Abstract:
A switch mechanism for a power tool switchable between an ON and OFF state including a support structure; a first actuator rotatably mounted on the support structure and which is rotatable between a first position and a second position, wherein the first actuator is capable of being releasably latched in either of the first or second positions. The first actuator includes a recess. A second actuator is a slidable button located inside of the recess of the first actuator so that the first actuator at least partially surrounds the second actuator, and is linearly slidable within the recess between a first position and a second position. The second actuator is biased towards its first position. Movement of the first actuator from its first position to its second position, when the second actuator is in its first position, switches the switching mechanism to its ON state; and movement of the first actuator from its second position to its first position, when the second actuator is in its first position, switches the switching mechanism to its OFF state. Movement of the second actuator from its first position to its second position, when the first actuator is latched in its second position, switches the switch mechanism to its OFF state.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims foreign priority under 35 U.S.C. §119(a) to Applicant&#39;s United Kingdom Patent Application No. 08 122 74.9 filed on Jul. 4, 2008. The entirety of this application is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a switch mechanism, in particularly to a switch mechanism for a power tool such as a power cutter. 
     A typical power cutter includes 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 gasoline tank which provides gasoline for the engine via a carburetor. An oil tank can also be provided, which provides lubricating oil to mix with the gasoline, 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 gasoline/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 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. Switches on existing designs do not provide for rapid operation and therefore the switching off operation of the engine can be slow and/or complicated, which is not desirable. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a switching mechanism which has a facility for being more rapidly switch to its OFF position. 
     According to a first aspect of the present invention there is provided a switch mechanism for a power tool switchable between an ON and OFF state including a support structure. A first actuator is rotatably mounted on the support structure and is rotatable between a first position and a second position. The first actuator is releasably latchable in either of the first or second positions. The first actuator includes a recess. A second actuator is a slidable button located inside of the recess of the first actuator so that the first actuator at least partially surrounds the second actuator, and is linearly slidable within the recess between a first position and a second position. The second actuator is biased towards its first position. Movement of the first actuator from its first position to its second position, when the second actuator is in its first position, switches the switching mechanism to its ON state; and movement of the first actuator from its second position to its first position, when the second actuator is in its first position, switches the switching mechanism to its OFF state. Movement of the second actuator from its first position to its second position, when the first actuator is latched in its second position, switches the switch mechanism to its OFF state. 
     According to a second aspect of the present invention there is provided a power tool including a switch mechanism wherein 1) when the switch mechanism is in the ON state and the power tool is deactivated, the power tool is able to be activated, and; 2) when the switch mechanism is in an OFF state and the power tool is deactivated, the power tool is prevented from being activated, and; 3) when the switch mechanism is switched from the ON state to the OFF state 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  illustrates a rear perspective view of the power cutter; 
         FIG. 2  illustrates a schematic view of the engine of the power cutter; 
         FIG. 3  illustrates the control system for the engine; 
         FIG. 4  illustrates the oil pump; 
         FIG. 5  illustrates the primer; 
         FIG. 6  illustrates the rotatable on/off switch; 
         FIG. 7  illustrates an exploded view of the switch; 
         FIGS. 8A to 8E  illustrate the switch cam and micro switch; 
         FIG. 9  illustrates a cut away view of the switch; 
         FIG. 10  illustrates the underside of the knob; 
         FIG. 11  illustrates the knob, bolt and spring; 
         FIGS. 12 and 13  illustrate rear views of the switch; and 
         FIGS. 14A and 14B  illustrate 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 ). 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a power cutter which includes a housing  800  in which is located a two stroke engine, a rear handle  802 , a support arm  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 is fed with an air/gasoline mixture from a carburetor  126 . The engine burns the mixture in 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 through an exhaust  146  to the surrounding atmosphere. The engine is started using a pull cord in well know manner. 
     The power cutter will include a gasoline tank  124  in which is located gasoline for driving the two stroke internal combustion engine  24 . Gasoline will pass from the tank  124  via passageway  144  through the carburetor  126  which will mix it with air from an air filter  890 , prior to being forwarded to the cylinder  120  where it will be burnt. A second tank  128  will also be mounted in the body as shown in which lubricating oil will be contained. 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  130  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/gasoline 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 gasoline/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 gasoline/air mixture. 
     The carburetor  126  is a standard design which, during normal operation, operates with out any external power input. However, the carburetor  126  includes a solenoid  714 . There are a number of ways a carburetor can use a solenoid, two of which are: 
     Firstly, the solenoid can open a channel within the carburetor which allows the gasoline to get direct access to the passageway leading to the cylinder. This provides the engine with an air/gasoline mixture which is richer in gasoline. 
     Secondly, 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/gasoline mixture is richer in gasoline. 
     The solenoid is used when the engine is cold to provide an air/gasoline mixture which is richer in gasoline to help start the engine. When the engine is warm, the solenoid is either not utilized or is switched off. The temperature of the engine is measure using a sensor  710  located on the engine block. The solenoid  714  is used to replace the choke on the carburetor whereby which an operator would manually adjust the valve to start the engine when it is cold. 
     An example of a carburetor which uses a solenoid in such a manner is disclosed in U.S. Pat. No. 7,264,230. 
     The engine ignition system is controlled by an 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 fly wheel  702  which contains a number of metal fins  704  which form an impeller. As the fly wheel  702  rotates, the impeller blows air around the outside 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 fly wheel  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 . The two generators  706 ,  708  will be off-the-shelf products. 
     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 fly wheel  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 and therefore further explanation of its construction will not be described in detail. 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 V1, the electric controller  716  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 a spring  854 , enlarging the 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 volge signal  892 , and hence the movement of the piston  850 , is double). The general operation of the oil pump 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. The primer is a manual pump. A pipe  738  connects from the gasoline 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 includes two valves  742 ,  744  located in series which allow the gasoline 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 allowing gasoline to enter the chamber  750 , the other only allowing gasoline to leave the chamber  750 . In order to use the primer, the operator, compresses 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 gasoline. As such, gasoline is ejected from the primer through the one of the valves  744 , as the second valve  742  remains closed, preventing gasoline from leaving the chamber  750  via that valve  742 . When the operator releases the dome  746 , the volume of the chamber  750  increases, causing gasoline to be sucked into the chamber  750  through the second valve  742  as the first valve remains closed  744  preventing gasoline from entering the chamber  750  through that valve  744 . Repetitive compressing and releasing of the dome  746  results in the gasoline being pumped through the primer  734 . The primer is arranged so that the operator can manually pump the gasoline 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 gasoline into the carburetor. Otherwise the operator has to spin the engine a number of times using the pull cord before a sufficient amount of gasoline 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 made easier as the amount compression of the gasoline/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 switch 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 centre 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 includes 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  includes a socket  772  into which is rigidly mounted a micro switch  774  (see  FIG. 8C ). 
     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 to it. 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 can rotate through a limited range of movement (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 movement 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 movement, 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  locates in a circular channel  790  formed on the underside of the knob  758  as best seen in  FIG. 10 . One end 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  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  includes a pin  838  which projects from the body of the micro switch  774 . The pin  838  is axially slidable in or out of the body of the micro switch  774  and 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 way the assembly for the ON/OFF switch works 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 cam switch  770  to rotate in unison as the rotary movement is transferred from the knob  758  to 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 way of switching the ON/OFF switch assembly to its OFF position. 
     The first method includes 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 includes 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  moved 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 gasoline 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 fly wheel  702  also rotates causing the two generators  706 ,  708  to produce sufficient electricity to operate the power cutter. 
     The electronic controller 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 supplies the power to the spark plug 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 fly wheel  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 predetermine 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 monitors the speed of the engine using the signal provided by the sensor  712  monitoring the angular position of the fly wheel  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 gasoline/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 predetermine 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 can not 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.