Reciprocating cutting tool

[0001] The invention described and claimed hereinbelow is also described in EP 07104239, filed on Mar. 14, 2007. This European Patent Application, whose subject matter is incorporated here by reference, provides the basis for a claim of priority of invention under 35 U.S.C. 119 (a)-(d).

[0002] The present invention relates to cutting tools. In particular, it relates to control systems for vegetation cutters especially hedge cutters or trimmers.

[0003] Hedgecutters comprise two elongate blades which reciprocate with respect to each other. Either one blade is fixed and the other caused to reciprocate or both blades caused to reciprocate in an equal and opposite manner. The blades are driven by means of a cam element driven by the output shaft of an electric motor or small engine. One such cutter is described in JP2005269972, which hedge cutter also includes a switch allowing the user to reverse the direction of motion of one of the blades.

[0004] Hedgecutters frequently suffer in use from a problem of blocked cutting blades when they encounter a twig or branch whose size or density exceeds the capacity of the blades. The operator typically removes the power to the blades to remove the pressure of the moving blade on the branch and then attempts to remove the branch by pulling the branch. This can give rise to substantial safety considerations as the user places their fingers near to the blades. Similar problems are presented with other cutting tools. The present invention seeks to overcome this problem and assist in freeing blocked cutter blades.

[0005] The present invention provides a cutting tool having at least one driven blade movable between two predefined points of maximum travel, the tool further comprising a prime mover adapted to cause movement of the said at least one movable blade between said points of maximum travel; characterized in that the cutting tool is further provided with a control means which causes a change of direction in the movement of the at least one movable blade at a point intermediate the points of maximum travel in response to an event sensed by the control means.

[0006] Preferably, the prime mover is an electric motor. More preferably, the change of direction is achieved by a change in the direction of motor rotation.

[0007] Preferably, the control means comprises a control circuit.

[0008] In one embodiment, the event is an increase in cutting load. Suitably the control circuit includes a current sensing circuit such that, when the current drawn by the motor exceeds a predetermined level, the control circuit reverses the direction of rotation of the motor. Alternatively, the control circuit includes a speed sensor, adapted to sense when the speed of the driven blade falls below a predetermined level.

[0009] In an alternative embodiment, the event is removal of power to the driven blade. Suitably, the control circuit is adapted to cause rotation of the motor sequentially in a first direction upon a first application of electrical power to the motor and in a second direction upon a subsequent application of electrical power to the motor.

[0010] In one embodiment the control circuit is adapted to operate the motor in the reverse direction for a predetermined period whereupon the circuit then causes operation of the motor in the normal direction. Suitably, the predetermined period is less than 1 second, more suitably less than half a second, even more suitably from 200 ms to 400 ms.

[0011] Preferably, the cutting tool is a portable tool. More preferably, the cutting tool is a vegetation cutter, most preferably a hedge cutter or hedge trimmer.

[0012] Preferably the cutting tool includes a gearbox intermediate the motor and the at least one drivable blade. More preferably, the gearbox includes a low ratio reverse gear.

[0013] Preferably, the cutting tool comprises an electric motor powered by means of a rechargeable battery or cell More preferably, the battery is a lithium ion battery.

[0021] A typical vegetation cutter in accordance with the present invention is in the form of an electric hedgecutter 10 having a bottom blade 11 and an upper blade 12 caused to reciprocate simultaneously and in an equal and opposite manner with respect to the housing and hence also to each other by means of an electric motor M housed in a housing unit 13. The housing unit 13 includes two grips 14, 15, each associated with a switch arrangement 16, 17. This switching configuration provides a safety feature requiring both left and right hands of the operator to be in position and actuating their respective switches in order to supply an electric current to the motor. Accordingly, the blades cannot be put into an operating configuration unless both hands are away from the blades.

[0022] The motor acts to reciprocate the moving blades 11,12 in a conventional manner through suitable gearing as required.

[0023] In accordance with the present invention, it is possible to drive the reciprocating blade 11 in a reverse direction, compared with its normal cutting direction. It will be appreciated that, in the context of a reciprocating action, the term ‘reverse’ is used in a relative sense rather than an absolute sense. In use, a hedgecutter frequently suffers from jamming when it encounters a branch or twig whose diameter or density exceeds the capacity of the blades. The present invention overcomes this jamming in one of two ways.

[0024] In one arrangement, the reciprocating blade is caused to reverse direction for a brief period, sufficient to release the jammed branch, before resuming its conventional direction automatically, without the operator releasing the switches.

[0025] In preferred arrangements, the reciprocating blade is caused to reverse its direction and maintain the reversed direction until such time as a further jammed situation is detected, whereupon the direction is reversed again, to the original direction, and so on automatically, without the operator releasing the switches. With this arrangement, upon encountering a jammed or stalled situation, the blade is caused to oscillate back and forth, thereby cutting the branch from both sides and achieving a cut which would not otherwise have been possible with the hedgecutter. Accordingly, the present invention not only provides a solution to the problem of stalled blades due to jamming but also extends the cutting capability of the cutting tool

[0026] Referring to FIG. 2, there is shown schematically a control circuit CC enabling this to be achieved in the context of an AC motor 18 (see FIG. 2) having zero commutator/brush shift. The drive motor has an armature 20 and brushes 21. Reverse rotation is achieved by reversing the direction of current through armature 20 by means of a double pole changeover relay 24. In an alternative embodiment, a 4-triac bridge is used.

[0027] Relay changeover state can be changed by means of further electronic control circuitry, the precise configuration of which would be readily apparent to the person skilled in the art.

[0028] In the case of motor M that is an AC motor 18 (see FIG. 2) having commutator/brush shift in addition to the electrical switchover, the brushes are mechanically moved, suitably by means of an electromechanical switching arrangement, thereby replicating the commuter/brush shift for the alternative direction of running.

[0029] In the case of motor M that is a DC motor 19 (see FIG. 3) powered vegetation cutter, suitably zero brush shift is incorporated. Reverse action of the motor, and thus reciprocating blade 12, is achieved by reversal of the motor polarity achievable with solid state or electromechanical switching.

[0030] In a preferred embodiment, the vegetation cutter is battery-powered, more preferably by means of a Li-ion battery unit. Devices using Li-ion batteries typically include an over-current protection circuit. This can be adapted to include the control circuit for the reverse drive operation of the present invention. FIG. 3 shows a suitable schematic control circuit CC showing that the change of motor direction can be readily accomplished by means of a MOSFET bridge 22. If a brushless DC motor is utilized, the commutation electronics can advantageously be programmed to reverse the motor direction.

[0031] The blade reversal event is advantageously triggered by detection of an increase in or high values of cutting load. In preferred embodiments, the control circuit includes a current sensing circuit for this purpose, adapted to initiate motor reverse when the current drawn by the motor exceeds a predetermined level. Typically, this would be the value at stall, although a value less than stall can be used such that the motor is not permitted to reach a stall condition. Accordingly, the battery charge is prolonged as it is not permitted to supply a full stall current, acting as further protection, in addition to the normal protection, to prevent abuse from overcurrent.

[0032] Alternatively, blade reversal can be triggered on each occasion electrical power is applied to the motor. When the motor is re-energized in a stall situation, the reciprocating blade(s) therefore reverses to clear the stall.

[0033] The vegetation cutter may include gearing adapted to allow a lower ratio geared reverse from that of the forward direction. This acts to aid unblocking of the blades and is particularly suitable for embodiments in which the reversal of blade movement is only temporary.

[0034] As mentioned above, the electric hedgecutter 10 includes a gearbox 100 positioned intermediate the electric motor M and at least one of the blades 11,12 as schematically shown in FIG. 4. The gearbox 100 includes a forward gear 101 and a reverse gear 102. The reverse gear 102 has a lower ratio than the forward gear 101. Further as mentioned above and as schematically shown in FIG. 5, the control circuit CC includes a speed sensor 200. Also as mentioned above and as schematically shown in FIG. 6, the electric hedgecutter 10 includes the rechargeable battery 300.

[0035] The present invention provides two solutions to the same technical problem of blade jamming in hedgecutters and other vegetation cutting apparatus. In one arrangement, when a jammed or stalled situation is sensed, the powered cutter blade is automatically put into reverse to free the vegetation causing the stall In the second arrangement it is recognized that the user will typically recognize a jammed or stalled situation and will instinctively release some or both switches and so cut power to the motor. This leaves the cutter in the jammed configuration. Accordingly, the blade is put into a reverse direction automatically on re-application of power to the motor. Those skilled in the art will be readily able to design suitable circuits to include the above features together with such other features as they require for a particular vegetation cutting apparatus.