Abstract:
A drilling device prevents recurrence of an overload condition after occurrence of the overload condition, thereby improving operability and safety in the drilling device. A motor for rotating a drill is connected to an AC power source through a motor control unit, a current detector, and a power switch. A magnet is also connected to the AC power source through the power switch and a full-wave rectifier. The motor control unit rotationally drives the motor on the basis of a signal sent from a main control unit according to a state in which a motor start switch is on. The main control unit controls the motor control unit to gradually reduce a supply voltage to the motor when the motor becomes overloaded, to gradually increase the voltage to the normal power supply condition when the overload condition is vanished, and to stop power supply to the motor if the overload condition continues for a predetermined period.

Description:
TECHNICAL FIELD 
     The present invention relates to a portable drilling device having a structure in which a body thereof can be secured to a workpiece by means of an electromagnet or the like, and a cutting tool, such as a drill, attached to an output shaft of a motor is manually moved toward and away from the workpiece. 
     BACKGROUND ART 
     As machine tools for drilling holes in a workpiece, there exist portable drilling devices having portability and a structure in which a body thereof can be secured to a workpiece by means of an attracting force of an electromagnet, a vise, or the like. Generally, drilling devices have a structure in which a drill is connected directly to a drive motor (hereinafter, referred to as “a motor” for short). A load acting on the drill significantly fluctuates according to a contact condition of the drill with the workpiece, a downward force applied to the drill, material properties of the workpiece, and so on. This fluctuation is transmitted directly to the motor. Therefore, in a case where a low-power motor is used, if an overload condition continues for a long period, the motor may burn out due to an overcurrent generated in the overload condition. 
     There is known a drilling device in which, for example, different first and second reference levels are defined for preventing overload of a motor. When a load current exceeds the first reference level, an alarm is generated. When the load current exceeds the second reference level, an alarm is generated and power supply to the motor is stopped. Such a drilling device is disclosed in, for example, Patent Document 1 below. 
     There is also known an electric drilling device in which a current flowing through a motor is interrupted when a load current exceeds a reference value, and power supply to the motor is automatically resumed after a lapse of a predetermined period after the load current is reduced to the reference value or less. Such an electric drilling device is disclosed in, for example, Patent Document 2 below.
     Patent Document 1: Japanese Examined Patent Application Publication No. 62-6295   Patent Document 2: Japanese Unexamined Patent Application Publication No. 2005-52914   

     DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention 
     In the configuration of the drilling device in Patent Document 1, if the load current exceeds the second reference level, a relay of a power supply circuit of the motor is turned off to stop power supply, and this state is maintained. Therefore, unless a power switch is turned off to reset the motor to the initial condition, power cannot be supplied again to the motor. In other words, it is troublesome to restart the motor, and thus there is room for improvement in workability. 
     In the configuration of the electric drilling device in Patent Document 2, after a lapse of the predetermined period after current interruption due to detection of an overload condition, a power of the same level as that before the current interruption is supplied to the motor even if the overload condition remains unchanged. In other words, a torque of the same level as that before the current interruption is generated, which is undesirable for safety reasons. 
     Further, if the motor is suddenly stopped by interrupting power supply to the motor according to detection of an overload condition, a rotational driving force by the motor is removed from a cutting edge of a cutting tool, while an inertia force and an urging force (an axial force of the drill) which urges the cutting edge against the workpiece are applied. In the resultant force of these forces applied from the cutting edge to the workpiece, the ratio of the axial component to the rotational component is larger than that in a state in which power is supplied to the motor. Thus, a reaction force applied from the workpiece to the cutting edge might cause breakage of the cutting edge, resulting in impossibility of continuous drilling operation. 
     In view of the foregoing, it is an object of the present invention to prevent recurrence of an overload condition by automatically restoring the motor if the overload condition is vanished after occurrence of the overload condition and by not restoring the motor if the overload condition continues for a predetermined period, thereby improving operability, safety, and workability. 
     Means for Solving the Problems 
     A portable drilling device according to the present invention includes a motor as a driving source for rotating a cutting tool such as a drill, a fixing unit for fixing a body including the motor to a workpiece, a motor control unit for rotationally driving the motor according to an on state of a motor start switch, and a main control unit for controlling the motor control unit. The main control unit includes a first control unit for controlling the motor control unit so as to reduce power supply to the motor when the motor becomes overloaded, and a second control unit for controlling the motor control unit so as to normally supply power to the motor when the overload condition is vanished. When the motor becomes overloaded, the first control unit is operated to reduce a supply voltage to the motor, while when the overload condition is vanished, the second control unit is operated to automatically restore power supply to the normal condition, thereby preventing recurrence of the overload condition. 
     Specifically, the first control unit can gradually reduce the supply voltage to the motor when the motor becomes overloaded. 
     More specifically, the second control unit can gradually increase the supply voltage to the motor to the normal power supply condition when the overload condition is vanished. 
     The main control unit may further include a third control unit for controlling the motor control unit so as to stop power supply to the motor if the overload condition continues for a predetermined period. 
     Specifically, the main control unit can control the motor control unit so as to start power supply to the motor when the motor start switch is turned on in a state in which the third control unit is in operation. 
     More specifically, when the first, second, and third control units are operated, the main control unit can execute display control such that a light-emitting element is turned on in respective different colors and/or different ways according to respective control states of the first, second, and third control units. 
     The main control unit may further include a fourth control unit. When a load current of the motor is not less than a predetermined reference value and the supply voltage to the motor becomes a predetermined value according to operation of the first control unit, the fourth control unit maintains a motor drive state for a predetermined period with the supply voltage to the motor being at the predetermined value. 
     Specifically, the main control unit can define the predetermined value of the motor supply voltage to be a value at which the motor does not burn out even in a locked state. 
     More specifically, the main control unit can gradually increase the supply voltage to the motor to the normal power supply condition by means of the second control unit when the load current of the motor is less than the reference value, in a case where the fourth control unit is operated. 
     Furthermore, the fixing unit is made of magnet for fixing the body including the motor to a workpiece by means of an electromagnetic force. The main control unit can rotationally drive the motor by means of the motor control unit according to a state in which the magnet is energized. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an external view of a magnetic base drilling device according to an embodiment of the present invention. 
         FIG. 2  is a block diagram showing a structure of the magnetic base drilling device according to the present invention. 
         FIG. 3  is a flow chart showing the operation of the magnetic base drilling device according to the present invention. 
         FIG. 4  is a flow chart showing processes following  FIG. 3 . 
         FIG. 5  is a flow chart showing the operation of a magnetic base drilling device according to another embodiment of the present invention. 
         FIG. 6  is a flow chart showing processes following  FIG. 5 . 
     
    
    
     EXPLANATION OF REFERENCE SYMBOLS 
     
         
           1  magnetic base drilling device 
           2  body 
           3  chuck 
           4  operating handle 
           6  carrying handle 
           10  AC power source 
           12  motor 
           14  main control unit 
           14   a  ROM 
           16  magnet 
           20  MG disconnection detector 
           22  step-down transformer 
           24  zero-cross detector 
           28  motor control unit 
           30  current detector 
           32  signal amplifier 
           34  display 
           36  power switch 
           38  motor start switch 
           40  motor stop switch 
       
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A magnetic base drilling device according to an embodiment of a portable drilling device of the present invention will now be described with reference to  FIGS. 1 to 4 .  FIG. 1  is an external view of a magnetic base drilling device according to the present invention.  FIG. 2  is a block diagram showing a control structure of the magnetic base drilling device. The magnetic base drilling device  1  is mainly composed of a body  2 , and a supporting part  6  for supporting a cutting tool held by a chuck  3  such that the cutting tool can be moved toward and away from a workpiece by rotating a operating handle  4 . The magnetic base drilling device  1  includes a motor  12  powered by, for example, a 100 V AC power source  10 , a main control unit  14  for motor control and display control of the motor control status, a magnet (MG)  16  for, when energized, generating a predetermined magnetic force, a full-wave rectifier  18  for supplying to the magnet  16  a DC output obtained by full-wave rectifying of the AC power source  10 , a MG disconnection detector  20  for detecting disconnection of the magnet  16 , a step-down transformer  22  for transforming the AC power source  10  to a predetermined low voltage, a zero-cross detector  24  for detecting zero-cross of a low-voltage output of the step-down transformer  22 , a DC power source  26  for supplying DC power to the main control unit  14  and the like, a motor control unit  28  for controlling the rotation of the motor  12 , a current detector  30  for detecting a current flowing through the motor  12 , a signal amplifier  32  for amplifying a detection signal generated by the current detector  30 , a display  34  connected to the main control unit  14  and performing alarm display by means of an LED, a power switch  36  for turning on/off the power source of the whole of the magnetic base drilling device  1 , a motor start switch  38  for turning on power supply to the motor  12 , and a motor stop switch  40  for turning off power supply to the motor  12 . 
     The main control unit  14  is, for example, a peripheral interface controller (PIC) composed of a one-chip microcomputer which incorporates a CPU and an A/D converter. A built-in ROM  14   a  stores programs for executing processes shown in  FIG. 3 , for example. The PIC series by MICROTIP TECHNOLOGY is an example of commercially available PIC. 
     The magnet  16  has a core and a winding which, when applied with DC power from the full-wave rectifier  18 , can generate a magnetic attracting force for fixing the body of the magnetic base drilling device  1  to a workpiece. The magnet  16  is disposed in a base portion or the like of the magnetic base drilling device  1 . 
     The MG disconnection detector  20  has a circuit which, in conjunction with a power supply switch for the magnet  16 , detects whether or not the magnet  16  is energized to detect disconnection of the magnet  16 , so that it is possible to give an alarm for disconnection of the magnet  16 . 
     The step-down transformer  22  is a low-power transformer which has a primary winding connected to the AC power source  10  and a secondary winding for supplying a low AC voltage to the zero-cross detector  24  and the DC power source  26 . 
     The zero-cross detector  24  has a circuit configuration in which the timing at which a sine wave of the AC power source  10  crosses the zero level is detected by means of a photocoupler or the like and the detected timing is transmitted to the main control unit  14 . 
     The DC power source  26  includes a full-wave rectifier for full-wave rectifying of an output of the secondary winding of the step-down transformer  22 , and a smoothing circuit and a voltage stabilizing circuit for respectively smoothing and stabilizing a DC output full-wave rectified by the full-wave rectifier. The DC power source  26  supplies the generated DC output to the main control unit  14  and other circuits. 
     The motor control unit  28  includes, for example, a triac, which is one of semiconductor controlling elements, and a controlling circuit for controlling a gate of the triac. 
     The current detector  30  includes, for example, a current transformer (CT) connected in series to the motor  12 . A detection signal generated by the current detector  30  is amplified by means of the signal amplifier  32  having an operational amplifier and then transmitted to the main control unit  14 . The detection signal transmitted to the main control unit  14  is converted to a digital value by means of the A/D converter incorporated in the main control unit  14 . 
     The display  34  includes, for example, an LED which can emit green, yellow, and red light. Under control of the main control unit  14 , this LED can emit light in any one of the above colors, as well as emit light in various lighting modes, such as emitting light continuously, blinking at long intervals, and blinking at short intervals. In this embodiment, the following lighting modes are provided for the display  34 . 
     Pattern 1: lighting in green (in operation) 
     Pattern 2: lighting in red (alarm) 
     Pattern 3: lighting in yellow (warning) 
     Pattern 4: high-speed blinking in red (alarm) 
     Pattern 5: blinking in green (alarm) 
     Pattern NG: blinking in red (alarm) 
       FIG. 3  is a flow chart showing the operation of the magnetic base drilling device  1 .  FIG. 4  is a flow chart showing processes following  FIG. 3 . In  FIG. 3 , steps S 101  to S 107  show processes for energizing the magnet  16  and confirming the energization, while steps S 201  to S 213  show processes for operating the motor  12  in the normal condition. In  FIG. 4 , steps S 301  to S 309  show processes for operating the motor  12  in the overload condition. Steps S 303  and S 304  show processes for measuring an overload time. In the processes described below, four reference values Vref (Vref 1 , Vref 2 , Vref 3 , Vref 4 ) are used, which have the relationship of Vref 1 &gt;Vref 2 &gt;Vref 3 &gt;Vref 4 . 
     First, the main control unit  14  determines whether or not the power switch  36  is turned on (S 101 ). If the power-on state is not confirmed, the main control unit  14  stands by, while if the power is on (S 101 : yes), the main control unit  14  energizes the magnet  16  (S 102 ). The main control unit  14  then determines whether or not the magnet  16  is energized, according to an output of the MG disconnection detector  20  (S 103 ). If the output of the MG disconnection detector  20  is not NG, i.e., the energization is confirmed (S 103 : no), the main control unit  14  turns on the LED of the display  34  in green (S 104 ). If the energization is not confirmed (S 103 : yes), the main control unit  14  blinks the LED of the display  34  in red (S 105 ). A user can notice abnormality according to this light, thereby taking action, such as turning off the power, inspecting the device, and the like. Then, the main control unit  14  determines whether or not the power switch  36  is turned off (S 106 ). If the power-off state is confirmed (S 106 : yes), the main control unit  14  ends the process (END). If the power is not off (S 106 : no), the main control unit  14  determines whether or not the motor start switch  38  is on (S 107 ). If the switch is on (S 107 : yes), the main control unit  14  activates the motor control unit  28  to start the motor  12  (S 201 ). If the on state of the motor start switch  38  is not confirmed (S 107 : no), the main control unit  14  returns the process back to step S 103  and then executes subsequent processes. 
     The main control unit  14  activates the motor control unit  28  to rotationally drive the motor  12  by phase control based on a predetermined energization angle. In this case, the main control unit  14  uses zero-cross pulse signals detected by the zero-cross detector  24  as external interrupt signals, thereby executing a process for activating motor control by the motor control unit  28  at every half cycle. Then, the main control unit  14  again determines whether or not the magnet  16  is energized, according to the output of the MG disconnection detector  20  (S 202 ). If the energization is confirmed (S 202 : no), the main control unit  14  turns on the LED of the display  34  in green (S 203 ), while if the energization is not confirmed (S 202 : yes), the main control unit  14  blinks the LED of the display  34  in red (S 212 ). After confirming the energization of the magnet  16 , the main control unit  14  then reads a load current value IL detected by means of the current detector  30  and the signal amplifier  32  (S 204 ). 
     Then, the main control unit  14  determines whether or not the motor stop switch  40  is manipulated (S 205 ). If the motor stop switch  40  is manipulated, the main control unit  14  interrupts power supply to the motor  12  (S 213 ), and then returns the process back to step S 103 . If the motor stop switch  40  is not manipulated, the main control unit  14  determines whether or not the read load current value IL is larger than the reference value Vref 1  (S 207 ). The reference value Vref 1  is a level at which the motor  12  should be immediately stopped, i.e., a reference current value by which whether or not the motor  12  is overloaded is determined 
     At step S 207 , if the condition of IL≦Vref 1  is satisfied, i.e., the motor is not overloaded (S 207 : no), the main control unit  14  now compares the load current value IL with the reference value Vref 2  (S 208 ). The reference value Vref 2  is a current value corresponding to a high load condition. The motor  12  is not necessarily stopped at this current value. If the condition of IL&gt;Vref 2  is satisfied at step S 208  (S 208 : yes), the main control unit  14  turns on the LED of the display  34  in red to warn the user of possibility of an overload condition (S 210 ), and then advances the process to step S 204  and executes subsequent processes. 
     If the condition of IL≦Vref 2  is satisfied at step S 208  (S 208 : no), the main control unit  14  compares the load current value IL with the reference value Vref 3  (S 209 ). The reference value Vref 3  is a reference current value by which whether or not the motor is operated in the normal load condition is determined. If the condition of IL&gt;Vref 3  is satisfied (S 209 : yes), the main control unit  14  turns on the LED of the display  34  in yellow to warn the user that the load is large (S 211 ), and then advances the process to step S 204 . If the condition of IL≦Vref 3  is satisfied (S 209 : no), the main control unit  14  does not activate the display  34  because the motor is operated in the normal load condition, and then advances the process to step S 203  and executes subsequent processes. 
     If the condition of IL&gt;Vref 1  is satisfied at step S 207  (S 207 : yes), the main control unit  14  executes control for forcibly reducing the supply voltage to the motor  12  in order to avoid the overload condition (S 301  in  FIG. 4 ). The main control unit  14  then blinks the LED of the display  34  in red at a high frequency (S 302 ) to warn the user that the driving force of the motor has been changed. The main control unit  14  now receives zero-cross signals from the zero-cross detector  24 , and then starts zero-cross counting (S 303 ). Then, the main control unit  14  compares the zero-cross count value at step S 303  with a predetermined value n (S 304 ). For example, a zero-cross count value of n represents a lapse of several seconds after occurrence of the overload condition. 
     If the condition of “zero cross count value≧n” is not satisfied at step S 304  (S 304 : no), the main control unit  14  compares the load current value IL with the reference value Vref 4  (S 305 ). The reference value Vref 4  is a reference current value by which whether or not the load is reduced is determined, after the supply voltage is once forcibly reduced due to detection of the overload condition by the current detector  30  and the signal amplifier  32 . If the condition of IL&lt;Vref 4  is satisfied at step S 305 , the main control unit  14  controls the motor control unit  28  to gradually increase the motor supply voltage (gradual increase in motor supply voltage) (S 306 ), and then advances the process to step S 201  in  FIG. 3 . With the control at step S 306 , it is possible to prevent the motor  12  from suddenly rotating at a high speed, whereby operability and safety can be improved. 
     If the condition of “zero-cross count value≧n” is satisfied at step S 304  (S 304 : yes), the main control unit  14  sends to the motor control unit  28  a signal for completely stopping the motor  12 . In response to the signal, the motor control unit  28  stops power supply to the motor  12  (S 307 ). Then, the main control unit  14  blinks the LED of the display  34  in green (S 308 ) to warn the user that the motor  12  has been temporarily forcibly stopped. Further, the main control unit  14  determines whether or not the motor start switch  38  is turned on (S 309 ). If the motor start switch  38  is on (S 309 : yes), the main control unit  14  advances the process to step S 201  in  FIG. 3  to execute control for operating the motor in the normal condition. As described above, after a lapse of a predetermined period after occurrence of the overload condition, the motor  12  cannot be activated unless the user manipulates the motor start switch  38 , whereby safety is improved. If the motor start switch  38  is off (S 309 : no), the main control unit  14  advances the process to step S 307  and then executes subsequent processes. 
     With the above-described control, it is possible to improve operability, to prevent a cutting tool such as a drill from being damaged, to prevent the motor from burning out, and to improve safety in the magnetic base drilling device  1 . 
       FIG. 5  is a flow chart showing processes in a magnetic base drilling device according to a second embodiment of the portable drilling device according to the present invention.  FIG. 6  is a flow chart showing processes following  FIG. 5 . In  FIGS. 5 and 6 , steps having the same processes as those in steps in  FIGS. 3 and 4  are denoted by the same numerals. 
     First, the main control unit  14  determines whether or not the power switch  36  is turned on at step S 101  in  FIG. 5 . If the power-on state is not confirmed, the main control unit  14  stands by, while if the power is on, the main control unit  14  energizes the magnet  16  (S 102 ). The main control unit  14  then determines whether or not the magnet  16  is energized (S 103 ). If the energization of the magnet  16  is not confirmed (S 103 : no), the main control unit  14  blinks the LED of the display  34  in red (S 105 ). These processes are the same as those in the embodiment shown in  FIG. 3 . In the second embodiment shown in  FIG. 5 , if the energization of the magnet  16  is confirmed (S 103 : no), the main control unit  14  advances the process to step S 108  for determining whether a frequency of the AC power source  10  is 50 Hz or 60 Hz. Specifically, the main control unit  14  detects the power source frequency according to a count value obtained by counting zero-cross pulses detected by the zero-cross detector  24  for 0.2 second. 
     Then, the main control unit  14  turns on the LED of the display  34  in green (S 104 ), and determines whether or not the power switch  36  is turned off (S 106 ). If the power-off state is confirmed (S 106 : yes), the main control unit  14  ends the process (END). If the power is not off (S 106 : no), the main control unit  14  determines whether or not the motor start switch  38  is on (S 107 ). If the switch is not on (S 107 : no), the main control unit  14  returns the process back to step S 103 . 
     If the switch is on at step S 107  (S 107 : yes), the main control unit  14  advances the process to a process loop for operating the motor in the normal condition. As is the case with  FIG. 3 , the main control unit  14  activates the motor control unit  28  to start the motor  12  (S 201 ). Then, the main control unit  14  determines whether or not the magnet  16  is energized (S 202 ). If the energization is confirmed (S 202 : no), the main control unit  14  turns on the LED of the display  34  in green (S 203 ), while if the energization is not confirmed (S 202 : yes), the main control unit  14  blinks the LED of the display  34  in red (S 212 ). After confirming the energization of the magnet  16 , the main control unit  14  now reads the load current value IL (S 204 ), and then determines whether or not the motor stop switch  40  is manipulated (S 205 ). If the motor stop switch  40  is manipulated (S 205 : yes), the main control unit  14  stops the motor  12  (S 213 ) and then returns the process back to step S 103 . If the motor stop switch  40  is not manipulated (S 205 : no), the main control unit  14  compares the read load current value IL with the reference values Vref (S 207  to S 209 ). The reference value Vref 1  used at step S 207  is a current value by which whether or not the motor  12  is overloaded is determined. If the condition of IL≦Vref 1  is satisfied, i.e., the motor is not overloaded, the main control unit  14  then compares the load current value IL with the reference value Vref 2  (S 108 ). The reference value Vref 2  is a current value corresponding to a high load condition. The motor  12  is not necessarily stopped at this current value. If the condition of IL&gt;Vref 2  is satisfied at step S 208 , the main control unit  14  turns on the LED of the display  34  in red to warn the user of possibility of an overload condition, and then advances the process to step S 204  and executes subsequent processes. 
     If the condition of IL≦Vref 2  is satisfied at step S 208  (S 208 : no), the main control unit  14  compares the load current value IL with the reference value Vref 3  (S 209 ). The reference value Vref 3  is a reference current value by which whether or not the motor is operated in the normal load condition is determined. If the condition of IL&gt;Vref 3  is satisfied (S 209 : yes), the main control unit  14  turns on the LED of the display  34  in yellow to warn the user that the load is large (S 211 ), and then advances the process to step S 204 . If the condition of IL≦Vref 3  is satisfied (S 209 : no), the main control unit  14  does not activate the display  34  because the motor is operated in the normal load condition, and then advances the process to step S 203  and executes subsequent processes. 
     If the load current exceeds the reference value Vref 1  at step S 207  (S 207 : yes), the main control unit  14  executes processes for operating the motor in the overload condition in  FIG. 6 . First, the main control unit  14  gradually reduces the supply voltage to the motor  12 , for several seconds (for example, four seconds), to X % of the supply voltage (for example, 35% of the rated voltage) at which there is no risk of burning out of the motor even if the motor  12  comes into a locked state (S 310 ). The main control unit  14  then blinks the LED of the display  34  in red at a high frequency (S 302 ) to warn the user that the driving force of the motor has been changed. 
     Then, the main control unit  14  compares the load current value IL with the reference value Vref 4  (S 311 ). The reference value Vref 4  is a reference value by which whether or not the load is reduced is determined, after the motor  12  is determined to be overloaded. If the condition of IL&lt;Vref 4  is satisfied (S 311 : yes), the main control unit  14  advances the process to step S 306  to execute control for gradually increasing the motor supply voltage. If the condition of IL≧Vref 4  is satisfied (S 311 : no), the main control unit  14  determines whether or not the control at step S 310  is completed, i.e., whether or not the motor supply voltage is reduced to X % (S 312 ). If the motor supply voltage is not reduced to X % (S 312 : no), the main control unit  14  returns the process back to step S 310  and then executes subsequent processes. If the motor supply voltage is reduced to X % (S 312 : yes), the main control unit  14  controls the motor control unit  28  so as to maintain the motor supply voltage at X % (S 313 ). 
     Then, the main control unit  14  receives zero-cross signals from the zero-cross detector  24  and performs zero-cross counting (S 303 ) in order to determine whether or not the motor control with the motor supply voltage maintained at X % is kept for a predetermined period (for example, several seconds). The main control unit  14  now compares the zero-cross count value at step S 303  with the predetermined value n (S 304 ). If the condition of “zero-cross count value≧n” is not satisfied (S 304 : no), the main control unit  14  determines whether or not the condition of IL&lt;Vref 4  is satisfied (S 305 ). 
     If the condition of IL≧Vref 4  is satisfied (S 305 : no), the main control unit  14  returns the process back to step S 313  to continue to maintain the motor supply voltage at X %. If the condition of IL&lt;Vref 4  is satisfied (S 305 : yes), i.e., the load is reduced after the motor  12  is determined to be overloaded, the main control unit  14  controls the motor control unit  28  to gradually increase the motor supply voltage (S 306 ). The main control unit  14  then determines whether or not the motor supply voltage reaches 100% (S 315 ). If the motor supply voltage does not reach 100%, the main control unit  14  returns the process back to step S 306  to control the motor control unit  28  to gradually increase the motor supply voltage. If the motor supply voltage reaches 100%, the main control unit  14  advances the process to step S 201  in  FIG. 5  and then executes subsequent processes. 
     If the condition of “zero-cross count value≧n” is satisfied (S 304 : yes), i.e., the predetermined period has past, the main control unit  14  stops power supply to the motor  12  (S 307 ). The main control unit  14  then blinks the LED of the display  34  in green (S 308 ) to warn the user that the motor  12  has been temporarily forcibly stopped. Then, the main control unit  14  determines whether or not the motor start switch  38  is turned on (S 309 ). If the motor start switch  38  is on (S 309 : yes), the main control unit  14  advances the process to step S 201  in  FIG. 5  to execute control for operating the motor in the normal condition. If the motor start switch  38  is off (S 309 : no), the main control unit  14  advances the process to step S 307  and then executes subsequent processes. 
     As described above, according to the processes in the second embodiment, the motor supply voltage is gradually reduced at step S 310 , whereby the force of the motor is reduced to prevent the cutting edge of the cutting tool from being damaged. Further, as is the case with the first embodiment, the motor supply voltage is maintained at the predetermined value at steps S 312  and S 313  in a state in which the load current value IL is larger than the reference value Vref 4 . Then, the motor is stopped or operated in the normal condition, according to the overload condition. Therefore, even if the process at step S 307  for completely stopping the motor  12  is executed, damage on the cutting edge of the cutting tool can be prevented because the force has already been reduced. Further, as is the case with the first embodiment, it is possible to improve workability and to prevent the motor from burning out in the magnetic base drilling device. 
     Although some embodiments of the magnetic base drilling device according to the present invention have been described above, the main control unit  14  is not necessarily limited to a PIC. Alternatively, the main control unit  14  may be an integrated circuit (IC) or a circuit specifically designed to execute the processes in  FIG. 3 . In the motor control unit  28 , a triac, which is suitable for simplifying a circuit configuration, is used as a semiconductor controlling element. Alternatively, other elements such as a gate turn off thyristor (GTO), and an insulated gate bipolar transistor (IGBT) may be used. 
     Further, the magnetic base drilling device  1  may be provided with an acceleration sensor for detecting occurrence of sideslip and the like of the magnetic base drilling device  1  in order to give an alarm of occurrence of the sideslip and the like. In the embodiments described above, a single LED is used for giving an alarm and a warning. Alternatively, three LEDs may be used, each of which emits monochromatic light of green (or blue), red, or yellow (or orange). Instead of the LED, text messages or pictographic characters of warning may be displayed on a liquid crystal display or the like. Further, instead of the light-emitting element, an acoustic device (such as alarm call, warning sound, and voice message) may be used. Furthermore, the process at step S 108  in  FIG. 5  may be added between steps S 103  and S 104  in  FIG. 3 .