Screwing device and method for automatically carrying out a screwing process

Abstract

Screwing device (2) for automatically carrying out a screwing process, in which a screw element (4) with a thread (8) and a predetermined thread pitch is screwed into a counter-thread (10) by rotating the screw element (4) in the screwing direction (26), wherein the thread (8) and the counter-thread (10) each have a beginning (24A,B), with - a rotating screwing tool (16) - an electromotive feed unit (12) for the automatic and controlled feed of the screwing tool (16) in an axial direction (18) - wherein the feed unit (12) has a position sensor (22) for detecting a current position of the screwing tool (16) and for emitting a position signal (S) corresponding to the current position, - a control device (28) which controls the feed of the screwing tool (16) on the basis of the position signal (S) and which is further designed to detect an optimal relative position between the thread (8) and the counter-thread (10) on the basis of the position signal (S).

Description

[0001] The invention relates to a screwing device and a method for automatically carrying out a screwing process in which a screw element with a thread is screwed into a counter thread.

[0002] Especially in automatic screwdriving processes, correct alignment of the screw partners is often necessary for reliable and error-free screwing. The thread and the counter thread each have a thread start. This thread start is also referred to as the thread chamfer. For the most error-free screwdriving possible, it is desirable that the start of the thread and the start of the counter thread are in a defined starting position with both starts at the same angular position so that when the screw element is turned in the screwing direction, the thread engages directly with the counter thread. This desired starting position is also referred to below as the optimal relative position between the thread and counter thread.In the present case, this is understood in particular to mean that the beginning of the thread is located in an angular range of 0-20°, preferably 0-10° or 0-5°, viewed in the screwing direction, before the beginning of the counter thread, i.e. before the two threads engage with each other during rotation in the screwing direction, i.e. before a thread engagement occurs.

[0003] In hand-held screwdriving devices, axial feed is manually operated. In automatic screwdriving devices, axial feed is achieved by means of a feed unit, which is often pneumatically operated, particularly with the option of adjusting or limiting the contact force, e.g., by means of spring compensation.

[0004] US 10 173 305 B2 describes a hand-held screwing device in which the optimal relative position is detected using a vibration sensor. Initially, the screw element is rotated in the opposite direction to the screwing direction, so that the thread and the counter-thread slide along each other with their leading thread flanks in opposite directions and are pushed apart. When the optimal relative position is reached, the beginning of the thread falls onto a thread turn below, causing vibrations that can then be identified.

[0005] US 10 399 193 B2 describes another method for detecting the optimal relative position. This arrangement uses an acoustic sensor to also evaluate the situation when the screw element slips forward during rotation counter to the screwing direction.

[0006] US 6 021 555 A also describes a method for detecting the optimal relative position, in which the axial movement is evaluated by means of a separate motion sensor.

[0007] JP H02 - 224 934 A describes a screwing device in which, when a nut is screwed onto a screw, the screw is first rotated in the opposite direction until a starting position detected by a sensor is reached. The screwing process is then started.

[0008] Based on this, the invention is based on the object of specifying a screwing device and a method for automatically carrying out screwing processes, wherein the optimal relative position between the thread and counter thread is detected easily and reliably.

[0009] The object is achieved according to the invention by a device and by a method for the automatic, in particular repeated, execution of a screwing process in which a screw element with a thread and a predetermined thread pitch is screwed into a counter thread by turning the screw in the screwing direction.

[0010] The device and the method are explained in parallel below. Preferred embodiments and advantages of the screwing device also apply to the method, and vice versa.

[0011] The mating thread and the thread each have a beginning. The screwing device has a rotatable screwing tool and an electromotive feed unit designed for the automatic and controlled feed of the screwing tool in an axial direction. The feed unit has a position sensor for detecting a current, axial position of the screwing tool and for emitting a position signal corresponding to the current position. The screwing device also has a control device that controls the feed of the screwing tool during a screwing process based on the position signal. Furthermore, the control device is designed to detect the optimal relative position between the thread and the mating thread based on the position signal.

[0012] The position signal is typically a position value curve that is angle-dependent. The position value correlates with the axial position of the screwdriving tool, specifically as a function of the screwdriving tool's angle of rotation. During operation, the screwdriving tool transmits a torque to the screw being tightened.

[0013] The particular advantage is that the optimal relative position is identified solely with the aid of the position sensor, which is an integral component of the feed unit. This means that the feed unit uses the position sensor for the automatic and controlled advance of the screwdriving tool. The position signal detected by this existing position sensor is now also used to detect the optimal relative position. Therefore, no additional sensors or units are required to detect the optimal relative position.

[0014] The position sensor is, for example, part of a linear position measuring system in which an axial displacement of a feed carriage of the feed unit is detected, for example with the help of a magnetic, optical or other sensor.

[0015] Alternatively, the position sensor is part of a commutation system of an electric drive for the feed movement.

[0016] The screwdriving device is generally characterized by the fact that the position sensor, and specifically the position signal it provides, is used directly to detect the optimal relative position. Preferably, no additional sensors are used for this detection.

[0017] The electric motor-driven feed unit generally comprises an electric (feed) drive for the linear feed movement of the screwdriving tool. Specifically, the feed unit comprises a rotating electric motor or an electric linear motor to generate the feed movement. In the case of a rotating electric motor, the rotating movement of the motor is converted into a linear feed movement via suitable mechanical elements, such as a threaded spindle.

[0018] The electric (feed) drive has controllable magnetic coils that are specifically energized, i.e., commutated. This commutation is performed by controlling the drive. Specifically, this is a brushless motor, where the required commutation is performed by such a drive control. Commutation typically requires (position) data that provide information about the current position of the magnetic coils, especially in relation to permanent magnets. In a rotary motor, which is the preferred method, this data provides information about the relative (angular) position of a rotor with respect to a stator.

[0019] For a stepper motor, for example, this data provides direct information about an axial position.

[0020] Preferably, these data, which are used for controlled commutation, are also used to record the current axial position of the screwdriving tool or at least the axial position of a feed slide. The screwdriving tool is axially fixed to the feed slide.

[0021] The position encoder therefore supplies and / or evaluates this data and derives the current position from this data – particularly taking into account the other design features of the feed unit – and transmits the corresponding position signal to the control device. The design features include, for example, a linear motor or a rotary motor with mechanical deflection elements (e.g., a threaded spindle). Based on the commutation data, the axial adjustment movement can be derived via a clear relationship. This is preferably done absolutely using an absolute encoder or also relatively.

[0022] In the first case, the position sensor detects the position of the magnetic coils or the relative position between the stator and rotor, for example, like a sensor. In an alternative, the data is determined sensorlessly based solely on the electrical characteristics of the electric motor.

[0023] In this design, the screwdriving device is characterized by the fact that the recorded or determined data used for the required commutation is also used to record the current axial position for the automatically controlled feed and also to detect the optimal relative position. No additional position encoders or sensors are used to record the optimal relative position.

[0024] Overall, the equipment complexity is therefore minimal. In particular, no design changes are required for existing, commercially available automatic screwdriving devices with electric feed drives. Rather, it is sufficient to additionally evaluate the data already available and recorded during operation in connection with the feed movement, for example, for commutation, to identify the optimal relative position.

[0025] The screw element is preferably a screw or, alternatively, a nut. The corresponding counterpart with the counter thread is, for example, a component, such as a sheet metal with a counter thread incorporated into it, a nut with a counter thread, or even a bolt with a counter thread.

[0026] “Automatic execution” of the screwing process means that the screwing device itself carries out the screwing process without manual interaction from an operator. For this purpose, the screwing device automatically moves to a required position, for example with the help of a robot, where the screwing process is to take place. Alternatively, the screwing device is stationary and a component with the mating thread is moved to the required position. Furthermore, an automatic feed for the screw elements or a gripping system for picking up the respective screw element from a ready position is typically designed. Furthermore, the control device is designed to automatically control the screwing process and monitors it, for example, using torque control or feed control.This means, among other things, that the screwdriving process is automatically terminated when a specified target value (torque, feed position, screw depth, angle of rotation) is reached. The screwdriving device is specifically designed to perform such a screwdriving process repeatedly. The individual steps of a screwdriving cycle are: advancing the screwdriving tool using the feed unit to a defined axial position, identifying the optimal relative position, screwing in the screw element, automatically terminating the screwdriving process, retracting the screwdriving tool using the feed unit, and automatically feeding or gripping another screw element. This cycle is performed repeatedly.

[0027] In a useful further development, the current angular position of the screw element is continuously recorded during the screwing process. For this purpose, the evaluation of the data from the electric rotary drive, in particular the data for commutation, is preferably also used. The screwing device is therefore generally designed to detect the current angular position. In a preferred variant, both the current angular position, in particular from the data of the rotary drive, and the current axial position, in particular from the data of the electric drive of the feed unit, are therefore made available to the control device, and the control device is thus designed to detect and evaluate an angle-dependent change in the position value of the position signal.Due to the angle-dependent evaluation, periodically recurring events during one revolution of the screw element can be used to assess the position signal.

[0028] Generally, the screwdriving device has a rotary drive to perform the rotary motion during the screwdriving process and, specifically, to apply the required torque. This is also typically an electric motor-driven rotary drive. According to one design variant, the rotary drive and the feed unit are independent of each other, thus creating two separate motor units. For example, the feed unit is arranged next to the rotary drive.

[0029] In an alternative, the feed unit and rotary drive form a single structural unit, i.e., the feed unit is simultaneously configured to perform a rotary movement, or the rotary drive is simultaneously configured to perform a linear movement. Such a combined electric motor, which simultaneously performs a linear movement and a rotary movement, is described, for example, in EP 2 733 830 B1.

[0030] After detecting the optimal relative position—that is, after detecting the optimal angular position of the thread start relative to the counter-thread start—the rotary drive switches from a start phase to a tightening phase based on the position signal. During the start phase, the tightening tool, and thus the screw element, is rotated relative to the counter-thread in the opposite direction to the actual tightening direction. Once the optimal relative position is reached, the system switches to the tightening phase and changes the direction of rotation, so that the rotary drive rotates in the tightening direction.

[0031] Preferably, the control device is configured such that a predetermined, sudden change in the detected instantaneous axial position and thus a sudden change in the position value of the position signal is detected to the optimal relative position.

[0032] In the start phase, the first few threads slide along each other and the two screw partners are pressed apart in the axial direction. When the start of the thread reaches the start of the mating thread, the screw element is abruptly offset in the axial direction. This abrupt offset is expressed in the sudden change of the detected instantaneous axial position. A sudden change is understood to be a change of at least 20% and preferably of at least 40% of a theoretical total position value difference, which correlates to the thread pitch. In other words, the known thread pitch corresponds to a theoretical maximum difference in the position value during one revolution. This maximum difference forms the total position value difference. The sudden increase occurs within an angular range of less than 5°, specifically less than 3° or even less than 1°.

[0033] What is crucial here is that the optimal relative position is only detected in the case of a predefined change, i.e., only when the position value changes within a predefined magnitude and a predefined time interval (angular range). The predefined change in the position value therefore represents a trigger criterion for detecting the optimal relative position. This predefined position value range is defined by the position value at the beginning of the sudden change and the position value at the end of the sudden change.

[0034] The position value change correlates with the thread pitch. The specified change used as the trigger criterion is preferably less than or equal to the thread pitch. Thread pitch is generally understood to be the axial distance between two consecutive thread turns. For standardized screws, the thread pitch is fixed and therefore known. Depending on the screw type, the thread pitch is often in the range between 0.3 mm and 2 mm.

[0035] In a practical design, the optimal relative position is only detected when the sudden change corresponds to 20-80% and in particular 40-60% of the thread pitch.

[0036] Studies have shown that the axial position change is significantly smaller than the thread pitch itself and preferably lies only in the range between 20-80%, specifically between 40 and 60% of the thread pitch. The restriction to a predefined value range for the position value change is also based on the realization that other events can also contribute to a sudden change, and that this measure enables a clear identification of the optimal relative position. This specifically prevents false results.

[0037] In a preferred embodiment, the control device is therefore also designed to infer and differentiate between different events based on the degree of the sudden change, i.e. based on the value of the position value change of the position signal in the range of a sudden change. In particular, a distinction is made between an axial relative movement between the screw partners, i.e. between the thread and the counter thread on the one hand, and an axial relative movement between a rotation transmission element and the screw element on the other. A torque is transmitted to the screw element by means of the rotation transmission element. The rotation transmission element is, for example, a screwdriver bit which engages in a corresponding receptacle in the screw element during the screwing process. Alternatively, it is a socket wrench which grips around a head of the screw element.This rotation transmission element is part of the screwing tool or is formed by the screwing tool. An axial misalignment between the rotation transmission element and the screw element occurs, for example, when the screw element and the rotation transmission element are not initially aligned in the correct relative angular position and only slide into each other during the screwing process.

[0038] In one embodiment, the feed unit is force-controlled. By means of the linear feed unit, a force acting in the axial direction is initially applied to the screw element, generally during the feed and in particular during the screwing process. This force is expediently controlled, in particular such that the force acting on the screw element remains constant. Preferably, only a characteristic value of the electric drive motor, in particular the motor current, is evaluated for this force control. The motor current of the electric drive can be used to determine the currently exerted force. Alternatively, a measuring element is used to measure the force.

[0039] The force exerted by the feed unit also causes the sudden change in the position signal, as the screw element is pushed forward in the axial direction.

[0040] Alternatively, the feed unit is position-controlled.

[0041] According to the invention, a computer program is further provided to achieve the object. This program has commands that, when executed by the control device, cause the previously described screwing device to execute the described method. Such a program can therefore be implemented and loaded into the control device as a program module. This makes it possible, in particular, to retrofit existing screwing devices with the computer program, enabling the detection of the optimal relative position even with existing devices.

[0042] Specifically, the program causes the control device to perform the following steps: a) Detection of the position signal b) Evaluating the position signal with regard to an abrupt change, c) Detecting the optimal relative position based on given criteria, d) Sending a signal to change the direction of rotation to the rotary actuator.

[0043] An embodiment of the invention is explained in more detail below with reference to the figures, which show, in partially simplified representations: Fig. 1 a side view of a screwing device, Fig. 2A a partial sectional view of two screw partners in a first relative rotational position to each other, Fig. 2B the two screw partners in a second relative rotational position to each other, which defines an optimal relative position and Fig. 3 an example angle-dependent course of a position signal.

[0044] One in the Fig. 1 The screwing device 2 shown as an example serves to automatically carry out a screwing process in which a screw element 4 is screwed into a screw partner, in the exemplary embodiment a nut 6 (cf. Fig. 2A, Fig. 2B). The screw element 4 has a thread 8, and the nut 6 has a counter-thread 10. The screwing device 2 has a feed unit 12 with an electric drive 14a for the feed movement. By means of the feed unit 12, a rotatable screwing tool 16 is moved in and counter to an axial direction 18 in a controlled manner. The screwing tool 16 is, in particular, a rod-shaped element which carries, at its front end, a particularly replaceable rotation transmission element such as a screwdriver bit or a socket wrench.

[0045] The feed unit 12 has a movable feed carriage 12a and a stationary part 12b. The stationary part 12b is attached, for example, to a tool carrier or a robot hand. In the illustrated embodiment, the stationary part 12b is arranged laterally next to an additional rotary drive 20, which is preferably oriented coaxially with the screwing tool 16. The rotary drive 20 also has an electric drive 14b for the rotational movement.

[0046] A tool head 17 is attached to the front end of the fixed part 12b, in particular to a bent mounting flange. In the exemplary embodiment, this tool head comprises a feed device 17a for feeding screw elements 4 and a front guide element 17b for the respective screw element 4.

[0047] An integrated position sensor 22 is arranged within the feed unit 12, in particular within the electric drive 14a of the feed unit 12.

[0048] The screwing device 2 shown is designed either as a stationary system to which the components to be screwed are fed. Alternatively, the screwing device 2 is designed as a movable screwing device 2 and is attached, for example, to a robot with which it is moved to a desired position.

[0049] The screwing process is fully automated, without requiring any user interaction, such as manually guiding the screwing device 2 or starting a specific screwing process. The screwing device 2 and the screwing process are controlled by a control device 28. This typically has several functions implemented in different control units or in a common control unit. These control functions include, in particular, controlling the rotational movement of the screwing tool 16, the feed movement of the screwing tool 16, and controlling the overall process as a higher-level control function.

[0050] For the reliable execution of the screwing process, the two screw partners 4,6 must be aligned in a desired rotational orientation to each other. This is carried out automatically by the screwing device 2. The two screw partners 4,6 are aligned in an optimal rotational position to each other, as shown in the Fig. 2B. The Fig. 2A shows a situation in which the relative rotational position of the two screw partners 4,6 is not yet in the optimal relative position.

[0051] In general, the thread 8 and the counter thread 10 each have a beginning 24A, B.

[0052] In the Fig. 2A, Fig. Figure 2B shows the nut 6 in a sectional view and the screw element 4 (screw) in a side view. The two screw partners 4, 6 are aligned along a common center axis parallel to the axial direction 18. The threads 8, 10 each have thread turns that begin at the respective beginning 24A, B. Fig. 2A shows a situation in which the two screw partners 4,6 are in a relative position to each other, in which the first thread turns lie on top of each other in the area of ​​the respective beginning 24A, B. The optimal relative position or starting position from which the actual screwing process is carried out is the position shown in the Fig. 2B. In this case, the two beginnings 24A, B of the threads 8, 10 are positioned at the same angular position. This also means that the beginning 24A of the thread 8—viewed in a screwing direction 26 (direction of rotation)—is a few degrees before the beginning 24B of the counter-thread 10. This ensures that when the screw element 4 is rotated in the screwing direction 26, the thread 8 engages directly with the counter-thread 10 with the first thread turn.

[0053] The respective screwing process is designed in two stages. During the initial phase, the screw element 4 is rotated in the opposite direction to the actual screwing direction 26. With a right-hand thread, the screwing direction 26 corresponds to a clockwise rotation, so in this case a counterclockwise rotation occurs in the initial phase. During this rotation, the screw element 4 rotates from the Fig. 2A shown position into the Fig. 2B. When the optimal relative position is reached, the screw element 4 is pushed forward in the axial direction 18 by the feed unit 12, so that the screw element 4 is abruptly displaced axially.

[0054] As soon as this optimal relative position is detected by the control device 28, it controls the rotary drive 20 to switch the direction of rotation of the rotary drive 20 so that it now rotates in the screwing direction 26 and the screw element 4 is screwed into the nut 6. The switchover preferably occurs when the optimal relative position is detected for the first time. Alternatively, it is also possible for several, for example up to three, opposite rotations to be performed before the switchover occurs. The switchover of the direction of rotation occurs, in particular, instantaneously, e.g., within an angular range of up to a maximum of 20° or of up to a maximum of 10° after the abrupt axial offset of the screw element 4.

[0055] During the start-up phase, the current axial position of the feed unit 12, and thus also of the screwing tool 16 and the screw element 4, is continuously recorded by the position sensor 22. This transmits a corresponding position signal S to the control device 28, where it is evaluated.

[0056] During the start phase, a typical course of the position signal S is recorded, as shown in the Fig. 3. In the Fig.3, the position value of the position signal S is plotted against the angle of rotation φ. Initially, an approximately constant minimum position value Min can be seen, before an abrupt increase in the position value to a maximum position value Max occurs, which then continuously drops again to the minimum position value Min. In the exemplary embodiment, the position value remains at the minimum value over a certain angular range. This curve depicts the situation of the counter-directional rotational movement during the start-up phase, in which the two threads 8, 10 slide along one another with their first thread turns and the screw element 4 is pressed back against the axial direction 18, so that the position value as a measure of the axial position continuously decreases. When the screw element 4 reaches the optimal relative position, it is abruptly displaced axially forward, which leads to the sudden change Δ.This sudden change Δ corresponds to the difference between the maximum position value Max and the minimum position value Min. The constant minimum position value Min before the sudden change Δ is explained in this case by the fact that the respective beginning 24A, B of the respective thread is chamfered, i.e. has a chamfer, ie its radial depth increases with increasing angle of rotation.

[0057] The control device 28 is designed to evaluate the position signal S and, upon identification of the sudden change Δ, detects the existence of the optimal relative position.

[0058] The sudden change Δ correlates with the thread pitch P. Studies have shown that the value of the sudden change Δ corresponds to only 20-80% and, in particular, only 40-60% of the thread pitch P. The control device 28 is accordingly configured to detect the optimal relative position only when the sudden change Δ lies within this predetermined range of, in particular, 40-60% of the thread pitch P. The thread pitch P is generally defined by the distance in the axial direction 18 between two consecutive thread turns of the thread 8 or counter-thread 10.

[0059] Of particular importance for the screwing device 2 is that the position sensor 22 is an integrated component of the feed unit 12 and that the position sensor 22 simultaneously provides the required data for several different functions of the screwing device 2. The position sensor 22 is, in particular, a sensor for controlling the electric drive 14, which transmits information about the current position of at least one of the coils of the electric drive 14. Based on this data, the required commutation (polarity reversal) of the electromagnetic coils of the drive 14 is carried out by means of a motor control unit.

[0060] At the same time, the data from the position sensor 22 is used for the controlled feed of the feed unit 12. For example, during the automatic screwing process, the screw element 4 must first be advanced by means of a feed movement toward the nut 6. Furthermore, the feed is also monitored during the screwing process by means of the position sensor 22.

[0061] Finally, the third use of the data from position sensor 22 is the detection of the optimal relative position described here. This detection of the optimal relative position is also referred to as thread cut detection.

[0062] The present invention is not limited to the embodiment described in the figures. Rather, the underlying principle can be applied to different screw devices and even different screw pairs. List of reference symbols 2 screw device. 4 screw element 6 Mother 8 threads 10 counter threads 12 feed unit 12a Feed carriage 12b fixed part 14a electric drive for feed movement 14b electric drive for rotational movement 16 screwing tools 17 Tool head 17a Feeding device 17b Guide element 18 Axial direction 20 rotary drive 22 position sensors 24A, B beginning of thread 26 Screw direction 28 Control device S position signal Min minimum position value Max maximum position value Δ sudden change P thread pitch

Claims

[1] Screwing device (2) for automatically performing a screwing operation in which a screwing element (4) with a thread (8) and a predetermined thread pitch is screwed into a mating thread (10) by rotating the screwing element (4) in the screwing direction (26), wherein the thread (8) and the mating thread (10) each have a beginning (24A,B), with - a rotating screwdriver (16) - an electromechanical feed unit (12) for automatic and controlled feed of the screwdriving tool (16) in an axial direction (18) - wherein the feed unit (12) has a position sensor (22) for detecting an instantaneous position of the screw tool (16) and for outputting a position signal (S) corresponding to the instantaneous position, - a control device (28) which controls the feed of the screw tool (16) on the basis of the position signal (S) and which is further designed to recognize an optimal relative position between the thread (8) and the mating thread (10) on the basis of the position signal (S). [2] Screw device (2) according to claim 1, in which no sensors are used to detect the optimal relative position in addition to the position sensor (22). [3] Screwing device (2) according to one of the preceding claims, wherein the feed unit (12) has an electric drive (14), and data are used to detect the instantaneous position, which are also used to control a commutation of the electric drive (14). [4] Screwing device (2) according to one of the preceding claims, in which characteristic values ​​of the electric drive (14) of the feed unit (12) are evaluated to detect the current position. [5] Screw device (2) according to one of the preceding claims, wherein the control device (28) is designed to evaluate an angle-dependent position value of the position signal (S). [6] Screwing device (2) according to one of the preceding claims, which further comprises a rotary drive (20) for rotating the screwing tool (16), wherein the control device (28) for controlling the rotary drive (20) depending on the position signal (S) is configured to switch the rotary drive (20) from a start phase to a screwing phase after detection of the optimal relative position, wherein in the start phase at the beginning of each screwing operation the screwing tool (16) is rotated against the screwing direction (26) and in the screwing phase in the screwing direction (26). [7] Screw device (2) according to one of the preceding claims, wherein the control device (28) is configured such that it detects the optimal relative position upon a predetermined abrupt change (Δ) of the detected instantaneous axial position. [8] Screw device (2) according to the preceding claim, wherein the predetermined change (Δ) is correlated with the thread pitch (P) and is less than or equal to the thread pitch (P). [9] Screw device (2) according to one of the two preceding claims, wherein the predetermined change (Δ) is in the range of 20-80%, in particular in the range of 40-60% of the thread pitch (P). [10] Screwing device (2) according to one of claims 7 to 9, in which the control device (28) is configured to distinguish between different events based on the degree of abrupt change (Δ), in particular to distinguish between an axial relative movement between a rotary transmission element and the screw element (4) and an axial relative movement between thread (8) and mating thread (10). [11] Screwing device (2) according to one of the preceding claims, wherein the feed unit (12) is force-controlled or position-controlled. [12] Method for automatically performing a screwing operation in which a screwing element (4) with a thread (8) and a predetermined thread pitch is screwed into a mating thread (10) by rotating the screwing element (4) in the screwing direction (26), wherein the thread (8) and the mating thread (10) each have a start (24A,B), wherein - the screw element (4) is rotated using a screw tool (16), - the screw tool (16) is automatically and controlled in the axial direction (18) by means of a feed unit (12), - a position sensor (22) of the feed unit (12) detects the current axial position of the screwdriving tool (16) and generates a position signal (S) which correlates with the current axial position, - based on the position signal (S) an optimal relative position between the thread (8) and the mating thread (10) is identified, in which the beginning (24A) of the thread (8) and the beginning (24B) of the mating thread (10) are at the same angular position. [13] Computer program comprising commands which, when the program is executed by a control device (28) of a screw device (2) according to any one of claims 1 to 11, cause the screw device (2) to execute the method according to claim 12.

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