Abstract:
A power tool comprising a housing displaceably mounted to a foot plate, a blade, a motor, an electric switch and an interlock for controlling movement of the housing with respect to the foot plate. The interlock has a control button mounted to one of the housing or foot plate which cooperates with a channel fixed in relation to the other of the housing or foot plate. The control button can select between a plunge cut mode wherein the blade can protrude below the foot plate, or a blade change mode wherein the blade is prepared for a blade change operation. The channel comprises a major leg in communication with a minor leg. Cooperation between the control button and the major leg selects the plunge cut mode. Cooperation between the control button and the minor leg selects the blade change mode. The interlock may also comprise a housing lock and a spindle lock.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a power tool with a motor housing movably mounted on a foot plate. The power tool is normally biased by a plunge spring into a “parked” position where the housing is retracted away from the foot plate. The power tool&#39;s electric motor is connected to a power source via an on-off switch activated by a switch button. 
       BACKGROUND OF THE INVENTION 
       [0002]    If the power tool is a plunge-cut circular saw its circular saw blade is shielded by a guard, except for when the housing is pushed towards the foot plate so that the saw blade protrudes through an aperture in the foot plate to perform a cutting operation. The housing and saw blade of a plunge-cut circular saw move relative to the foot plate about a pivot connecting the housing and foot plate. 
         [0003]    If the power tool is a router it normally does not have a guard because its router bit is at least partially shielded by a pair of support columns, but the router blade does protrude through an aperture in the foot plate to perform a cutting operation when the housing is pushed towards the foot plate. The housing and router blade move relative to the foot plate in a straight line path guided by the pair of support columns which connect the housing and foot plate. 
         [0004]    At the point of maximum protrusion of the circular saw blade, or the router bit, as the case may be, through the footplate, the housing is said to be in the “plunged” position. 
         [0005]    An interlock may be used to control the extent to which the housing can move towards the foot plate and the conditions under which such movement occurs, like, for example, the isolation or de-isolation of the on-off switch, or locking or unlocking of the housing&#39;s movement or the blade&#39;s rotation. 
         [0006]    FIG. 3 of patent publication DE 40 23 101 discloses an embodiment of a plunge saw comprising a tool housing displaceably mounted to a foot plate, a rotatable blade the rotational axis of which is fixed in relation to the housing, an electric motor for driving the blade, an electric switch manually operable by a switch button to energize the motor; and an interlock for controlling movement of the housing with respect to the foot plate. The interlock has a manually operable control button mounted to the housing and the control button cooperates with a side of a rail which is fixed in relation to the foot plate. This enables the control button to select between a plunge cut mode whereby movement of the housing is limited to between a parked position whereat the blade is entirely above the foot plate and a plunged position whereat the blade protrudes below the foot plate, or a blade change mode whereby movement of the housing is limited to between the parked position and a “blade change” position whereat the blade is prepared for a blade change operation. 
         [0007]    When blade change mode is selected, the control button maintains isolation of the on-off switch by blocking manual movement of the switch button. This prevents energization of the motor. The housing is moveable into the blade change position. However, if ever the housing is accidentally released it naturally returns to the parked position under the bias of a plunge spring. The control button adopts a default mode which prevents the housing from leaving the parked position. The user must take care to maintain pressure on the housing otherwise blade change mode is automatically de-selected. It can be re-selected, but this requires repetition of a two-stage process which may require two hands: one hand to operate the control button and another hand to push the housing toward the foot plate. No hand is free to steady the plunge saw. 
         [0008]    When plunge cut mode is selected, the control button de-isolates the on-off switch by unblocking movement of the switch button. Subsequent manual movement of the switch button energizes the motor and latches the switch button behind the control button to hold the latter in situ. The housing can move between the parked and plunge cut positions. However, if ever the switch button is accidentally released it naturally de-energizes the motor and unlatches the control button. The blade stops rotating and, if the control button is not manually held in situ, the control button adopts either the blade change mode, or, if the housing hasn&#39;t yet descended towards the plunged position, a default mode which prevents from leaving the parked position. Again, the user must take care to maintain pressure on the switch button otherwise plunge cut mode is automatically de-selected. It can be re-selected, but, again, this requires a two-stage process which may require two hands: one hand to operate the control button and another hand to operate the switch button. No hand is free to steady the plunge saw or a work piece upon which the plunge saw is performing a cutting operation. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    The object of the present invention is to provide a power tool with an improved interlock for controlling movement of the housing towards the foot plate and the conditions under which such movement occurs. 
         [0010]    This object is achieved according to the present invention which is a power tool comprising a tool housing displaceably or pivotably mounted to a foot plate, a rotatable blade the rotational axis of which is fixed in relation to the housing, an electric motor for driving the blade, an electric switch manually operable by a switch button to energize the motor, and an interlock for controlling movement of the housing with respect to the foot plate, wherein the interlock has a manually operable control button mounted to one of the housing or foot plate and the control button cooperates with a channel fixed in relation to the other of the housing or foot plate to enable the control button to select between a plunge cut mode whereby movement of the housing is limited to between a parked position whereat the blade is entirely above the foot plate and a plunged position whereat the blade protrudes below the foot plate, or a blade change mode whereby movement of the housing is limited to between the parked position and a blade change position whereat the blade is prepared for a blade change operation, characterized in that the channel comprises a major leg and a minor leg in communication with the major leg, wherein cooperation between the control button and the major leg selects the plunge cut mode and cooperation between the control button and the minor leg selects the blade change mode. 
         [0011]    Cooperation between the control button and the channel is a basic feature of the present invention. This cooperation may be directly between part of the control button and the channel, or via an intermediary, like, for example, a linkage or an arm. The blade may be any power tool cutting implement, like, for example, a circular saw blade or a router bit. 
         [0012]    Typically, the channel is in part of a body fixed in relation to the foot plate while the control button is mounted to the housing. This has the advantage that the same hand that operates the control button to select plunge or blade change modes can also grasp the handle and push the housing towards the foot plate. However, the location of the control button and the groove can be reversed. 
         [0013]    The channel may be a closed groove in a body fixed in relation to the housing or foot plate. This has the advantage that cooperation between control button and channel is maintained by the circumscribing nature of a closed groove. The channel need not be closed and could, instead, be relatively open like, for example, a path around the outer sides of a rail. The minor leg could be one side of the rail and the major leg could be the opposing side of the rail. The rail option works properly if the control button is biased into contact with the rail at all times, and provided that the rail is shaped to define the points at which the channel terminates. The termination points could be protruding pockets adapted to arrest the control button as it slides against the rail. Any combination of features that maintains cooperation between the channel and control button throughout relative movement between the housing and foot plate is acceptable. Once the mode of operation has been selected the limits of movement of the housing relative to the foot plate are positively defined until another mode of operation is positively selected. Thus, accidental release of the switch button does not unintentionally reverse the mode selection. This allows the user to concentrate on correctly manipulating the housing&#39;s movement in relation to the foot plate and steadying the power tool. 
         [0014]    Preferably the major leg terminates at a point closer to the foot plate than the point at which the minor leg terminates. As a result, a blade change position can be located where the blade does not protrude below foot plate. If the power tool has have a blade guard, or other protection means, the blade remains shielded unless blade change mode is positively selected. 
         [0015]    Preferably, the channel has a face substantially opposing the direction in which the housing moves towards the foot plate such that cooperation between the control button and the face selects a parked mode whereby movement of the housing from the parked position is blocked. The parked mode is beneficial because it allows the control button to immobilize movement between the housing and the foot plate when the power tool is not in operation. 
         [0016]    Preferably, the control button is biased to cooperate with the face. This is in addition to the natural bias of the housing towards the parked position. As such, the parked mode is preferred and, in the interests of safety, the plunge cut or blade change modes require positive selection on the part of the user before the housing can be manually moved towards the foot plate. 
         [0017]    Preferably, the face has a detent at a location where the face communicates with the rest of the channel. The detent, or detents, can be a small protrusion at a point, or points, of entry to the face&#39;s portion of the channel. The protrusion can be elastically deformable. The detent acts as a gateway to the face which must be overcome prior to cooperation between the control button and the face. As such, the control button is biased to cooperate with the face and this maintains the parked mode until one of the plunge cut or blade change modes is positive selected. Conversely, the detent, or detents, prevents unintentional selection of the parked mode. 
         [0018]    Preferably, the face is part of a web in the channel and wherein the web connects the major and minor legs at a location where the housing is in the parked position. This arrangement forms an inverted U-shaped channel. This is a simple shape of channel which is well suited for the purposes of the interlock. 
         [0019]    Preferably, the control button cooperates with the channel via an arm. A relatively small control button can be employed which is equipped with a relatively long reach by virtue of the arm. Thus, a compact control button need not be located aside the channel. This permits greater flexibility in the design of the interlock. 
         [0020]    Preferably, the control button pivots about a control button axis fixed in relation to one of the housing or foot plate, wherein the arm is pivotally coupled at one end to the control button and pivotally connected part way along its length to an arm axis fixed in relation to the control button axis. Pivotal movement of the control button is a simple and conventional means of varying its position. Coupling the arm at one end to the control button translates this movement to the arm. The amplitude and direction of the arm&#39;s movement depends on the position of its pivotal connection to the arm&#39;s axis. For example, the closer the arm axis is to the control button the greater the amplitude of the arm&#39;s pivotal movement. As such, this arrangement is a simple and flexible means of facilitating cooperation between the control button and the channel. 
         [0021]    Preferably, the arm has a control pin at a free end opposite the end of the arm coupled to the control button, wherein the control pin cooperates with the channel. If the channel is a closed groove then the pin can slide therein. If the channel is a path around the outer sides of a rail then the pin can slide against the rail. The pin is an adaptable means of cooperating with the channel. 
         [0022]    Preferably, the control button is moveable into the path of the switch button, preventing actuation thereof, when the blade change mode is selected. This ensures that the motor cannot be energized, and the blade rotated, if the user has decided to perform a blade change operation. Blade change is almost impossible if the blade rotates. Optionally, the control button is also moveable into the path of the switch button, preventing actuation thereof, when the parked mode is selected. This ensures that the motor can only be energized, and the blade rotated, when plunge cut mode is selected. 
         [0023]    Preferably the interlock comprises a manually moveable slider mounted to one of the housing or foot plate, wherein the slider is biased by a slider spring into a blocking position such that the slider blocks selection of the blade change mode by the control button. This is an optional feature which complements the bias of the control button in preventing accidental selection of the blade change mode. 
         [0024]    Preferably, the interlock comprises a housing lock for locking the housing in the blade change position and a manually displaceable and rotatable control shaft for operating the housing lock, wherein the blocking position of the slider blocks operation of the control shaft. The housing lock is only used when the blade change position has been adopted and a blade change operation is desired. The housing lock secures the housing in relation to the foot plate at a location specially adapted for blade detachment i.e. the blade change position of the housing. The housing lock is deliberately inoperable until the slider is moved from its blocking position. 
         [0025]    Preferably, the housing lock comprises a slideable dowel mounted to one of the housing or foot plate and coupled to the control shaft, and a recess fixed in relation to the other of the housing or foot plate, wherein movement of the plate from the blocking position enables (a) selection of the blade change mode, (b)movement of the housing into the blade change position such that the dowel is aligned with the recess, and (c) one of rotation or displacement of the control shaft which causes engagement of the dowel with the recess thereby operating the housing lock. 
         [0026]    Preferably, the dowel is biased into contact with a cam fixed to the control shaft, and rotatable therewith, whereby the housing lock is operable by rotation of the control shaft. 
         [0027]    Preferably, the slider is prevented from leaving the blocking position when the plunge cut mode is selected by the control button. This isolates the housing lock by shielding the dowel&#39;s leading end with the slider. Risk of overriding proper functioning of the interlock by deliberately jamming the dowel against the housing or the foot plate is thereby prevented. 
         [0028]    Preferably, the slider has a manually operable push button which blocks operation of the control shaft when the slider is the blocking position. This also isolates the housing lock and provides further means for ensuring proper functioning of the interlock. 
         [0029]    Preferably, the control button is prevented from selecting the plunge cut mode when the slider moves from the blocking position. This provides still further means for ensuring proper functioning of the interlock. 
         [0030]    Preferably, the interlock comprises a spindle lock for locking the blade against rotation, wherein the spindle lock is operable by the control shaft after operation of the housing lock. The spindle lock is only used when the blade change position has been adopted and a blade change operation is desired. The spindle lock immobilizes the blade against rotation to enable efficient detachment. The spindle lock is inoperable prior to operation of the housing lock thereby ensuring that energization of the motor is prevented and that the blade and housing are locked in situ. When a blade change operation is complete, the housing and spindle locks can be released and the housing and the blade can return, through upward bias of a plunge spring, to the parked position. 
         [0031]    Preferably the one of rotation or displacement of the control shaft overcomes a barrier to operation of the spindle lock. In other words, spindle lock is inoperable until the housing lock has been operated. More preferably, the barrier is a wall that physically blocks operation of the spindle lock by the other of rotation or displacement of the control shaft. 
         [0032]    Preferably, the spindle lock comprises a slideable bolt mounted to one of the housing or foot plate and coupled to the control shaft, and a pocket in a spindle arrangement coupled to the blade, wherein the other of rotation or displacement of the control shaft causes engagement between the bolt and the pocket thereby operating of the spindle lock. The term spindle arrangement is used to describe the group of components used to transmit the motor&#39;s driving rotation to the blade, like, for example, a motor spindle, a gear box, a main spindle, and a coupling. The spindle arrangement has a spindle plate which typically comprises the pocket. Engagement of the bolt with the pocket immobilizes the whole spindle arrangement, as well as the motor and the blade. 
         [0033]    Preferably, the power tool is a plunge-cut circular saw, wherein the tool housing is pivotably mounted to the foot plate and the blade is a circular saw blade, wherein the blade is shielded by a guard while the housing is in the blade change position or the parked position. Alternatively, the power tool is a router, wherein the tool housing is displaceably mounted to the foot plate and the blade is a router bit. 
         [0034]    The foot plate is for use with a guide positioned upon a work-piece. The guide has a rail extending in the longitudinal direction of the guide on its upper side. In the case of a plunge-cut circular saw, the footplate has on its lower side channel for engagement of the rail, so that the tool may be positioned on the guide and guided along the rail when the rail engages the channel. In the case of a router, the footplate may be coupled to a guide member with a channel for engagement of the rail. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0035]    An embodiment of the invention will be described with reference to the drawings, in which: 
           [0036]      FIG. 1  is a perspective view of a plunge-cut circular saw upon a guide; 
           [0037]      FIG. 2  is a perspective view of the saw with its blade exposed at one side; 
           [0038]      FIG. 3  is a rear end view of the saw and the guide; 
           [0039]      FIG. 4  is a side view of the saw; 
           [0040]      FIGS. 5A &amp; 5B  are schematic views of the saw with its blade performing a straight cut and a bevel cut; 
           [0041]      FIG. 6  is a schematic view of the saw with its blade moving between a “parked position” and a “blade change position” or a “plunged position”; 
           [0042]      FIG. 7  is a perspective view of part of a bevel hinge and a plunge pivot; 
           [0043]      FIG. 8  is a perspective view of the saw without the parts shown in  FIG. 7 ; 
           [0044]      FIG. 9A  is a perspective view of the saw showing its depth stop mechanism; 
           [0045]      FIG. 9B  shows a main and alternative scale in detail; 
           [0046]      FIG. 10  is a perspective view of the saw without blade and guard; 
           [0047]      FIG. 11  is a perspective view of the saw without the blade, guard, housing and half a handle; 
           [0048]      FIG. 12  is a side view of the saw without the blade, housing and handles; 
           [0049]      FIG. 13  is a perspective view of the saw in  FIG. 12 ; 
           [0050]      FIG. 14  is a perspective view of the saw in  FIG. 12  turned through 90° in relation to  FIG. 13 ; 
           [0051]      FIGS. 15A to 15C  show different operation stages of an interlock for controlling blade movement; 
           [0052]      FIGS. 16A to 16C  show detail of the interlock, in particular a spindle lock and a housing lock; 
           [0053]      FIG. 17A to 17D  show detail of the interlock, in particular an alternative slider and push button. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0054]    Referring to  FIG. 1 to 4 , a hand-held tool in the form of a plunge-cut circular saw  10  is designed to slide along a guide  12 . The saw comprises a foot plate  14 , a blade guard  16  and a housing  18 . The housing is pivotally mounted to the guard at a bevel hinge  20 . The guard is pivotally mounted to the foot plate at a plunge pivot  22 . The housing has an electric motor  24  connected, via and on-off switch  26 , to a power source (not shown). The manually operable on-off switch has a switch button  30 . The motor&#39;s driving rotation is transmitted via a motor spindle  31 , a gear box, a main spindle, and a coupling  32 . These transmission components are collectively called a spindle arrangement  33 . The motor is coupled to the spindle arrangement via the motor spindle. A circular saw blade  34  is coupled to the spindle arrangement via the coupling. 
         [0055]    Referring to  FIGS. 5A and 5B , the bevel hinge  20  is so-called because it enables bevel cutting by the blade. This is when the housing  18 , blade  34  and guard  16  pivot relative to the foot plate  14  about an axis X-X which is parallel to the direction of cut A of the blade. This allows the inclination of the blade to be varied between a plane orthogonal to the foot plate, to perform so-called “straight cuts”, (bevel angle α=0°) and any predetermined oblique plane, to perform so-called “bevel cuts” (bevel angle α greater than 0° up to 47°). 
         [0056]    Referring to  FIG. 6 , the plunge pivot  22  is so-called because it enables the blade  34  to descend, or “plunge”, into a work piece  28 . The housing and blade can pivot relative to the foot plate and the guard about an axis Y-Y perpendicular to axis X-X. The housing is biased to pivot upwards and away from the foot plate by a plunge spring  36  so that the blade is normally retracted within the guard i.e. the blade is in the “parked” position  34   a  where it is shielded by the guard. The housing may be manually moved, via a front handle  38  and a rear handle  40 , against the bias of the plunge spring to pivot downwards and towards the foot plate. The blade protrudes through an aperture  42  on the underside of the guard and the foot plate i.e. the blade is in the “plunged” position  34   b  where it is able to cut into the work piece. Alternatively, the blade may stop part way between parked and plunged positions at a “blade change” position  34   c.    
         [0057]    In use, the blade rotates in the direction of arrow W. Viewed from one end, the centre of the blade follows an arc-shaped path BP as the blade pivots about the plunge pivot between parked and plunged positions. The housing also follows an arc-shaped path corresponding to path BP. The exact length of path BP, and thus the depth of cut of the blade in the plunged position, is determined by a depth-stop mechanism  44 . The bevel hinge, the plunge pivot and the depth-stop mechanism are discussed in more detail below. 
         [0058]    Returning to  FIG. 1 , the guide has a straight rail  46  on its upper surface and the foot plate has a correspondingly-shaped middle channel  48  in its lower surface. The rail is accommodated within the middle channel when the foot plate is on the guide. The saw can be pushed forwardly, again via the handles, so that the foot plate slides along the guide in a straight line determined by the rail. The blade cuts a straight line in the work piece in the direction of cut A. The guide is made of strong material, like, for example, metal. Sliding motion is assisted by low-friction strips on the upper surface of the guide. 
         [0059]    Returning to  FIG. 4 , the plunge pivot  22  is a double swing-arm hinge like the one disclosed by DE 196 35 527. The plunge pivot comprises a lower arm  52  and an upper arm  54 . Both arms link the housing to the rear of the guard. The lower arm is pivotally mounted to the guard at a point directly beneath where the upper arm is pivotally mounted to the guard. The two arms are the same length. The lower arm is pivotally mounted to the body at a point directly beneath where the upper arm is pivotally mounted to the body. Thus, the orientation of the housing  18  in relation to the foot plate  14  remains constant when it moves towards the base plate. 
         [0060]    Referring to  FIGS. 3 ,  5 A and  5 B, the bevel hinge  20  has a virtual pivot axis X-X like the one disclosed by EP0247417. The virtual pivot axis is in the inner blade plane  55  and is located a distance Z below the foot plate equal to the thickness of the guide. This ensures that the blade engagement line, which is determined by the intersection of the inner blade plane with the very top of the work piece  28 , and which coincides with the virtual pivot axis, maintains its position with respect to the work piece, whether straight or bevel cuts are performed. 
         [0061]    Referring to  FIGS. 7 and 8 , the bevel hinge  20  comprises two hinges  56   a,    56   b.  The hinges oppose each other and are set apart in the direction of cut A. Each hinge comprises an outer hinge part  58   a,    58   b  fixed to the foot plate  14  and an inner hinge part  60   a,    60   b  which is integral with the guard  16 . Each outer hinge part has a pair of circular arc grooves  62   a,    62   b,  the centre points of which coincide with the virtual pivot axis. Each inner hinge part has a pair of arcuate tongues  64   a,    64   b  accommodated within the circular arc grooves  62   a,    62   b  in which they slide, with centre points which coincide with the virtual pivot axis. The arc of the bevel hinge  20  sweeps from straight cut to maximum bevel cut. 
         [0062]    The inner and outer hinge parts have facing circular arc slots  66 , the centre points of which also coincide with the virtual pivot axis. Each hinge  56   a,    56   b  has a bolt and thumb screw arrangement  68  through its circular arc slots. Manual loosening of both thumb screws unfastens the bolts so that the tongues are free to slide within the grooves and allow the blade to pivot about the virtual pivot point, and vice versa. 
         [0063]    Referring to  FIGS. 7 and 9A , the depth stop mechanism  44  comprises a circular arc groove  70  in a curved molding  72  fixed to the guard, a stop assembly  74  slidably mounted in the circular arc groove and a finger  76  protruding from the housing. 
         [0064]    The stop assembly  74  has a flange (not shown) on the interior of the molding and a manually operable thumb screw  80  passing through the circular arc groove  70  to engage a threaded hole in the flange. The bolt also passes through a carriage with a needle  84  located between the thumb screw and the molding. Loosening of the thumb screw unfastens the flange from the interior of the molding until the stop assembly is free to slide within the groove, and vice versa. The flange has a finger  86  protruding though an elongate slit  88  on the inner curved side of the molding. The flange&#39;s finger  86  is located within in the arcuate path of the housing&#39;s finger  76  as the blade and housing move from the parked position to the plunged position. Thus, the stop assembly determines the depth to which the blade can cut a work piece because the flange&#39;s finger limits the depth to which the housing can be plunged. 
         [0065]    Referring to  FIG. 9B , the depth stop mechanism  44  further comprises a main scale  90  on the outer curved side of the molding. The needle  84  points at the main scale. Blades used on the saw always have the same diameter and the guide rail&#39;s thickness does not change. Thus, the needle&#39;s position on the scale main indicates the maximum blade&#39;s depth into a work piece as permitted by the depth stop mechanism i.e. the blade&#39;s depth-of-cut when the housing&#39;s finger abuts the flange&#39;s finger and when movement of the blade any further into the plunged position is arrested. For example, when the needle points to 0 the blade cannot cut into the work piece. When the needle points to the number  45  the blade can cut up to a maximum of 45 mm deep into the work piece. 
         [0066]    Sometimes, a plunge-cut circular saw is used as a regular circular saw i.e. the foot plate is placed directly upon a work piece, instead of on the guide. In this case, the blade will cut into the workpiece sooner than it would do if the guide was present. Thus, an alternative scale  94  accompanies the main scale on the outer curved side of the molding. The needle also points at the alternative scale. The alternative scale is shifted upward by an amount equivalent to the thickness of the guide so that needle arrives at the 0 mark sooner than it does for the main scale. This compensates for earlier cutting into the work piece as the blade is moved into the plunged position. 
         [0067]    The arcuate path of the housing, and therefore the path of the housing&#39;s finger, traveling between parked and plunged positions, are determined by the plunge pivot  22  and thus correspond to the arc-shaped path BP of the coupling  32 . Initially, angular travel along this path results in relatively shallow increase in the blade&#39;s depth of cut because it is at the top of its arc. Naturally, the rate of increase in the blade&#39;s depth of cut accelerates as this path approaches the side of its arc. This is reflected in both the main scale and the alternative scale where the graduations are increasingly compressed as the path of the housing&#39;s finger approaches the side of its arc. 
         [0068]    An interlock  98  controls the conditions under which the housing moves about the plunge pivot and relative to the foot plate. The interlock does not interact with the bevel hinge. 
         [0069]    Referring to  FIGS. 15A to 15C , the interlock comprises a control button  100  which can be pivoted clockwise about a control button axis  102  into a “blade change” mode position  104 , or, alternatively, counter-clockwise about the control button axis into a “plunge cut” mode position  106 . When no force is exerted, either clockwise or counter-clockwise, the control button is biased to return to a central, or “parked”, mode position  108  by a primary spring  110 . 
         [0070]    Referring to  FIGS. 10 to 14 , the interlock comprises a control arm  112  which is pivotally coupled at one end to the control button and pivotally mounted in the middle to an arm axis  114 . The control button axis is fixed to the rear handle  40  of the housing  18 . The arm axis is fixed to another part of the housing. The arm is in two portions: an upper arm portion  112   a  between the control button and the arm axis, and a lower arm portion  112   b  between the arm axis and a control pin  116  at the free end of the arm. 
         [0071]    The switch button  30  is pivotally mounted upon a switch axis  118  fixed to the rear handle  40 . The switch is biased open to disconnect the power source from the motor  24 . When the switch button is depressed it pivots counter-clockwise in direction S and closes the on/off switch  26  to connect the power source to the motor, as is shown in best  FIG. 12 . When the switch button is released it pivots clockwise, the switch re-opens and the motor is disconnected from the power source. 
         [0072]    The switch button has a finger  120  which pivots about the switch axis  118  with the switch button. In certain circumstances, the control button blocks the finger&#39;s path of pivotal movement so that the switch button cannot be depressed and the motor cannot be energized by the off/off switch. For example, the switch button cannot be depressed when the control button is in the blade change mode position  104 , as is shown in  FIG. 15C . 
         [0073]    However, when the control button returns to the parked mode position  108 , as is shown in  FIG. 15A , or the control button has been pivoted counter-clockwise into the plunge cut mode position  106 , as is shown in  FIG. 15B , the control button does not block movement of the finger  120 . Thus, the switch button can be depressed and the motor can be energized. 
         [0074]    Optionally, a minor modification to the shape of the control button  100  can change this scenario so that the control button blocks the finger&#39;s path of pivotal movement in all but the plunge cut mode position. 
         [0075]    The tip of the pin  116  slides within an up-side-down U-shaped channel  122  formed in the side of the guard  16 . The pin  116  is part of the interlock and all but the tip of the pin is located inside the housing. The path selected for the pin  116  determines the extent to which the housing and the blade may move in relation to the guard and the foot plate. The channel includes three sections: a minor leg  122   a;  a major leg  122   b  and a web  122   c  joining the upper ends of the major and minor legs. 
         [0076]    The blade is normally in the parked position  34   a  and the control button  100  is in the parked mode position  108 , as is shown in  FIG. 15A , under the bias of the primary spring  110 . 
         [0077]    The control button can be pivoted counter-clockwise into the plunge cut mode position, as is shown in  FIG. 15B . The arm pivots about the arm axis in a clockwise direction and against the bias of the primary spring  110 . The pin leaves the web and enters the major leg  122   b  of the channel. The switch button can be depressed to energize the motor and rotate the blade. The blade can be moved from the parked position  34   a  (the pin at top end of the major leg) towards the plunged position  34   b  (the pin at bottom, or blind end, of the major leg) as the pin is freely moveable along the major leg of the channel. The blade and the housing move together, of course. 
         [0078]    When the downward force on the handles is relaxed, the blade returns to the parked position  34   a  under the bias of the plunge spring  36 . Release of the control button causes it to pivot clockwise into the parked mode position  108  under the bias of the primary spring  110 . The arm pivots counter-clockwise about the arm axis. The pin  116  leaves the top end of the major leg of the channel and enters the web. The blade cannot be accidentally moved from the parked position  34   a  as downward movement of the pin is blocked by a lower face  1220   c  of the web. 
         [0079]    Optionally, and as best shown in  FIGS. 15A to 15C , the lower face  1220   c  is slightly concave-shaped thus providing a small protrusion, or detent, at each point of entry to the web. The detent acts as a gateway to the face which must be overcome prior to cooperation between the pin  116  and the face. 
         [0080]    The control button  100  can also be pivoted clockwise into the blade change mode position  104 , as is shown in  FIG. 15C . The arm pivots counter-clockwise about the arm axis and against the bias of the primary spring. The pin  116  leaves the web and enters the minor leg of the channel. The blade can be moved from the parked position  34   a  (the pin at top end of the minor leg) towards the blade change position  34   c  (the pin at bottom, or blind end, of the minor leg) as the pin is freely moveable along the minor leg of the channel. 
         [0081]    Referring to  FIGS. 10 to 14 , the interlock comprises a slider  124  which is pivotally mounted upon a slider axis  126 . The slider is biased into a raised position (as shown) by a slider spring  128 . The slider is located in the housing. The slider axis is fixed to the housing. The slider has a manually operable push button  130  connected to the slider via a straight stem  132  protruding outside the housing. The slider has a nose  133  in abutment with the pin  116  when the slider is in the raised position and the control pin  116  is the web  122   c  of the U-shaped channel as is shown best in  FIG. 12 . Abutment between the nose and the pin blocks the latter from moving into the minor leg  122   a  of the channel and so it prevents unintentional pivoting of the control button into the blade change mode position  104 . 
         [0082]    The interlock comprises a rotatable slideable control shaft  134  with a manually operable control knob  142  protruding outside the housing. The control shaft is mounted to the housing. 
         [0083]    The interlock comprises a housing lock  138  formed by a slideable dowel  140  nested within the housing  18 . The dowel is biased away from the guard by a spring  141 . The path between the dowel and the guard is partially blocked by the slider when the slider is in a raised position. 
         [0084]    The control shaft  134  has a cam  144  in abutment with the dowel&#39;s trailing end  140   b.  The cam is shown in the  FIG. 16A  in a position where it does not push the dowel towards the guard. Manual clockwise rotation of the control knob through 90 degrees (when viewed from above, like in  FIG. 16C ) causes corresponding rotation of the cam which pushes the dowel towards the guard. However, this is only possible once the slider is in a lowered position where it does not block the path between the dowel and the guard. 
         [0085]    Returning to  FIG. 12 , the interlock comprises a spindle lock  146  formed by a slideable bolt  148  coupled to the control shaft. The control shaft, control knob and bolt are all upwardly biased by a control spring  150 . 
         [0086]    Returning to  FIGS. 16A to 16C , the control knob has a lever  156  under which a wall  158  on the housing is accommodated when the control knob and the control shaft are in the position shown in all the Figures. The wall blocks the control shaft from sliding downwards into a lowered position. 
         [0087]    A stop  160  on the housing limits counter-clockwise rotation of the control knob in the area of the wall. Clockwise rotation of the control knob through approximately 90 degrees leads to a wall-free area  161  where there is nothing to inhibit the control knob from being pushed downward causing corresponding downward movement of the control shaft  134 , the cam  144  and the bolt  148 . The cam is long enough in an axial direction to maintain abutment with the dowel&#39;s trailing end  140   b  during downward movement of he control shaft. 
         [0088]    A hook  162  is formed between the nose  133  and the stem  132  of the slider  124 . When the push button  130  moves the slider into a downward position the nose moves out of abutment with the pin. The pin is free to move into the minor leg  122   a  of the channel. This allows clockwise rotation of the control button into the blade change mode position  104  which causes the pin to enter the minor leg  122   a  of the channel and, at the same time, approach the mouth of the hook  162 . The push button is released and the slider  124  raises slightly under the bias of the slider spring  128  to capture the pin  116  within the hook. Meanwhile, the housing and the blade are free to move from the parked position to the blade change position where the dowel is aligned with a blind recess  168  in the guard. The slider has moved into a downward position which fully unblocks the path of the dowel toward the recess. 
         [0089]    Clockwise rotation of the control knob  142  and the cam  144  pushes the dowel&#39;s leading end  140   a  into the recess  168 , thereby locking the housing to the guard while the blade is in the blade change position. The housing lock is thus operated. The rotated control knob is above the wall-free area  161 . Downward movement of the control knob  142  causes the bolt  148  to engage a pocket  152  in a spindle plate  154  on the spindle arrangement  33 . The spindle arrangement is locked against rotation by the bolt for as long as downward pressure is applied to the control knob. The spindle lock is thus operated. A blade change operation can now be performed because the blade is locked against rotation. 
         [0090]    Meanwhile, the pin  116  cannot leave the hook, or the minor leg of the channel, because the slider is biased upwards by the slider spring  128 . The control button  100  is held in the blade change mode position  104  where it blocks any movement of the switch button  30 . The motor cannot be energized because the switch button cannot operate the on/off switch  26 . 
         [0091]    Release of downward pressure on the control knob  142  causes the cam  144  and the bolt  148  to return to a raised position under the bias of the control spring  150 . This initiates reversal of the above process as the bolt  148  disengages the pocket  152  to free the spindle plate  154 , the spindle arrangement  33  and the blade  34 . The spindle lock is no longer operated. 
         [0092]    Counter-clockwise rotation of the control knob  142  through 90 degrees until it abuts the wall  158  causes the cam to retreat from the dowel&#39;s trailing end  140   b.  The dowel retracts into the housing and its leading end  140   a  exits the recess  168  under the bias of the spring  141 . The housing  18  is no longer locked to the guard and the blade can leave the blade change position  34   c.  The housing lock is no longer operated. 
         [0093]    The pin  116  is free to move upwards from the blind end of the minor leg of the channel so that the blade can re-adopt the parked position  34   a.  The push button  130  must be moved down slightly against the bias of the slider spring  128  before the pin is released from the hook  162  and is free to leave the minor leg  122   a  of the channel. Once this has occurred, the control button  100  is free to pivot into the parked mode position  108  causing the pin to move into the web  122   c  of the channel. 
         [0094]    Referring to  FIGS. 17A to 17D , the interlock may comprise an alternative slider  1240 , an alternative push button  1300  and an alternative primary spring (not shown). Other features of the interlock, in particular the control button  100 , the control arm  112 , the control pin  116 , the housing lock  138  and a spindle lock  146  are unchanged. Like the slider  124  described above (hereafter “the original slider  124 ”), the alternative slider  1240  forms a link between the control button, control arm and pin, on the one hand, and the housing lock and spindle lock, on the other hand. 
         [0095]    The alternative slider has a different shape to the original slider. The alternative push button  1300  has also been re-shaped with a chamfer  1300   a  on its top surface. The alternative push button is connected to the alternative slider via the straight stem  132  protruding outside the housing. 
         [0096]    The alternative slider  1240  is mounted centrally upon the slider axis  126  and the stem  132  has been re-located to the alternative slider&#39;s rear end. The hook  162  is no longer formed between the nose  133  and the stem  132 . Instead, an alternative hook  1620  has been re-shaped and re-located below the nose. 
         [0097]    The alternative slider is biased, by the slider spring  128 , to tilt clockwise so that it rests in a substantially horizontal position such that the alternative push button is in a raised position, as is shown in  FIG. 17A . The body of the alternative slider prevents operation of the control shaft  134  and the housing lock by shielding the leading end  140   a  of the dowel  140 . The nose  133  blocks the pin  116  from entering the minor leg  122   a.  This prevents unintentional selection of the blade change mode by the control button  100 . 
         [0098]    Like the primary spring  110  described above (hereinafter the “original primary spring  110 ”) the alternative primary spring acts upon the control arm  112 . However, instead of biasing the control arm so that the control button  100  returns to the parked mode position  108 , the alternative primary spring biases the control button towards the blade change mode position  104 . 
         [0099]    Operation of the alternative slider is now described with reference to the two different scenarios which can occur when the pin is in the web  122   c.    
         [0100]    In a first scenario, the alternative push button  1300  is manually moved to a lowered position, as is shown in  FIG. 17B , which tilts the alternative slider counter-clockwise so that the pin enters the mouth of the alternative hook  1620 . Here, the pin is blocked from moving into the major leg  122   b  by the end  1620   a  of the alternative hook. The pin is biased to move, by the alternative primary spring, in the direction of an arrow Y, to the minor leg  122   a  and the control button pivots clockwise into the blade change mode position  106  where it remains. Should this occur, interaction between alternative slider and pin maintains the alternative slider in situ thereby exposing the dowel&#39;s leading end  140   a.  Referring to  FIGS. 17C and 17D , the chamfer  1300   a  clears the path for clockwise rotation of the control knob  142 . Clockwise rotation of the control knob causes the cam  144  to push the dowel towards the blade guard to engage the recess  168 , thereby operating the housing lock. This first scenario is reversible up to the point where the blade change mode position is to be de-selected. This is done by manually pivoting the control button counter-clockwise to the parked mode position whereupon the alternative slider tilts clockwise to its position of rest under the bias of the slider spring  128 . 
         [0101]    In a second scenario, the alternative push button  1300  is stationary. The pin can move, against the bias of the alternative primary spring and in the direction of an arrow X, to the major leg  122   b  upon selection of “plunge cut” mode by the control button, as is shown in  FIG. 17A . Should this occur, the alternative push button is locked in its raised position and the alternative slider cannot tilt counter-clockwise because the pin abuts the end  1620   a  of the alternative hook. The dowel&#39;s leading end  140   a  remains shielded by the slider. The control knob  142 , which operates the housing lock, cannot be rotated clockwise because its path is blocked by the alternative push button. This isolates the housing lock. This second scenario is reversible.