KR20000012975A - Device of cutting load indication and overload cut-off for electromotive machine tool - Google Patents

Abstract

PURPOSE: A device of cutting load indication and overload cut-off for an electromotive machine tool is provided to prevent safety accidents and to minimize the damage of a motor due to over current. CONSTITUTION: The device comprises:a converter(206) to supply driving power to a motor and to output the driving power changed according to the torque value of the motor after inducing; a rectifying piece(207) to rectify the voltage induced in the converter to D.C. voltage; a current variable value detecting piece(208) to detect the variable degree of the current attained in the rectifying piece sequentially and then to output the value; and an indication device(212) to indicate the detected variable degree of the current sequentially and to control the load given to a cutting blade(202). Thereby the cutting load state is easily identified by optical colors and lighted numbers of an indication device.

Description

Cutting load indication and overload breaker of power tool

The present invention relates to a manual grinder for cutting (hereinafter referred to as "power tool"), and more specifically to a variety of construction materials (hereinafter referred to as "work material"), such as granite plate, tile, brick, concrete block The present invention relates to a cutting load display and an overload cut-off device of a power tool which displays the cutting load state according to various working conditions during manual cutting operation to select an appropriate working condition and prevent safety accidents caused by the overload of the power tool. .

In general, a manual grinder for cutting a workpiece has a reference current value for determining an overcurrent in a microcomputer provided in the motor drive device in order to prevent burnout due to overcurrent of the motor.

That is, the motor driving apparatus compares the overcurrent detection circuit for detecting the load current of the motor and the load current value of the motor detected by the overcurrent detection circuit with a predetermined reference current value so that the detected load current value is equal to the reference current value. If the value is exceeded, it is equipped with a microcomputer that outputs an overcurrent detection signal by judging it as an overcurrent, and is configured to temporarily stop the motor through a drive circuit when an overcurrent of the motor occurs, thereby preventing burnout by overcurrent of the motor.

A device as shown in FIG. 1 is known as an example of a conventional device including a microcomputer and an overcurrent detection circuit in a general motor driving device to prevent burnout of the motor due to overload.

A grinder for cutting a workpiece in the motor driving apparatus of the grinder according to the prior art will be described as an electric tool.

The conventional power tool 10 is a tool body 101 that is largely held by the operator with both hands, and performs work such as cutting and grinding workpieces, and the tool body 101 is connected to the tool body 101 by a cable 115. It consists of a control device 100 for controlling the.

Tool body 101 is provided with a cylindrical housing 116 is configured in a so-called port shape.

The stator 102 and the rotor 103 are incorporated in the housing 116 to form one motor 105.

The rotating shaft 104 to which the rotor 103 is fixed is coupled to the inside of the housing 116 by the bearings 106 and 114 to be freely rotated. One end of the rotary shaft 104 protrudes outward of the housing 116 and is fastened to the outermost end of the rotary shaft 104 with the support 107 and the cutting blade 108 mounted on the protruding end thereof. 109).

In addition, the upper portion of the housing 116 is covered by the mounting of the cover 110.

On the other hand, a printed circuit board 111 is disposed in the inner space surrounded by the cover 110, the light emitting element 112 is fixed on the printed circuit board 110 by soldering or the like. At this time, the upper portion of the light emitting element 112 is protruded to the upper surface of the cover 110 so that the operator using the tool body 101 can be checked from above.

In addition, the printed circuit board 111 is connected to a signal line 113 for transmitting a signal supplied from the outside to control the point, the light-off state of the light emitting element 112, the signal line 113 is the tool body 101 It is connected to the control apparatus 100 via the cable 115 which supplies a drive electric power to it.

The control device 100 includes an overcurrent detection circuit for detecting a current generated by the motor 105, a microcomputer for determining an overcurrent by comparing the detected current value with a set reference current value, and the motor by the determined current value. And a drive circuit for controlling the driving of the 105. As shown in FIG.

Looking at the operation of the motor drive device of the power tool according to the prior art configured as described above are as follows.

First, when a commercial AC power source, for example, 220V is applied to cut or polish the workpiece, the control device 100 of the power tool 10 receives the input commercial AC voltage through the cable 115 to the motor 105. To the rotor 103.

Accordingly, the rotor 103 of the motor 105 is rotated about the stator 102 by the input AC voltage, and the rotating shaft 104 coupled with the rotor 103 is rotated.

When the rotary shaft 104 rotates, the cutting edge 108 mounted at the outer end rotates at a high speed to cut or polish the workpiece.

As described above, when the cutting or polishing operation is performed by using the tool body 101, the overcurrent detection circuit of the control device 100 periodically detects a current state of the motor 105 generated according to the work condition of the workpiece and performs a logical operation. The voltage is supplied to the microcomputer.

The microcomputer compares the value of the current detected by the overcurrent detection circuit with the value of the reference current set in advance, and determines that the current exceeds the value of the reference current, and outputs an overcurrent detection signal.

The overcurrent detection signal is supplied to the printed circuit board 111 through the signal line 113 to turn on the light emitting device 112 and is also supplied to the drive circuit.

Accordingly, the operator who is working with the tool body 101 for cutting or polishing, etc., recognizes that the overcurrent state has occurred through the recognition of the lighting state of the light emitting device 112, and the drive circuit of the control device 100 The overcurrent detection signal generated by the microcomputer stops the driving of the motor 105 to prevent burnout due to the overcurrent of the motor 105.

The motor driving apparatus of the power tool according to the related art described above, when the motor of the tool main body rotates by the driving power from the control device to perform the operation, the control device detects the overload of the motor and transmits the light to the light emitting element of the tool main body. Notice and stop the motor.

Therefore, there is a problem in that the user cannot appropriately and promptly respond to the overcurrent generation only by the configuration of notifying the main body of the power tool when the overcurrent of the motor occurs and stopping the motor.

In addition, when using the conventional power tools as described above, there is a problem that the operator must manually adjust the following parameters in applying the cutting load to the workpiece.

(1) Variables of cutting load should be adjusted according to physical characteristics and work habits of each worker.

(2) Variables of cutting load are to be adjusted according to various dimensions such as hardness and thickness of workpiece.

(3) Variables of cutting load depending on the quality difference of the diamond attached to the outer edge of the cutting edge, that is, the various quality differences of the diamond cutting edge and the softness of the bond (sintered body of metal powder holding diamond particles). Must be adjusted accordingly.

According to the above three variables, general workers as well as professional workers should rely on the human sense only to impose appropriate working loads. There are many risk factors in the cutting operation itself such that the cutting edge (the sintered body of the diamond-containing metal powder) is broken during the operation, resulting in a fatal accident for the operator's safety.

In addition, as a way to prevent safety accidents caused by cutting loads imposed on the workpiece, each reference current value is set differently according to various working conditions rather than fixed by setting one reference current value in the microcomputer. Although the method may be considered, there is a problem in that it is difficult to develop a separate program, and also have a difficulty in arranging and designing a plurality of light emitting devices in the tool body according to each working condition.

Therefore, there is a need for the development of cutting load display devices for power tools that are built or attached to existing power tools to prevent the occurrence of safety accidents caused by various working conditions and minimize the damage of the motor due to overcurrent. It's happening.

Accordingly, the present invention is a method for preventing burnout of the motor due to overload and human accidents caused by incorrectly applied cutting loads according to various working variables. By detecting the current load state of the motor and expressing it through a plurality of display means (usually light emitting diodes) installed on the body of the power tool to be visually recognized by the operator, the operator adjusts the cutting load appropriately. It is an object of the present invention to provide a cutting load display device for a power tool that enables working.

Another object of the present invention is to detect the overload state of the motor from the amount of change in the input current to automatically cut off the drive power of the motor when the overload of the motor reaches a dangerous level to protect the operator and to prevent damage caused by overcurrent of the motor It is an object of the present invention to provide an overload blocking device for power tools.

Another object of the present invention is to facilitate the re-operation of the power tool after the interruption of the driving power due to the overload of the motor.

1 is a cross-sectional schematic diagram showing an electrical configuration for explaining the motor drive device of the grinder according to the prior art,

Figure 2 is a block diagram showing an embodiment provided for the description of the cutting load display and overload cut-off device of the power tool according to the invention,

3 is a circuit diagram showing the cutting load display and the overload cut-off window of the power tool in FIG. 2 in more detail.

<Description of Symbols for Main Parts of Drawings>

200: power tool 201: tool body

202: cutting edge 203: main switch

204: reset switch 206: current transformer

207: rectifier 208: first current change value detector

212 display device 213 voltage control unit

214: voltage interrupter 215: reset part

Cutting load display device of the power tool according to the present invention for achieving the above object, according to the torque value of the motor by the friction force applied to the cutting edge when cutting the workpiece to supply the drive power to the motor of the tool body A current transformer for inducing and outputting the changed driving power; Rectifying means for rectifying a voltage obtained by being induced in the current transformer into a DC voltage; Current change value detecting means for detecting and outputting a change amount of current obtained by the rectifying means step by step; And a display device for controlling the load imposed on the cutting edge by displaying the degree of change of the current detected by the current change value detecting means in stages.

The current change value detecting means is a means for detecting a change amount of a threshold value, an appropriate value change amount, an intermediate value change amount and a no load change amount by comparing a change value of the current obtained by the rectifying means with a reference current value set at different levels. It is preferable to configure to include.

More preferably, the detection means of any one of the threshold value detection means, the titration value detection means, the intermediate value detection means and the no-load value detection means,

(1) an input current varying means for satisfying the conditions of the cutting load by variably setting the value of the current obtained by varying in the rectifying means according to the cutting load;

(2) reference current setting means for setting a reference current to be compared with a change value of the current; And

(3) In the input current varying means, a current comparing means for comparing the change value of the current with the reference current value and supplying the resultant value to the display device.

In addition, it is preferable that the display device includes at least three light emitting elements in order to display the degree of change of the current detected by the current change value detecting means in stages.

In addition, if the degree of change of the current detected from the current change value detecting means is a current value that reaches a dangerous level may be configured to additionally include a voltage interrupting means for cutting off the drive power supplied from the current transformer to the tool body.

In order to achieve the above objects, an overload interruption device of a power tool according to another embodiment of the present invention supplies a driving power to a motor of a tool body and torques of the motor due to a friction force applied to a cutting edge when cutting a workpiece. A current transformer for inducing and outputting the driving power that varies according to a value; Rectifying means for rectifying a voltage obtained by being induced in the current transformer into a DC voltage; Current change value detecting means for detecting and outputting a change amount of current obtained by the rectifying means step by step; A voltage interrupt control means for selecting a maximum change amount value from the detected change amount of the current and generating a control signal for cutting off the driving power according to the change amount of the selected current; Voltage interrupting means for interrupting driving power supplied from the current transformer to the tool body by the voltage interrupting control means; And a display device for adjusting the cutting load imposed on the cutting edge by displaying the degree of change of the current detected by the current change value detecting means in stages.

It may be configured to additionally include a reset means for supplying the drive power of the current transformer to the tool body by resetting the voltage interrupt control means after the driving power is cut off by the voltage interrupting means.

With this configuration, the operator can adjust the cutting load of the power tool by detecting and expressing the overload condition of the motor in multiple stages with the change of input current according to the working condition and cutting edge state when cutting with the power tool. Work, and can automatically cut off the driving power supplied to the power tool.

As a result, even when cutting with excessive load, the driving power is automatically cut off to protect the operator, and the cutting load is expressed in multiple stages, which improves work efficiency and increases the service life of the cutting edge. There is an advantage.

In addition, there may be a plurality of embodiments of the present invention. Hereinafter, the most preferred embodiments will be described in detail.

Through this preferred embodiment, the objects, features and advantages of the present invention will be more clearly understood.

Hereinafter, with reference to the accompanying drawings will be described in detail a preferred embodiment of the cutting load display and overload blocking device of the power tool according to the present invention.

In addition, the technical idea described in the claims of the present invention may be applied to various types of power tools, such as a grinder for cutting and polishing a workpiece using driving power.

Therefore, the power tools shown in FIGS. 2 and 3 are not limited to a specific power tool, but use cutting power to manually cut, polish, and drill various construction materials including granite sheets, tiles, bricks, concrete blocks, and the like. Drilling tools include various types of power tools.

In addition, in each drawing used for description, the same code | symbol is attached | subjected and shown about the same component.

In addition, in the following description, the manual grinder with the cutting edge which consists of a sintered compact containing a diamond is used as a specific example of a power tool.

Figure 2 is a block diagram showing an embodiment provided for the description of the cutting load display and overload cut-off device of the power tool according to the present invention.

The power tool 200 according to the present embodiment includes a motor, a cutting blade 202 and a main switch 203 connected to a rotating shaft of the motor and a commercial AC power source input through the plug 205. The tool main body 201 and the plug AC 205 are supplied to the motor of the tool main body 201 via a primary coil, and the cutting edge A current transformer 206 that induces and outputs the AC voltage changed according to the torque value of the motor due to the frictional force imposed on the second side, and rectifies the DC voltage induced by the current transformer 206 into DC voltage. And detecting and outputting a step value, an appropriate value, an intermediate value, and a no load value step by step from the change value of the rectifying unit 207 to be outputted and the current rectified by the rectifying unit 207 according to the change in the torque value of the motor. 1st to 4th war A voltage interruption which selects and inputs the maximum change value from the change value detection unit 208 to 211 and the change value of the stepwise detected current and generates a control signal for cutting off the main power according to the change value of the selected current; A voltage interrupter 214 for disconnecting commercial AC power supplied from the current transformer 206 to the motor of the tool body 201 by a control signal generated by the control unit 213 and the voltage interrupt control unit 213; The degree of change in current detected by the first to fourth current change value detectors 208 to 211 is displayed on the cutting edges 202 of the tool body 201 by displaying the amount of change in the current on the first to fourth light emitting elements 212a to 212d in steps. After the commercial AC power is cut off by the display device 212 and the voltage interrupter 214 to adjust the applied load, the voltage interrupt control unit 213 is reset by a switching action of the reset switch 204. Supply commercial AC power of current transformer 206 It consists of the reset part 215 which consists of a capacitor | condenser and a resistance supplied to the said motor of the old-body main body 201, and in figure, reference numeral VR1 is a variable resistor, and R1 is a resistor.

As shown in FIG. 3, the first current change value detection unit 208 variably sets the change value of the current rectified by the rectifier 207 and is input to the variable resistor VR2, the capacitor C1, and the resistor R2. A first reference current setting unit 208b including a first input current varying unit 208a, a resistor R3 to R6 for setting and outputting a first reference current to be compared with the change value of the current; A first current comparison for comparing the change value of the current input from the first input current variable part 208a with the first reference current value and supplying the resulting value to the first light emitting device 212a of the display device 212. Element 208c, and the second to fourth current change value detectors 209 to 211 are the same as the first current change value detector 208, respectively, to the second to fourth input current variable portions 209a and 210a. 211a, second to fourth reference current setting units 209b, 210b, and 211b, and second to fourth current comparison devices 209c, 210c, and 211c. Since their functions and configurations are the same as those of the first current change value detector 208, they will be omitted.

In addition, as shown in FIG. 3, the voltage interrupt control unit 213 may include a diode D1 for shaping the change value of the current input from the first current change value detector 208 and the change value of the standardized current. It is turned on according to the clock outputted from the output of the flip-flop 213a and the output terminal Q2 of the flip-flop 213a by maintaining the clock on the rising edge of the next clock and outputting it through the output terminals Q1 and Q2. It is composed of a fifth light emitting element (213b) which turns off and informs the state of supply of the commercial AC power supply to the tool body (201).

In addition, the voltage interrupter 214 may include a switching element TR1 output from the output terminal Q1 of the flip-flop 213a and switched on / off according to a control pulse input through the resistor R22. The first relay 214a which is excited in accordance with the on / off switching of the switching element TR1 and shorts its a contact S1, and its b contact S2 is opened by driving the first relay 214a. It is composed of a second relay (214b) for cutting off the commercial AC power supplied to the tool body 201.

The cutting load display and overload cut-off device of the power tool according to the present invention thus made performs the following operations with respect to the overload detection and display of the motor with respect to the cutting load imposed on the workpiece and the interruption of the main power supply.

Hereinafter, with reference to FIGS. 2 and 3 will be described in more detail.

First, as shown in FIGS. 2 and 3, when the main switch 203 of the tool body 201 is pressed to cut the workpiece with the power tool 200, the commercial AC power source AC through the plug 205 is the tool body. Supplied to the motor in the tool body 201 through a contact S2 of the second relay 214b applied to the motor in the 201 and configured in the primary side coil of the current transformer 206 and the voltage interrupter 214. The motor rotates at high speed.

When the motor starts to operate, the disk-shaped cutting edge 202 fixed to its rotational shaft rotates at high speed, thereby cutting, grinding or drilling the workpiece.

At this time, the cutting tool 202 is imposed on the cutting edge 202 according to the working conditions (cutting, feed rate, the material of the workpiece) and the cutting edge 202 (abrasive state, quality of the abrasive grain) during the cutting operation with the power tool 200. The friction force affects the motor torque value of the power tool 200 which rotates at high speed, and thus the commercial AC power input to the primary coil of the current transformer 206 is changed to be induced into the secondary coil of the current transformer 206. .

The power induced by the secondary coil of the current transformer 206 is converted into a DC power through a rectifier 207 such as a bridge diode and input to the first to fourth current change value detectors 208 to 211, respectively.

The first to fourth current value detectors 208 to 211 detect the change value of the current input through the rectifying unit 207 in multiple stages, and are derived from the outside of the tool body 201 and installed in the first display device 212. The fourth to fourth light emitting devices 212a to 212d are selected and turned on.

That is, as shown in FIG. 3, the first to fourth current change value detectors 208 to 211 respectively include the first to fourth input current variable parts 208a to 211a and the first to fourth reference current setters 208b. To 211b) and first to fourth current comparison elements 208c.

Accordingly, the current output from the rectifier 207 is controlled by the variable resistor VR1 and the resistor R2 and the capacitor C1 of the first input current variable part 208a, and then the same as the differential amplifier. 1 is input to the non-inverting terminal (+) of the current comparing element 208c.

The first reference current setting unit 208b of the first current change value detection unit 208 drops the current input from the power supply terminal Vcc through the fixed resistors R3 and R4 to the first reference current value. And the set first reference current value is input to the inverting terminal (-) of the first current comparison element 208c.

The first current comparator 208c compares the change value of the current that is variably set by the first input current variable part 208a with the first reference current that is set and input by the first reference current setting part 208b. When the change value of the current is greater than the first reference current, a high potential signal is output to light the corresponding light emitting device of the display device 212, that is, the first light emitting device 212a.

In addition, the second to fourth current change value detectors 209 to 211 also detect the change value of the current rectified by the rectifier 207 in multiple stages in the same manner as described above, so that the corresponding light emitting device 212b of the display device 212 is provided. To 212d), the detailed description thereof will be omitted.

In addition, the first reference current set in the first reference current setting unit 208b of the first current change value detector 208 and the second to fourth of the second to fourth current change value detectors 209 to 211. The second to fourth reference currents set in the reference current setting units 209b to 211b have the following relationship, respectively.

That is, assuming that the first reference current is Iref-1, the second reference current is Iref-2, the third reference current is Iref-3, and the fourth reference current is Iref-4, Iref-1> Iref-2> Iref- Has a relationship of 3> Iref -4.

Accordingly, the first to fourth current change value detectors 208 to 211 detect the change value of the current rectified by the rectifier 207 in multiple stages, and the detected current value is equal to the reference current Iref-1 to. If the value of Iref-4) is greater than any one of the reference current values, the light emitting device of any one of the first to fourth light emitting devices 212a to 212d of the display device 212, for example, a threshold value, an appropriate value, and an intermediate value of a cutting load. By lighting the corresponding light emitting elements having no-load values and light colors different from each other and expressing the cutting load state to the worker, the load applied to the cutting edge 202 of the power tool 200 can be appropriately adjusted.

On the other hand, when the current change amount of a threshold value larger than the first reference current Iref-1 is detected from the first current change value detector 208, that is, when the overload reaches a dangerous level, the voltage interruption control unit 213 determines the first current change value. The change value of the current detected by the detector 208 is shaped through the diode D1 and provided to the clock terminal CK of the flip-flop 213a.

The flip-flop 213a is shaped until a falling and reset signal of the change value of the input current is input, that is, a logic low signal is input from the first current change value detector 208 and will be described later. Until the reset signal is input from the reset unit 215, a high signal is output through its output terminal Q1 and provided to the voltage interrupter 214 such as a relay, and a low signal to the output terminal Q2. Will print

The voltage interrupter 214 conducts the switching element TR1 with the high signal output from the output terminal Q1 of the flip-flop 213a.

When the switching element TR1 is turned on, the coil of the first relay 214a is excited by the power supply B1 to short the a contact S1 thereof.

When the contact a (S1) of the first relay 214a is shorted, the coil of the second relay 214b is excited by the power supply B2 to open its b contact S2, and consequently through the current transformer 206. The commercial AC power supplied to the motor in the tool body 201 is cut off to prevent worker safety and burnout of the motor. In addition, the fifth light emitting device 213b of the voltage interrupt control unit 213 is turned on to notify the worker that the commercial AC power supply AC is blocked by the voltage interrupter 214.

In this way, when the operator presses the reset switch 204 to restart work after the power is cut off due to the overload, the voltage of the power supply terminal Vcc is corrected by the capacitor C9 and the resistor R23 of the reset part 215. Generated by the instantaneous reset signal by the number to initialize the flip-flop 213a.

After the flip-flop 213a of the voltage interrupt control unit 213 is deinitialized and a low signal is input from the first current change value detector 208, the flip-flop 213a outputs a high signal to its output terminal Q2. The fifth light emitting element 213b is turned off and a low signal is output to the output terminal Q1 to cut off the switching element RT1 of the voltage interrupting unit 214.

When the switching element TR1 is interrupted, the coil of the first relay 214a is de-excited to open its contact a (S1), and the coil of the second relay 214b is also de-energized to short its b contact S2. In addition, the commercial alternating current (AC) through the current transformer 206 is supplied to the motor in the tool body 201 again to perform a cutting operation.

On the other hand, as a comparative example, unlike the prior art, that is, to detect any specific overcurrent by fixing only one reference current value to the microcomputer regardless of the working conditions, the present invention multi-stages the cutting load state according to the working conditions. By detecting and marking with the operator, the operator can adjust the cutting load appropriately, and the main power is automatically cut off when the cutting load reaches the dangerous level.

And while specific embodiments of the present invention have been described and illustrated above, the present invention may be embodied in various modifications by those skilled in the art.

Such modified embodiments should not be individually understood from the technical spirit of the present invention, and such modified embodiments should be determined to fall within the appended claims of the present invention.

As described above, according to the cutting load display and overload cut-out device of the power tool of the present invention, the cutting load state is easily identified by the light color and lighting number of the display device during cutting, rather than when working on the senses. In spite of various variables of the work (worker, workpiece, cutting tool), it is possible to work with a proper load at all times, so that ordinary workers who have no experience in cutting work can easily perform work.

In addition, there is an advantage that the safety of the work can be secured by automatically shutting off the power in the event of overload as well as protecting the worker from the risk of a fatal safety accident due to breakage of the diamond cutting tool through excessive work.

In addition, since the operator always performs uniform and proper work on the basis of the display device, it does not affect the power tool and the cutting tool, thereby increasing the service life of the power tool and the cutting tool.

Claims (6)

A current transformer which supplies driving power to the motor and induces and outputs the driving power which is changed according to the torque value of the motor due to the frictional force imposed on the cutting edge when the workpiece is polished or cut; Rectifying means for rectifying the voltage induced in the current transformer into a DC voltage; Current change value detecting means for detecting and outputting a change amount of current obtained by the rectifying means step by step; And And a display device for controlling the load imposed on the cutting edge by displaying the amount of change in the current detected by the current change value detecting means in stages. The method of claim 1, The current change value detecting means detects an amount of change of a threshold value, an amount of change of an appropriate value, an amount of change of an intermediate value and an amount of no load change by comparing a change of current obtained by the rectifying means with a value of a reference current set at different levels. Cutting load display device for a power tool comprising a means. The method of claim 2, The detecting means of any one of the highest value detecting means, the proper value detecting means, the intermediate value detecting means and the lowest value detecting means, (1) an input current varying means for varying the value of the current obtained by the rectifying means according to the cutting load of the workpiece to satisfy the conditions of the cutting load; (2) reference current setting means for setting a reference current to be compared with a change value of the current; And (3) A cutting load display device for a power tool, comprising: a current comparing means for comparing the change value of the current in the input current varying means with the value of the reference current and supplying the resultant value to the display device. The method according to claim 1 or 3, And the display device includes a light emitting element capable of expressing at least different light colors in order to display the degree of change in current detected by the current change value detecting means in stages. The method of claim 1, The cutting load display of the power tool further comprises a voltage interrupting means for interrupting the drive power supplied from the current transformer to the motor if the degree of change of the current detected from the current change value detecting means reaches a dangerous level. Device. A current transformer which supplies driving power to the motor and induces and outputs the driving power which is changed according to the torque value of the motor due to the frictional force applied to the cutting edge when cutting the workpiece; Rectifying means for rectifying the voltage induced in the current transformer into a DC voltage; Current change value detecting means for detecting and outputting a change amount of current obtained by the rectifying means step by step; Voltage interruption control means for selecting a maximum change amount value from the detected change amount of the current and generating a control signal for cutting off the driving power according to the change amount of the selected current; Voltage interrupting means for interrupting the driving power supplied from the current transformer to the motor by the voltage interrupting control means; And And a display device for controlling the load imposed on the cutting edge by displaying the degree of change in the current detected by the current change value detecting means in a stepwise manner.