably held in a cooked position to which it is returned by the force of the explosion. F the Applicants Br. Yitzhak Hess