Abstract:
A switch mechanism includes a support structure and an electric switch having an activator moveable between a first position where the electric switch is ON and a second position where the electric switch is OFF. A cam is connected to a first actuator, moveably mounted on the support structure, so that movement of the first actuator results in movement of the cam. A bar is connected to a second actuator, moveably mounted on the support structure, so that movement of the second actuator results in movement of the bar. The cam engages the activator so that movement of the cam by movement of the first actuator results in the activator moving between its two positions. The bar engages the activator so that movement of the bar by movement of the second actuator results in the activator moving between its two positions.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to GB 0910774.9 filed Jun. 23, 2009. GB 0910774.9 is incorporated by reference in its entirety. 
     TECHNICAL FIELD 
     The present invention relates to a switch mechanism, in particularly to a switch mechanism for a power tool such as a power cutter. 
     BACKGROUND OF THE INVENTION 
     A typical power cutter comprises a housing in which is mounted a two stroke internal combustion engine. Attached to the side of the housing is a support arm which extends forward of the housing. Rotatably mounted on the end of the support arm is a cutting blade, usually in the form of a grinding disk. The motor is drivingly connected to the cutting blade via a drive belt. The rotary output of the engine rotatingly drives the cutting blade via the drive belt. The drive belt is driven via a centrifugal clutch which enables the output drive spindle of the engine to disengage from the belt when the engine is running at a slow speed, to allow the engine to continue running, whilst allowing the blade to be stationary. 
     Also mounted in the housing is a fuel tank which provides fuel for the engine via a carburetor. An oil tank can also be provided, which provides lubricating oil to mix with the fuel, to lubricate the engine. 
     Mounted on the rear of the housing is a rear handle for supporting the power cutter, which contains a trigger switch for accelerating the engine upon depressing. Depression of the trigger switch causes more of the aerated fuel/oil mixture to be injected into the engine which in turn causes the speed of the engine to accelerate. 
     GB2232913 and WO2005/056225 show such power cutters. 
     Power cutters are typically started using a pull cord. Once started, the engine will continue to run in an idle mode until stopped. It is important to provide a switching mechanism which prevents the power cutter from being started when it is in the OFF position, and which allows it to be started when it is in the ON position. The switching mechanism is also used to stop the engine when it is running by being switched from its ON position to its OFF position. However, it is desirable to be able to switch the engine off quickly during an emergency situation. 
     Unpublished UK patent application No. 0812274.9 discloses a power cutter having such an ON/OFF switching mechanism and which is described in detail below. However, the problem associated with such a design of the ON/OFF switching mechanism is that it is complex and difficult to assemble. Furthermore, such a design is prone to failure due to dust and debris, created during the operation of the power cutter, penetrating the switch mechanism and interfering with the operation of the various component parts, such as the relative movement of the ramps. 
     The present invention provides a simplified design of an ON/OFF switching mechanism to that disclosed in UK patent application no. 0812274.9 and which is less prone to failure due to dust and debris. 
     BRIEF SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention, there is provided a switch mechanism comprising a support structure. An electric switch is mounted on the support structure and comprises an activator moveable between a first position where the electric switch is switched on and a second position where the electric switch is switched off. A first actuator is moveably mounted on the support structure. A cam, having a cam surface, is connected to the first actuator so that movement of the first actuator results in movement of the cam. A second actuator is moveably mounted on the support structure; and a bar is connected to the second actuator so that movement of the second actuator results in movement of the bar. The cam engages the activator so that movement of the cam by movement of the first actuator results in the activator moving between its two positions. The bar engages the activator so that movement of the bar by movement of the second actuator results in the activator moving between its two positions. The bar passes through or alongside the surface of the cam when it engages the activator. 
     According to a second aspect of the present invention there is provided a power tool comprising a switch mechanism wherein 1) when the electric switch is on and the power tool is deactivated, the power tool is able to be activated; 2) when the electric switch is off and the power tool is deactivated, the power tool is prevented from being activated; and 3) when the electric switch is switched from being on to being off when the power tool is activated, the power tool is deactivated. 
     The power tool can be a power cutter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An embodiment of the present invention will now be described with reference to the accompanying drawings of which: 
         FIG. 1  shows a rear perspective view of the power cutter; 
         FIG. 2  shows a schematic view of the engine of the power cutter; 
         FIG. 3  shows a control system for the engine; 
         FIG. 4  shows an oil pump; 
         FIG. 5  shows a primer; 
         FIG. 6  shows an example of a rotatable on/off switch; 
         FIG. 7  shows an exploded view of the switch; 
         FIGS. 8A to 8E  show the switch cam and micro switch; 
         FIG. 9  shows a cut away view of the switch; 
         FIG. 10  shows the underside of the knob; 
         FIG. 11  shows the knob, bolt, and spring; 
         FIGS. 12 and 13  show rear views of the switch; 
         FIGS. 14A and 14B  show the electric signal sent to the oil pump from the electronic controller operating at two speeds, a slow speed ( FIG. 14A ) and a high speed ( FIG. 14B ); 
         FIG. 15  shows an exploded view of an embodiment of the on/off switch according to the present invention; 
         FIG. 16  shows a partial cross sectional view of the on/of switch of  FIG. 15 ; 
         FIG. 17A  shows the rotatable knob in the OFF position; 
         FIG. 17B  shows the position of cam wheel when the rotatable knob is in the position shown in  FIG. 17A ; 
         FIG. 18  shows the cam wheel and micro switch; 
         FIG. 19A  shows the rotatable knob in the ON position; 
         FIG. 19B  shows the position of cam wheel when the rotatable knob is in the ON position; 
         FIG. 20  shows the under side of the knob; 
         FIG. 21  shows a cross sectional view of the ON/OFF switch; 
         FIG. 22A  shows the rotatable knob in the ON position but with the stop button depressed; 
         FIG. 22B  shows the position of cam wheel when the rotatable knob is in the ON position with end of the tongue extended; 
         FIG. 23A  shows a side view of the cam wheel having a three-sided aperture formed in the peripheral cam; 
         FIG. 23B  shows a side view of the cam wheel having a four-sided aperture formed in the peripheral cam; 
         FIG. 23C  shows a side view of the cam wheel with the end of the tongue alongside of the peripheral cam; and 
         FIG. 24  shows an alternative design of the stop button. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to  FIGS. 1 to 14 ,  FIG. 1  shows a power cutter which comprises a housing  800  in which is located a two stroke engine  116 , 118 , a rear handle  802 , a support aim  804  and a front handle  806 . A cutting blade  808  is rotatably mounted on the support arm and which can be driven by the engine. A guard  810  surrounds the top part of the blade  808 . 
     Referring to  FIG. 2 , the two stroke internal combustion engine  116 , 118  is fed with an air/fuel mixture from a carburetor  126 . The engine  116 , 118  burns the mixture in a well known manner to generate rotary motion of its crank shaft  114 , which connects to an output shaft. The exhaust gases are then expelled from the engine  116 , 118  through an exhaust  146  to the surrounding atmosphere. The engine  116 , 118  is started using a pull cord in well know manner. 
     The power cutter comprises a fuel tank  124  in which is located fuel for driving the two stroke internal combustion engine  116 , 118 . Fuel will pass from the tank  124  via passageway  144  through the carburetor  126  which will mix the fuel with air from an air filter  890 , prior to being forwarded to the cylinder  120  where it will be burnt. A second tank  128 , as shown in which lubricating oil will be contained, will also be mounted in the body. The oil will be pumped out of the tank  128  via an oil pump  700 . The oil pump  700  will pump the oil through the oil passageways indicated by lines  142  from the oil tank  128  via the pump  700  into the passageway  132  between the carburetor  126  and the cylinder  120 , in a suitable form, for example, as a spray or atomized, which is then mixed with the air/fuel mixture generated by the carburetor  126 . A sensor  140  is mounted within the passageway  132  between the carburetor  126  and cylinder  120 . The sensor monitors the amount of oil being added to the fuel/air mixture and sends a signal, via an electric cable  701 , indicative of the amount of oil in the passageway  132  back to an electronic controller  716  (see  FIG. 3 ). Such a sensor can be of a capacitance type whereby the sensor monitors the change in capacitance between two plates, the capacitance being a function of the amount of oil there is in the fuel/air mixture. 
     The carburetor  126  is a standard design which, during normal operation, operates with out any external power input. However, the carburetor  126  comprises a solenoid  714 . There are a number of ways a carburetor can use a solenoid. Two ways are: 
     (1) The solenoid can open a channel within the carburetor which allows the fuel to get direct access to the passageway leading to the cylinder. This provides the engine with an air/fuel mixture which is richer in fuel; and 
     (2) The solenoid can close an air channel within the carburetor, which passes clean air around the carburetor to the passageway. With the airflow closed by the solenoid (or substantially closed), the air/fuel mixture is richer in fuel. 
     The solenoid is used when the engine is cold to provide an air/fuel mixture which is richer in fuel to help start the engine. When the engine is warm, the solenoid is either non-utilized or is switched off. The temperature of the engine is measured using a sensor  710  located on the engine block. The solenoid  714  is used to replace the choke on the carburetor whereby an operator could manually adjust the valve to start the engine when it is cold. 
     The engine ignition system is controlled by the electronic controller  716 , the function of which is described in more detail below with reference to  FIG. 3 . 
     Mounted on the end of the end of the crank shaft  114  is a flywheel  702  which contains a number of metal fins  704  which form an impeller. As the flywheel  702  rotates, the impeller blows air around the out side of the engine. Adjacent the impeller  702  are two generators  706 ,  708 . The two generators generate electricity using magnets and the change of inductance caused by the rotating flywheel  702 . As the flywheel  702  rotates, it causes the two generators  706 ,  708  to produce electricity. The first generator  706  is used to provide electricity for the ignition system of the engine and the electronic controller  716 . The second generator  708  is used to provide electricity for the oil pump  700  and the solenoid  714  in the carburetor. Both are connected to the electronic controller  716  via cables  717 . 
     Also mounted adjacent the flywheel are two sensors  710 ,  712 . The first sensor  710  monitors the temperature of the engine block and sends a signal via an electric cable  711  indicative of the temperature to the electronic controller  716 . The second sensor  712  monitors the angular position of the flywheel  702  and sends a signal via an electric cable  713  indicative of the angular position of the flywheel  702  back to the electronic controller  716 . This signal can also be used by the electronic controller  716  to determine the rate of rotation of the flywheel  702 , as well as its angular position. 
     The oil pump  700  is an electrically powered oil pump  700 , the power for which is supplied by the electronic controller  716  via electric cable  715 . The oil pump is shown in  FIG. 4 . This type of oil pump is described in EP1236894. The oil pump  700  is driven by the electronic controller  716  which sends a square shaped voltage signal  892  to the oil pump (see  FIG. 14A ) When the voltage is at V 1 , it causes the piston  850  of the pump to move, reducing the size of the oil chamber  852 . This causes a preset amount of oil to be pumped out of the chamber  852 . When voltage is “0”, the piston returns to its starting position due to the biasing of the spring  854 , enlarging the oil chamber  852  and allowing the chamber  852  to fill with oil. The higher the frequency of the square shaped voltage signal  892 , the more oil the oil pump  700  pumps per unit of time. The oil pump is capable of running at two speeds (the first speed shown in  FIG. 14A , the second speed being shown in  FIG. 14B  where the frequency of the square shaped voltage signal  892 , and hence the movement of the piston  850 , is double) and its general operation is described in more detail below. 
     A spark plug  730  is connected to the electronic controller  716  via a cable  732 . Ignition of the spark plug is controlled by the electronic controller  716 . 
     A primer  734  is mounted on the rear wall  736  of the housing  800  of the power cutter. See  FIG. 1 . The primer is a manual pump. A first pipe  738  connects from the fuel tank  124  to the primer  734 . A second pipe  740  connects from the primer to the carburetor  126 . A brief description of the principle of how the primer works will now be described with reference to  FIG. 5 . The primer has two valves  742 ,  744  located in series which allow the fuel to flow one way through them only (indicated by Arrows A and B). Located between the two valves  742 ,  744  is a chamber  750  having a rubber dome  746  forming a wall which is accessible to the user of the power cutter. One valve  742  only allows fuel to enter the chamber  750 , while the other only allows fuel to leave the chamber  750 . In order to use the primer, the operator, presses the rubber dome  746  (shown as dashed lines  748 ). This reduces the amount of volume in the chamber  750  formed between the valves and hence the amount of space which can contain fuel. As such, fuel is ejected from the primer through the one of the valves  744 , as the second valve  742  remains closed, preventing fuel from leaving the chamber  750  via that valve  742 . When the operator releases the dome  746 , the volume of the chamber  750  increases, causing fuel to be sucked into the chamber  750  through the second valve  742  as the first valve remains closed  744  preventing fuel from entering the chamber  750  through that valve  744 . Repetitive pressing and releasing of the dome  746  results in the fuel being pumped through the primer  734 . The primer is arranged so that the operator can manually pump the fuel from the tank  124  to the carburetor  126  through the pipes  738 ,  740 . 
     The purpose of the primer is to enable the operator to place fuel into the carburetor. Otherwise the operator has to spin the engine a number of times using the pull cord before a sufficient amount of fuel is sucked through into the carburetor  126 . 
     A DECO valve  752  is mounted on the side of the cylinder  120 . The valve  752  is opened manually by the operator prior to starting the engine. When opened, the DECO valve reduces the pressure within the cylinder  120  prior to ignition. This enables the starting of the engine using the pull cord to be easier as the amount of compression of the fuel/air mixture required is reduced. When the engine is started, the DECO valve automatically closes. 
     The electronic controller  716  has an on/off switch  754  in the form of a rotatable knob  758 . The on/off switch  754  is connected to the electronic controller via an electric cable  756 . 
     The knob  758  has a pointer  764  integrally formed on its periphery. The rotatable knob  758  has two angular positions between which it can rotate. In the first position, the switch is ON. In this position, the pointer  764  points to an ON label  762  (see  FIG. 1 ). In the second position, the switch is OFF. In this position, the pointer  764  points to an OFF label  760 . When the rotatable knob is in the ON position, the operator can start the engine and use the power cutter. When the rotatable knob  758  is in the OFF position, the engine is prevented from being started. If the rotatable knob  758  is moved from the ON to the OFF position when the engine is running, the engine is automatically switched off 
     A stop button  766  is located in the center of the knob  758 . If the engine is running (i.e., the knob is in the ON position), depression of the stop button  766  will result in the engine being switched off. The knob  758  then automatically returns to the OFF position. If the knob  758  is prevented from returning to the OFF position after the stop button has been depressed, the engine will not be able to be started until the knob  758  has been allowed to return to the OFF position. 
     The construction of the assembly for the ON/OFF switch  754 , which includes the knob  758  and stop button  766 , will now be described. 
     The ON/OFF switch assembly comprises the rotatable knob  758 , a crank  768 , a switch cam  770  and the stop button  766 . 
     The crank  768  is rigidly fixed into the rear wall  736  of the housing  800  and prevented from rotation. The crank  768  comprises a socket  772  into which is rigidly mounted a micro switch  774  (see  FIG. 8C ). 
     As shown in  FIG. 7 , rotatably mounted on the outside of the crank  768  is the knob  758 . Rotatably mounted on the inside of the crank  768  is the switch cam  770 . A bolt  778 , which passes through the base of a tubular recess  776  formed in the knob  758 , screws into the switch cam  770  and is rigidly attached thereto. Sandwiched between the head of the bolt  778  and the base of the recess  776  is a spring  780 . The bolt  778  and spring  780  hold the knob  758  and switch cam  770  onto the crank  768 , biasing them towards each other as the spring biases the head of the bolt  778  away from the base of the recess  776 . The knob  758  can rotate through a limited range of positions (between the ON and OFF positions) relative to the crank  768 . The range of positions is limited by pegs  786  formed on the underside of the knob engaging with recesses  788  formed in the edge of the rear wall  736  of the housing. The switch cam  770  can also rotate through a limited range of positions relative to the crank  768 . In addition, the switch cam  770  can axially slide relative to the crank  768  in a direction parallel to the longitudinal axis of the bolt  778  over a limited range of positions, the range being limited by the length of the bolt  778  within the recess  776 . The bolt  778  rotates and slides with the switch cam  770 . 
     The stop button  766  is mounted within the tubular recess  776  formed in the knob  758  and encloses the end of the bolt  778  located in the recess  776  and the spring  780 . (See  FIG. 9 ). The stop button  766  can axially slide within the recess  776  towards or away from the switch cam  770 . The range of outward axial movement of the stop button is limited by stops  782  each engaging with an inner step of the knob  758 . The head of the bolt  778  directly abuts the underside of the stop button  766 . Depression of the stop button, causes the bolt  778  to be pushed through the base, compressing the spring  780 , moving the switch cam  770  away from the crank  768  and knob  758 . 
     Connected between the knob  758  and the crank  768  is a long helical spring  784 . The helical spring  784  is located in a circular channel  790  formed on the underside of the knob  758  as best seen in  FIG. 10 . One end of the helical spring  784  abuts against a wall  792  at the end of the channel  790 . The other end abuts against a stop (not shown) formed on the crank  772 . The spring  784  rotationally biases the knob  758  relative to the crank to its OFF position. 
     Connected between the switch cam  770  and the crank  768  is a leaf spring  794  as best seen in  FIGS. 12 and 13 . One end of the leaf spring  794  is connected using a small bolt  796  to the switch cam  770 . The other end abuts a stop  798  on the crank  768 . The leaf spring  794  rotationally biases the switch cam  770  relative to the crank to an OFF position. 
     Formed on the underside of the knob  758  are two ramps  820 , each ramp having a ramp end  822  as best seen in  FIG. 10 . Formed on the side of the switch cam  770  which faces the knob  758  are ramp recesses  824  which have ramp recess ends  826  as best seen in  FIG. 9 . When the switch assembly is in the OFF position i.e. when both the knob  758  and the switch cam  770  are in their OFF positions under the biasing force of their respective springs  784 ,  794 , each of the two ramps  820  is located in a corresponding ramp recess  824  with the ramp ends  822  of each ramp  820  abutting directly against the ramp recess ends  826  of the corresponding ramp recess  824 . 
     Formed on the underside of the crank  768  are two crank ramps  828 , each ramp  828  having a crank ramp end  830  as best seen in  FIG. 8C . Formed on the side of the switch cam  770  which faces the knob  758  are switch cam crank ramps  832  which have switch cam crank ramp ends  834  as best seen in  FIG. 9 . When the switch assembly is in the OFF position i.e. with both the knob and the switch cam  770  in their OFF positions under the biasing force of their respective springs  784 ,  794 , each of the two switch cam crank ramps  832  are located against the low end (the end of the crank ramp  828  away from the crank ramp end  830 ) of the corresponding crank ramp  828  as shown in  FIG. 8C . 
     Formed around the edge of the switch cam  770  is a peripheral cam  836  as best seen in  FIGS. 8A and 8B . The micro switch  774  comprises a pin  838  which projects from the body of the micro switch  774 . The pin  838  slides axially in or out of the body of the micro switch  774  and is biased to its outer most position by a spring (not shown) inside the micro switch  774 . The pin  838  engages the peripheral cam  836 . Rotation of the switch cam  770  causes the pin  838  to slide along the peripheral cam  836 , which causes it to be pushed into the body of the micro switch  774  against the biasing force of the spring, or allows it to slide out of the body of the micro switch  774  under influence of the spring. When the switch cam  770  is in its OFF position, the pin is pushed into the body of the micro switch  774  as shown in  FIG. 8A . When switch cam is rotated to its ON position, the pin  838  extends to its outer most position as shown in  FIG. 8B . 
     The operation of the assembly for the ON/OFF switch will now be described. Initially, the knob  758  and the switch cam  770  are both located in their OFF positions. The operator of the power cutter desires to turn the unit on using the ON/OFF switch. The operator uses their hand to rotate the knob  758  from its OFF position to its ON position. When the knob  758  is rotated, it causes the switch cam  770  to rotate in unison as the rotary movement is transferred from the knob  758  to the switch cam  770  by the ramp ends  822  of each ramp  820  pushing the ramp recess ends  826  of each corresponding ramp recess  824 , against which it abuts, in the direction of Arrow M in  FIG. 9 , to cause the switch cam  770  to rotate with the knob  758 . As the switch cam  770  rotates, the two switch cam crank ramps  832 , which are initially located against the low end of the crank ramps  828  (shown in  FIG. 8C ), ride up the crank ramps  828  (shown in  FIG. 8D ), which are stationary. As the switch cam crank ramps  832  ride up the crank ramps  828  due to the rotation of the switch cam  770 , the switch cam  770  is forced to axially slide away from the knob  758  (direction of Arrow N in  FIG. 9 ), causing the spring  780  to be compressed and the head of the bolt  778  to move towards the base of the recess  776 . When the switch cam has rotated sufficiently that the crank ramp ends  830  and the switch cam crank ramp ends  834  become aligned, the switch cam  770  axially slides under the biasing force of the spring  780  towards the knob  758 , ensuring that the crank ramp end  830  and the switch cam crank ramp ends  834  abut against each other as shown in  FIG. 8E . When the crank ramp ends  830  and the switch cam crank ramp ends  834  abut each other as shown in  FIG. 8E , the switch cam  770  is in its ON position and is prevented from returning to its OFF position, under the influence of the leaf spring  794 , as the crank ramp ends  830  and the switch cam crank ramp ends  834  prevent relative movement as they are jammed against each other. The knob  758  is prevented from returning to its OFF position under the influence of the spring  784  by the ramps  820  being held within the ramp recesses  824  by the action of the spring  780  which overrides the spring  784 . When the switch cam  770  rotates from the OFF position (see  FIG. 8A ) to the ON position ( FIG. 8B ), the peripheral cam  836  rotates, which in turn allows the pin  838  to extend from the body of the micro switch  774 . This in turn makes a connection which allows the electric controller  716  to activate the power cutter and allow it to start when the pull cord is pulled. 
     As such, the assembly of the ON/OFF switch is now ON with the knob  758  and the switch cam  770  both in their ON positions, allowing the pin  838  to extend from the body of the micro switch  774 . There are two ways of switching the ON/OFF switch assembly to its OFF position. 
     The first method comprises the depression of the stop button  766 . Depression of the stop button  766  causes the head of the bolt  778  to slide towards the base of the recess  776  of the knob  758 , compressing the spring  780 , which in turn causes the switch cam  770  to axially slide away from the knob  758 . As the switch cam  770  axially slides, the switch cam  770  moves away from the crank  768 , which in turn causes the crank ramps  828  and the switch cam crank ramps  832  to move away from each other, and thus causes the crank ramp ends  830  and the switch cam crank ramp ends  834  to disengage. As such, the switch cam  770  can now rotate back to its OFF position under the influence of the leaf spring  794 . As the knob is held in its ON position by the ramps  820  being held within the ramp recesses  824 , the knob  858  will also return to its OFF position as the ramp recesses  824  rotate with the switch cam  770 . Should the ramps  820  become disengaged from the ramp recesses  824  due to the sliding movement of the switch cam  770  relative to the knob  758 , the knob  758  will return to its OFF position under the influence of the spring  784  between the knob  758  and the crank  768 . 
     The second method of switching the ON/OFF switch assembly OFF comprises the rotation of the knob  758 . The operator rotates the knob  758  to its OFF position. As the ramps  820  are held within the ramp recesses  824 , rotation of the knob  758  urges rotation of the switch cam  770 . However, the switch cam  770  is prevented from rotating as the crank ramp ends  830  and the switch cam crank ramp ends  834  abut each other. Therefore, the ramps  820  slide out of the ramp recesses  824 , the ramp ends  822  moving away from ramp recess ends  826 . As the ramps  820  slide out of the ramp recesses  824 , the switch cam  770 , which is prevented from rotating, axially slides away from the knob  858  by the camming action of the ramps  820  and ramp recesses  824 . When the switch cam  770  has slid sufficiently far enough away from the knob  758 , the crank ramp ends  830  and the switch cam crank ramp ends  834 , which are sliding away from each other, become disengaged. Thus, the switch cam  770  can rotate under the influence of the leaf spring  794  to its OFF position. The knob  758  will move under the influence of the operator and/or the spring  784 . As such, both the knob  758  and the switch cam  770  return to their OFF position where they are held by the springs  784 ,  794 . 
     When both the knob and switch cam  770  move to their OFF positions, the ramps  820  engage with the ramp recesses  824  so that the switch can be used again to switch on the power cutter. 
     The operation of the power cutter will now be described. 
     The operator first activates the DECO valve  752  and then pumps some fuel into the carburetor  126  using the primer  734 . The operator then switches the ON/OFF switch to ON by rotation of the knob  758  to its ON position. The operator then pulls the pull cord to rotate the crank  114  of the engine. As the crank  114  rotates, the flywheel  702  also rotates causing the two generators  706 ,  708  to produce sufficient electricity to operate the power cutter. 
     The electronic controller  716  checks the temperature of the engine using sensor  710 . If the engine is cold, the electronic controller uses the electricity from the second generator  708  to power the solenoid  714  in the carburetor to set the “automatic choke”. The second generator  708  is not powerful enough to power both the oil pump  700  and solenoid  714  at the same time. Therefore, when the electronic controller  716  is operating the solenoid  714 , it switches off the oil pump  700 . It has been found that the period during which lubricating oil is not required before the engine is damaged is greater than that required to heat up the engine. 
     The electronic controller  716  supplies the power to the spark plug  730  to cause combustion in the engine, the power being provided by the first generator  706 , the timing being determine by the electronic controller  716  based on the signal provided by the sensor  712  in relation to the angular position of the flywheel  702 . 
     Once the engine commences firing, the DECO valve automatically closes. The electronic controller  716  continues to monitor the engine temperature and when it has reached a predetermined temperature, the electronic controller  716  switches the solenoid  714  in the carburetor  126  off. The electronic controller  716  then commences supplying a square shape voltage signal to the oil pump to commence pumping oil. The electronic controller  716  monitors the speed of the engine using the signal provided by the sensor  712  monitoring the angular position of the flywheel  702  to calculate the rotational speed. If the rotational speed is below a predetermined value, the electronic controller  716  sends a signal ( FIG. 14A ) to the oil pump  700  to cause it to pump at a slow speed. If the rotational speed is above a predetermined value, the electronic controller  716  sends a signal ( FIG. 14B ) to the oil pump  700  to cause it to pump at a higher speed. The speed of the engine is dependent on the operator squeezing a trigger switch which connects to the carburetor via a cable. 
     While the engine is running, the electronic controller  716  monitors the oil being added to the fuel/air mixture using the sensor  140 . If the sensor  140  detects that the rate of flow of the oil being pumped by the oil pump  700  has dropped below a predetermined amount (e.g. there is a blockage in the oil pipe  142  or the tank  128  is empty), the electronic controller places the engine into an idle mode using the ignition system so that the engine runs, but at a minimal rate. The operator cannot speed up the engine using the trigger until the sensor  140  detects the flow of oil. This protects the engine from damage due to a lack of lubrication. It has been found that the engine can run in idle mode for a considerable period of time before damage to the engine results. 
     In order for the operator to stop the power cutter, the operator either depresses the stop button  766  or rotates the knob  758  to its OFF position. 
     An embodiment of an ON/OFF switch according to the present invention will now be described with reference to  FIGS. 15 to 24 . This embodiment provides an alternative design of the ON/OFF switch to the example described above and can be substituted for that design in a power cutter as described above. 
     Where the same features in this embodiment are the same as those disclosed in the previous example of the ON/OFF switch described above, the same reference numbers have been used. 
     Except for the design of the ON/OFF switch  754 , the design of the rest of the power cutter is that same as that described above with reference to  FIGS. 1 to 14 . 
     The switch  754  comprises a rotatable knob  758 . The knob  758  has a pointer  764  integrally formed on its periphery. The rotatable knob  758  has two angular positions between which it can rotate. In the first position, the switch is ON. In this position, the pointer  764  points to an ON label  762  (see  FIG. 19A ). In the second position, the switch is OFF. In this position, the pointer  764  points to an OFF label  760  (see  FIG. 17A ). When the rotatable knob is in the ON position, the operator can start the engine and use the power cutter. When the rotatable knob  758  is in the OFF position, the engine is prevented from being started. If the rotatable knob  758  is moved from the ON to the OFF position when the engine is running, the engine is automatically switched off. 
     A stop button  766  is located in the center of the knob  758 . If the engine is running (i.e. the knob is in the ON position), depression of the stop button  766  will result in the engine being switched off. 
     A circular hole  900  is fowled through the rear wall  736  of the housing  800 . The rotatable knob  758  is mounted onto the outside of the rear wall  736  adjacent the circular hole  900 . A cam wheel  902  is rotatably mounted on the inside of the rear wall  736  adjacent the circular hole  900 . Two screws (not shown) connect the rotatable knob  758  to the cam wheel  902 , the screws passing through the circular hole  900 . The peripheries of the rotatable knob  758  and the cam wheel  902  sandwich the periphery of the circular hole  900  to hold the rotatable knob  758  and the cam wheel  902  in place on the rear wall over the circular hole  900 . The rotatable knob  758  and the cam wheel  902  can rotate about the central axis of the circular hole, the rotatable knob  758  and the cam wheel  902  rotating in unison. 
     Formed on the part of the rear wall  736  sandwiched between the rotatable knob  758  and the cam wheel  902  is a curved rib  904  (see  FIG. 16 ). Formed on the rear of the rotatable knob  758  is a correspondingly shaped groove  906  (see  FIG. 20 ). When the rotatable knob  758  is mounted on the rear wall  736 , the rib  904  locates inside the groove  906 . The length of the groove  906  is longer than the rib  904  allowing the rib  904  to slide within the groove. This restricts the amount of pivotal movement of the knob, and hence the cam wheel  902 , to sixty degrees, allowing the pointer  764  to pivot between its angular positions between its ON and OFF positions. 
     Also formed on the rear of the rotatable knob  758  are two pockets  908 ,  910 . Located in each pocket  908 ,  910  are pins  912  and springs  914  which bias the pins  912  out of the pockets  908 ,  910 . The pins  912  are prevented from completely exiting the pockets  908 ,  910  by the springs  914 . Formed on the part of the rear wall  736  sandwiched between the rotatable knob  758  and the cam wheel  902  are two pairs of recesses  916 , each recess  916  in each pair being located on the opposite side of the circular hole  900  to the other recess in that pair. The location of each pair of recesses  916  corresponds to an angular position of the knob  758 . When the knob  758  is rotated to one of its angular positions, the pins  912  locate within the recesses  916  of the first corresponding pair, due for the biasing force of the springs  914 , to latch the knob  758  in that angular position and hold it there. When the knob  758  is rotated, the pins  912  ride out of the recesses  916  by being pushed into the pockets  910 . As the knob  758  rotates, the pins  912  remain in the pockets  910 . When the knob  758  moves to its second angular position, the other pair of recesses  916  align with the pins  912 , which then move out of the pockets  910  and enter the recesses  916  under the biasing force of the springs  914 . The knob is then latched in its second angular position. 
     Mounted with a bracket  926  on the inside of the rear wall  736 , adjacent the cam wheel  902  is a micro switch  918  (see  FIG. 17B ). The micro switch  918  comprises a pin  920  which projects from the body of the micro switch  918 . The pin  920  slides axially in or out of the body of the micro switch  920  and biased to its outer most position by a spring (not shown) inside the micro switch  920 . A lever  922  is pivotally attached to the micro switch  918 . The lever  922  lies across the end of the pin  920 . Pivotal movement of the lever  922  towards the micro switch  918 , pushes the pin  920  into the micro switch  918 . 
     Formed around the edge of the cam wheel  902  is a peripheral cam  924  as best seen in  FIG. 17B . The peripheral cam  924  engages with the lever  922 . Rotation of the cam wheel  902  causes the lever  922  to slide along the peripheral cam  924 , which causes it to be pivoted towards the body of the micro switch  918 , which in turn pushes the pin  920  into the micro switch  918 , against the biasing force of the spring, or to be pivoted away from the body of the micro switch  918 , due to biasing force on the pin  920 , allowing the pin  920  to slide out of the body of the micro switch  918  wider influence of the spring. When the cam wheel is in its OFF position ( FIG. 17B ), the pin  920  is pushed into the body of the micro switch  918 . When the cam wheel  902  is rotated to its ON position, the pin  920  extends to its outer most position as shown in  FIG. 19B . 
     The micro switch  918  can be switched on and off by the depression or release of the pin  920  by the rotation of the knob  758  between its two angular positions. 
     The stop button  766  is mounted within a tubular recess  934  formed in the knob  758  and slides within the recess  934  towards or away from the base  936  of the recess  934 . A spring  928  is sandwiched between the stop button  766  and the base  936  of the recess and biases the stop button  766  out of the recess  934 . The range of outward movement of the stop button  766  is limited by four stops  930  mounted on legs  932  which engage with the underside of the knob  758  when the stop button has reached its maximum outward position. The range of inward movement of the stop button  766  is limited by the base  936  of the recess  934 . 
     Attached to the underside of the stop button  766  is a flexible tongue  938 . The tongue extends through an aperture in the base  936  of the recess  934  and then is curved by a guide  940  formed on the inner wall  942  of the wheel cam  902  through  90  degrees towards an aperture  923  formed in the peripheral cam  924  (see  FIG. 23A ). As can be seen, the aperture  923  is formed at a side edge  925  of the peripheral cam  924 , the peripheral cam  924  providing three sides to the aperture  921  However, it will appreciated that the aperture  923  can be formed in the middle of the peripheral cam  924  as shown in  FIG. 23B . Alternatively, the end  944  of the tongue can pass along side of the peripheral cam  924  as shown in  FIG. 23C , thus avoiding the need to form an aperture in the peripheral cam  924 . When the stop button  766  is located in its most outward position, the end  944  of the tongue  938  is located just inside of the guide  940  adjacent the aperture  923  in the peripheral cam  924 . When the stop button  766  is located in its most inward position, the end  944  of the tongue  938  passes through and projects from the aperture  923  in the peripheral cam  924 . Depression of the stop button  766  causes the end  944  of the tongue  938  to exit the aperture  923  in the peripheral earn  924 . 
     When the knob  758  is in its ON position as shown in  FIG. 19A , the aperture  923  in the peripheral cam  924  faces the lever  922 . When the knob  758  is in its ON position, the peripheral cam  924  allows the lever  922  to pivot away from the micro switch  918 , in turn allowing the pin  920  to slide out of the micro switch (see  FIG. 19B ). The stop button  766  is biased towards its outward position by the spring  928 . As such, the end  944  of the tongue  938  is located within the cam wheel  902 . However, when the stop button is pushed into the recess, the end  944  of the tongue is pushed through the aperture in the peripheral cam  924 , away from the cam wheel  902 , into engagement with the lever  922 . As the end  944  of the tongue continues to move away from the cam wheel  902 , it pivots the lever  922  toward the micro switch, pushing the pin  920  on the micro switch  918  into the micro switch. When the knob  758  is in its OFF position as shown in  FIG. 17A , the aperture in the peripheral cam  924  is located away from the lever  922  and therefore depression of stop button has no effect on the lever as the end of the tongue can not engage it. 
     Initially, the ON/OFF switch is in its OFF position, with the rotatable knob  758  in its first angular position and the pointer  764  pointing to the OFF label  760 . The peripheral cam  924  on the cam wheel  902 , is located in a position where it pushes the pivotal lever  922  towards the micro switch  918  which in turn pushed the pin  920  into the micro switch (see  FIG. 17A ). When the pin is in this position, it provides a signal to the electric controller  716 , when an electrical power supply is provided to the micro switch  918  and electric controller  716  by the operation of the pull cord of the power cutter that the engine can not be started. In order for the power cutter to be started, the rotatable knob  758  is rotated to its first angular position, the pointer  764  pointing to the ON label  762  to put the ON/OFF switch in its ON position. As the rotatable knob  758  rotates, the cam wheel  902 , and hence the peripheral cam  924  also rotate in unison. The peripheral cam  924  rotates to a position where it allows the pivotal lever  922  to pivot away from the micro switch  918  which in turn allows the pin  920  to extend from the micro switch (see  FIG. 19B ). When the pin  920  is in this position, it provides a signal to the electric controller  716 , when an electrical power supply is provided to the micro switch  918  and electric controller  716  by the operation of the pull cord of the power cutter, that the engine can be started. Whilst the engine is running, the pin  920  must remain extended from the micro switch  918 . In order to switch the power cutter off, the pin  920  of the micro switch must be pushed into the micro switch  918 . This is achieved in one of two ways. Firstly, the rotatable knob  758  can be rotated to its OFF position. Rotation of the knob  758  results in rotation of the cam wheel and peripheral can  924 , causing the peripheral cam  924  to push the lever  922  towards the micro switch  918  and hence the pin into the micro switch  918 . Secondly, the stop button  766  can be pushed into the recess  934 , causing the end  944  of the tongue  938  to project out of the aperture  923  in peripheral cam  924  and into engagement with the lever  922 , causing it pivot towards the micro switch  918  hence pushing the pin into the micro switch  918 . When the pin  920  is pushed inside of the micro switch  918 , it provides a signal to the electric controller  716  that the engine should be stopped. If the engine is stopped by depression of the stop button  766 , release of the stop button will allow to return to its outer most position under the biasing force of the spring  928 . When this happens the end  944  of the tongue  938  is retracted through the aperture in the peripheral cam  924  inside of the cam wheel  902  allowing the lever  922  to pivot away from the micro switch and hence the pin  920  to slide out of the micro switch  91 . 
     The operation of the power cutter is the same for this embodiment of ON/OFF switch as for the previous example of ON/OFF switch. 
     It will be appreciated by a person skilled in the art that an additional safety feature could be added whereby, when the engine has been stopped by depression of the stop button  766 , the rotatable knob  758  has to be first rotated to its OFF position and then back to its ON position before the engine can be started again. 
       FIG. 24  shows an alternative design of stop button  766  where the tongue  938  is integrally formed with the button  766 .