Work machine

Abstract

Provided is a work machine that allows for quicker fastening of a self-drilling screw. A calculation unit 95 is configured to reduce the motor rotation speed from a first rotation speed to a second rotation speed lower than the first rotation speed when detecting that a thread penetrating a mating material has been formed by a self-drilling screw or detecting that a transmission mechanism has switched from an intermittent rotation state to a continuous rotation state in a self-drilling screw fastening mode. The calculation unit 95 is configured to reduce the motor rotation speed from the second rotation speed to a third rotation speed lower than the second rotation speed or stop a motor 3 when detecting that the self-drilling screw has been seated in the mating material or detecting that the transmission mechanism has switched from the continuous rotation state to the intermittent rotation state after reducing the motor rotation speed from the first rotation speed to the second rotation speed in the self-drilling screw fastening mode.

Description

Work equipment

[0001] The present invention relates to a work machine capable of tightening screws.

[0002] Patent Document 1 discloses an impact tool as a work machine capable of executing a Teks Mode suitable for tightening self-drilling screws, also known as Teks Screws (registered trademark). Patent Document 2 discloses an impact tool as a work machine configured to reduce the rotational speed of a motor after detecting multiple impacts by an impact mechanism.

[0003] Patent No. 6024446 Patent No. 6044707

[0004] The impact tool in Patent Document 1 reduces the duty ratio when the current exceeds a predetermined value, but the timing for this reduction is set before the start of impact. This means that the duty ratio is reduced before the self-drilling screw forms a thread that penetrates the mating material, which increases the time required to form the thread.

[0005] The impact tool of Patent Document 2 reduces the duty ratio after multiple impacts have occurred, which is expected to shorten the time required to form a thread when tightening a self-drilling screw, but there is a risk of over-tightening the self-drilling screw.

[0006] The present invention aims to solve at least one of the following problems 1 and 2: Problem 1: To provide a work machine that can speed up the tightening of self-drilling screws. Problem 2: To provide a work machine that can reduce the risk of over-tightening self-drilling screws.

[0007] One aspect of the present invention is a work machine capable of screw tightening, comprising: a motor; a tool holder; a transmission mechanism for transmitting rotation of the motor to the tool holder, the transmission mechanism being configured to switch from a continuous rotation state in which a continuous rotation force is generated in the tool holder to an intermittent rotation state in which an intermittent rotation force is generated in the tool holder when a load on the tool holder increases; a control unit for controlling the motor; and an operation unit configured to instruct starting and stopping of the motor, wherein the control unit is capable of executing a self-drilling screw tightening mode, and is configured to reduce the rotational speed of the motor from a first rotational speed to a second rotational speed lower than the first rotational speed when it detects that a thread that penetrates a mating material has been formed by a self-drilling screw in the self-drilling screw tightening mode or when it detects that the transmission mechanism has switched from the intermittent rotation state to the continuous rotation state.

[0008] The present invention may be expressed as an "electric working machine," "electric tool," "electrical equipment," etc., and such expressions are also valid as aspects of the present invention.

[0009] According to the present invention, at least one of the above problems 1 and 2 can be solved.

[0010] 6A and 6B are side cross-sectional views of a work machine 1 according to an embodiment; a circuit block diagram of the work machine 1; a schematic diagram showing the process of tightening a self-drilling screw 17 by the work machine 1; a state transition diagram of the work machine 1; a control flowchart of the work machine 1; graphs showing an example of the change over time in current flowing to the motor 3 of the work machine 1; a graph enlarged from the horizontal axis of FIG. 6 from 7.5 seconds onwards; (A) is a graph showing an example of the change over time in current flowing to the motor 3 of the work machine 1; (B) is a graph showing an example of the change over time in the rotational speed of the motor 3; and (C) is a graph showing an example of the change over time in duty in PWM control of the motor 3.

[0011] Fig. 1 is a side cross-sectional view of a work machine 1 according to an embodiment of the present invention. Fig. 1 defines the mutually perpendicular front-rear and up-down directions of the work machine 1. The work machine 1 is a work machine capable of tightening screws, and specifically, is an impact driver.

[0012] The work machine 1 has a housing 2. The housing 2 has a motor housing portion 2a, a handle portion 2b, and a battery mounting portion 2c.

[0013] The motor housing 2a is a cylindrical portion whose central axis is substantially parallel to the front-to-rear direction. The housing 2 includes a hammer case 11 made of, for example, metal, connected to the front of the motor housing 2a. The front surface of the hammer case 11 is covered with a front cap 12, which is a protective member made of elastomer or the like.

[0014] The handle portion 2b has an upper end connected to a middle portion of the motor housing portion 2a in the front-rear direction and extends downward from the middle portion. The work machine 1 has a trigger switch 6 and a rotation direction selector switch 13 at the upper end of the handle portion 2b. The trigger switch 6 is an operating unit configured to allow the operator to start and stop the motor 3 (switch the drive state of the motor 3). The trigger switch 6 is an infinitely variable speed switch. The rotation direction selector switch 13 is a rotation direction selector that allows the operator to switch between forward and reverse rotation of the motor 3, i.e., between forward and reverse rotation of the anvil 10 (described below).

[0015] The battery mounting section 2c is provided at the lower end of the handle section 2b, and a battery pack 7 can be detachably mounted thereon. The work machine 1 operates using power from the battery pack 7. The work machine 1 has an operation panel 20 (switch panel) on the front upper surface of the battery mounting section 2c. The work machine 1 has a control board 30 inside the battery mounting section 2c.

[0016] The work machine 1 has a motor 3, a reduction gear mechanism 4, a spindle 5, a hammer 8, a spring 9, and an anvil 10 as a tool holder, housed within a motor housing 2a and a hammer case 11. The reduction gear mechanism 4, the spindle 5, the hammer 8, and the spring 9 constitute a transmission mechanism that transmits the rotation of the motor 3 to the anvil 10. This transmission mechanism is an impact mechanism that, when the load applied to the anvil 10 increases, switches from a continuous rotation state in which a continuous rotational force is generated in the anvil 10 to an intermittent rotation state in which an intermittent rotational force (impact force) is generated in the anvil 10.

[0017] The motor 3 is an inner rotor type brushless motor. The reduction mechanism 4 reduces the rotation speed of the motor 3 and transmits it to the spindle 5. The spindle 5 rotates the hammer 8. The hammer 8 is movable forward and backward relative to the spindle 5. A spring 9 biases the hammer 8 forward. The hammer 8 rotates or rotary strikes the anvil 10. That is, when the load applied to the anvil 10 increases, the hammer 8 switches from a continuous rotation state to an intermittent rotation state in which it generates an intermittent rotational force as an impact force on the anvil 10. The anvil 10 is rotatably supported by a hammer case 11 and is located in front of the hammer 8. The anvil 10 has a tool attachment hole 10a into which a tool 14 such as a bit can be attached.

[0018] The work machine 1 has lighting LEDs 16 that illuminate the area around the work location around the front of the hammer case 11. The work machine 1 also has a sensor board 15. The sensor board 15 is equipped with a magnetic sensor 84 shown in FIG. 2 that detects the rotation of the motor 3. The sensor board 15 is supported in front of the main body of the motor 3 (the portion of the motor 3 excluding the motor shaft 3a) in a position that is approximately perpendicular to the motor shaft 3a.

[0019] 2 is a circuit block diagram of the work machine 1. The work machine 1 includes an inverter circuit 82, a control signal output circuit 83, a magnetic sensor 84, a rotor position detection circuit 85, a rotation speed detection circuit 86, a mode selector switch 87, a control circuit voltage supply circuit 88, a battery voltage detection circuit 89, a motor current detection circuit 91, an illumination LED drive circuit 92, a control circuit voltage detection circuit 93, a display LED drive circuit 94, and a calculation unit 95.

[0020] The inverter circuit 82 includes semiconductor switching elements Q1 to Q6 connected in a three-phase bridge. The inverter circuit 82 converts the DC power output from the battery pack 7 into AC power for driving the motor 3 and supplies it to the motor 3. The control signal output circuit 83 applies a drive signal, for example a PWM (Pulse Width Modulation) signal, to each gate of the switching elements Q1 to Q6 under the control of the calculation unit 95.

[0021] The magnetic sensor 84 detects the magnetic field generated by the rotor of the motor 3 and transmits the result to a rotor position detection circuit 85. The rotor position detection circuit 85 detects the rotor position of the motor 3 based on the signal from the magnetic sensor 84 and transmits the result to a calculation unit 95. The rotation speed detection circuit 86 detects the rotation speed of the motor 3 (hereinafter referred to as "motor rotation speed") based on the signal from the rotor position detection circuit 85 and transmits the result to the calculation unit 95.

[0022] The mode selector switch 87 is provided on the operation panel 20 in Fig. 1. The mode selector switch 87 detects a mode switching operation by the operator and transmits the result to the calculation unit 95. The drive modes of the work machine 1 that can be selected by the mode selector switch 87 include a self-drilling screw tightening mode, which will be described later.

[0023] The control circuit voltage supply circuit 88 steps down the output voltage of the battery pack 7, converts it into a power supply voltage for the calculation unit 95, etc., and supplies it to the calculation unit 95, etc. The battery voltage detection circuit 89 detects the output voltage of the battery pack 7 and sends it to the calculation unit 95. The motor current detection circuit 91 detects the motor current from the voltage of a resistor R provided in the path of the current flowing through the motor 3 (hereinafter referred to as the "motor current"), and sends it to the calculation unit 95.

[0024] 1 under the control of the calculation unit 95. The control circuit voltage detection circuit 93 detects the output voltage of the control circuit voltage supply circuit 88 and sends it to the calculation unit 95. The display LED drive circuit 94 supplies a drive current to the display LEDs provided on the operation panel 20.

[0025] The calculation unit 95 is a control unit that controls the motor 3. The calculation unit 95 controls the inverter circuit 82 via the control signal output circuit 83, for example, by PWM control, in accordance with the drive mode selected by the mode selector switch 87, the rotation direction set by the rotation direction selector switch 13 (hereinafter referred to as the "set rotation direction"), and the operation of the trigger switch 6, to control the drive of the motor 3. The calculation unit 95 can control the effective value of the voltage applied to the motor 3 by the duty of the PWM control (hereinafter referred to as the "duty").

[0026] The calculation unit 95 can detect the load on the motor 3 based on the motor current. The calculation unit 95 can distinguish between forward and reverse rotation of the motor 3 based on the signal from the rotor position detection circuit 85, i.e., the signal from the magnetic sensor 84.

[0027] FIG. 3 is a schematic diagram showing the process of tightening the self-drilling screw 17 by the work machine 1.

[0028] The tightening of the self-drilling screw 17 proceeds in the following order: drilling, threading, thread groove passage, and seating. Drilling is the process of drilling a hole with the tip of the self-drilling screw 17 into a mating material 20 that does not already have a hole. Threading is the process of cutting a thread into the inner circumference of the hole using the threaded portion on the shank of the self-drilling screw 17 after drilling. When the threaded portion on the shank of the self-drilling screw 17 enters the entrance of the hole and reaches the exit of the hole, a thread is formed that penetrates the mating material 20, and the threading is complete. Thread groove passage is the process of the shank of the self-drilling screw 17 progressing through the threaded hole. Seating is the state in which the head of the self-drilling screw 17 comes into contact with the mating material 20.

[0029] The calculation unit 95 can execute a self-drilling screw fastening mode. In the self-drilling screw fastening mode, the calculation unit 95 is configured to increase the duty from the start of drilling until completion of threading is detected, to increase the duty from completion of threading to seating detection, and to decrease the duty after seating detection.

[0030] In the self-drilling screw tightening mode, the calculation unit 95 is configured to reduce the motor rotation speed from a first rotation speed to a second rotation speed lower than the first rotation speed when it detects that a thread that penetrates the mating material 20 has been formed by the self-drilling screw 17 (thread cutting completed) or that the transmission mechanism has switched from an intermittent rotation state to a continuous rotation state.

[0031] In the self-drilling screw tightening mode, after reducing the motor rotation speed from the first rotation speed to the second rotation speed, the calculation unit 95 is configured to reduce the motor rotation speed from the second rotation speed to a third rotation speed lower than the second rotation speed when it detects that the self-drilling screw has seated on the mating material or that the transmission mechanism has switched from a continuous rotation state to an intermittent rotation state. Instead of reducing the motor rotation speed to the third rotation speed, the calculation unit 95 may stop the motor 3.

[0032] The calculation unit 95 is configured to reduce the motor rotation speed from the first rotation speed to a fourth rotation speed that is lower than the second rotation speed when it detects that the self-drilling screw has seated on the mating material while the motor 3 is being driven at the first rotation speed in the self-drilling screw tightening mode. The calculation unit 95 may stop the motor 3 instead of reducing the motor rotation speed to the fourth rotation speed.

[0033] The calculation unit 95 is configured to set the applied voltage, lead angle, or conduction angle of the motor 3 by open-loop control or closed-loop control when controlling the motor 3 at the first rotational speed, the second rotational speed, the third rotational speed, or the fourth rotational speed in the self-drilling screw tightening mode.

[0034] The first rotation speed, the second rotation speed, the third rotation speed, and the fourth rotation speed do not need to be constants, and may be rotation speeds that change depending on the load condition, etc. The third rotation speed and the fourth rotation speed may be the same rotation speed.

[0035] FIG. 4 is a state transition diagram in the self-drilling screw tightening mode.

[0036] During the period from the start of drilling to the detection of thread cutting completion, the calculation unit 95 controls the upper limit of the duty to, for example, 87.5% and the motor rotation speed under no load to, for example, 23,200 RPM (S1). When the thread cutting completion detection condition is satisfied in the S1 state, the calculation unit 95 transitions to the S3 state.

[0037] The conditions for detecting completion of thread cutting are that the amount of pulling of the trigger switch 6 is 88.6% or more of the maximum amount (100%) (condition 1), that after detecting a maximum motor current of 20 A or more, a motor current of 70% or less of the maximum current is detected continuously for 10 ms (detecting an intermittent rotation state) (condition 2), and that after condition 2 is satisfied, a state in which the difference between the maximum and minimum motor current values ​​over the past 30 ms is less than 10 A is detected for 30 ms (detecting the end of the intermittent rotation state) (condition 3).

[0038] The calculation unit 95 controls the duty cycle to have an upper limit of, for example, 35% during the period from when the thread cutting completion detection condition is satisfied until seating detection (S3). When the seating detection condition is satisfied in the state of S3, the calculation unit 95 transitions to state S5. The seating detection condition is to detect an intermittent rotation state.

[0039] After the seating detection condition is satisfied, the calculation unit 95 controls the upper limit of the duty to, for example, 16% (S5).

[0040] When the no-load detection condition is satisfied in states S3 and S5, the calculation unit 95 transitions to state S1. The no-load detection condition is that the 10 ms moving average of the motor current is detected to be 7.5 A or less for 500 ms.

[0041] When the seating detection condition is satisfied in the S1 state, the calculation unit 95 transitions to the S5 state. For example, when a tightening operation is started after thread cutting is completed, the seating detection condition is satisfied in the S1 state. In this case, the seating detection condition is that a motor current of 50 A or more is detected for 3 ms.

[0042] 5 is a control flowchart for the self-drilling screw tightening mode, which starts when the trigger switch 6 is turned on.

[0043] The calculation unit 95 controls the rotation of the motor 3 in accordance with the pulling amount (operation amount) of the trigger switch 6 (S11). If the pulling amount of the trigger switch 6 is not equal to or greater than the threshold (No in S13), the calculation unit 95 continues to control the rotation of the motor 3 in accordance with the pulling amount of the trigger switch 6 (S11).

[0044] If the pulling amount of the trigger switch 6 is equal to or greater than the threshold (Yes in S13), the calculation unit 95 controls the rotation of the motor 3 at the first duty (S15) and detects the motor current (S17). If the calculation unit 95 does not detect a motor current equal to or greater than the fourth current threshold (e.g., 50 A) for a fourth time threshold (e.g., 3 ms) or longer (No in S19), the calculation unit 95 proceeds to S21.

[0045] When the calculation unit 95 detects that the maximum value of the motor current is equal to or greater than a second current threshold (e.g., 20 A) (Yes in S21) and that the motor current is equal to or less than a first percentage (e.g., 70%) of the maximum value for a period equal to or greater than a second time threshold (e.g., 10 ms) (Yes in S23), it determines that the motor has entered an intermittent rotation state and sets an impact detection flag (S25).

[0046] If either or both of the conditions in S21 and S23 are not satisfied (No in S21 or No in S23), the calculation unit 95 continues to control the rotation of the motor 3 at the first duty (S15).

[0047] When the impact detection flag is set and the calculation unit 95 detects that the difference between the maximum and minimum motor current values ​​within the past third time threshold (e.g., 30 ms) is less than the third current threshold (e.g., 10 A) for a third time (e.g., 30 ms) (Yes in S27), the calculation unit 95 determines that the intermittent rotation state has ended (thread cutting has been completed) and clears the impact detection flag (S29). If the condition in S27 is not satisfied (No in S27), the calculation unit 95 continues to control the rotation of the motor 3 at the first duty (S15). Note that if the impact detection flag is set, after the No in S19, the calculation unit 95 skips the branches in S21 and S23 and proceeds directly to S27 (not shown).

[0048] After clearing the impact detection flag (after S29), the calculation unit 95 controls the rotation of the motor 3 at a third duty cycle lower than the first duty cycle (S31). The calculation unit 95 detects the motor current (S35) after a period of time during which the motor current is not detected, which period is equal to a third time threshold equivalent to approximately one impact (S33). If the calculation unit 95 detects a motor current less than the first current threshold (e.g., 7.5 A) for a period equal to or greater than the first time threshold (e.g., 500 ms) (Yes in S37), the process returns to S15. By providing a period during which the motor current is not detected in S33, the risk of erroneously determining that the motor is in an unloaded state due to a sudden decrease in motor current accompanying a sudden decrease in duty cycle and proceeding to Yes in S37 is reduced.

[0049] If the calculation unit 95 does not detect a motor current less than the first current threshold for the first time threshold or more (No in S37), but the difference between the maximum and minimum motor current values ​​within the past third time threshold is equal to or greater than the third current threshold (Yes in S39), the calculation unit 95 determines that the vehicle has entered an intermittent rotation state (the driver has been seated), and proceeds to S41. Note that, in the case of the condition in S39, the calculation unit 95 also proceeds to Yes if the motor current suddenly increases without entering an intermittent rotation state. The condition in S39 may also be that the calculation unit 95 detects a motor current equal to or greater than the fifth time threshold.

[0050] The calculation unit 95 controls the rotation of the motor 3 at a second duty that is lower than the third duty (S41) and detects the motor current (S43). If the calculation unit 95 does not detect a motor current that is less than the first current threshold for the first time threshold or more (No in S45), the calculation unit 95 continues to control the rotation of the motor 3 at the second duty (S41). If the calculation unit 95 detects a motor current that is less than the first current threshold for the first time threshold or more (Yes in S45), the calculation unit 95 returns to S15.

[0051] If the calculation unit 95 detects a motor current equal to or greater than the fourth current threshold for a fourth time threshold or longer in S19 (Yes in S19), it determines that a person is seated and controls the rotation of the motor 3 at the second duty (S47). If the calculation unit 95 does not detect a motor current less than the first current threshold for a first time threshold or longer (No in S49), it continues to control the rotation of the motor 3 at the second duty (S47). If the calculation unit 95 detects a motor current less than the first current threshold for a first time threshold or longer (Yes in S49), it returns to S15.

[0052] FIG. 6 is a graph showing an example of the change in motor current over time in the self-drilling screw tightening mode.

[0053] At time 0 s, the trigger switch 6 is turned on, causing a temporary large starting current to flow. The motor current then begins to rise around time 1 s. The steep slope of the motor current around time 7 s is due to the start of thread cutting at around time 7 s. In the example of FIG. 6 , the load on the anvil 10 is still light even after thread cutting begins, so the anvil does not enter an intermittent rotation state, and no vibration in the motor current due to impact occurs. As another example, the intermittent rotation state may begin from the drilling stage or the beginning of thread cutting.

[0054] FIG. 7 is a graph in which the period from 7.5 seconds onward on the horizontal axis of FIG. 6 is extracted and enlarged.

[0055] Around time 8 s, the load on the anvil 10 increases due to threading, causing the anvil to change from a continuous rotation state to an intermittent rotation state, and the motor current oscillates. After that, when threading is completed, the load on the anvil 10 decreases, causing the anvil to change from an intermittent rotation state to a continuous rotation state, and the motor current oscillates. After that, the calculation unit 95 detects completion of threading and reduces the duty, causing the motor current to decrease. After that, the load on the anvil 10 increases due to seating, causing the anvil to change from a continuous rotation state to an intermittent rotation state, and the motor current oscillates. After that, the calculation unit 95 detects seating and further reduces the duty, and impacts at a low duty (intermittent rotation state) continue.

[0056] 8A to 8C are graphs showing an example of the changes over time in the motor current, motor rotation speed, and duty in the self-drilling screw tightening mode.

[0057] The threading completion detection time is around 1.5 seconds, and the seating detection time is around 3.2 seconds. The calculation unit 95 reduces the duty and the motor rotation speed at the threading completion detection time (around 1.5 seconds) and the seating detection time (around 3.2 seconds).

[0058] This embodiment has the following advantages.

[0059] (1) In the self-drilling screw tightening mode, the calculation unit 95 is configured to reduce the motor rotation speed from a first rotation speed to a second rotation speed lower than the first rotation speed when it detects that the self-drilling screw has formed a thread that penetrates the mating material (thread cutting completion) or that the transmission mechanism has switched from an intermittent rotation state to a continuous rotation state. This reduces the time required to form a thread in the mating material and speeds up the tightening of the self-drilling screw compared to when the motor rotation speed is reduced before the self-drilling screw has formed a thread that penetrates the mating material or before the transmission mechanism has switched from an intermittent rotation state to a continuous rotation state. Furthermore, this reduces the risk of overtightening the self-drilling screw and reduces power consumption and noise when tightening the self-drilling screw compared to when the motor rotation speed is not reduced until multiple impacts have occurred.

[0060] (2) In the self-drilling screw tightening mode, after the calculation unit 95 has reduced the motor rotational speed from the first rotational speed to the second rotational speed, if the calculation unit 95 detects that the self-drilling screw has seated on the mating material or that the transmission mechanism has switched from a continuous rotation state to an intermittent rotation state, the calculation unit 95 reduces the motor rotational speed from the second rotational speed to a third rotational speed that is lower than the second rotational speed, or stops the motor 3. Therefore, compared to when the motor rotational speed is maintained at the second rotational speed after seating detection, the risk of overtightening the self-drilling screw can be reduced, and power consumption and noise when tightening the self-drilling screw can be reduced.

[0061] (3) In the self-drilling screw tightening mode, when the calculation unit 95 detects that the self-drilling screw has seated on the mating material while the motor 3 is being driven at the first rotational speed, the calculation unit 95 is configured to either reduce the rotational speed of the motor 3 from the first rotational speed to a fourth rotational speed that is lower than the second rotational speed, or stop the motor 3. Therefore, for example, if an operator uses the operation unit to instruct the motor to stop after forming a thread that penetrates the mating material with a self-drilling screw, and then uses the operation unit to instruct the motor to start again, the risk of overtightening the self-drilling screw can be reduced, and power consumption and noise when tightening the self-drilling screw can be reduced.

[0062] Although the present invention has been described above using the embodiments as examples, the present invention is not limited to the embodiments. Various modifications can be made to the details specifically described in the embodiments within the scope of the claims.

[0063] The transmission mechanism that transmits the rotation of the motor to the tool holder is not limited to an impact mechanism, but may be an oil pulse mechanism that generates torque pulses using hydraulic pressure. Oil pulse mechanisms are built into work machines such as silent impact drivers and soft impact drivers.

[0064] The duty, current, time, and the like given as specific numerical values ​​in the embodiments do not limit the scope of the invention in any way, and can be changed as desired to suit the required specifications.

[0065] 1...work machine, 2...housing, 2a...motor accommodating section, 2b...handle section, 2c...battery mounting section, 3...motor, 3a...motor shaft, 4...reduction mechanism, 5...spindle, 6...trigger switch (operating section), 7...battery pack, 8...hammer, 9...spring, 10...anvil (tip tool holding section), 10a...tip tool mounting hole, 11...hammer case, 12...front cap (protective member), 13...rotation direction change switch (rotation direction change section), 14...tip tool, 15...sensor board, 16...illumination LED, 17...self-drilling screw, 82...inverter circuit, 83...control signal output circuit, 84...magnetic sensor, 85...rotor position detection circuit, 86...rotation speed detection circuit, 87...mode change switch, 88...control circuit voltage supply circuit, 89...battery voltage detection circuit, 91...motor current detection circuit, 92...illumination LED drive circuit, 93...control circuit voltage detection circuit, 94...display LED drive circuit, 95...calculation section

Claims

1. A work machine capable of screw tightening, comprising: a motor; a tool bit holder; a transmission mechanism that transmits the rotation of the motor to the tool bit holder, the transmission mechanism being configured to switch from a continuous rotation state in which a continuous rotational force is generated in the tool bit holder to an intermittent rotation state in which an intermittent rotational force is generated in the tool bit holder when the load on the tool bit holder increases; a control unit that controls the motor; and an operation unit configured to instruct the motor to start and stop, wherein the control unit is capable of executing a self-drilling screw tightening mode, and is configured to reduce the rotational speed of the motor from a first rotational speed to a second rotational speed lower than the first rotational speed when it detects that a thread that penetrates a mating material has been formed by a self-drilling screw in the self-drilling screw tightening mode, or when it detects that the transmission mechanism has switched from the intermittent rotation state to the continuous rotation state.

2. A work machine as described in claim 1, wherein the control unit is configured to, in the self-drilling screw tightening mode, after reducing the rotational speed of the motor from the first rotational speed to the second rotational speed, reduce the rotational speed of the motor from the second rotational speed to a third rotational speed lower than the second rotational speed, or stop the motor, when it detects that the self-drilling screw has seated on the mating material, or detects that the transmission mechanism has switched from the continuous rotation state to the intermittent rotation state.

3. A work machine as described in claim 1, characterized in that the control unit is configured to reduce the rotational speed of the motor from the first rotational speed to a fourth rotational speed lower than the second rotational speed, or to stop the motor, when it detects that the self-drilling screw has seated on the mating material while the motor is being driven at the first rotational speed in the self-drilling screw tightening mode.

4. A work machine according to claim 1, characterized in that the control unit is configured to set the applied voltage, advance angle or conduction angle of the motor by open-loop control or closed-loop control when controlling the motor at the first rotational speed or the second rotational speed in the self-drilling screw tightening mode.

5. A work machine as claimed in claim 1, characterized in that the transmission mechanism is an impact mechanism, and includes a hammer which is rotationally driven by the motor and which, when the load applied to the anvil serving as the tool holder increases, switches from the continuous rotation state to the intermittent rotation state in which an impact force is generated on the anvil as the intermittent rotation force.

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