Abstract:
The invention relates to a powered reciprocating saw, in particular, to a pruning saw. To simplify use of the saw, a clamping mechanism is provided which holds an object in position while it is sawed. The clamping mechanism includes a clamping arm that is mounted at the front portion of the saw housing near the blade by a one way rotary clutch such that the arm can freely rotate in only one direction. During use, the saw vibrates due to its reciprocating parts. Since the clamping arm has a center of mass spaced apart from the axis of rotation provided by the one way rotary clutch, a progressive rotational movement of the clamping arm results which thereby automatically closes to clamp the object to be cut.

Description:
The invention relates to a hand held powered reciprocating saw, and in particular to a mechanism for holding an object while it is cut by the blade of the reciprocating saw. 
     A typical reciprocating saw is disclosed in GB 2 234 033. The reciprocating saw disclosed therein comprises a housing having a handle portion and a front portion from which a saw blade extends. The saw blade is guided for reciprocating movement and is connected to a drive mechanism within the housing which drives the saw blade to reciprocate. A typical example of saws of this kind are pruning saws used for cutting the branches of trees, bushes and the like. In this field of application the user holds the saw at the handle portion and brings the reciprocating saw blade into contact with the branch to be cut. The reciprocating saw blade is slowly moved through the branch while cutting it. The reciprocating movements of the saw blade exert forces on the object which is sawn, i.e. the object, for example a branch, tries to follow the reciprocating movement of the saw blade since there is a considerable friction between the reciprocating saw blade and the branch. Therefore, the user has to grip the branch and hold it tightly with one hand against the saw, while the pruning saw is held in the other hand in order to achieve an accurate cut. The need to hold the object, e.g. a branch, to be sawn with the reciprocating saw can make the saw very inconvenient to use. 
     Another problem or inconvenience arises due to the fact that the user has to absorb the oscillating forces which act between the reciprocating saw and the object which is sawn. When the user grips the branch to be cut and holds the reciprocating saw with the other hand he has to absorb the reciprocating forces acting between the branch and the saw with his arms. On the one hand this makes use of the saw tiring, and on the other hand, since the user cannot hold the branch completely fixed, the cut can be inaccurate if the saw is not steadily held in the same position with respect to the branch. 
     The problems outlined above do not apply to pruning saws only but to reciprocating saws in general. Whenever the object to be sawn has a certain flexibility or is not fixed and so has to be held by the user, for example when a rod has to be cut into pieces of shorter length, problems as described above arise. 
     The problems are further exacerbated when the branches of trees and bushes high above a user are cut using a reciprocating saw mounted on an extension arm or pole. In this case a user cannot reach the branch to be cut to hold it while it is cut. Thus, the reciprocating movement of the blade is transferred to the branch and the branch reciprocates also. This reduces the cutting performance of the saw, can lead to inaccurate cuts being made and tends to increase the length of time required to complete the task in hand. 
     It is therefore desirable to have a clamping device to hold the object to be saw stationary to prevent it from following the reciprocating movement of the saw blade whilst allowing the saw blade to cut through the object. 
     One such clamping device for a compass saw is disclosed in GB 2,182,282 and comprises a clamping holder in the form of a part circular bracket which is pivotally mounted at its centre on the body of the saw. On one arm of the part circular bracket is a clamping jaw having two inwardly facing clamping surfaces. On the other arm of the part circular bracket is a clamping part which comprises a threaded shaft which screws through a corresponding threaded hole, the direction of travel of the shaft as it screws into or out of the hole being within the plane of the clamping device either towards or away from the clamping jaw. In operation, the workpiece to be cut is inserted into the region bounded by the part circular bracket and is clamped between the clamping part and the clamping jaw by screwing the clamping part towards the clamping jaw and trapping the workpiece in between. As the blade of the saw cuts through the workpiece, the clamping device pivots to allow the workpiece to remain in contact with the cutting edge of the saw blade. 
     The clamping device disclosed in GB 2,182,282 must be firmly attached to the object to be sawn by the user prior to commencing the sawing operation. This can be difficult if the object to be sawn is located in an awkward position, for example, if it is a branch high up in a tree. The user is required to use both hands whilst attaching the clamp, one to hold the clamping device against the object to be cut whilst the other tightens the clamping part to hold the object to be cut. Furthermore, as the clamping device is firmly attached to the object to be cut, the only movement relative to the object to be cut which the body of the saw can make whilst cutting through the object to be cut is a pivotal movement. This may be undesirable in certain situations where there is insufficient room for the body of the saw to pivot. 
     DE 4429408 also disclosed a clamping device for a compass saw. The clamping device comprises a structure similar to a pipe wrench which is pivotally mounted on the saw housing. The object to be sawn is clamped by turning a knob which in turn rotates a threaded spindle to bring two clamping surfaces of the wrench into engagement with and thus hold the object. The clamping device disclosed in DE 4429408 suffers from the same problems as those suffered by the clamping device in GB 2,182,282. 
     It is an object of the present invention to overcome the problems outlined above. In particular it is an object to provide a hand held reciprocating saw which allows the user to hold the saw with one hand without the need to hold the object to be cut with the other hand in order to prevent an oscillating movement of that object while it is cut. 
     According to a first aspect of the present invention there is provided a hand-held powered saw which saw has a reciprocating cutting blade and comprises: 
     a clamping arm which is rotatably mounted on the saw by means of a one way rotary clutch such that the arm can freely rotate in one direction only; and 
     a support member which co-operates with the clamping arm to hold an object to be cut; 
     wherein the centre of mass of the clamping arm is spaced apart from the axis of rotation of said rotary one way clutch so that the vibration of the saw when the saw is in use induces a progressive rotation of the clamping arm. 
     The vibration of the saw induces a stepwise progressive rotation of the clamping arm due to its moment of inertia. Since the clamping arm can rotate in one direction only, movements caused by the moment of inertia while the saw as a whole is vibrating results in stepwise rotational movements in one direction only. Thus, when the saw is activated and held by the user with the saw blade reciprocating, the clamping arm performs a progressive rotational movement. If the user holds the saw close to a branch so that the branch contacts the support member the rotational movement of the clamping arm has the effect that the clamping arm closes and clamps the branch resting on the support member. If the object to be cut is not properly clamped when it comes into contact with the saw blade, the initial movement of the object as it follows the reciprocating movement of the blade causes greater vibrations which quickly accelerates the clamp into place. Thus, the user only has to position the saw such that the branch can be held between support member and clamping arm, and the clamping mechanism will close automatically driven by the reciprocating movement of the saw. The clamping arm and support member can be arranged so that they co-operate with each other to firmly clamp the object to be cut between the two. However, the clamping arm and support can alternatively be arranged so that they co-operate with each other to hold the object to be cut against the cutting edge of the saw blade. In either arrangement the saw blade can then be moved relative to the object, by a variety of means, in the direction of cut so that the reciprocating saw blade can move through the object held between the support member and clamping arm to cut the object. Objects such as branches can be sawn while holding the reciprocating saw with one hand without having to hold the branch as well. 
     The forces in the direction of blade reciprocation between the object and the saw which are caused by the reciprocating movement of the saw blade through the object can be absorbed by the holding of the object between the clamping arm and support member. Thus, the reciprocating saw according to the invention is easier and more convenient to use than prior art saws. 
     As the blade cuts through the object to be cut, the clamping arm may rotate in order to maintain hold with the co-operation of the support member, on the object to be cut and thus ensure that any reciprocating movement of the object to be cut is prevented or substantially reduced whilst the object is being cut. 
     A preferred means of moving the saw blade relative to an object held between the clamping arm and support member uses a slide element which is slideably mounted on the saw for sliding movement in a direction across the blade wherein the clamping arm is mounted on the slide element so that the sliding movement of the slide element and clamping arm guides an object held between the support member and the clamping arm across the cutting blade. Preferably, the support member is also mounted on the slide element so that the object held between the support member and the clamping arm does not have to move across the support member as it is guided across the cutting blade. 
     The support member may comprise the reciprocating saw blade. The shape of the clamping arm can be designed so that it grips and holds the object to be cut against the cutting edge of the blade without the assistance of any other part of the saw. The clamping arm would prevent or substantially reduce the reciprocating movement of the object to be cut caused by the reciprocating movement of the saw blade. As the blade cut through the object to be cut, the clamping arm would rotate ensuring that the object to be cut is securely held by the clamping arm to prevent or substantially reduce the reciprocating movement of the object. By using the saw blade as the support, it is possible to design a reciprocating saw with a clamping mechanism so that the object to be cut can be held against any part of the length of the reciprocating blade. 
     Where the clamping arm co-operates with the support member to hold the object to be cut against the cutting edge of the saw blade, ideally, the shape of curvature of the surface of the clamping arm which engages an object to be cut (hereinafter referred to as the holding surface) and location of the axis of rotation of the clamping arm are such that when an object of circular cross section is first cut by the reciprocating blade, the tangent to the surface of the object at the point on the surface where the clamping arm first makes contact with the object is at an angle of between 30° and 60° and preferably between 40° and 50° to a plane which passes through the length of the blade which plane is 90° to the plane of the blade. This has the benefit of providing a clamping arm which can securely hold objects to be cut against the cutting edge of the blade for a variety of such objects which vary greatly in their cross sectional areas whilst preventing or severely reducing the reciprocal movement of the object caused by the reciprocal movement of the blade. 
     In a preferred embodiment of the present invention, the clamping arm is mounted on the saw via a reverse slip clutch which allows rotation of the clamping arm in the reverse direction to the direction of free rotation of the rotary one way clutch if a reverse torque is exerted exceeding a specified threshold. This embodiment allows the user to remove the object from the clamping mechanism in a simple manner by manually turning the clamping arm in a direction opposite to the direction of rotation of the rotary one way clutch. Ideally, the one way rotary clutch and the reverse slip clutch are co-axially mounted to simplify the construction. 
     Furthermore, it is preferable that the axis of rotation of the rotary one way clutch is oriented essentially perpendicular to the plane of the blade. 
     Ideally, the clamping arm is made from a plastics material having a metal weight located towards the end of the clamping arm distant from the rotary one way clutch. By making the clamping arm from plastic, it makes the manufacture of the clamping arm cheaper and easier. Furthermore, it reduces the weight of the clamping arm which in turn reduces the overall weight of the saw making it easier for the user to use. By adding a metal weight towards the end of the clamping arm away from the one way rotary clutch, the centre of mass of the clamping arm is moved a greater distance from the axis of rotation of the clamping arm. This results in a greater moment of inertia of the clamping arm about the axis of rotation and therefore increases its rate of rotation. 
     In the plane of the clamping arm, the direction of curvature of the end of the clamping arm distant from the one way rotary clutch should preferably be opposite to that of the holding surface of the clamping arm, for example the clamping arm may be ‘S’ shaped. Changing the direction of curvature of the tip region of the clamping arm prevents the clamping arm from snagging on larger branches and makes it easier to pull the saw away from a branch whilst it is still being held by the clamping mechanism. 
     According to the second aspect of the present invention there is provided a clamping mechanism which is mountable on a hand-held powered reciprocating saw which saw has a reciprocating cutting blade, said clamping mechanism comprising: 
     a clamping arm which is rotatably mounted on the clamping mechanism by means of a one way rotary clutch such that the arm can freely rotate in one direction only; 
     wherein the centre of mass of the clamping arm is spaced apart from the axis of rotation of said rotary one way clutch. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following the invention will be described in more detail with reference to the accompanying drawings in which: 
     FIG. 1 shows a sectional view of a reciprocating saw having a clamping mechanism according to the first embodiment of the invention; 
     FIG. 2 shows a schematic front view of the reciprocating saw of FIG. 1; 
     FIGS. 3 a ,  3   b  show schematic side views of the clamping mechanism of FIGS. 1 and 2 in two positions; 
     FIG. 4 shows a clamping mechanism according to a second embodiment of the invention which is releasably attachable to a reciprocating saw; 
     FIG. 5 shows the attachable clamp mechanism of FIG. 4 when it is attached to the front end of a reciprocating saw; 
     FIG. 6 a  shows a schematic view of a dismantled clamping mechanism according to the embodiment of the invention shown in FIGS. 4 and 5; 
     FIG. 6 b  shows a close up exploded view of the “C” clip and the slot across which it is located as shown in FIG. 6; 
     FIG. 7 shows a view of an adapter of the clamping mechanism comprising two resilient arms which clip into the front of a reciprocating saw; 
     FIG. 8 shows a view of the adapter of the clamping mechanism according to the embodiment shown in FIGS. 4 to  6 ; 
     FIG. 9 shows a view of the clamping mechanism with a circular clip holding the clamping arm on the adapter; 
     FIG. 10 shows the plane of the blade of a reciprocating saw; 
     FIG. 11 shows a schematic view of the second embodiment of the invention in which the clamping mechanism is holding a branch; 
     FIG. 12 shows a schematic view of the second embodiment of the clamping mechanism holding a branch which has been partially cut through by the blade of the saw; 
     FIG. 13 shows the geometry of the shape of the clamping arm, in a first position; 
     FIG. 14 shows the geometry of the shape of the clamping arm in a second position; 
     FIG. 15 shows the geometry of the shape of the clamping arm in a third position; 
     FIG. 16 shows a plane which passes through the length of the blade; 
     FIG. 17 shows a clamp mechanism according to the third embodiment of the present invention; 
     FIG. 18 shows a sectional, fragmentary view of a portion of a clamping arm mounted via a reverse slip clutch to the rotary one way clutch; 
     FIG. 19 shows an perspective view of an alternative reverse slip clutch; 
     FIGS. 20 a  and  20   b  show an end view of the alternative reverse slip clutch in two positions; 
     FIG. 21 shows a clamping arm with the tip region having a reverse direction of curvature to that of the holding part of the arm; and 
     FIG. 22 shows a clamping arm holding a large branch having a tip region with a reverse direction of curvature to that of the holding part of the arm. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1 of the drawings, the saw has a housing  2  having a handle portion  3  and front portion  4  from which a reciprocating saw blade  20  projects through a slot in the front portion  4 , for reciprocation in the directions shown by the arrow R. An electric motor  6  is mounted within the housing which drives a driving gear  8 . A switch  5  is provided to turn the motor on and off. The driving gear  8  is in engagement with gear wheel  10 . An eccentric pin  12  is attached to the gear wheel  10 . The eccentric pin  12  is in engagement with a transverse groove in which it can slide in a direction transverse to the direction of reciprocating movement of the saw blade  20 . The groove is formed in a member which is connected to a reciprocating shaft  14 . Rotation of the gear wheel  10  thus drives the reciprocating shaft  14  in a reciprocating movement. At the front end of the reciprocating shaft  14  a blade holder  16  is attached which holds the saw blade  20 . 
     A first embodiment of the clamping mechanism as disclosed in FIGS. 1 to  3  is provided at the front portion of the housing and comprises a sliding element  30  and a clamping arm  40 . The clamping arm  40  is mounted on the sliding element  30  via a rotary one way clutch  42  so that the clamping arm can freely rotate in one direction only (in a counter-clockwise sense in the representation of FIG.  1 ). 
     The clamping mechanism is shown in a front view in FIG.  2 . The clamping mechanism comprises the clamping arm  40  and the slide element  30 . The slide element  30  presents a supporting surface portion  32  which faces away from the front portion of the housing and which serves to support an object in a position below the saw blade  20  and while it is cut. A rod  44  is attached to the slide element  30  at a lower end portion thereof. The rotary one way clutch  42  is fitted to the outer end portion of the rod  44 . The clamping arm  40  has a bore with an inner diameter so that it can be fitted onto the outer circumference of the one way rotary clutch  42 . As will be described in more detail below, it is preferred that the clamping arm  40  is not directly connected to the rotary one way clutch  42  but via a further intermediate clutch. 
     As can be seen from FIG. 2, the clamping arm  40  is held by the rod  44  in a position so that the plane of rotation of the clamping arm  40  is beyond the envelope of the housing, and the clamping arm  40  can freely rotate around 360° without interfering with parts of the housing. The slide element  30  may be a precision ball slide having an inner guiding part  31  which is attached to a guard member of the front portion of the housing. The slide element or table  30  is slideable with respect to the guide element  31  by two sets of ball rollers (not shown) which are retained in raceways on both sides of the guide element  31  between the guide element  31  and the slide element  30  which can thereby perform a linear sliding movement (in the directions of the arrow L in FIGS.  1  and  2 ). The direction of linear movement is aligned with the cutting direction (shown by the arrow C in FIGS. 1 and 2) of the saw blade  20 . 
     The rotary one way clutch can be of a type known as a drawn cup roller clutch. These clutches are annular rings which can be fitted over a shaft. They have a thin walled drawn outer ring which has a series of ramps on its inside diameter. Needle rollers, which are retained and guided by a plastic cage, form the clamping elements. The needle rollers are held in the locked/unlocked position by springs. The frictional moment during idling of drawn cut roller clutches is very low so that the clutch can very easily be rotated in one direction whereas it can withstand a relatively much higher reverse torque before the clutch is damaged. 
     The operation of the clamping mechanism will be described in connection with FIGS. 3 a  and  3   b . In FIGS. 3 a  and  3   b  only the slide element  30  and the clamping arm  40  are shown whereas the guide element  31  and the housing of the saw have been omitted. The clamping arm  40  can freely rotate in clockwise direction by means of one way rotary clutch  42  which is attached by rod  44  to the slide element  30 . 
     In operation of the saw, the reciprocating movement of the reciprocating shaft  14  and the saw blade  20  induces vibration of the saw as a whole because of the inertia of the reciprocating parts. This vibration is even stronger when the gripping portion  3  is not aligned with the reciprocating parts so that an oscillating torque is induced when the user holds the saw at its gripping portion  3 . The vibration of the saw is also transferred to the slide element  30 . Thus, the one way rotary clutch  42  also vibrates. Since the clamping arm  40  has a moment of inertia with respect to this axis of rotation of the one way rotary clutch  42 , the vibrations are transformed into a stepwise, progressive rotational movement of the clamping arm  40 , the clamping arm  40  making one incremental rotational movement in each back and forth movement of the vibration of the saw. Therefore, when the saw is in operation, the clamping arm  40  progressively turns in its direction of free rotation (clockwise in FIGS. 3 a  and  3   b ). 
     When the saw is held to a object to be sawn, e.g. a branch  50 , so that the branch  50  abuts to the supporting surface  32  below the saw blade  20 , the rotational movement of the clamping arm  40  induced by the vibration of the saw, automatically closes the clamping mechanism, thereby clamping the branch  50  between clamping arm  40  and supporting surface  32  as shown in FIG. 3 b . In this position further rotational movement of the clamping arm  40  is prevented by the branch  50 . Then, the user moves the saw as a whole down with respect to the branch  50  in the direction of cut (C) to bring the sawing blade  20  into contact with the branch  50  and to move the reciprocating saw blade through the branch  50 . During this movement the sliding element  30  performs its linear sliding movement relative to the cutting blade  20  in a direction opposite to the direction of cut since it is clamped to the branch  50 . Thus, the branch is guided across the blade  20  and is accurately cut. 
     With the clamping mechanism described, the user simply holds the saw as usual with the object to be sawn near the front portion below the reciprocating saw blade. When the drive mechanism of the saw is activated, the vibration of the saw as a whole induces the rotational movement of the clamping arm as described and rapidly closes the clamping mechanism so that the object is clamped between the clamping arm and the supporting surface of the slide element. Thus, the user can saw branches of trees while holding the pruning saw in one hand without the need to grip the branch with the second hand. 
     A second embodiment of the clamping mechanism is disclosed in FIGS. 4 to  12 . As shown in FIGS. 4 and 5, the clamping mechanism is releasably attachable to the front of a reciprocating saw,  105 . 
     The clamping mechanism comprises an adapter  100  on which is mounted a clamping arm  101  via a one way clutch  102  (see FIG.  6 ). The adapter  100  is designed so that it is able to slide into two slots  103 ,  104  on the reciprocating saw  105  formed by two plates  128 ,  129  which form the front  115  of the reciprocating saw  105  and attach to the saw  105  as shown in FIG.  5 . 
     An alternative design of adapter  100 ′ comprising means for attachment to a reciprocating saw  105  is shown in FIG.  7 . The adapter  100 ′ comprises two resilient arms  130 ,  131  which project away from the clamping arm  101 ′ substantially parallel to the axis  112  of rotation of the clamping arm  101 ′. A ramp  132 ,  133  is formed on the outer edge of each arm  130 ,  131 . The ramps  132 ,  133  commence near the end of each arm  130 ,  131  and curve outwardly towards the clamping arm  101 ′. The ends of the ramps  132 ,  133  form ledges  134 ,  135 . The distance between the two arms  130 ,  131  corresponds to the width of the slots  103 ,  104 . 
     The adapter  100 ′ is attached to the front of the reciprocating saw  105  by sliding the two arms  130 ,  131  into the two slots  103 ,  104 . As the arms  130 ,  131  slide into the slots  103 ,  104  the metal plate  128  which forms the edge of the first slot  103  engages with and slides along the ramps  132 ,  133 , causing the two arms  130 ,  131  to bend inwardly towards each other. When the ramps  132 ,  133  on the two arms  130 ,  131  have passed through the first slot  103  the two arms  130 ,  131  spring outwards, the ramps  132 ,  133  entering into the space between the two metal plates  128 ,  129  to clip the adapter onto the front of reciprocating saw  105 . The adapter  100 ′ is prevented from sliding any further by a connecting section  136  of the adapter  100 ′ abutting against the housing of the reciprocating saw  105 ′. 
     The adapter  100 ′ is prevented from sliding out of the slots  103 ,  104  by the edge of the metal plate  128  which forms the first slot  103  blocking the path of the ledges  134 ,  135 . In order for the adapter  100 ′ to be removed or unclipped from the slots  103 ,  104 , the two arms  130 ,  131  must be bent inwardly towards each other in order to move the ledges  134 ,  135  to positions where the edge of the metal plate  128  would not block their paths when the adapter  100 ′ is withdrawn from the slots  103 ,  104 . 
     A slot  106 ,  106 ′ has been formed in the adapter  100 ,  100 ′ so that when the clamping mechanism is attached to the reciprocating saw  105 , the blade  107  of the saw  105  is able to freely pass through the slot  106 . At the side of the adapter  100 ,  100 ′ is a hole  108  (see FIG. 8) through which a spindle  109  passes. The clamping arm  101  is attached to the spindle  109  via the one way clutch  102 . The clamping arm  101  is mounted onto the one way clutch  102  and the one way clutch  102  is press fitted onto the spindle  109  so that the clamping arm  101  is not able to rotate relative to the spindle  109  in the direction opposite to the direction of free rotation of the one way clutch  102 . A part circular clip  137 , as shown in FIG. 9, fits into a corresponding groove  138  formed around the circumference of the end of the spindle  109  to hold the clamping arm  101  and the one way clutch  102  on the spindle  109 . The end  110  of the spindle  109  is covered by a cap  111 . As an alternative to the part circular clip  137 , two resilient protrusions (not shown) integrally formed with the clamping arm  101  can project into the groove  138  to hold the clamping arm  101  and the one way clutch  102  on to the spindle  109 . The spindle  109  is able to rotate within the hole  108  when sufficient rotational torque is applied to the spindle  109 . The spindle  109  is unable to slide axially within the hole  108 . The spindle  109  and the hole  108  together form a secondary reverse slip clutch which shall be described in greater detail further below. 
     The clamping arm  101  is only able to rotate in one direction (shown by arrow A in FIG. 4) in relation to the adapter  100  due to the one way clutch  102 . 
     When the clamping mechanism is attached to the reciprocating saw  105 , the clamping arm  101  is held by the adapter  100  so that it is beyond the envelope of the housing of the saw  105  so that the clamping arm  101  can freely rotate around 360° without interfering with parts of the housing. The axis of rotation  112  of the clamping arm  101  is perpendicular to the plane  113  of the blade  107 . The plane  113  of the blade is shown in FIG.  10 . 
     The rotary one way clutch  102  is a drawn cup roller clutch of the type previously described. 
     The operation of the clamping mechanism will now be described in relation to FIGS. 11 and 12. The two figures shown are schematic diagrams. For simplicity, the figures only show the saw blade  107 , a branch  114  and a representation of the front housing  115  of the saw  105 . 
     In operation, the clamping mechanism is attached to a reciprocating saw. When the saw is activated, the saw  105  as a whole vibrates because of the inertia of the reciprocating parts. The vibration of the saw  105  is transferred to the adapter  100 . Thus, the adapter  100  together with the one way rotary clutch  102  also vibrates. The vibrational movement of the adapter  100  is transferred to the clamping arm via the one way rotary clutch  102 . Since the clamping arm  101  has a moment of inertia about the axis rotation  112  of the one way rotary clutch  102 , the vibrations are transformed into a stepwise, progressive rotational movement of the clamping arm  101 , the clamping arm  101  making one incremental rotational movement in each back and forth movement of the vibration of the saw  105 . Therefore, when the saw  105  is in operation, the clamping arm  101  progressively turns in its direction of free rotation (shown by arrow A). 
     The saw  105  is placed near the branch  114  to be cut so that it lies near to the cutting edge  116  of the blade  107 . When the saw  105  is operated, the arm  101  rotates until it holds the branch  114  against the front  115  of the housing and the cutting edge  116  of the blade  105 . The clamping arm  101  is then hindered from any further rotation by the branch  114 . 
     FIG. 11 shows the branch  114  being held against the cutting edge  116  of the saw blade  107 , by the clamping arm  101  and the front  115  of the saw housing. The clamping arm  101  is only able to rotate in the direction indicated by arrow A and therefore the branch  114  is prevented from reciprocation along the axis  117  with the reciprocating blade  107  by the clamping arm  101  and the front  115  of the saw housing. 
     As the reciprocating blade  107  is moved through the branch  114 , it cuts the branch  114  which moves upwardly relative to the blade  107  as shown in FIG.  10 . As the branch  114  moves upwards, the clamping arm  101  rotates due to vibration of the saw  105  ensuring that the branch  114  remains held fast by the clamping arm  101  and the front  115  of the housing against the cutting edge  116  of the blade  107  and so is prevented from reciprocating along the direction of the axis  117  with the blade  107 . 
     Referring now to FIGS. 13 to  15 , the curvature of the holding surface  118  of the clamping arm  101  and the location of the axis  112  of rotation of the clamping arm  101  are such that when a pole  119  of circular cross section is first cut by the reciprocating blade  107 , the tangent  120  to the surface  121  of the pole  119  at the point  122  on the surface  121  where the clamping arm  101  first makes contact with the pole  119  is at an angle  123  of 45° to a plane  124  (as shown in FIG. 16) which passes through the length of the blade  107 . The pole  119  is used as it represents a branch  114  with an ideal shaped cross-section i.e. circular. The plane  124  which passes through the length of the blade  107  is shown in FIG.  16  and is at 90° to the plane  113  of the blade  107 . When a large diameter pole is cut, a different part of the holding surface  118  of the clamping arm  101  makes contact with the pole  119  when it is initially cut by the cutting edge  116  of the blade  107 . However, due to the shape of the holding surface  118  of the arm, the tangent  120  of the circumference  121  at the point  122  where the clamping arm  101  first makes contact with the pole  119  is always 45° to the plane  124  through the length of the blade  107 . The pole shown in FIG. 11 has a smaller diameter than that shown in FIG. 14 which in turn is smaller than that in FIG.  15 . 
     The clamping arm  101  shown in FIGS. 13 to  15  is made from plastic. A metal pin  125  is inserted into the tip of the clamping arm. The rate of rotation of the arm is dependent upon the moment of inertia induced into the clamping arm  101  which is in turn dependent upon the distance between the centre of mass of the clamping arm  101  and the axis  112  of rotation of the clamping arm  101 . By adding a relatively heavy metal pin  125  to the tip of the clamping arm  101 , the distance between the axis  112  of rotation of the clamping arm  101  and the centre of mass is increased and hence the rate of rotation of the clamping arm  101  is increased. 
     In the previously described embodiments after the object has been sawn the user can pull the object out of its clamped position between the supporting surface  32 ; 115  and the clamping arm  40 ; 101 , whereupon the clamping arm  40 ; 101  starts its rotational movement again if the reciprocating movement of the saw blade continues. Then, the user can position the saw near the next branch to be cut, and the foregoing procedure is repeated. 
     A third embodiment of the clamping arm mechanism is disclosed in FIG.  17 . The method of construction and operation of the third embodiment is similar to that of the first and second embodiments except that the clamping arm  200  is shaped so that the branch  201  is held against the cutting edge  202  of the saw blade  203  by only the clamping arm  200 . 
     The clamping arm  200  is mounted on a one way rotary clutch and is therefore only able to rotate in the direction shown by arrow B. The shape of the clamping arm  200  is such that, when it holds the branch  201  against the cutting edge  202  of the saw blade  203 , the arm prevents the branch from reciprocating along the direction of the axis  205  with the blade  203 . As the blade  203  cuts through the branch  201 , the branch  201  moves upwards relative to the blade  203  allowing the clamping arm  200  to rotate due to the vibrations of the saw and to continue to hold the branch  201  against the cutting edge  202  of the blade  203 . 
     In any of the embodiments described above, in order to simplify removal of the object being held either by the clamping arm  200 , or between the clamping arm  40 ; 101  and the supporting surface portion  32 ; 115 , a reversing mechanism can be provided which allows rotation of the clamping arm  40 ; 101 ; 200  in the opposite direction to the direction of free rotation of the one way rotary clutch  42 ; 102 . Such a mechanism is provided by a reverse slip clutch acting between the clamping arm  40 ; 101 ; 200  and the fixed part  44 ; 100 ; 203 . The reverse slip clutch can be provided between the rod  44  or spindle  109  and the rotary one way clutch  42 ; 102  or between the outer circumference of the rotary one way clutch  42 ; 102  and the bore of the clamping arm  40 ; 101 ; 200 . In its simplest form the reverse slip clutch can be provided by using a predetermined press fit of the bore of the clamping arm  40  on the outer circumference of the rotary one way clutch  42  as shown in FIGS. 1 to  3 . If a certain torque is exerted on the clamping arm  40  the frictional forces between the bore of the clamping arm  40  and the outer surface of the rotary one way clutch are overcome, and the clamping arm may be turned backwards. 
     An alternative form of reversing mechanism is disclosed in the second embodiment (see FIGS. 4 to  9 ). The hole  108  in the adapter  100  into which the spindle  109  is inserted has a slot  127  cut from the hole  108  to the edge of the adapter (see FIG. 6 a ,  6   b  and  7 ). The spindle  109  is located within the hole  108 . The diameter of the hole  108  is slightly greater than the diameter of the spindle  109 . A ‘C’ clip  126  is then placed across the end of the slot  127 . See FIG. 6 b  which shows an exploded view of the “C” clip and the slot. The ‘C’ clip  126  exerts a pressure across the slot  127  causing it to close. This reduces the diameter of the hole  108  and thus the edge of the hole squeezes the spindle  109 . This generates a frictional force between the spindle  109  and the edge of the hole  108 . The spindle  109  is therefore only able to rotate when a sufficient rotational torque is applied to the spindle  109  to overcome the frictional force between the spindle  109  and the edge of the hole  108 . The amount of frictional force between the spindle  109  and the edge of the hole  108  is dependent on the strength of the ‘C’ clip  126 . 
     However, from a manufacturing point of view, it creates difficulties to provide these reversing mechanisms which hold a reverse torque of a specified value in a reproducible manner. Therefore, a reverse slip clutch mechanism can be utilised, as shown in a simplified and schematical manner in FIG.  18 . In FIG. 18, the rotary clutch  300  is fitted onto the rod  301 . The rod  301 , is rigidly attached to a saw or adapter. On the cylindrical outer surface of the rotary one way clutch  300 , a part  302  can be fixed made, for example of glass filled nylon or by diecasting or sintering, having a corrugated outer surface. The inner diameter of the bore of the clamping arm  303  is larger than the largest outer diameter of the reverse slip clutch part  302 . From the inner surface  304  of the bore of the clamping arm  303  flexible protrusions  305  extend which interact with the corrugations  306  of the reverse slip clutch part  302 . The protrusions extend non-radially into the bore and are flexible so that the clamping arm  303  may slip in one direction when a torque is applied above a predetermined torque in threshold value (in counter clockwise direction in FIG. 18) by bending the protrusions  305  so that they can move across the corrugations  306 , whereas rotation in the other direction would require a much higher torque. By specifying the dimensions and flexibility of the protrusions  305  the characteristics of the reverse slip clutch, in particular the torque threshold value, beyond which a slipping rotation of the clutch is possible can be adjusted to the desired values. The threshold torque value of the reverse slip clutch should exceed the maximum torque values which could be exerted on the clamping arm by the object while it is sawn. 
     An alternative design for a reverse slip clutch is disclosed in FIGS. 19 and 20 a  and  20   b.    
     The reverse slip clutch comprises a shaft  400  which is rotatable about its longitudinal axis  401 . The shaft  400  is connected to a clamping arm via a rotary one way clutch. A groove  402  runs along part of the length of the shaft  400 . A flat leaf spring  403  is located in and is attached at its ends to the adapter  100 . The flat leaf spring  403  is bent to form a tooth  404 . The tooth  404  is arranged so that it projects into the groove  402  when the shaft  400  is rotated so that the groove  402  faces the tooth  404 . When the tooth  404  is located within the groove  402  and a low rotational torque is applied to the shaft  400  the tooth  404  restrains the movement of the groove  402  and thus prevents the shaft  400  from rotating (see FIG. 20 a ). However, when a rotational torque greater than a predetermined level is applied to the shaft  400  the tooth  404  is forced out of the groove  402  flexing the spring  403  and allowing the shaft  400  to rotate (see FIG. 20 a ), The predetermined level of rotational torque is determined by the stiffness of the spring  403 . 
     In addition to the reversing mechanism, the tip region  501  of the clamping arm  500  can have a curvature in the plane of the clamping arm which is in a reverse direction to that of the holding surface  506  of the clamping arm, as shown in FIG.  21 . This enables branches to be removed from the clamping arm  500  more easily. The direction of curvature (Y) of the tip region  501  of the clamping arm  500  is opposite to the direction of curvature (X) of the holding surface  506 . This prevents the clamping arm  500  from being snagged on larger branches  503  being cut by the reciprocating blade  504  of the saw  505  as shown in FIG.  22 .