Abstract:
A reciprocating saw comprising a cutting arrangement; a rotatable drive shaft with a drive shaft axis, wherein the drive shaft comprises a first eccentric; a timing actuator capable of performing reciprocating motion, wherein the timing actuator is associated with the cutting arrangement so that, in use, the reciprocating motion of the timing actuator is transmitted to the cutting arrangement; and a counterweight capable of performing reciprocating motion, wherein the counterweight is coupled to the first eccentric so that rotation of the first eccentric drives the reciprocating motion of the counterweight, wherein the reciprocating motion of the counterweight is out of phase with the reciprocating motion of the cutting arrangement, characterised in that the drive shaft comprises a second eccentric, wherein the second eccentric is associated with the timing actuator so that, in use, the rotation of the second eccentric controls the reciprocating motion of the timing actuator.

Description:
[0001]    The present invention relates to a reciprocating saw and, in particular, to an improved drive mechanism for a reciprocating saw.  
         BACKGROUND OF THE INVENTION  
         [0002]    Reciprocating saws are well known in the art. Some reciprocating saws are stationary power tools, whereas other reciprocating saws are portable power tools. Certain reciprocating saws are robustly designed to cut through large pieces of wood or steel bars. Certain other reciprocating saws, such as a jigsaw, are designed for cutting through a smaller workpiece in fine detail. A reciprocating saw in the form of a conventional jigsaw is shown in FIG. 1. A jigsaw of this type is used for cutting a workpiece in a cutting direction shown by arrow A. All reciprocating saws have the common feature of a saw blade performing reciprocating motion in order to cut a workpiece.  
           [0003]    German utility model publication no. DE29912907U1 discloses one such reciprocating saw comprising a cutting arrangement capable of performing reciprocating motion, a rotatable drive shaft with a drive shaft axis, wherein the drive shaft comprises a first eccentric, a timing actuator capable of performing reciprocating motion, wherein the timing actuator is associated with the cutting arrangement so that, in use, the reciprocating motion of the timing actuator is transmitted to the cutting arrangement, and a counterweight capable of performing reciprocating motion, wherein the counterweight is coupled to the first eccentric so that, in use, rotation of the first eccentric drives the reciprocating motion of the counterweight, wherein the reciprocating motion of the counterweight is out of phase with the reciprocating motion of the cutting arrangement.  
           [0004]    The reciprocating saw disclosed by document DE29912907U1 has only one single eccentric to drive the counterweight and control the timing actuator. While the aim of having one single eccentric is to make the reciprocating saw more compact, the single eccentric nonetheless gives rise to certain disadvantages. In particular, the freedom to design the single eccentric in accordance with the timing actuator is comprised by the need to make the single eccentric also compatible with the requirements of the counterweight, and vice versa. Such compromises may require expensive component materials, complex component arrangement, additional components, or other compromise solutions.  
         BRIEF DESCRIPTION OF THE INVENTION  
         [0005]    The present invention provides a reciprocating saw as described at the outset, characterised in that the drive shaft comprises a second eccentric, wherein the second eccentric is associated with the timing actuator so that, in use, the rotation of the second eccentric controls the reciprocating motion of the timing actuator. This has the advantage that the first eccentric can be designed solely for the purpose of operating the reciprocating motion of the counterweight without considering the timing actuator, and the second eccentric can be designed solely for the purpose of controlling the timing actuator without considering the counterweight. This freedom promotes efficiency in the design of the first eccentric, second eccentric, the timing actuator, or cutting arrangement, each of which can therefore be made smaller, lighter or less complex. Also, the most suited materials can be chosen for these components to prove their efficiency.  
           [0006]    Preferably, the reciprocating motion of the timing actuator is superimposed upon the reciprocating motion of the cutting arrangement. This creates two components to the reciprocating motion of the cutting arrangement which can improve the cutting action of the reciprocating saw, provided that these two components of the reciprocating motion of the cutting arrangement are in a plane parallel to the cutting direction. For example, the first component can be the up and down reciprocating motion of the cutting arrangement, and the second component can be the backward and forward reciprocating motion of the cutting arrangement. In this case, the cutting action may be performed during the upward motion of the cutting arrangement and the forward motion is time to coincide with the upper motion in order to counteract cutting frictional forces acting upon the or cutting arrangement in the opposite sense to the cutting direction. By doing this, the forward motion eliminates, or significantly reduces, deflection of the cutting arrangement in the opposite sense to the cutting direction thereby improving the cutting action of the reciprocating saw. The two components of the reciprocating motion of the cutting arrangement can be in phase, or out of phase, depending on the requirements of the reciprocating saw.  
           [0007]    Preferably, the first eccentric is a first cam arranged eccentrically with respect to the drive shaft axis, wherein the first cam is yoked by an aperture in the counterweight. The transverse elongate counterweight aperture surrounds the substantially cylindrical first cam in the manner of a yoke. In use, the first cam rotates about the drive shaft axis. The horizontal component of this rotational motion is accommodated by the counterweight aperture, which permits side to side motion of the first cam within its confines without causing side to side motion of the counterweight. Whereas the vertical component of this rotational motion is not accommodated by the counterweight aperture and instead of the counterweight moves up and down in time with the up and down motion of the first cam. This is a simple means of translating the rotational movement of the drive shaft into the reciprocating rectilinear motion of the counterweight.  
           [0008]    Preferably, the drive shaft comprises a driven gear and the second eccentric is located between the driven gear and the counterweight. The second eccentric can be integral with the driven gear, or the second eccentric can be a separate component fixed to the driven gear.  
           [0009]    Preferably, the second eccentric is a second cam arranged eccentrically with respect to the drive shaft axis. Rotation of the second cam about the drive shaft axis can be used as a simple means of converting its rotational motion into reciprocating motion, as is described in more detail below.  
           [0010]    Preferably, the timing actuator comprises a first part and a second part, wherein the first part engages the second cam so that, in use, the second cam drives the reciprocating motion of the timing actuator, and wherein the second part is associated with the cutting arrangement so that, in use, the reciprocating motion of the timing actuator is transmitted to the cutting arrangement. The first part and the second part can be integral components of the timing actuator.  
           [0011]    Preferably, the first part is coupled to the second part. In this case, the first part and the second part may not necessarily be integral components of the timing actuator. Instead, the first part can be coupled to the second part by a mechanical, electrical, pneumatic, or hydraulic link, depending on the special requirements of the area of the jigsaw accommodating the timing actuator. Alternatively, the first part is coupled to the second part by at least one intermediate part. In this case, the at least one intermediate part may be a mechanical link comprising one component, again depending on the special requirements of the area of the jigsaw accommodating the timing actuator.  
           [0012]    Preferably, the first part is a cam follower contacting the second cam. A cam follower is a simple component that abuts the eccentric peripheral face of the second cam to perform reciprocating rectilinear motion or oscillating motion when the second cam rotates. Preferably, the cam follower is pivotally mounted on a cam follower axis for oscillating motion.  
           [0013]    Preferably, the second part is a bearing arranged to bear against the cutting arrangement. In this case, the reciprocating motion of the timing actuator is transmitted to the cutting arrangement through abutment between the bearing and the cutting arrangement. The bearing can abut any part of the cutting arrangement. The bearing may be a sliding plate or bracket. Preferably, the bearing is pivotally mounted on a bearing support axis for oscillating motion.  
           [0014]    Preferably, the bearing comprises a wheel rotatably mounted on the bearing. In this case, it is the wheel that abuts the cutting arrangement because it can rotate to follow the reciprocating motion of the cutting arrangement, thereby reducing friction between the bearing and the cutting arrangement.  
           [0015]    Preferably, the cutting arrangement comprises a shaft and a saw blade clamped to the shaft. In this case, a used saw blade can be replaced at the end of its life span.  
           [0016]    Preferably, the reciprocating motion of the counterweight is 180° out of phase with the reciprocating motion of the cutting arrangement. This has the advantage that the inertial forces created by the reciprocating motion of the cutting arrangement can be substantially counteracted by the reciprocating motion of the counterweight.  
           [0017]    Preferably, the reciprocating saw is a jigsaw supported by a shoe. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    A preferred embodiment of the present invention will now be described by way of example only, with reference to the accompanying illustrative drawings in which:  
         [0019]    [0019]FIG. 2 shows a perspective view from one side of an electric motor and a drive mechanism of a reciprocating saw;  
         [0020]    [0020]FIG. 3 shows an exploded perspective view of the drive mechanism, from the opposite side as shown in FIG. 2;  
         [0021]    [0021]FIG. 4 shows an exploded perspective view of the drive mechanism, from the same side as shown in FIG. 2;  
         [0022]    [0022]FIG. 5 shows a side elevational view of the electric motor and the drive mechanism inside one half of a housing of the reciprocating saw, from the same side as shown in FIG. 2; and  
         [0023]    [0023]FIG. 6 shows a cross-sectional view of the drive mechanism along the section X-X. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]    Referring to FIGS.  2  to  6 , the reciprocating saw  2  comprises a housing  4  and a shoe  6  which supports the housing  4  and, in use, rests upon a workpiece (not shown). The housing  4  encapsulates an electric motor  8  and a drive mechanism  10 . The electric motor  8  is coupled to a drive gear  12 . The drive mechanism  10  comprises a rotatable drive shaft  14  with a drive shaft axis  16 , a timing actuator  18 , a counterweight  20 , a cutting arrangement  22 , and a driven gear  24  mounted on the drive shaft  14 . The drive gear  12  intermeshes with the driven gear  24  so that, in use, the electric motor  8  rotates the drive shaft  14  about the drive shaft axis  16 .  
         [0025]    The majority of the cutting arrangement  22  is mounted within the housing  4 . In use, the cutting arrangement  22  performs vertical rectilinear reciprocating motion in the direction of the double arrow B. The cutting arrangement  22  comprises a shaft  26  and a saw blade  28  releaseably clamped to the shaft  26  by a clamp  30 . The plane of the saw blade  28  is parallel to the cutting direction A and the saw blade  28  projects downward through a recess  32  in the shoe  6 . The saw blade  28  has a row of saw teeth  34  on a leading side oriented in the cutting direction A. The saw blade  28  has a planar face  36  on a trailing side orientated counter to the cutting direction.  
         [0026]    A crank plate  38  is mounted upon the drive shaft  14  and is adjacent the cutting arrangement  22 . A pin  40  projects from a side of the crank plate  38  facing the cutting arrangement  22 . The pin  40  is rotatable with respect to the crank plate  38 . The pin  40  is eccentric with respect to the drive shaft axis  16 . The shaft  26  of the cutting arrangement  22  has a transverse elongate shaft aperture  42  which surrounds the pin  40  in the manner of the yoke. In use, the pin  40  rotates about the drive shaft axis  16 . The horizontal component of its rotational motion is accommodated by the shaft aperture  42  which permits side to side motion of the pin  40  within its confines without causing side to side motion of the cutting arrangement  22 . Whereas the vertical component of this rotational movement is not accommodated by the shaft aperture  42  and instead the cutting arrangement  22  moves vertically up and down in time with the vertical up and down motion of the pin  40 . This is a simple means of translating the rotational movement of the drive shaft  14  into reciprocating rectilinear motion of the cutting arrangement  22 .  
         [0027]    The counterweight  20  is arranged upon two guide pins  44 ,  46  each penetrating a respective guide slot  48 ,  50  in the counterweight  20 . The guide pins  44 ,  46  are fixed in relation to the jigsaw housing  4 . The guide slots  48 ,  50  are elongate and are orientated to permit the counterweight  20  to perform vertical rectilinear reciprocating motion, also the direction of the double arrow B. A first cam  52  having a cylindrical peripheral face  54  is mounted upon the drive shaft  14 . The cylindrical peripheral face  54  of the first cam  52  is arranged eccentrically with respect to the drive shaft axis  16 . As is most clearly shown in FIG. 6, the axial length and D of the first cam  52  corresponds to the thickness D of the counterweight  20 . A transverse oval shape to aperture  56  in the counterweight  20  surrounds the first cam  52  in the manner of the yoke. In use, the first cam  52  rotates about the drive shaft axis  16  and this rotational motion is translated into the reciprocating motion of the counterweight  20  in the same way as rotational motion of the pin  40  is translated into the reciprocating motion of the cutting arrangement  22 , as described above. The counterweight  20  is provided to counteract the inertial forces created by the reciprocating motion of the cutting arrangement  22 .  
         [0028]    The pin  40  is arranged on the crank plate  38  such that its eccentricity, with respect to the drive shaft axis  16 , is opposed to the eccentricity of the first cam  52 , with respect to the drive shaft axis  16 . I.e. the centres of the pin  40  and the first cam  52  are offset by 180° about the drive shaft axis  16 , so that the centres are located on diametrically opposed sides of the drive shaft axis  16 . Accordingly, the reciprocating motion of the counterweight  20  is 180° out of phase with the reciprocating motion of the cutting arrangement  22 . The effect of the reciprocating motion of the counterweight  20  counteracting the inertial forces created by the reciprocating motion of the cutting arrangement  22  is thereby achieved.  
         [0029]    A second cam  58  is integral with the driven gear  24  and is located between the driven gear  24  and the counterweight  20 . The second cam  58  has a cylindrical outer peripheral face  60  which is arranged eccentrically with respect to the drive shaft axis  16 . As is most clearly shown by FIG. 6, the second cam  58  has an axial thickness F.  
         [0030]    The timing actuator  18  comprises a sickle shape cam follower  62 , a link member  64 , a wheel  66 , and a wheel support  68 . The wheel  66  is rotatably mounted on a lower arm  70  of the wheel support  68 .  
         [0031]    The cam follower  62  has a finger  72  located at one end. The finger  72  abuts the outer peripheral face  60  of the second cam  58 . The cam follower  62  is pivotally mounted on a cam follower axis  44  at an end a remote from the finger  72 . In the present embodiment the cam follower axis is also the guide pin  44 , although this need not be the case and alternative arrangements are possible. The cam follower axis  44  is fixed in relation to the jigsaw housing  4  and extends substantially in the cutting direction A. Like the second cam  58 , the cam follower  62  is located between the driven gear  24  and the counterweight  20 . The cam follower  62  has a thickness in the direction of the drive shaft axis  16  which is marginally inferior to thickness F. This allows movement of the cam follower  62  relative to the counterweight  20  and the driven gear  24  without frictional contact therebetween. The cam follower  62  has a foot  76  adjacent the cam follower axis  44 .  
         [0032]    The link member  64  is guided for reciprocating rectilinear motion and by a bracket (not shown) fixed in relation to the jigsaw housing  4 . An end  80  of the link member  64  abuts the foot  76  of the cam follower  62 . An opposite end  82  of the link member  64  abuts an upper arm  84  of the wheel support  68 . Accordingly, the link member  64  forms a mechanical link between the cam follower  62  and the wheel support  68 .  
         [0033]    The wheel support  68  is pivotally mounted on a wheel support axis  86  of approximately the midpoint of the wheel support  68 . The wheel support axis  86  is fixed with respect to the jigsaw housing  4  and is substantially perpendicular to the cutting direction A. The wheel support  68  is arranged to perform oscillating motion about the wheel support axis  86  in the direction of the curved double arrow C.  
         [0034]    The motion of the timing actuator  18  is controlled by the second cam  58  in the following manner. In use, the drive shaft  14  rotates at the second cam  58 . The finger  72  follows the path of the outer peripheral face  60  of the second cam  58  causing the cam follower  62  to oscillate about the cam follower axis  44 . In turn, the foot  76  acts against the link member  64  such that the oscillating motion of the cam follower  62  is translated into the reciprocating rectilinear motion of the link member  64 . Next, the link member  64  acts upon the upper arm  84  of the wheel support  68  such that the rectilinear reciprocating motion of the link member  64  is translated into oscillating motion of the wheel support  68  about the wheel support axis  86  in the direction of the curved double arrow C.  
         [0035]    As described above, the cutting arrangement  22  performs a reciprocating motion in the direction of the double arrow B. The saw blade teeth  34  are arranged to cut the workpiece when the cutting arrangement  22  travels in the upward direction. Conversely, the saw blade teeth  34  glide past the workpiece substantially without cutting the workpiece when the cutting arrangement  22  travels in the downward direction. Frictional force between the saw blade  28  and the workpiece is greatest when the saw blade teeth  34  are cutting the workpiece. The cutting frictional force tends to push the saw blade  28  away from the workpiece and in the opposite sense to the cutting direction A. This is undesirable because movement of the saw blade  28  away from the workpiece diminishes the cutting action of the saw blade teeth  34 .  
         [0036]    The purpose of the timing actuator  18  is to counteract the cutting frictional force so that good cutting action of the saw blade teeth  34  is maintained. The wheel  66  bears against the trailing planar face  36  of the saw blade  28  and rotates as the saw blade  28  follows the reciprocating motion of the cutting arrangement  22 . The wheel  66  also oscillates with the wheel support  68  under the control of the second cam  58 . The contact between the wheel  66  and the saw blade  28  causes the oscillating motion of the wheel support  68  to be superimposed on the reciprocating motion of the cutting arrangement  22 . This occurs in the following manner. In use, the cutting arrangement  22  moves downwardly and the wheel support  68  oscillates counter to the cutting direction A so that the saw blade  28  cam glide past the workpiece. Conversely, as the cutting arrangement  22  moves upwardly the wheel support  68  oscillates with the cutting direction A to counteract the frictional force acting against the saw blade  28  in the opposite sense to the cutting direction A.