KR20060011799A - Method for detecting drill breakage and device for the same - Google Patents

Abstract

The present invention relates to a drill damage detection method and detection device,

Drill in a number of holes to be arranged in the roughly plate-like workpiece, the drill moves along the position where the holes are to be arranged, and the drill depth is gradually increased by dividing the depth of the entire hole by a predetermined number of times when drilling each hole CLAIMS 1. A method of detecting whether a drill is damaged in a drilling apparatus to be processed, the method comprising: making an electrical insulation state between a workpiece and a bed of a drilling apparatus using an insulating material; Connecting both terminals of the resistance measuring means to the workpiece and the drill side, respectively; Measuring resistance between the workpiece and the contact surface of the drill at each step of the drill for forming each hole through the resistance measuring means; And determining that the drill is broken when the measured resistance value is larger than the set reference resistance value. The present invention provides a detection method including a detection device configured as another aspect.

Drill, breakage, detection

Description

Method for detecting drill breakage and device for the same}

1 is a view showing an example of a conventional drilling apparatus,

2 is a step processing flowchart by a drill in a steady state;

3 is a step processing flowchart with a drill in a broken state;

4 is a view of a drilling apparatus according to an embodiment of the present invention,

5 is a view showing the principle of the damage detection method by the voltage measurement according to an embodiment of the present invention,

6 is a view showing the principle of the damage detection method by measuring the current in accordance with an embodiment of the present invention,

7 is a work flow diagram according to an embodiment of the present invention.

<Description of Symbols for Main Parts of Drawings>

10: drill 20: drive unit

30: moving part 40: frame

50: Bed 60: Workpiece

70: insulation material 100: resistance measuring means

110: resistance measurement circuit unit 120: display unit

130: control unit 140: arithmetic unit

300: drilling control unit

The present invention relates to a drill damage detection method and a detection device, and more particularly, to detect whether or not the drill damage of the drilling apparatus for step processing so that a plurality of holes are arranged in the roughly plate-like workpiece, in particular the workpiece and the drill The present invention relates to a drill breakage detection method and a detection device for measuring whether a drill is broken by measuring a resistance formed by a contact surface and comparing the measured resistance value with a reference resistance value.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a view showing an example of a conventional drilling apparatus, Fig. 2 is a step machining flow chart by a drill in a steady state, and Fig. 3 is a step machining flow chart by a drill in a broken state.

The illustrated drilling apparatus includes a bed 50 for loading a roughly plate-shaped workpiece 60, a drill 10 for machining a hole in a direction perpendicular to the workpiece 60, and a drill up and down (E in Fig. 1). Direction) and back and forth (AB direction in FIG. 1) according to the arrangement of the holes 20 to be installed and processed on the driving unit 20, the frame 40, and the upper portion of the frame 40 while providing rotational driving force (F direction in FIG. 1). Or the moving part 30 which moves the drive part 20 to the left and right (CD direction of FIG. 1). In addition, the drilling apparatus is generally provided with a drilling control unit 300 for automatically controlling the operation of the drive unit 20 and the moving unit 30.

Such a drilling apparatus is drilled so that a plurality of holes are arranged in a roughly plate-like workpiece such as a shower head used in a semiconductor or LCD process, and drills through the moving part 30 along the position where the holes are to be arranged ( 10) is moved, and as illustrated in FIG. 2, when drilling the holes, step machining is performed by dividing the total hole depth by a predetermined number of times to gradually increase the drilling depth. If the step is processed as described above, the chip is more smoothly discharged than when the hole is completed by one drilling operation, so that the surface of the workpiece 60 can be prevented from being roughened by the chip, thereby roughening the workpiece 60. Has the advantage of getting better.

For example, the drill 10 is separated from the workpiece 60 after the primary machining as shown in step (a) of FIG. 2, and the secondary machining is performed more deeply with the hole depth as shown in step (b) of FIG. 2. After the drill 10 is separated from the workpiece 60, the drill 10 is completely removed from the workpiece 60 and moved to the next hole position after completely drilling the hole as in step (c) of FIG. The same process is repeated. As in the above example, the drill 10 and the workpiece 60 are separated at every step, so that the chips stuck in the drill 10 are discharged, thereby improving the roughness of the workpiece 60.

By the way, when the drill diameter is small, breakage of the drill (for example, cutting at an intermediate point) often occurs during such step machining.

In this case, as illustrated in FIG. 3, since the workpiece 60 is processed with the broken drill 14, even if the work is completed up to the last step, the entire hole is not penetrated, resulting in a machining failure state. . In addition, if the worker is left undetected and left unattended, all the holes to be processed afterwards are in a bad state, and as a result, there is a problem that the product itself must be newly processed. In addition, since the broken part of the drill is not in the state of a normal cutting edge, processing heat due to friction is generated between the broken drill 14 and the workpiece 60, resulting in a risk of fire.

For this reason, conventionally, the operator visually identified during the drilling process and detected whether the drill 10 was broken. However, when the worker visually detects whether the drill 10 is broken, there is a problem that the worker should always wait by the drilling device during the working process because the drill 10 must be continuously monitored without knowing when it will be broken. This, in turn, has become a major source of cumulative worker fatigue.

In addition, even if the worker detects the damage of the drill, if the hole is already found after processing a large number of defective state, there was still a problem that the product itself must be newly processed.

In addition, when the drill 10 is broken when the worker confirms the breakage of the drill 10, there is a possibility that the injury of the worker may be caused by the fragmentation of the drill 10.

The present invention has been made in view of the above problems, and is to detect whether or not the drill damage of the drilling apparatus for step machining so that a plurality of holes are arranged in a roughly plate-like workpiece, in particular the resistance between the workpiece and the drill surface The object of the present invention is to provide a drill breakage detection method for detecting whether a drill is broken by measuring the measured resistance value and comparing the measured resistance value with a reference resistance value.

In addition, another object of the present invention is to provide a drill failure detection device that implements the above technical features in another aspect.

Drill breakage detection method of the present invention for achieving the above object, while drilling a plurality of holes to be arranged in a roughly plate-like workpiece, the drill moves along the position where the holes are arranged, when drilling each hole A method of detecting whether a drill is broken in a drilling device that is stepping by dividing the depth of the entire hole by a predetermined number of times to gradually increase the drilling depth, and making an electrical insulation state between the workpiece and the bed of the drilling device using an insulating material. ; Connecting both terminals of the resistance measuring means to the workpiece and the drill side, respectively; Measuring resistance between the workpiece and the contact surface of the drill at each step of the drill for forming each hole through the resistance measuring means; And determining that the drill is broken when the measured resistance value is larger than the set reference resistance value.

More preferably, the resistance measurement is made by applying a constant current to the workpiece and the drill side, respectively, through both terminals of the resistance measuring means, and measuring the voltage change. According to another aspect, the resistance measurement is made by applying a constant voltage to the workpiece and the drill side, respectively, through both terminals of the resistance measuring means, and measuring the current change.

More preferably, the drill breakage detection method of the present invention determines that the drill is broken when the resistance value measured at the time of the step processing is ∞.

Drill failure detection device of the present invention according to another aspect, the drill processing so that a plurality of holes are arranged in a roughly plate-like workpiece, the drill moves along the position where the holes are arranged, the depth of the entire hole when drilling each hole A device for detecting whether a drill is damaged in a drilling device for stepping in a manner of gradually increasing the drilling depth by dividing a predetermined number of times, the insulating material is installed between the workpiece and the bed of the drilling device to make them electrically insulated; Both terminals are electrically connected to the workpiece and the drill side, respectively, and the resistance between the workpiece and the drill surface is measured at each step of drilling to form each hole, and the measured resistance is greater than the set reference resistance. Resistance measuring means for determining that the drill is broken when large; It is configured to include.

More preferably, the resistance measuring means, the resistance measuring circuit unit for measuring the resistance value of the contact surface of the workpiece and the drill; A calculator configured to determine whether the drill is broken by comparing the measured resistance value with a reference resistance value set to a predetermined value; A display unit which displays a result determined by the operation unit; And a control unit for performing at least one of operations for stopping a hole machining operation or generating an alarm when it is determined that the drill is broken as a result of the determination of the operation unit.

More preferably, the resistance measuring circuit section includes a current source for applying a constant current to the workpiece and the drill side through both terminals of the resistance measuring means, and a potential difference measuring section for measuring a potential difference therebetween.

More preferably, the resistance measuring circuit unit includes a voltage source for applying a constant voltage to the workpiece and the drill side through both terminals of the resistance measuring unit, and a current measuring unit for measuring a current flowing therebetween.

More preferably, the resistance measuring means determines that the drill is broken in the case where the resistance value measured at the step machining is ∞.

On the other hand, the 'drill side' is to include at least one portion of the bed or drive unit, the moving unit, the frame of the drilling device is formed of a metal material and the electrical conduction state with the drill, including a drill.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

Figure 4 is a view of a drilling apparatus according to an embodiment of the present invention, Figure 5 is a view showing the principle of the damage detection method by the voltage measurement according to an embodiment of the present invention, Figure 6 is an embodiment of the present invention The principle of the damage detection method by the electric current measurement by this figure is shown.

Drilling apparatus for processing a plurality of holes in a roughly plate-like workpiece such as a showerhead used in a semiconductor or LCD process generally has the following configuration as illustrated in FIG.

That is, the bed 50 for stacking the substantially plate-shaped workpiece 60, the drill 10 for processing a hole in the vertical direction to the workpiece 60, and providing a rotational driving force while moving the drill up and down Drilling device including a drive unit 20, the frame 40, and a moving unit 30 for moving the drive unit 20 in front or rear or left and right according to the arrangement of the holes to be installed and processed on the frame 40 Is composed. In addition, the drilling apparatus is provided with a drilling control unit 300 for automatically controlling the operation of the drive unit 20 and the moving unit 30.

In this drilling device, the drill 10 moves through the moving part 30 along the position where the holes are to be arranged, and as illustrated in FIG. 2, the drilling depth is divided by a predetermined number of times when drilling the respective holes. Step machining is carried out in an incremental manner.

Drill breakage detection device of the present embodiment is installed in such a drilling device to detect whether the drill breaks during the working process.

The drill damage detection device of this embodiment is provided between the work piece 60 and the bed 50 of the drilling device, the insulating material 70 to make them electrically insulated, and the work piece 60 and the drill side 90. Both terminals are electrically connected to each other, and the resistance between the workpiece 60 and the contact surface of the drill 10 is measured at each step of the drill 10 for forming each hole, and the measured resistance is measured by resistance. It comprises a resistance measuring means 100 to determine that the drill 10 is broken if it is larger than the reference resistance value stored in the means (100).

Each time the step 10 of the drill 10 for forming each hole to be processed, the workpiece 60 and the drill 10 to form a contact surface in the state as shown in (a), (b), (c) of FIG. If breakage occurs in (10), as shown in (a), (b) and (c) of FIG. 3, a contact surface smaller than the expected contact surface is formed for each step.

Typically, the workpiece 60 is made of a metal material such as aluminum, and thus has a property of conducting electricity well, and the drill 10 is also made of a metal material and has a property of conducting electricity well. Therefore, if the drill 10 is to cut the workpiece 60 in a state in which power is connected to the workpiece 60 and the drill 10, the energized state is achieved through their contact surfaces at the moment of cutting.

From an electrical point of view, it can be seen that an energization circuit is formed through a variable resistor having a resistance value inversely proportional to the area of the contact surface of the workpiece 60 and the drill 10 in this state.

Therefore, the average resistance value formed by the contact surface in the normal state as shown in (a), (b), and (c) of FIG. 2 is measured in advance, and is set as the reference resistance value, and the cutting instant workpiece 60 If the resistance value of the contact surface of the drill 10 and the measurement is made, the measured value has a value larger than the reference resistance value, that is, less than when the contact surface of the workpiece 60 and the drill 10 is normal It is possible to judge when the contact itself is not made as a broken state.

On the other hand, the drilling apparatus, including the drill 10 and the frame 40, the components such as the drive unit 20, the moving unit 30, the bed 50 is made of a metal material such as iron or steel, these All components are electrically connected to each other to achieve a state of energization.

In view of this, in the present embodiment, the bed 50, the drill 10, the drive unit 20, the moving unit 30, and the frame 40 of the drilling device, which electrically constitute a single conductor, are collectively referred to as' It is called the drill side 90 '.

Since the drill side 90 is electrically connected, when the workpiece 60 is placed on the bed 50 without the insulating material 70, the workpiece 60 is connected to both terminals of the resistance measuring means 100. ) And the drill side (90) is always in an electrically connected state, which makes it meaningless to measure the resistance between the workpiece 60 and the contact surface of the drill (10) during the step machining of the drill (10) .

In order to solve this problem, an insulating material 70 is installed between the workpiece 60 and the bed 50 to make them electrically insulated, and during the stepping of the drill 10, the workpiece 60 and the drill ( Only 10) is to be electrically connected to measure the resistance generated by the contact surface of the workpiece 10 and the drill 10 at the moment of contact.

As the insulating material 70, conventional materials such as synthetic resin, rubber, and wood may be used, and any material may be applied as long as the material can maintain the insulating state between the bed 50 and the workpiece 60.

The resistance measuring means 100 is a part for measuring the electrical resistance generated between the workpiece 60 and the drill side 90 at the time of step machining of the drill 10, and the portion of the drill 10 for forming each hole. The resistance formed between the contact surface of the workpiece 60 and the drill 10 is measured at each step machining, and when the measured resistance value is larger than the reference resistance value stored in the resistance measuring means 100, it is determined that the drill 10 is broken. Done. Preferably, it is determined that the drill 10 is broken when the resistance value measured at the step machining is ∞ (when no contact is made).

In consideration of the accuracy of the resistance measurement value, the terminals of the resistance measuring means 100 may be directly connected to the drill 10, and in operation, the drill 10, the driving unit 20, and the moving unit 30 are formed. Since the terminal moves along the hole position to be installed, the terminal of the resistance measuring means 100 may be installed to be connected to the bed 50 or the frame 40 of the drilling apparatus.

The resistance measuring means 100, the resistance measuring circuit unit 110 for measuring the resistance value formed between the contact surface of the workpiece 60 and the drill 10, and the measured resistance value compared with the reference resistance value set to a predetermined value And a calculation unit 140 for determining whether the drill 10 is damaged, and a display unit 120 for displaying the result determined by the calculation unit 140.

In addition, the resistance measuring unit 100, the control unit 130 for performing at least one of the operation to stop the hole machining operation or generate an alarm when it is determined that the drill 10 is damaged as a result of the determination of the operation unit 140 It is provided.

The reference resistance value set to the predetermined value in the calculation unit 140 may be, for example, a value obtained by measuring a large number of resistance values when machining the corresponding hole in the corresponding step in the state where the drill 10 is normal and using the average value thereof. have. When the reference resistance value is measured and used in this manner, there is an advantage that accurate detection is possible even when the drill 10 is partially damaged.

On the other hand, considering that the drill 10 is still slightly broken, substantially hole machining is still possible, and when the resistance value measured at the step machining is ∞ without measuring the reference resistance value as described above (contact is Only when it is determined that the drill 10 is broken, it is possible to detect a sufficient damage.

In this case, if a partial breakage of the drill 10 occurs during the machining of the previous step hole, the non-contact state as shown in FIG. It is measured and it is possible to detect that the drill 10 is broken.

The resistance measuring circuit 110 measures a resistance generated between the workpiece 60 and the drill 10 and may have, for example, the following configuration.

First, as shown in FIG. 5, a current source 240 for applying a constant current is provided between the workpiece 60 and the drill 10, and a potential difference measuring unit (voltmeter 250) for measuring a potential difference therebetween is provided. .

In such a configuration, when a constant current is generated in the current source 240, the contact surface of the workpiece 60 and the drill 10 becomes a state of a kind of variable resistance, and the potential difference between both ends is changed according to the change of the resistance value of the variable resistance. Will change. Therefore, if the potential difference generated by the contact surface of the workpiece 60 and the drill 10 is measured, the resistance can be measured by the following equation (1).

Next, as shown in FIG. 6, a voltage source 220 for applying a constant voltage is installed between the workpiece 60 and the drill 10, and a current measuring unit (ammeter 230) for measuring a current flowing therebetween is installed. Configuration.

In such a configuration, when the voltage source 220 generates a constant voltage, the contact surface of the workpiece 60 and the drill 10 is in a state of a kind of variable resistor, and the current flowing at both ends according to the change in the resistance value of the variable resistor. The value will change. Therefore, if the current flowing through the contact surface of the workpiece 60 and the drill 10, the resistance can be measured by the above equation (1).

The calculating unit 140 determines whether the drill 10 is damaged by comparing the resistance value measured by the resistance measuring circuit unit 110 with a reference resistance value set to a predetermined value. The resistance value measured as described above is ∞. It may be determined that the drill 10 is broken.

When machining with the drill 12 in the steady state, the workpiece 60 and the drill 10 are contacted with each step, and a predetermined resistance value is measured. However, when machining with the broken drill 14, a step occurs in which the workpiece 60 and the broken drill 14 do not come into contact with each other, thereby preventing the workpiece 60 and the broken drill 14 from touching. In the step, the resistance value is measured as ∞. In this way, the calculation unit 140 determines that the drill 10 is broken when the resistance value of ∞ is measured.

The display unit 120 is a portion that displays the resistance value measured by the resistance measurement circuit unit 110 or the result determined by the operation unit 140, and various monitors such as an LCD and LED display means may be applied.

The controller 130 is a part for stopping the hole machining operation or generating an alarm when it is determined that the drill 10 is damaged as a result of the determination of the calculator 140.

To this end, the control unit 130 of the drill damage detection device is preferably connected to the drilling control unit 300 is configured to transmit the control signal as described above.

Through such a configuration, when the resistance value is measured as ∞ and the drill 10 is determined to be broken, the drilling device stops the hole machining operation or generates an alarm to the operator to directly operate the drilling device. By making it possible to stop, it is possible to prevent product defects that may occur when continuing to work with the broken drill 14.

On the other hand, the resistance measurement circuit unit 110, the calculation unit 140, the display unit 120, the control unit 130 of the above-described resistance measurement means 100, for example, various operation keypad, alarm speaker, LCD monitor, A CPU having an EEPROM or the like may be configured as a combined microcomputer circuit unit, and connected to the drilling control unit 300 through a conventional signal transmission unit.

7 is a work flow diagram according to an embodiment of the present invention. Hereinafter, a drill damage detection method will be described with reference to FIG. 7.

When the worker places the workpiece 60 with the insulating material 70, the worker makes an electrical insulation state between the workpiece 60 and the bed 50 of the drilling apparatus and avoids both terminals of the resistance measuring means 100. It is connected to the workpiece 60 and the drill side 90, respectively, and the drilling control unit 300 is operated to start the hole processing.

The electrical resistance of the contact surface of the workpiece 60 and the drill 10 is measured during the hole processing. That is, the workpiece 60 and the drill side 90 connected to both terminals of the resistance measuring means 100 are electrically connected each time the drill 10 processes the hole, and the contact surface is a kind of variable as described above. As the resistance is in the same state, the resistance is measured.

Such a resistance value can be measured by the following method.

First, as shown in FIG. 5, a constant current is applied to the workpiece 60 and the drill side 90 through both terminals of the resistance measuring means, and the measurement may be performed by measuring a voltage change.

Next, as shown in FIG. 6, a constant voltage may be applied to the workpiece 60 and the drill side 90 through both terminals of the resistance measuring means, and measured by measuring a change in current.

By comparing the measured resistance value with the reference resistance value, it is determined that the breakage of the drill 10 occurs when the measured resistance value exceeds the reference resistance value. Since the process of obtaining the reference resistance value is the same as described above, duplicate description thereof will be omitted.

Considering the fact that the drill 10 is still slightly broken, it is still possible to drill a hole substantially. If the resistance value measured at the step machining is ∞ without measuring the reference resistance value as described above (contact is not made). If it is not), sufficient damage detection is possible only by judging that the drill 10 is broken. This process is also the same as described above, so duplicate description thereof will be omitted.

Meanwhile, in the above description, the configuration of comparing and determining the resistance value measured by the calculation unit 140 provided in the drill breakage detection device and the reference resistance value is illustrated, but the resistance measurement circuit unit 110 and the display unit 120 in the drill breakage detection device are illustrated. ) So that the measured resistance value is displayed on the display unit 120, and a method of detecting breakage of the drill 10 in a manner in which the operator visually checks it is also possible.

This method may be viewed as connecting the both ends of the measuring unit of the conventional ohmmeter to the drill 10 and the workpiece 60, and the operator checks the resistance value detected by the ohmmeter, such as determining whether the damage and work control Although the process is inconvenient to be done manually by the operator, it provides the advantage of using a conventional ohmmeter as it is.

If the measured resistance value is less than or equal to the reference resistance value or not ∞, the drilling control unit 300 determines whether the entire hole depth is processed, and proceeds to the processing of the next hole or the next step normally. That is, when it is determined that the steady state is completed, after completing all the steps for machining one hole, the drill 10 is moved to the next hole, and when the step is not completed, the processing of the next step of the hole is completed. Will be

When the measured resistance value is greater than or equal to the reference resistance value, the control unit 130 of the drill breakage detection device described above performs an operation of stopping hole processing or generating an alarm, and the control unit 130 receives the control signal. The controller stops the work.

At this time, when the operator manually performs the process of determining whether or not the damage, the operation control, etc., the operator who checks the damage state of the drill through the display unit 120 of the resistance measuring means 100 by manually operating the drilling control unit 300 The hole machining will be interrupted.

The present invention described above can be embodied in many different forms without departing from the spirit or main features thereof. Therefore, the above embodiments are merely examples in all respects and should not be construed as limiting.

Since the present invention as described above measures the resistance value between the workpiece and the drill side to detect the breakage of the drill, it is possible to check whether the drill is broken even if the operator does not check with the naked eye, and the worker's safety can be achieved.

In addition, since the worker can know the breakage of the drill while doing other work, it is possible to multi-worker.

In addition, when the drill break occurs, it can be detected at least in the next machining step of the hole, thereby minimizing the hole machining defect rate of the product.

Claims (9)

Drill in a number of holes to be arranged in the roughly plate-like workpiece, the drill moves along the position where the holes are to be arranged, and the drill depth is gradually increased by dividing the depth of the entire hole by a predetermined number of times when drilling each hole As a method of detecting the damage of the drill of the drilling device to be processed, Making an electrically insulating state between the workpiece and the bed of the drilling apparatus using an insulating material; Connecting both terminals of the resistance measuring means to the workpiece and the drill side, respectively; Measuring resistance between the workpiece and the contact surface of the drill at each step of the drill for forming each hole through the resistance measuring means; Determining that the drill is broken when the measured resistance value is greater than the set reference resistance value; Drill breakage detection method, characterized in that configured to include. The method of claim 1, The resistance measurement, Drill breakage detection method characterized in that by applying a constant current to the workpiece and the drill side through both terminals of the resistance measuring means, and measuring the voltage change. The method of claim 1, The resistance measurement, Drill breakage detection method, characterized in that by applying a constant voltage to the workpiece and the drill side through both terminals of the resistance measuring means, and measuring the change in current. The method according to any one of claims 1 to 3, And a drill breakage detection method, characterized in that it is determined that the drill is broken when the resistance value measured at the step machining is ∞. Drill in a number of holes to be arranged in the roughly plate-like workpiece, the drill moves along the position where the holes are to be arranged, and the drill depth is gradually increased by dividing the depth of the entire hole by a predetermined number of times when drilling each hole As a device for detecting the damage of the drill of the drilling device to be processed, An insulating material installed between the workpiece and the bed of the drilling device to make them electrically insulated; Both terminals are electrically connected to the workpiece and the drill side, respectively, and the resistance between the workpiece and the drill surface is measured at each step of drilling to form each hole, and the measured resistance is greater than the set reference resistance. Resistance measuring means for determining that the drill is broken when large; Drill breakage detection device, characterized in that configured to include. The method of claim 5, The resistance measuring means, A resistance measuring circuit unit measuring a resistance value of the contact surface between the workpiece and the drill; A calculator configured to determine whether the drill is broken by comparing the measured resistance value with a reference resistance value set to a predetermined value; A display unit which displays a result determined by the operation unit; A control unit for performing at least one of operations for stopping a hole machining operation or generating an alarm when it is determined that the drill is broken as a result of the determination of the operation unit; Drill breakage detection device, characterized in that configured to include. The method of claim 6, The resistance measuring circuit unit, And a current source for applying a constant current to the workpiece and the drill side through both terminals of the resistance measuring means, and a potential difference measuring unit for measuring the potential difference therebetween. The method of claim 6, The resistance measuring circuit unit, Drill breakage detection device comprising a voltage source for applying a constant voltage to the workpiece and the drill side through both terminals of the resistance measuring means, and a current measuring unit for measuring the current flowing between them. The method according to any one of claims 5 to 8, The resistance measuring means is a drill breakage detection device, characterized in that it is determined that the drill is broken when the resistance value measured during the step machining is ∞.

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