Abstract:
A hammer drill comprising: a motor; a drive transmission capable of operating in at in at least a rotary mode and a reciprocating mode; a mode change mechanism; a switch which, when closed, provides power to the motor; and a mechanical lock-on mechanism which, when locked mechanically holds the switch closed to maintain the power to the motor. Additionally, the hammer drill includes a first sensor, which indicates the mode of operation of drive transmission; and a second sensor which indicates whether the lock-on mechanism is locked; and a controller which is capable of interfering with the operation of the motor, when the controller detects that the drive transmission is in the rotary mode of operation and the lock-on mechanism has been locked.

Description:
FIELD OF THE INVENTION 
   The present invention relates to hammer drills which are capable of being operated in at least two modes of operation, in particular, a hammer drill which has a hammer only mode, and more in particular, to hammer drills which are capable of being operated in three modes of operation, one being hammer only mode, the second being drill only mode and the third being a combined hammer and drilling mode. 
   BACKGROUND OF THE INVENTION 
   Hammer drills are power tools that generally have three modes of operation, i.e. a hammer only mode, a drill only mode and a combined hammer and drilling mode. In general, the motor of a hammer drill is operated by the user depressing a spring-loaded trigger, and deactivated by the user releasing the trigger such that it is necessary to hold the trigger down during operation of the tool. 
   U.S. Pat. No. 6,109,364 describes a rotary hammer drill which has three modes of operation, namely a purely drilling mode, a purely hammering mode and a combination of drilling and hammering mode. A mechanism is provided by which the rotary hammer can be switched between the three modes of operation. 
   It is desirable for such tools to be able to be “locked on” in the pure hammering mode only. This means that when the pure hammer mode is selected and the trigger button is depressed, the hammer can be “locked on” so that the removal of the fingers from the trigger button does not cause the tool to switch off but it in fact continues operating within the pure hammer mode until the “lock on” mechanism is deactivated. However, it is undesirable that such a feature is capable of being activated when in either the rotary only mode of operation or in the combination of the rotary and hammering mode of operation. Therefore, rotary hammers are constructed so that they can only be “locked on” when in the pure hammer mode only. GB2314288 describes one such mechanism whereby the trigger button is mechanically locked on in the hammer only mode. 
   The present invention provides an alternative design to the “lock on” mechanism in GB2314288. 
   BRIEF SUMMARY OF THE INVENTION 
   Accordingly there is provided a hammer drill comprising: 
   a motor; 
   a tool holder capable of holding a cutting tool; 
   a drive transmission, capable of operating in at least two modes of operation, which, when a cutting tool is held by the tool holder, is capable of converting the drive output of the motor into a rotary drive for the cutting tool and/or repetitive impacts which are imparted to the cutting tool  8  depending on the mode of operation of the drive transmission; 
   a mode change mechanism which is capable of switching the drive transmission between the at least two modes of operation; 
   a switch which, when activated, provides power to the motor; and 
   a lock on mechanism which, when activated, locks the switch in its activated state to maintain the power to the motor; 
   characterised in that there is further provided: 
   a first sensing apparatus which indicates the mode of operation of drive transmission; 
   a second sensing apparatus which indicates whether the lock on mechanism has been activated; and 
   a controller which is capable of interfering with the operation of the motor when power is provided to it by the switch and which monitors the signals from the first and second sensing apparatuses wherein, when the controller detects that the gear transmission is in at least one certain predetermined mode of operation and the lock on mechanism has been activated, it interferes with the operation of the motor. 
   In the proposed new design, the switch will be capable of being locked on in any mode of operation. However, when the rotary hammer is in certain predetermined modes of operation, such as, either pure drilling mode or combination drilling and hammering mode, the sensing apparatus will detect when an operator tries to “lock on” the hammer and deactivate or at least interfere with the running of the motor, for example by altering at least one of the amperage, the voltage, and the frequency of the electric current to the motor. 
   It will be appreciated that the drive transmission can be moved into the certain mode of operation or one of the several certain modes of operation and the lock on mechanism can be activated in any order, or simultaneously, prior to and in order for the controller to interfere with the operation of the motor. 
   It will also be appreciated that the first and second sensing apparatuses can either be single sensors or a plurality of sensors. Furthermore, the signals generated by the first and second sensing apparatuses can be transmitted to the controller using mechanical method or electrical, optical or radio signals or any other suitable method of transmission. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The lock on system according to the present invention will now be described with reference to the accompanying drawings of which: 
       FIG. 1  shows a side view of a hammer drill; 
       FIG. 2  shows a plan view of the latch mechanism; 
       FIG. 3  shows a side view of the latch mechanism; 
       FIG. 4  shows a perspective view of the latch mechanism; 
       FIG. 5  shows an exploded view of the latch mechanism; and 
       FIG. 6  shows a circuit diagram of the lock on system. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1 , the hammer drill comprises a body  2 , having a handle  4  attached to its rear. A tool holder  6  is mounted on the end of a spindle (not shown) on the front of the body  2  and which drivingly supports a drill bit  8  in well known manner. A motor  20  is mounted within the body  2  which drives the hammer drill. The motor is powered by a mains electricity supply which is supplied to the hammer drill via an electric cable  24 . 
   The hammer drill can operate in three different modes of operation. In the first mode, the motor rotatingly drives the spindle, which in turn drives the tool holder  6 , which in turn rotatingly drives the drill bit  8 . This is referred to as drill only mode. In the second mode, the motor reciprocatingly drives a ram (not shown) which is slideably mounted within the spindle and which repetitively strikes the end of the drill bit  8  via a striker (not shown). This is referred to as hammer only mode. In the third mode, the motor rotatingly both drives the spindle, which in turn drives the tool holder  6 , which in turn rotatingly drives the drill bit  8 , and reciprocatingly drives the ram, which is slideably mounted within the spindle and which repetitively strikes the end of the drill bit  8  via the striker. This is referred to as the combined hammer and drilling mode. 
   The mechanisms by which a hammer drill is able to perform the three modes of operation and is able to be changed between the three modes of operation are well known in the art and as such, are not described in any further detail. 
   The mode of operation of the hammer drill as shown in  FIG. 1  is altered by adjusting a knob  10  to select one of the three modes of operation  18 ,  14 ,  16  and then depressing the trigger button  12  which activates an electric motor  20  to drive the tool within that mode of operation. The release of the trigger button  12  cuts the power to the motor  20  and thus stops the tool from operating. 
   The electrical circuit which provides power to the motor  20  comprises an electrical switch  22 , which, is mechanically connected to the trigger button  12 , and a control switch  52  which switches are both in series with each other and the motor  20  (as best seen in  FIG. 6 ). The control switch  52  is operated by a controller  40 . The control switch  52  is normally maintained in a closed position allowing current to pass through it. Therefore, depression of the trigger button  12  closes the electric switch  22  allowing current to pass through it and thus activate the motor  20  (as the control switch is normally closed). 
   The three modes of operation are the drill only mode  14 , the combined hammer and drilling mode  16  and the hammer only mode  18 . 
     FIGS. 2 to 5  show the latch mechanism. The latch mechanism  26  comprises a casing  28  in which is slideably mounted a slider  30 . The slider can slide in the direction of arrow (E) within the casing  28 . A spring  32  biases the slider  30  towards the bottom end  34  of the casing  28 . Mounted within the casing  28  towards the bottom end  34  is a micro-switch  36 . When the slider is allowed to travel under the biasing force of the spring  32  to its maximum extent within the casing  28 , it engages with the micro-switch  36  and switches it on. The micro-switch is electrically connected to the central control unit  40  and sends a signal to the control unit  40  indicating whether it is switched on or off. An elongate slot  38  is formed within the casing  28 . A finger pad  42  is integrally formed with the slider  30  and when the slider is located within the casing  28 , projects through the elongate slot  38 . A user of the power tool can slide the slider  30  within the casing  28  by placing their finger on the finger pad  42  and sliding it along the length of the elongate slot  38 . Formed on one end of the slider  30  is a latch  44  which, when the slider  30  is slid to its maximum extent to the top end  46  the casing  28  projects through a hole formed in the top end  46  of the casing. The casing  28  is sealed with a lid  48  which keeps the slider and micro-switch and spring within the casing. 
   The latch mechanism  26  is located within the handle  4  of the rotary hammer below the trigger button  12  (see  FIG. 1 ). The finger pad  42  projects through a hole formed in the clamshell of the handle  4  and is accessible to a user and is located immediately below the trigger button  12 . In normal conditions, the finger pad  42  is biased to the bottom end  34  of the casing (downwardly in  FIG. 1 ), the latch  44  of the slider  30  being located entirely within the casing  28 . In order to use the power tool, an operator sets the mode switch  10  to an appropriate mode of operation  14 ,  16 ,  18  and then depresses the trigger button  12  to activate the rotary hammer. Upon release of the trigger button  12  which is biased outwardly by a spring (not shown), the rotary hammer is deactivated. However, when the trigger button  12  is depressed, the operator can then slide the slider  30  within the casing  28  by sliding the finger pad  42  towards the top end  46  of the casing causing the latch  44  to project from the casing  28  and engage with the trigger button  12 . When the finger pad  42  and hence slider  30  are at their maximum top position, the operator can release the trigger button  12  which engages with the latch  44  and thus is held in a depressed position and hence the rotary hammer is “locked on”. The slider  30  is prevented from returning to its bottom-most position by the force acting on the latch  44  by the trigger button  12  due to the biasing spring acting on the trigger button and a small ridge formed at the end of the latch  44 . 
   The latch mechanism  26  is capable of being operated when the rotary hammer switch  10  is located in any of the three modes of operation  14 ,  16 ,  18 . A sensor  50  is located adjacent the mode switch knob  10  and detects which mode the rotary hammer is in and communicates this information to the controller  40 . When the latch mechanism is operated, the slider  30  disengages from the micro-switch  36  thus sending a signal to the controller  40  that the “lock on” is being activated. The controller  40  then checks to determine what mode of operation the mode switch  10  is in by determining the output signal of the mode switch knob sensor  50 . If the sensor  50  indicates that the hammer is in the hammering only mode  18 , the hammer is able to continue normal operation. However, if the controller  40  detects that the latch mechanism  26  is being operated and that the rotary hammer is in either the drilling only mode  18  or the combined hammer and drilling mode  16 , it automatically switches off the motor  20  and prevents the rotary hammer from being used until either the latch mechanism  26  is deactivated or the rotary hammer is set into the purely hammer mode  18 . 
   In an alternative design, instead of completely switching the motor off, the controller  40  interferes with the running of the motor, possibly by altering at least one of the amperage, the voltage, and the frequency of the electric current to the motor from the values of the amperage, voltage, and frequency supplied by the tool&#39;s source of electric current. For example, the motor could be driven at a different speed, such as an extremely slow speed, to indicate to the operator that something is wrong. This can be achieved by introducing a high resistance into the power circuit by the controller  40  when the latch mechanism is operated and the hammer drill is not in the hammer only mode. Alternatively, the controller  40  could alter the drive torque, for example, by reducing it. The electric motor is normally capable of producing a rotational torque sufficient to drive the hammer drill in all of its normal operational requirements. If the drive torque is altered, preferably by being reduced, it would result in the motor slowing or stalling if a torque greater than that which the motor is capable of delivering after its drive torque had been altered, is applied to the motor.