A thread cutting method for a sewing machine

Abstract

The present invention provides a thread cutting method for a sewing machine, belonging to the technical field of sewing machines. It solves the problem of insufficient anti-bird nest groove effect of the existing thread cutting method. The thread cutting method includes the following steps: A. Driving the sewing needle to move downward with the thread end of the surface thread through the lower needle hole on the moving knife and then move upward above the needle plate, and the thread end of the surface thread remains in the lower needle hole; B. The thread cutting power mechanism starts to work, driving the moving knife to move away from the needle dropping hole with the thread end of the surface thread, and then the thread cutting power mechanism pauses; C. The needle bar power mechanism drives the sewing needle to sew several stitches; D. The thread cutting power mechanism works again, driving the moving knife and the auxiliary knife to move together towards the direction close to the needle dropping hole until the moving knife stops moving when the cutting edge of the moving knife is directly below the needle dropping hole on the needle plate, and the auxiliary knife continues to move to bite with the cutting edge of the moving knife to cut off the thread end of the surface thread. The present invention has the advantages of high thread cutting efficiency and good anti-bird nest effect.

Description

Technical Field

[0001] The present invention belongs to the technical field of sewing machines and relates to a thread cutting method for a sewing machine. Background Art

[0002] A sewing machine is a device that uses one or more sewing threads to form one or more stitch types on fabric, interweaving or sewing together two or more layers of fabric. To improve sewing efficiency, a sewing machine is usually equipped with a thread cutting mechanism that automatically cuts off the upper thread end remaining on the fabric at the starting end of the fabric sewing stitch. Otherwise, there will be a long thread end, which will form a "bird's nest" phenomenon after knotting, resulting in poor aesthetics. After the last stitch is sewn, it is also necessary to cut off the upper thread and the bobbin thread.

[0003] The existing thread cutting method of a sewing machine, such as the thread cutting method of the double-acting knife thread cutting mechanism disclosed in the patent document (application number: 202411329869.0), in order to cut off the upper thread end, first moves a thread cutting moving knife one towards the direction of the sewing needle. After the first needle inserting action is completed, the thread cutting moving knife one retreats towards the position close to the thread cutting moving knife two. When the thread hole groove passes through the thread clamp, the upper thread in the thread hole groove is pressed by the thread clamp, and the sewing needle performs the sewing action of the second needle. Then, the thread cutting moving knife one continues to reset to the first position. During this process, the thread cutting moving knife one swings downward, and the thread cutting moving knife two continues to move forward to the fourth position; when the second cutting edge of the thread cutting moving knife two and the first cutting edge of the thread cutting moving knife one coincide, the sewing thread located in the thread hole groove is cut off. This thread cutting method has the following deficiencies in the actual production and manufacturing process:

[0004] 1. When it is necessary to cut off the upper thread end at the start of sewing, since it cuts off the upper thread end by the thread cutting moving knife one biting with the thread cutting moving knife two during the process of retreating downward from the second position (swinging upward to the position below the sewing needle). The thread cutting position of this thread cutting mechanism is far from the needle hole of the needle plate, so that the cut end of the upper thread is relatively long from the fabric, that is, the upper thread end remaining on the fabric is relatively long, and the anti-bird's nest effect is poor.

[0005] 2. It uses a transmission rod two sleeved outside the transmission rod one, and the first driving member and the second driving member drive the transmission rod one and the transmission rod two to rotate respectively, and then drive the thread cutting moving knife one and the thread cutting moving knife two to swing up and down respectively, that is, this thread cutting mechanism uses two motors to drive the thread cutting moving knife one and the thread cutting moving knife two to swing respectively. At this time, since the two motors operate independently, the two motors require more complex control algorithms to coordinate the cooperation relationship between the thread cutting moving knife one and the thread cutting moving knife two. This complexity easily increases the risk of potential failures and easily causes the cooperation relationship between the thread cutting moving knife one and the thread cutting moving knife two to change, thus affecting the working stability and reliability of thread cutting. Summary of the Invention

[0006] The object of the present invention is to address the above problems in the existing technology and propose a thread cutting method for a sewing machine. On the basis of ensuring the thread cutting efficiency of the sewing machine, the present invention solves the problem of insufficient anti-bird groove effect of the existing thread cutting method.

[0007] The object of the present invention can be achieved by the following technical solutions: A thread cutting method for a sewing machine, the sewing machine includes a machine base with a workbench surface, a needle plate with a needle dropping hole, a moving knife, an auxiliary knife and a rotary hook. The moving knife has a moving knife edge and a lower needle hole. A thread cutting power mechanism for driving the moving knife and the auxiliary knife to reciprocate and a needle bar power mechanism for driving the sewing needle to move up and down are provided on the machine base. The characteristic is that this thread cutting method includes the following steps:

[0008] A. Place the material to be sewn on the workbench surface. The needle bar power mechanism starts to work, driving the sewing needle to move downward with the top thread end through the lower needle hole on the moving knife and then move upward above the needle plate. The top thread end remains in the lower needle hole.

[0009] B. The thread cutting power mechanism starts to work, driving the moving knife with the top thread end to move away from the needle dropping hole. After moving, the moving knife and the auxiliary knife are located on the same side of the rotary hook. Then the thread cutting power mechanism pauses working.

[0010] C. The needle bar power mechanism drives the sewing needle to sew several stitches, and the top thread is locked with the bobbin thread in the rotary hook.

[0011] D. The thread cutting power mechanism works again, driving the moving knife and the auxiliary knife to move together towards the direction close to the needle dropping hole, and making the moving speed of the auxiliary knife less than that of the moving knife. Until the moving knife stops moving when its moving knife edge is directly below the needle dropping hole on the needle plate, the auxiliary knife continues to move to push the top thread end to the moving knife edge and cut the top thread end by the bite between the auxiliary knife and the moving knife edge.

[0012] Before starting the sewing work, first pass one end of the upper thread through the sewing needle to complete the threading of the sewing needle. The section of the upper thread passing through the needle hole of the sewing needle is the reserved upper thread end. In this thread cutting method, after driving the sewing needle to perform the sewing action for the set number of stitches, in order to cut off the upper thread end, in step D, the moving knife and the auxiliary knife are driven to move together in the direction close to the needle dropping hole. When the moving knife moves to a position where its cutting edge is directly below the needle dropping hole on the needle plate, the moving knife stops and remains stationary, waiting for the auxiliary knife to approach for thread cutting. This can make the cutting position of the upper thread end closer to the needle dropping hole of the sewing needle, thereby shortening the upper thread end remaining on the fabric as much as possible and achieving a better anti-nest effect. Moreover, compared with the method of moving both the moving knife and the auxiliary knife to bite and cut the upper thread end, this thread cutting method uses a "one static and one moving" method for thread cutting. In this way, the cutting position only depends on the position accuracy of the moving knife, and has no direct relationship with the timing of the moving knife and the auxiliary knife biting and the moving speed of the auxiliary knife approaching the cutting edge of the moving knife. Therefore, it can simplify the control algorithm of the thread cutting mechanism of the sewing machine, and thus can more accurately control the cutting position below the needle dropping hole of the needle plate to ensure a better anti-nest effect.

[0013] In addition, when driving the moving knife to move in the direction close to the needle dropping hole in step D, the auxiliary knife also moves along with it. In this way, when the moving knife moves to the thread cutting position, the auxiliary knife has already moved a certain distance in advance, thereby shortening the waiting time of the moving knife and further improving the thread cutting efficiency. Since the moving knife and the auxiliary knife move to the thread cutting position to cut the thread instantaneously, rather than slowly approaching the needle dropping hole for thread cutting, therefore, in step D, by designing the moving speed of the auxiliary knife to be less than that of the moving knife, the distance between the cutting edge of the moving knife and the auxiliary knife can be gradually increased during the movement of the moving knife. This can improve the error tolerance and avoid premature thread cutting. For example, even if mechanical components wear after long-term operation of the equipment, or there are slight deviations in the control parameters of the auxiliary knife, resulting in the moving speed of the auxiliary knife becoming faster than the preset speed, it will only shorten the distance between the auxiliary knife and the moving knife that is widened, and there will be no situation where the moving knife and the auxiliary knife move quickly, causing the cutting edge of the moving knife to bite the upper thread end prematurely before reaching directly below the needle dropping hole. Therefore, on the basis of ensuring high thread cutting efficiency of the sewing machine, the cutting position is always directly below the needle dropping hole of the needle plate to achieve a better anti-nest effect.

[0014] In the above thread cutting method of the sewing machine, in step B, driving the moving knife to move away from the needle dropping hole with the upper thread end specifically means: driving the moving knife to swing around the outer periphery of the bobbin from the upper part of the bobbin to the side part of the bobbin with the upper thread end.

[0015] In the step D, the driving of the moving knife and the auxiliary knife to move together in the direction close to the needle punching hole specifically is: driving the moving knife and the auxiliary knife to swing together around the outer periphery of the rotary hook from the side part of the rotary hook to the upper part of the rotary hook. When the moving knife and the auxiliary knife swing upward, the moving knife and the auxiliary knife approach the needle punching hole of the needle plate, and vice versa, they are away from the needle punching hole of the needle plate.

[0016] In the above-mentioned thread cutting method of the sewing machine, the moving knife further has a thread hooking groove, and this thread cutting method further includes the following steps:

[0017] E. The thread cutting power mechanism drives the moving knife and the auxiliary knife to swing downward, so that the moving knife and the auxiliary knife move to the same side of the rotary hook again. After that, the thread cutting power mechanism pauses working, and the needle bar power mechanism continuously drives the sewing needle to perform the sewing action until the sewing action ends, and then the needle bar power mechanism pauses working;

[0018] F. The thread cutting power mechanism works again, drives the moving knife to swing upward to a position where its thread hooking groove is vertically aligned with the needle punching hole on the needle plate, the upper thread and the bobbin thread fall into the thread hooking groove, and then drives the moving knife to swing downward to a position where its moving knife edge is vertically aligned with the needle punching hole on the needle plate. After that, it drives the auxiliary knife to swing upward to push the upper thread and the bobbin thread to the position of the moving knife edge, and cuts the upper thread and the bobbin thread by the biting of the auxiliary knife and the moving knife edge.

[0019] In the above step E, by swinging the moving knife and the auxiliary knife downward to the same side of the rotary hook, the needle position of the sewing needle can be avoided, so that the sewing needle will not hit the moving knife and the auxiliary knife when it penetrates the fabric, and then the sewing machine can continue the subsequent sewing until the sewing action ends. After the sewing action ends, by cutting the upper thread and the bobbin thread in step F, the material to be sewn can be taken off from the workbench surface. And because in step F, the moving knife also stops at a position where its moving knife edge is directly below the needle punching hole, the thread cutting position is closer to the needle punching hole, so that after sewing, the cut upper thread and bobbin thread are as short as possible, further ensuring a good sewing nest effect.

[0020] In the above-mentioned thread cutting method of the sewing machine, the thread cutting power mechanism includes a driving motor, a driving shaft, a moving knife cam, an auxiliary knife cam, a moving knife link assembly connected to the moving knife, and an auxiliary knife link assembly connected to the auxiliary knife. The moving knife cam and the auxiliary knife cam are both fixedly connected to the driving shaft. The moving knife link assembly has a first abutting part, and the auxiliary knife link assembly has a second abutting part;

[0021] The working circumference of the movable knife cam includes a first concave arc segment AB, a first convex arc segment BC, a second concave arc segment CD and a second convex arc segment DA which are connected end to end in sequence. The first convex arc segment BC has a trimming arc surface which is concentric with the drive shaft. The distances from the starting point B and the end point C of the first convex arc segment BC to the axis of the drive shaft are both greater than the distance from the trimming arc surface to the axis of the drive shaft. When the pushing and abutting portion of the working circumference of the movable knife cam moves in a direction away from the axis of the drive shaft, the movable knife swings downward.

[0022] The working circumference of the auxiliary knife cam has a needle starter thread trimming peach tip a, a to-be-trimmed peach tip b and a tail needle thread trimming peach tip c arranged in sequence along the circumferential direction, and the tail needle thread trimming peach tip c is connected to the needle starter thread trimming peach tip a through a connecting convex arc segment ca, and when the working circumference of the auxiliary knife cam pushes the abutting portion 2 to move in a direction away from the axis of the driving shaft, the auxiliary knife swings upward;

[0023] The step D is specifically as follows:

[0024] The thread trimming power mechanism starts working again, the driving motor drives the driving shaft to start rotating, the working surface of the moving knife cam slides from the starting point B of the first convex arc segment BC to the thread trimming arc surface against the abutment portion 1, so that the moving knife swings upward, and at the same time, the working surface of the auxiliary knife cam slides from the middle position of the connecting convex arc segment ca to the needle thread trimming peach tip a against the abutment portion 2, so that the auxiliary knife swings upward and the speed is lower than the upward swing speed of the moving knife;

[0025] The driving shaft continues to rotate so that the cutting arc surface on the moving knife cam slides over the abutment part 1, and the moving knife stops at a position where the moving knife edge is directly below the needle drop hole on the needle plate. At the same time, the needle and thread cutting peach tip a on the auxiliary knife cam slides over the abutment part 2, so that the auxiliary knife engages with the moving knife edge on the moving knife to cut the upper thread end.

[0026] The driving motor drives the moving knife cam and the auxiliary knife cam to rotate in a clockwise unidirectional cycle through the driving shaft. The thread trimming power mechanism adopts a cam-connecting rod structure, so that the position and speed of the moving knife and the auxiliary knife at a certain time can be determined by the design of the working surface shape of the moving knife cam and the auxiliary knife cam. Therefore, the shape design of the first convex arc segment BC and the connecting convex arc segment ca can be used to achieve that the speed of the auxiliary knife is lower than that of the moving knife during the upward swing of the moving knife and the auxiliary knife, thereby avoiding the situation in which the moving knife blade is engaged with the auxiliary knife before reaching the bottom of the needle drop hole during rapid movement, thereby cutting off the upper thread in advance.

[0027] On the moving knife cam, by providing a thread cutting arc surface concentric with the drive shaft on the first convex arc segment BC, since the distance from the first abutting part to the axis of the drive shaft (or the rotation center of the moving knife cam) does not change during the process of the thread cutting arc surface sliding past the first abutting part, the moving knife can remain stationary during this process. Therefore, the design of this thread cutting arc surface enables the auxiliary knife cam to rotate and drive the auxiliary knife to move towards the moving knife for thread cutting. Even though the moving knife cam must rotate with the drive shaft, the moving knife can remain stationary and wait for the auxiliary knife to approach, thus forming a "one static and one moving" thread cutting method to accurately control the thread cutting position below the needle plate needle dropping hole, shortening the upper thread end remaining on the fabric, and thus achieving a better anti-nest effect.

[0028] In the above thread cutting method of the sewing machine, the thread cutting power mechanism includes a moving knife motor for driving the moving knife to swing reciprocally and an auxiliary knife motor for driving the auxiliary knife to move reciprocally. Both the moving knife motor and the auxiliary knife motor are servo motors. The moving knife motor and the auxiliary knife motor are connected to the moving knife and the auxiliary knife through a linkage mechanism. The control system controls the moving knife motor and the auxiliary motor to work according to the set rotation time and rotation angle, thereby controlling the moving knife and the auxiliary knife to swing in the corresponding manner.

[0029] In the above thread cutting method of the sewing machine, the starting needle thread cutting peach tip a and the to-be-thread-cut peach tip b are connected by a connecting concave arc segment ab. In step E, the thread cutting power mechanism driving the moving knife and the auxiliary knife to swing downward is specifically as follows: the driving motor drives the drive shaft to rotate, and the working surface of the moving knife cam slides from the thread cutting arc surface to the end point C of the first convex arc segment BC along the first abutting part. At the same time, the working surface of the auxiliary knife cam slides from the starting needle thread cutting peach tip a to the middle position of the connecting concave arc segment ab along the second abutting part.

[0030] Since the distances from the starting point B and the end point C of the first convex arc segment BC to the axis of the drive shaft are both greater than the distance from the thread cutting arc surface to the axis of the drive shaft, during the process of the working surface of the moving knife cam sliding from the thread cutting arc surface to the end point C of the first convex arc segment BC along the first abutting part, the distance from the first abutting part to the axis of the drive shaft gradually increases, causing the moving knife to swing downward; during the process of the working surface of the auxiliary knife cam sliding from the starting needle thread cutting peach tip a to the middle position of the connecting concave arc segment ab along the second abutting part, the distance from the second abutting part to the axis of the drive shaft gradually decreases, causing the auxiliary knife to also swing downward.

[0031] In the above thread trimming method of the sewing machine, in step F, after the thread trimming power mechanism resumes operation, the drive motor drives the drive shaft to start rotating. The working surface of the moving knife cam slides along abutment portion 1 from the end point C of the first convex arc segment BC towards the inner concave inflection point of the second concave arc segment CD. At the same time, the working surface of the auxiliary knife cam slides along abutment portion 2 from the middle position of the connecting concave arc segment ab towards the thread trimming tip b, causing both the moving knife and the auxiliary knife to swing upward. When passing over abutment portion 1 at the inner concave inflection point of the second concave arc segment CD, the moving knife is in a position where its thread engaging groove is directly above and below the needle hole on the needle plate.

[0032] After the thread engaging operation is completed, the sewing machine needs to cut the upper thread and the bobbin thread in the thread engaging groove. During the upward swing of the moving knife that moves the thread engaging groove below the needle hole, by having the auxiliary knife swing upward together with the moving knife, when the upper thread and the bobbin thread need to be cut subsequently, the auxiliary knife can be closer to the thread trimming position, thereby shortening the waiting time of the moving knife and further improving the thread trimming efficiency.

[0033] In the above thread trimming method of the sewing machine, the second convex arc segment DA is a circular arc surface concentric with the drive shaft. The distances from the starting needle thread trimming tip a and the end needle thread trimming tip c to the axis of the drive shaft are equal. In step F, after the upper thread and the bobbin thread fall into the thread engaging groove, the working surface of the moving knife cam slides along abutment portion 1 from the inner concave inflection point of the second concave arc segment CD towards the second convex arc segment DA. At the same time, the auxiliary knife cam slides along abutment portion 2 from the thread trimming tip b towards the thread trimming tip a, causing the moving knife to swing downward and the auxiliary knife to swing upward;

[0034] The continuous rotation of the drive shaft causes the second convex arc segment DA of the moving knife cam to slide over abutment portion 1, stopping the moving knife at a position where its cutting edge is directly below the needle hole on the needle plate. At the same time, the end needle thread trimming tip c on the auxiliary knife cam slides over abutment portion 2, causing the auxiliary knife to engage with the cutting edge of the moving knife to cut the upper thread and the bobbin thread.

[0035] Since the second convex arc segment DA is a circular arc surface concentric with the drive shaft and the distances from the starting needle thread trimming tip a and the end needle thread trimming tip c to the axis of the drive shaft are equal, the auxiliary knife swings to the same position during both starting needle thread trimming and end needle thread trimming. Also, because the second convex arc segment DA is a circular arc surface concentric with the drive shaft, when the second convex arc segment DA slides over abutment portion 1, the moving knife will also stop at a position where its cutting edge is directly below the needle hole, and the auxiliary knife can also move to a position where it engages with the cutting edge of the moving knife. In this way, the thread trimming position is closer to the needle hole, making the cut upper thread and bobbin thread as short as possible after sewing is completed, further improving the sewing nest effect.

[0036] In the above thread trimming method of the sewing machine, before the step A, the thread trimming method further includes the following steps: The thread trimming power mechanism works in advance, the driving motor drives the driving shaft to start rotating, so that the inner concave inflection point of the first concave arc segment AB on the moving knife cam abuts against the abutting portion 1, and the moving knife is stopped at a position where its lower needle hole is directly opposite to the needle dropping hole on the needle plate. Then the thread trimming power mechanism pauses. Before the first needle is inserted, the lower needle hole on the moving knife is directly opposite to the needle dropping hole on the needle plate, so that the sewing needle can pass through the lower needle hole downward, then leave the lower needle hole upward and rise above the needle plate to complete the first needle inserting action of the sewing needle.

[0037] In the above thread trimming method of the sewing machine, the distances from the points on the connecting convex arc segment ca to the axis of the driving shaft are all smaller than the distance from the starting needle trimming peach tip a to the axis of the driving shaft. The step B is specifically as follows: The thread trimming power mechanism starts to work, the driving motor drives the driving shaft to start rotating, the connecting convex arc segment ca of the auxiliary knife cam slides along the abutting portion 2, and the working surface of the moving knife cam slides along the abutting portion 1 from the inner concave inflection point of the first concave arc segment AB to the starting point B of the first convex arc segment BC. Then the thread trimming power mechanism pauses.

[0038] During the process that the working surface of the moving knife cam slides along the abutting portion 1 from the inner concave inflection point of the first concave arc segment AB to the starting point B of the first convex arc segment BC, it will push the abutting portion 1 away from the axis of the driving shaft. Therefore, the moving knife can be swung downward until the starting point B of the first convex arc segment BC abuts against the abutting portion 1. At this time, the moving knife swings to the lower dead center. Since the distances from the points on the connecting convex arc segment ca to the axis of the driving shaft are all smaller than the distance from the starting needle trimming peach tip a to the axis of the driving shaft, during the process that the connecting convex arc segment ca of the auxiliary knife cam slides along the abutting portion 2, the auxiliary knife is lower than the thread trimming position and is on the same side of the rotating hook as the moving knife to avoid cutting the thread end of the surface thread in advance when the moving knife swings downward. Then, the sewing needle can perform the needle inserting action.

[0039] Compared with the prior art, the thread trimming method of the sewing machine has the following advantages:

[0040] 1. In this thread cutting method, since a double-cam type thread cutting power mechanism is adopted to achieve thread cutting, and both the moving knife cam and the auxiliary knife cam are fixedly connected to the drive shaft, a single motor can be used to drive the rotation of the moving knife cam and the auxiliary knife cam. This can eliminate the possible error accumulation between the two motors when using two motors to drive separately, simplify the control algorithm, and make the movements of the moving knife and the auxiliary knife more coordinated. Moreover, since the geometric shapes of the moving knife cam and the auxiliary knife cam are pre-designed and they are driven by the same drive shaft, the positions of the moving knife and the auxiliary knife and their cooperation relationship depend on the rotation angle of the drive shaft. That is to say, when the motor drives the drive shaft to rotate to a specific angle, the moving knife and the auxiliary knife will reach the corresponding positions and form the corresponding cooperation relationship. This design enables the moving knife and the auxiliary knife to perform thread cutting and post-thread cutting reset actions more accurately, improving the stability and reliability of the thread cutting operation.

[0041] 2. Since the position of the moving knife is directly controlled by the shape of the moving knife cam, after a thread cutting arc surface concentric with the drive shaft is provided on the moving knife cam, even if there is a slight error in the rotation angle of the motor driving the drive shaft, it will not affect the final position of the moving knife, ensuring that the moving knife always waits for thread cutting directly below the needle dropping hole. In contrast, if the motor directly drives the moving knife through a connecting rod, the rotation angle error of the motor will be directly transmitted to the moving knife, so any slight error will cause a change in the position of the moving knife. Therefore, it is impossible to ensure that the biting position of the moving knife and the auxiliary knife is always directly below the needle dropping hole. Therefore, the thread cutting method of the present invention can tolerate a certain error in the motor without affecting the precise thread cutting position, thus being able to more accurately control the thread cutting position below the needle dropping hole of the needle plate, achieving a better anti-nest effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 It is a schematic structural diagram of the thread cutting mechanism of a sewing machine.

[0043] Figure 2 It is a schematic structural diagram of the thread cutting mechanism of a sewing machine.

[0044] Figure 3 It is a schematic transmission structure diagram of the moving knife.

[0045] Figure 4 It is a three-dimensional structural diagram of the moving knife cam.

[0046] Figure 5 It is a schematic diagram of the corresponding part of the working surface of the moving knife cam.

[0047] Figure 6 It is a schematic diagram of the auxiliary knife cam.

[0048] Figure 7It is the front view when the thread cutting mechanism is in the initial state.

[0049] Figure 8 It is the partial schematic diagram when the thread cutting mechanism is in the initial state.

[0050] Figure 9 It is the schematic diagram of the motion relationship between the moving knife cam and the auxiliary knife cam when the thread cutting mechanism is in the initial state.

[0051] Figure 10 It is the front view when the thread cutting mechanism is in the wire pressing state.

[0052] Figure 11 It is the schematic diagram of the motion relationship between the moving knife cam and the auxiliary knife cam when the thread cutting mechanism is in the wire pressing state.

[0053] Figure 12 It is the front view of the process of the thread cutting mechanism cutting the head of the cross thread.

[0054] Figure 13 It is the partial schematic diagram of the process of the thread cutting mechanism cutting the head of the cross thread.

[0055] Figure 14 It is the schematic diagram of the motion relationship between the moving knife cam and the auxiliary knife cam during the process of the thread cutting mechanism cutting the head of the cross thread.

[0056] Figure 15 It is the front view of the process of the thread cutting mechanism in the thread hooking state.

[0057] Figure 16 It is the partial schematic diagram of the process of the thread cutting mechanism in the thread hooking state.

[0058] Figure 17 It is the schematic diagram of the motion relationship between the moving knife cam and the auxiliary knife cam during the process of the thread cutting mechanism in the thread hooking state.

[0059] Figure 18 It is the front view of the process of the thread cutting mechanism cutting the cross thread and the bobbin thread.

[0060] Figure 19 It is the partial schematic diagram of the process of the thread cutting mechanism cutting the cross thread and the bobbin thread.

[0061] Figure 20 It is the schematic diagram of the motion relationship between the moving knife cam and the auxiliary knife cam during the process of the thread cutting mechanism cutting the cross thread and the bobbin thread.

[0062] In the figure, 1 is the machine base; 2 is the needle plate; 21 is the needle dropping hole; 3 is the moving knife; 31 is the cutting edge of the moving knife; 32 is the lower needle hole; 33 is the thread hooking groove; 4 is the auxiliary knife; 41 is the cutting edge of the auxiliary knife; 5 is the driving shaft; 6 is the moving knife link assembly; 61 is the first abutting part; 7 is the auxiliary knife link assembly; 71 is the second abutting part; 8 is the moving knife cam; 81 is the thread cutting arc surface; 82 is the acceleration concave arc surface; 9 is the auxiliary knife cam; 10 is the rotating hook; 11 is the presser; 12 is the driving motor; 13 is the tool rest; 14 is the lower connecting rod; 15 is the upper connecting rod; 16 is the hinge shaft; 17 is the suction air duct. Detailed implementation mode

[0063] The following are specific embodiments of the present invention and in combination with the attached drawings, the technical solutions of the present invention are further described, but the present invention is not limited to these embodiments.

[0064] Embodiment 1

[0065] In the thread cutting method of this sewing machine, as Figure 1 and Figure 2 shown, the sewing machine includes a moving knife 3, an auxiliary knife 4, a rotating hook 10, a presser 11, a suction air duct 17, a machine base 1 with a workbench surface, and a needle plate 2 with a needle dropping hole 21. The moving knife 3 has a cutting edge 31 of the moving knife, a thread hooking groove 33, and a lower needle hole 32. The lower edge of the end face of the auxiliary knife 4 near one end of the sewing needle forms a cutting edge 41 of the auxiliary knife. A thread cutting power mechanism for driving the moving knife 3 and the auxiliary knife 4 to reciprocate and a needle bar power mechanism for driving the sewing needle to move up and down are provided on the machine base 1.

[0066] Among them, the thread cutting power mechanism includes a driving motor 12, a driving shaft 5, a moving knife cam 8, an auxiliary knife cam 9, a moving knife link assembly 6 connected to the moving knife 3, and an auxiliary knife link assembly 7 connected to the auxiliary knife 4. The moving knife cam 8 and the auxiliary knife cam 9 are both fixedly connected to the driving shaft 5. Figure 5 and Figure 6 From a certain perspective, the driving motor 12 drives the moving knife cam 8 and the auxiliary knife cam 9 to rotate clockwise in a one-way cycle through the driving shaft 5.

[0067] As Figure 3 shown, both the moving knife link assembly 6 and the auxiliary knife link assembly 7 include an annular tool rest 13, a lower connecting rod 14, and an upper connecting rod 15. The moving knife 3 and the auxiliary knife 4 are respectively installed on the corresponding tool rests 13. The annular tool rest 13 is rotatably arranged on the machine base 1, and the rotation center line coincides with the center line of the rotating hook 10 of the sewing machine. The moving knife link assembly 6 and the auxiliary knife link assembly 7 have similar structures. One end of the upper connecting rod 15 is hinged to the corresponding tool rest 13 of the moving knife 3 or the auxiliary knife 4, and the other end is hinged to one end of the lower connecting rod 14. The other end of the lower connecting rod 14 is hinged to the machine base 1 through a hinge shaft 16.

[0068] The middle part of the lower connecting rod 14 of the moving knife connecting rod assembly 6 has a first abutting portion 61 that abuts against the working peripheral surface of the moving knife cam 8. When the driving motor 12 drives the driving shaft 5 to rotate, power is transmitted by relying on the moving knife cam 8 and the moving knife connecting rod assembly 6, which can drive the moving knife 3 to swing up and down between the upper part and the side part of the rotary hook 10 around the outer periphery of the rotary hook 10. When swinging upward, the moving knife 3 approaches the needle dropping hole 21 of the needle plate 2, and vice versa, it moves away from the needle dropping hole 21 of the needle plate 2.

[0069] As Figure 2 shown, the middle part of the lower connecting rod 14 of the auxiliary knife connecting rod assembly 7 has a second abutting portion 71 that abuts against the working peripheral surface of the auxiliary knife cam 9. When the driving shaft 5 drives the auxiliary knife cam 9 to rotate, power is transmitted by relying on the auxiliary knife cam 9 and the auxiliary knife connecting rod assembly 7, which can drive the auxiliary knife 4 to swing up and down between the upper part and the side part of the rotary hook 10 around the outer periphery of the rotary hook 10. When swinging upward, the auxiliary knife 4 approaches the needle dropping hole 21 of the needle plate 2, and vice versa, it moves away from the needle dropping hole 21 of the needle plate 2.

[0070] Further, as Figure 4 and Figure 5 shown, the working peripheral surface of the moving knife cam 8 includes a first concave arc segment AB, a first convex arc segment BC, a second concave arc segment CD, and a second convex arc segment DA that are connected end to end in sequence. There is a thread cutting arc surface 81 concentric with the driving shaft 5 on the first convex arc segment BC. The distances from the starting point B and the ending point C of the first convex arc segment BC to the axis of the driving shaft 5 are both greater than the distance from the thread cutting arc surface 81 to the axis of the driving shaft 5. The second convex arc segment DA is a circular arc surface concentric with the driving shaft 5. When the working peripheral surface of the moving knife cam 8 pushes the first abutting portion 61 to move in a direction away from the axis of the driving shaft 5, the moving knife 3 swings downward.

[0071] As Figure 6 shown, the working peripheral surface of the auxiliary knife cam 9 has a starting needle thread cutting peach tip a, a waiting-for-thread-cutting peach tip b, and a tail needle thread cutting peach tip c arranged in sequence along the circumferential direction. The distances from the starting needle thread cutting peach tip a and the tail needle thread cutting peach tip c to the axis of the driving shaft 5 are equal. The tail needle thread cutting peach tip c is connected to the starting needle thread cutting peach tip a through a connecting convex arc segment ca. The distances from each point on the connecting convex arc segment ca to the axis of the driving shaft 5 are all less than the distance from the starting needle thread cutting peach tip a to the axis of the driving shaft 5. The starting needle thread cutting peach tip a and the waiting-for-thread-cutting peach tip b are connected through a connecting concave arc segment ab. When the working peripheral surface of the auxiliary knife cam 9 pushes the second abutting portion 71 to move in a direction away from the axis of the driving shaft 5, the auxiliary knife 4 swings upward.

[0072] The thread cutting power mechanism adopts a cam-linkage structure. In this way, by designing the working surface shapes of the moving knife cam 8 and the auxiliary knife cam 9, the positions and speeds of the moving knife 3 and the auxiliary knife 4 swinging at a certain time can be determined. Therefore, through the shape design of the first convex arc segment BC and the connecting convex arc segment ca, when the moving knife 3 and the auxiliary knife 4 swing upward, the speed of the auxiliary knife 4 can be made less than that of the moving knife 3, thus avoiding the situation that the cutting edge 31 of the moving knife bites with the auxiliary knife 4 and cuts off the thread end of the surface thread in advance before reaching directly below the needle dropping hole 21 during rapid movement. Specifically, as Figure 5 shown, on the moving knife cam 8, there is a concave accelerating concave arc surface 82 on the working surface between the starting point B of the first convex arc segment BC and the thread cutting arc surface 81, so that when the accelerating concave arc surface 82 slides past the abutting part 61, it can accelerate the upward swinging speed of the moving knife 3.

[0073] This thread cutting method includes the following steps:

[0074] 1. Initial state: As Figure 7 、 Figure 8 and Figure 9 shown, the thread cutting power mechanism works in advance, the driving motor 12 drives the driving shaft 5 to start rotating, so that the inner concave inflection point of the first concave arc segment AB on the moving knife cam 8 abuts against the abutting part 61, stopping the moving knife 3 at a position where its lower needle hole 32 is directly opposite to the needle dropping hole 21 on the needle plate 2, and then the thread cutting power mechanism pauses working.

[0075] 2. Needle lifting and needle dropping: Place the material to be sewn on the workbench surface, the needle bar power mechanism starts to work, driving the sewing needle to move downward with the thread end of the surface thread through the lower needle hole 32 on the moving knife 3 and then move upward above the needle plate 2, and the thread end of the surface thread remains in the lower needle hole 32;

[0076] 3. Pressing the thread: As Figure 10 and Figure 11 shown, the thread cutting power mechanism starts to work, driving the moving knife 3 with the thread end of the surface thread to swing around the outer periphery of the rotary hook 10 from the upper part of the rotary hook 10 to the side of the rotary hook 10. After swinging, the moving knife 3 and the auxiliary knife 4 are located on the same side of the rotary hook 10. The driving motor 12 drives the driving shaft 5 to start rotating, the connecting convex arc segment ca of the auxiliary knife cam 9 slides along the abutting part 71, and the working surface of the moving knife cam 8 slides along the abutting part 61 from the inner concave inflection point of the first concave arc segment AB to the starting point B of the first convex arc segment BC. The moving knife 3 and the auxiliary knife 4 both swing downward to move away from the needle dropping hole 21, and the moving knife 3 swings downward to the lower dead point, located on the same side of the rotary hook 10 as the auxiliary knife 4. The lower side surface of the moving knife 3 abuts against the thread pressing piece 11 to clamp the thread end of the surface thread passing through the lower needle hole 32, and then the thread cutting power mechanism pauses working;

[0077] 4. The needle bar power mechanism drives the sewing needle to perform sewing actions for a set number of stitches. The upper thread on the sewing needle is locked with the bobbin thread in the rotary hook 10. After the sewing needle sews the set number of stitches, such as two stitches, three stitches, or four stitches, etc.

[0078] 5. Cut the end of the upper thread: The drive motor 12 drives the drive shaft 5 to start rotating. The working surface of the moving knife cam 8 slides along the abutting part one 61 from the starting point B of the first convex arc segment BC to the thread-cutting arc surface 81, causing the moving knife 3 to swing upward. At the same time, the working surface of the auxiliary knife cam 9 slides along the abutting part two 71 from the middle position of the connecting convex arc segment ca to the starting needle thread-cutting tip a, causing the auxiliary knife 4 to swing upward at a speed slower than that of the moving knife 3 swinging upward.

[0079] As Figure 12 、 Figure 13 and Figure 14 shown, the continuous rotation of the drive shaft 5 causes the thread-cutting arc surface 81 on the moving knife cam 8 to slide past the abutting part one 61, stopping the moving knife 3 at a position where its cutting edge 31 is directly below the needle hole 21 on the needle plate 2. At the same time, the starting needle thread-cutting tip a on the auxiliary knife cam 9 slides past the abutting part two 71, causing the auxiliary knife 4 to engage with the cutting edge 31 of the moving knife 3 to cut the end of the upper thread. Additionally, during this process, before cutting the end of the upper thread, the suction air pipe 17 is already in the suction state and the presser 11 has released the end of the upper thread, so that the cut end of the upper thread is directly sucked away by the suction air pipe 17.

[0080] 6. Continue sewing: The drive motor 12 continues to rotate, driving the drive shaft 5 to rotate. The working surface of the moving knife cam 8 slides along the abutting part one 61 from the thread-cutting arc surface 81 to the end point C of the first convex arc segment BC. At the same time, the working surface of the auxiliary knife cam 9 slides along the abutting part two 71 from the starting needle thread-cutting tip a to the middle position of the connecting concave arc segment ab, causing the moving knife 3 and the auxiliary knife 4 to move to the same side of the rotary hook 10 again, avoiding the lower needle position of the sewing needle so that the lower needle of the sewing needle does not hit the moving knife 3 and the auxiliary knife 4. Then the thread-cutting power mechanism pauses working and the drive shaft 5 stops rotating; the needle bar power mechanism drives the sewing needle to continuously perform sewing actions until the sewing action ends, and the needle bar power mechanism pauses working.

[0081] 7. Cut the upper thread and the bobbin thread: The drive motor 12 drives the drive shaft 5 to start rotating. The working surface of the moving knife cam 8 slides along the abutting part one 61 from the end point C of the first convex arc segment BC to the inner concave inflection point of the second concave arc segment CD. At the same time, the working surface of the auxiliary knife cam 9 slides along the abutting part two 71 from the middle position of the connecting concave arc segment ab to the thread-to-be-cut tip b, causing both the moving knife 3 and the auxiliary knife 4 to swing upward, as Figure 15 、 Figure 16 and Figure 17As shown, when the moving knife 3 slides past the first abutting portion 61 at the inner concave inflection point of the second concave arc segment CD, the moving knife 3 is in a position where its thread hooking groove 33 is directly above and below the needle dropping hole 21 on the needle plate 2. The upper thread and the bobbin thread fall into the thread hooking groove 33. The moving knife cam 8 continues to rotate, and its working peripheral surface slides along the first abutting portion 61 from the inner concave inflection point of the second concave arc segment CD to the second convex arc segment DA. At the same time, the auxiliary knife cam 9 slides along the second abutting portion 71 from the tip b of the thread cutting lobe to the tip a of the thread cutting lobe, causing the moving knife 3 to swing downward and the auxiliary knife 4 to swing upward.

[0082] As Figure 18 , Figure 19 and Figure 20 shown, the drive shaft 5 continues to rotate, causing the second convex arc segment DA of the moving knife cam 8 to slide past the first abutting portion 61, stopping the moving knife 3 at a position where its moving knife edge 31 is directly below the needle dropping hole 21 on the needle plate 2. At the same time, the tail needle thread cutting tip c on the auxiliary knife cam 9 slides past the second abutting portion 71, causing the auxiliary knife 4 to engage with the moving knife edge 31 to cut the upper thread and the bobbin thread.

[0083] 8. Reset: The drive shaft 5 continues to rotate, causing the moving knife 3 and the auxiliary knife 4 to reset to the Figure 7 , Figure 8 and Figure 9 shown initial state, waiting for the next sewing. Of course, during the actual sewing process, the moving knife 3 and the auxiliary knife 4 can be reset to other positions. When the sewing machine needs to be started for the next sewing, the control system controls the drive motor 12 to act, causing the moving knife 3 and the auxiliary knife 4 to first swing to the Figure 7 , Figure 8 and Figure 9 shown initial state.

[0084] Embodiment 2

[0085] The method steps of this embodiment are basically the same as those of Embodiment 1. The difference is that before the sewing machine is turned on, the inner concave inflection point of the first concave arc segment AB on the moving knife cam 8 has already abutted against the first abutting portion 61, stopping the moving knife 3 at a position where its lower needle hole 32 is directly above and below the needle dropping hole 21 on the needle plate 2. Therefore, after the sewing machine is turned on, the step of the thread cutting power mechanism working in advance is omitted.

[0086] Embodiment 3

[0087] The method steps of this embodiment are basically the same as those of Embodiment 1. The difference lies in that: instead of using the cooperation of the moving knife cam 8 and the auxiliary knife cam 9 with the connecting rod structure to realize the up and down swing of the moving knife 3 and the auxiliary knife 4, a structure similar to the thread cutting method of the double moving knife thread cutting mechanism in the background art (application number: 202411329869.0) is adopted, including a moving knife motor for driving the reciprocating swing of the moving knife 3 and an auxiliary knife motor for driving the reciprocating movement of the auxiliary knife 4. Both the moving knife motor and the auxiliary knife motor adopt servo motors. The moving knife motor and the auxiliary knife motor are connected to the moving knife 3 and the auxiliary knife 4 through a connecting rod mechanism, and the control system controls the moving knife motor and the auxiliary motor to work according to the set rotation time and rotation angle, so as to control the moving knife 3 and the auxiliary knife 4 to swing in a corresponding manner.

[0088] The specific embodiments described herein are merely illustrative of the spirit of the present invention. Those skilled in the art of the present invention can make various modifications or supplements to the described specific embodiments or use similar methods for substitution, but will not deviate from the spirit of the present invention or exceed the scope defined by the appended claims.

[0089] Although terms such as 1. machine base; 2. needle plate; 21. needle dropping hole; 3. moving knife; 31. moving knife blade edge; 32. lower needle hole; 33. thread hooking groove; 4. auxiliary knife; 41. auxiliary knife blade edge; 5. drive shaft; 6. moving knife connecting rod assembly; 61. abutting part one; 7. auxiliary knife connecting rod assembly; 71. abutting part two; 8. moving knife cam; 81. thread cutting arc surface; 82. accelerating concave arc surface; 9. auxiliary knife cam; 10. rotary hook; 11. presser; 12. drive motor; 13. tool rest; 14. lower connecting rod; 15. upper connecting rod; 16. hinge shaft; 17. air suction pipe are used more in this article, the possibility of using other terms is not excluded. Using these terms is only to more conveniently describe and explain the essence of the present invention; interpreting them as any additional limitation is contrary to the spirit of the present invention.

Claims

1. A thread trimming method for a sewing machine, the sewing machine comprising a machine base (1) having a work surface, a needle plate (2) having a needle drop hole (21), a movable knife (3), an auxiliary knife (4) and a rotary hook (10), the movable knife (3) having a movable knife edge (31) and a lower needle hole (32), the machine base (1) being provided with a thread trimming power mechanism for driving the movable knife (3) and the auxiliary knife (4) to move back and forth and a needle bar power mechanism for driving the machine needle to move up and down, characterized in that: The thread trimming method comprises the following steps: A. Place the material to be sewn on the work surface, and the needle bar power mechanism starts to work, driving the needle to move the upper thread end downward through the lower needle hole (32) on the movable knife (3) and then move upward to above the needle plate (2), and the upper thread end remains in the lower needle hole (32) of the movable knife (3); B. The thread trimming power mechanism starts to work, driving the movable knife (3) to move the upper thread end away from the needle drop hole (21), so that the movable knife (3) and the auxiliary knife (4) are located on the same side of the rotary hook (10), and then the thread trimming power mechanism stops working; C. The needle bar power mechanism drives the needle to sew a few stitches, and the upper thread is locked with the lower thread in the rotary hook (10); D. The thread trimming power mechanism starts to work again, driving the movable knife (3) and the auxiliary knife (4) to move together in a direction close to the needle drop hole (21), and making the auxiliary knife (4) move at a speed lower than the movable knife (3), until the movable knife (3) is in a position where the movable knife edge (31) is located directly below the needle drop hole (21) on the needle plate (2), then the movable knife (3) stops moving, and the auxiliary knife (4) continues to move to push the upper thread end to the movable knife edge (31), and the upper thread end is cut by the engagement of the auxiliary knife (4) and the movable knife edge (31).

2. The thread trimming method for a sewing machine according to claim 1, characterized in that: In the step B, driving the movable knife (3) with the upper thread end moving in a direction away from the needle drop hole (21) is specifically: driving the movable knife (3) with the upper thread end swinging around the outer circumference of the rotary shuttle (10) from the upper part of the rotary shuttle (10) to the side of the rotary shuttle (10); In the step D, driving the movable knife (3) and the auxiliary knife (4) to move together in a direction close to the needle drop hole (21) specifically comprises: driving the movable knife (3) and the auxiliary knife (4) to swing together around the outer circumference of the rotary hook (10) from the side of the rotary hook (10) to the upper part of the rotary hook (10).

3. The thread trimming method for a sewing machine according to claim 2, characterized in that: The movable knife (3) also has a thread hooking groove (33). The thread trimming method further comprises the following steps: E. The thread trimming power mechanism drives the movable knife (3) and the auxiliary knife (4) to swing downward, so that the movable knife (3) and the auxiliary knife (4) move to the same side of the rotary hook (10) again, and then the thread trimming power mechanism stops working, and the needle bar power mechanism continues to drive the needle to perform sewing action until the sewing action is completed, and the needle bar power mechanism stops working; F. The thread trimming power mechanism starts working again, driving the movable knife (3) to swing upward to a position where the thread hooking groove (33) is vertically opposite to the needle drop hole (21) on the needle plate (2), and the upper thread and the lower thread fall into the thread hooking groove (33). Then, the movable knife (3) is driven to swing downward to a position where the movable knife edge (31) is vertically opposite to the needle drop hole (21) on the needle plate (2), and then the auxiliary knife (4) is driven to swing upward to push the upper thread and the lower thread to the movable knife edge (31), and the upper thread and the lower thread are cut by the engagement of the auxiliary knife (4) with the movable knife edge (31).

4. The thread trimming method for a sewing machine according to claim 3, characterized in that: The thread trimming power mechanism comprises a driving motor (12), a driving shaft (5), a moving knife cam (8), an auxiliary knife cam (9), a moving knife connecting rod assembly (6) connected to the moving knife (3), and an auxiliary knife connecting rod assembly (7) connected to the auxiliary knife (4); the moving knife cam (8) and the auxiliary knife cam (9) are both fixedly connected to the driving shaft (5); the moving knife connecting rod assembly (6) has a first abutment portion (61), and the auxiliary knife connecting rod assembly (7) has a second abutment portion (71); The working circumference of the movable knife cam (8) comprises a first concave arc segment AB, a first convex arc segment BC, a second concave arc segment CD and a second convex arc segment DA which are connected end to end in sequence; the first convex arc segment BC has a thread-cutting arc surface (81) which is concentric with the drive shaft (5); the distances from the starting point B and the end point C of the first convex arc segment BC to the axis of the drive shaft (5) are both greater than the distance from the thread-cutting arc surface (81) to the axis of the drive shaft (5); when the pushing abutment portion (61) of the working circumference of the movable knife cam (8) moves in a direction away from the axis of the drive shaft (5), the movable knife (3) swings downward; The working circumferential surface of the auxiliary knife cam (9) has a needle start thread trimming peach tip a, a to-be-trimmed peach tip b and a tail needle thread trimming peach tip c arranged in sequence along the circumferential direction, the tail needle thread trimming peach tip c is connected to the needle start thread trimming peach tip a via a connecting convex arc segment ca, and when the working circumferential surface of the auxiliary knife cam (9) pushes the abutment portion 2 (71) in a direction away from the axis of the drive shaft (5), the auxiliary knife (4) swings upward; The step D is specifically as follows: After the thread trimming power mechanism resumes operation, the drive motor (12) drives the drive shaft (5) to start rotating, and the working surface of the movable knife cam (8) slides from the starting point B of the first convex arc segment BC to the thread trimming arc surface (81) against the abutment portion 1 (61), so that the movable knife (3) swings upward. At the same time, the working surface of the auxiliary knife cam (9) slides from the middle position of the connecting convex arc segment ca to the needle thread trimming peach tip a against the abutment portion 2 (71), so that the auxiliary knife (4) swings upward at a speed less than the upward swinging speed of the movable knife (3). The driving shaft (5) continuously rotates so that the thread cutting arc surface (81) on the movable knife cam (8) slides over the abutment portion 1 (61), and the movable knife (3) stops at a position where the movable knife edge (31) is located directly below the needle drop hole (21) on the needle plate (2). At the same time, the needle thread cutting peach tip a on the auxiliary knife cam (9) slides over the abutment portion 2 (71), so that the auxiliary knife (4) engages with the movable knife edge (31) on the movable knife (3) to cut the upper thread end.

5. The thread trimming method for a sewing machine according to claim 1 or 2, characterized in that: The thread trimming power mechanism comprises a moving knife motor for driving a moving knife (3) to move back and forth and an auxiliary knife motor for driving an auxiliary knife (4) to move back and forth.

6. The thread trimming method for a sewing machine according to claim 4, characterized in that: The needle thread cutting peach tip a and the to-be-cut peach tip b are connected via a connecting concave arc segment ab. In the step E, the thread cutting power mechanism drives the movable knife (3) and the auxiliary knife (4) to swing downwards as follows: the driving motor (12) drives the driving shaft (5) to rotate, and the working circumference of the movable knife cam (8) slides from the thread cutting arc surface (81) against the abutment portion 1 (61) to the end point C of the first convex arc segment BC. At the same time, the working circumference of the auxiliary knife cam (9) slides from the needle thread cutting peach tip a against the abutment portion 2 (71) to the middle position of the connecting concave arc segment ab.

7. The thread trimming method for a sewing machine according to claim 6, characterized in that: In step F, after the thread trimming power mechanism resumes operation, the drive motor (12) drives the drive shaft (5) to start rotating, and the working circumference of the movable knife cam (8) slides along the abutment portion 1 (61) from the end point C of the first convex arc segment BC to the concave turning point of the second concave arc segment CD. At the same time, the working circumference of the auxiliary knife cam (9) slides along the abutment portion 2 (71) from the middle position connecting the concave arc segment ab to the tip b of the thread to be trimmed, so that the movable knife (3) and the auxiliary knife (4) both swing upward. When the movable knife (3) slides over the abutment portion 1 (61) at the concave turning point of the second concave arc segment CD, the movable knife (3) is in a position where its thread hooking groove (33) is vertically opposite to the needle drop hole (21) on the needle plate (2).

8. The thread trimming method for a sewing machine according to claim 7, characterized in that: The second convex arc segment DA is an arc surface concentric with the drive shaft (5), and the distances from the starting needle thread trimming peach tip a and the tail needle thread trimming peach tip c to the axis of the drive shaft (5) are equal. In the step F, after the upper thread and the lower thread fall into the thread hooking groove (33), the working circumference of the movable knife cam (8) slides from the concave turning point of the second concave arc segment CD to the second convex arc segment DA along the abutment portion 1 (61), and at the same time, the auxiliary knife cam (9) slides from the to-be-trimmed peach tip b to the trimming peach tip a along the abutment portion 2 (71), so that the movable knife (3) swings downward and the auxiliary knife (4) swings upward; The driving shaft (5) continuously rotates so that the second convex arc segment DA of the movable knife cam (8) slides over the abutment portion 1 (61), and the movable knife (3) stops at a position where the movable knife edge (31) is located directly below the needle drop hole (21) on the needle plate (2). At the same time, the tail needle thread cutting peach tip c on the auxiliary knife cam (9) slides over the abutment portion 2 (71), so that the auxiliary knife (4) engages with the movable knife edge (31) to cut the upper thread and the lower thread.

9. The thread trimming method for a sewing machine according to claim 4, 6, 7 or 8, characterized in that: Before step A, the thread trimming method further comprises the following steps: the thread trimming power mechanism starts working in advance, the driving motor (12) drives the driving shaft (5) to start rotating, so that the inner concave turning point of the first concave arc segment AB on the movable knife cam (8) abuts against the abutment portion 1 (61), and the movable knife (3) is stopped at a position where the lower needle hole (32) and the needle drop hole (21) on the needle plate (2) are vertically opposite, and then the thread trimming power mechanism stops working.

10. The thread trimming method for a sewing machine according to claim 4, 6, 7 or 8, characterized in that: The distances from each point on the connecting convex arc segment ca to the axis of the drive shaft (5) are all smaller than the distance from the needle thread trimming peach tip a to the axis of the drive shaft (5). The step B specifically comprises: after the thread trimming power mechanism starts to work, the drive motor (12) drives the drive shaft (5) to start rotating, the connecting convex arc segment ca of the auxiliary knife cam (9) slides against the abutment portion 2 (71), and the working circumference of the movable knife cam (8) slides against the abutment portion 1 (61) from the concave inflection point of the first concave arc segment AB to the starting point B of the first convex arc segment BC, and then the thread trimming power mechanism stops working.

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