Thread trimming method of sewing machine

Abstract

The invention provides a thread trimming method of a sewing machine, and belongs to the technical field of sewing machines. The problem that an existing thread trimming method is insufficient in bird groove preventing effect is solved. The thread trimming method comprises the following steps that A, a machine needle is driven to drive an upper thread end to move downwards to penetrate through a lower needle hole in a movable cutter and then move upwards to the position above a needle plate, and the upper thread end is kept in the lower needle hole; b, a thread trimming power mechanism starts to work, a movable cutter is driven to drive an upper thread end to move in the direction away from a needle falling hole, and then the thread trimming power mechanism stops working; c, the needle bar power mechanism drives the machine needle to sew a plurality of stitches; d, the thread trimming power mechanism works again, the movable cutter and the auxiliary cutter are driven to move in the direction close to the needle falling hole together, the movable cutter stops moving until the movable cutter is located under the needle falling hole in the needle plate, and the auxiliary cutter continues to move to be meshed with the movable cutter edge to cut off the upper thread end. The thread trimming device has the advantages of being high in thread trimming efficiency and good in bird nest prevention effect.

Description

Technical Field

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

[0002] A sewing machine is a device that uses one or more threads to form one or more stitches on fabric to interweave or sew two or more layers of fabric. In order to improve sewing efficiency, a thread trimming mechanism is usually provided on the sewing machine, which automatically cuts off the upper thread ends remaining on the fabric at the beginning of the sewing stitches. Otherwise, there will be longer thread ends, which will form a "bird's nest" phenomenon after clumping and look unsightly. After the last stitch is sewn, the upper and lower threads also need to be cut off.

[0003] The existing thread trimming method of the sewing machine is such as the patented double-moving knife thread trimming mechanism disclosed in the patent document (application number: 202411329869.0). In order to cut the upper thread end, the thread trimming knife 1 is first moved toward the direction of the machine needle. After completing the first needle movement, the thread trimming knife 1 retreats to a position close to the thread trimming knife 2. 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 machine needle performs the second needle sewing action. After that, the thread trimming knife 1 continues to reset to the first position. During this process, the thread trimming knife 1 swings downward, and the thread trimming knife 2 continues to move to the fourth position; when the second cutting edge of the thread trimming knife 2 and the first cutting edge of the thread trimming knife 1 overlap, the sewing thread in the thread hole groove is cut. This thread trimming method has the following shortcomings in the actual production process: 1. When the needle thread needs to be cut, the upper thread ends are cut off because the upper thread ends are engaged with the thread cutter moving knife 2 during the process of retreating downward from the second position (swinging upward to the position below the needle). The thread cutting position of the thread cutting mechanism is far away from the needle drop hole of the needle plate, so that the end of the upper thread that is cut is far away from the fabric, that is, the upper thread ends remaining on the fabric are long, and the anti-bird nest effect is poor.

[0004] 2. It adopts a transmission rod 2 sleeved on the outside of the transmission rod 1, and the transmission rod 1 and the transmission rod 2 are driven to rotate by the first driving member and the second driving member respectively, and then the thread cutting movable knife 1 and the thread cutting movable knife 2 are driven to swing up and down respectively, that is, the thread cutting mechanism uses two motors to drive the thread cutting movable knife 1 and the thread cutting movable knife 2 to swing respectively. At this time, since the two motors operate independently, the two motors need a more complex control algorithm to coordinate the matching relationship between the thread cutting movable knife 1 and the thread cutting movable knife 2. This complexity is likely to increase the risk of potential failures, and it is easy to cause the matching relationship between the thread cutting movable knife 1 and the thread cutting movable knife 2 to change, thereby affecting the working stability and reliability of the thread cutting. Summary of the invention

[0005] The purpose of the present invention is to solve the problem of insufficient bird-proof groove effect of the existing thread cutting method while ensuring the thread cutting efficiency of the sewing machine.

[0006] The object of the present invention can be achieved by the following technical scheme: a thread trimming method of a sewing machine, the sewing machine comprises a machine base with a work surface, a needle plate with a needle drop hole, a movable knife, an auxiliary knife and a rotary shuttle, the movable knife has a movable knife edge and a lower needle hole, the machine base is provided with a thread trimming power mechanism for driving the movable knife and the auxiliary knife 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 on the movable knife and then move upward to the top of the needle plate, and the upper thread end remains in the lower needle hole; B. The thread trimming power mechanism starts to work, driving the movable knife to move the upper thread end away from the needle drop hole, so that the movable knife and the auxiliary knife are on the same side of the rotary hook, 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; D. The thread trimming power mechanism works again, driving the movable knife and the auxiliary knife to move together in the direction close to the needle drop hole, and making the auxiliary knife move at a speed lower than the movable knife, until the movable knife is in a position where the movable knife edge is directly below the needle drop hole on the needle plate, and the movable knife stops moving. The auxiliary knife continues to move and pushes the upper thread end to the movable knife edge, and the upper thread end is cut by the engagement of the auxiliary knife and the movable knife edge.

[0007] Before sewing begins, first pass one end of the upper thread through the needle to complete the needle threading action. The section of the upper thread that passes through the needle hole is the reserved upper thread end. In this thread cutting method, after driving the needle to perform a set number of sewing actions, in order to cut off the upper thread end, in step C, the movable knife and the auxiliary knife are driven together to move toward the needle hole, and when the movable knife moves to the point where the movable knife edge is directly below the needle hole on the needle plate, the movable knife stops and remains motionless, waiting for the auxiliary knife to approach for thread cutting. In this way, the cutting position of the upper thread end can be closer to the needle hole of the needle, thereby shortening the upper thread end remaining on the fabric as much as possible, achieving a better anti-bird nest effect. Moreover, compared with the method in which both the movable knife and the auxiliary knife move to engage and cut the surface thread end, the present thread cutting method adopts a "static and dynamic" method to cut the thread. In this way, the thread cutting position only depends on the position accuracy of the movable knife, but has no direct relationship with the time point of engagement of the movable knife and the auxiliary knife and the movement speed of the auxiliary knife close to the moving knife edge. Therefore, the control algorithm of the thread cutting mechanism of the sewing machine can be simplified, so that the thread cutting position can be more accurately controlled below the needle hole of the needle plate to ensure a better anti-bird nest effect.

[0008] In addition, when the movable knife is driven to move toward the needle drop hole in step C, the auxiliary knife also moves with it, so that when the movable 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 movable knife, thereby further improving the thread cutting efficiency. The movement of the movable knife and the auxiliary knife to the thread-cutting position for thread-cutting is completed instantly, instead of slowly approaching the needle hole for thread-cutting. Therefore, in step C, the design of making the auxiliary knife move at a speed lower than the moving knife speed can gradually increase the distance between the movable knife edge and the auxiliary knife during the moving process of the movable knife. This can improve the fault tolerance rate and avoid premature thread cutting. For example, even if the mechanical parts of the equipment are worn out after a long period of operation, or there is a slight deviation in the control parameters of the auxiliary knife, resulting in the auxiliary knife moving faster than the preset speed, it will only shorten the distance between the auxiliary knife and the movable knife. The situation that the movable knife edge engages with the auxiliary knife before reaching the needle hole and cutting the upper thread end in advance will not occur due to the rapid movement of the movable knife and the auxiliary knife. Therefore, on the basis of ensuring the high efficiency of thread cutting of the sewing machine, the thread cutting position is always directly below the needle hole of the needle plate to achieve a better anti-bird's nest effect.

[0009] In the thread trimming method of the sewing machine, in step B, driving the movable knife with the upper thread end moving in a direction away from the needle drop hole is specifically: driving the movable knife with the upper thread end swinging around the outer circumference of the rotary hook from the upper part of the rotary hook to the side of the rotary hook; In step D, driving the movable knife and the auxiliary knife together to move toward the needle drop hole specifically includes: driving the movable knife and the auxiliary knife together to swing around the outer circumference of the rotary hook from the side of the rotary hook to the upper part of the rotary hook. When the movable knife and the auxiliary knife swing upward, the movable knife and the auxiliary knife are close to the needle drop hole of the needle plate, and vice versa.

[0010] In the above-mentioned thread trimming method for a sewing machine, the movable knife further has a thread hooking groove, and the thread trimming method further comprises the following steps: E. The thread trimming power mechanism drives the movable knife and the auxiliary knife to swing downward, so that the movable knife and the auxiliary knife move to the same side of the rotary hook again, and then the thread trimming power mechanism stops working, and the needle bar power mechanism continues to drive the needle to sew until the sewing action is completed, and the needle bar power mechanism stops working; F. The thread trimming power mechanism works again, driving the movable knife to swing upward to a position where the thread hooking groove is vertically opposite to the needle drop hole on the needle plate, and the upper thread and the lower thread fall into the thread hooking groove. Then the movable knife is driven to swing downward to a position where the movable knife edge is vertically opposite to the needle drop hole on the needle plate, and then the auxiliary knife is driven to swing upward to push the upper thread and the lower thread to the movable knife edge, and the upper thread and the lower thread are cut by the engagement of the auxiliary knife and the movable knife edge.

[0011] In the above step E, by swinging the movable knife and the auxiliary knife downward to the same side of the rotary hook, the needle position can be avoided, so that the needle position will not hit the movable knife and the auxiliary knife, and the sewing machine can continue to sew until the sewing action is completed. After the sewing action is completed, the upper thread and the lower thread are cut in step F, so that the sewing material can be removed from the work surface. In step F, the movable knife will also stop at a position where the movable knife edge is located directly below the needle drop hole, so that the thread cutting position is closer to the needle drop hole, so that after sewing, the upper thread and the lower thread after cutting are as short as possible, further ensuring a good effect of sewing bird's nests.

[0012] In the above-mentioned thread trimming method of a sewing machine, the thread trimming power mechanism includes a driving motor, a driving shaft, a moving knife cam, an auxiliary knife cam, a moving knife connecting rod assembly connected to the moving knife, and an auxiliary knife connecting rod 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 connecting rod assembly has a first abutment portion, and the auxiliary knife connecting rod assembly has a second abutment portion; 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. 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; The step D is specifically as follows: 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; 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.

[0013] 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.

[0014] On the moving knife cam, a cutting arc surface is provided on the first convex arc segment BC that is concentric with the drive shaft. Since the distance from the abutment portion 1 to the axis of the drive shaft (or the rotation center of the moving knife cam) does not change during the process of the cutting arc surface sliding over the abutment portion 1, the moving knife can remain stationary during the process. Therefore, the design of the cutting arc surface enables the auxiliary knife cam to rotate and drive the auxiliary knife to move toward the moving knife to cut the thread. Even if the moving knife cam must rotate with the drive shaft, the moving knife can remain stopped and wait for the auxiliary knife to approach, thereby forming a "static and dynamic" cutting method, so as to accurately control the cutting position below the needle hole of the needle plate, shorten the thread ends of the upper thread remaining on the fabric, and achieve a better anti-bird nest effect.

[0015] In the above-mentioned thread trimming method of a sewing machine, the thread trimming power mechanism includes a moving knife motor for driving the moving knife to swing back and forth and an auxiliary knife motor for driving the auxiliary knife to move back and forth. The moving knife motor and the auxiliary knife motor are both servo motors, and the moving knife motor and the auxiliary knife motor are connected to the moving knife and the auxiliary knife through a connecting rod 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 a corresponding manner.

[0016] In the above-mentioned thread trimming method of the sewing machine, the needle thread trimming peach tip a and the to-be-trimmed peach tip b are connected by a connecting concave arc segment ab. In the step E, the thread trimming power mechanism drives the movable knife and the auxiliary knife to swing downward as follows: the driving motor drives the driving shaft to rotate, and the working circumference of the movable knife cam slides from the thread trimming arc surface against the abutment portion 1 to the end point C of the first convex arc segment BC. At the same time, the working circumference of the auxiliary knife cam slides from the needle thread trimming peach tip a against the abutment portion 2 to the middle position of the connecting concave arc segment ab.

[0017] 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 trimming arc surface to the axis of the drive shaft, as the working circumference of the movable knife cam slides against the abutment portion 1 from the thread trimming arc surface to the end point C of the first convex arc segment BC, the distance from the abutment portion 1 to the axis of the drive shaft gradually increases, causing the movable knife to swing downward; as the working circumference of the auxiliary knife cam slides against the abutment portion 2 from the needle and thread trimming peach tip a to the middle position connecting the concave arc segment ab, the distance from the abutment portion 2 to the axis of the drive shaft gradually decreases, causing the auxiliary knife to also swing downward.

[0018] In the above-mentioned thread trimming method of the sewing machine, in the step F, after the thread trimming power mechanism works again, the driving motor drives the driving shaft to start rotating, and the working circumference of the moving knife cam slides against the abutment portion 1 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 slides against the abutment portion 2 from the middle position connecting the concave arc segment ab to the tip b of the thread to be trimmed, so that the moving knife and the auxiliary knife both swing upward, and when sliding over the abutment portion 1 at the concave turning point of the second concave arc segment CD, the moving knife is in a position where its thread hook groove is directly opposite to the needle drop hole on the needle plate.

[0019] After the thread hooking work is completed, the sewing machine needs to cut the upper thread and the lower thread in the thread hooking groove. When the movable knife swings upward to move the thread hooking groove below the needle drop hole, the auxiliary knife swings upward with the movable knife, so that when the upper thread and the lower thread need to be cut later, the auxiliary knife is closer to the thread cutting position, thereby shortening the waiting time of the movable knife and further improving the thread cutting efficiency.

[0020] In the above-mentioned thread trimming method for a sewing machine, the second convex arc segment DA is an arc surface concentric with the driving shaft, the distances between the starting needle thread trimming peach tip a and the tail needle thread trimming peach tip c and the axis of the driving shaft are equal, and in the step F, after the upper thread and the lower thread fall into the thread hooking groove, the working circumference of the movable knife cam slides from the concave turning point of the second concave arc segment CD to the second convex arc segment DA against the abutment portion 1, and at the same time, the auxiliary knife cam slides from the to-be-trimmed peach tip b to the trimming peach tip a against the abutment portion 2, so that the movable knife swings downward and the auxiliary knife swings upward; The driving shaft continues to rotate, causing the second convex arc segment DA of the moving knife cam to slide over the abutment portion 1, stopping the moving knife at a position where the moving knife edge is directly below the needle drop hole on the needle plate. At the same time, the tail needle thread cutting peach tip c on the auxiliary knife cam slides over the abutment portion 2, causing the auxiliary knife to engage with the moving knife edge to cut the upper thread and the lower thread.

[0021] Since the second convex arc segment DA is an arc surface concentric with the drive shaft, the distances from the needle thread trimming peach tip a and the tail needle thread trimming peach tip c to the axis of the drive shaft are equal, so that the auxiliary knife swings to the same position when the needle thread trimming and the tail needle thread trimming are performed. At the same time, since the second convex arc segment DA is an arc surface concentric with the drive shaft, when the second convex arc segment DA slides over the abutment portion 1, the movable knife will also stop at a position where the movable knife edge is located directly below the needle drop hole, and the auxiliary knife can also move to a position that engages with the movable knife edge, so that the thread trimming position is closer to the needle drop hole, so that after sewing, the upper thread and the lower thread after cutting are as short as possible, further improving the bird's nest sewing effect.

[0022] In the above-mentioned thread trimming method of the sewing machine, before 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 concave turning point of the first concave arc segment AB on the movable knife cam abuts against the abutting portion 1, and the movable knife is stopped at a position where its lower needle hole and the needle drop hole on the needle plate are vertically opposite, and then the thread trimming power mechanism is suspended. Before the first needle is inserted, the lower needle hole on the movable knife and the needle drop hole on the needle plate are vertically opposite, so that the machine needle can pass through the lower needle hole downward, then leave the lower needle hole upward and lift it above the needle plate, completing the first needle insertion action of the machine needle.

[0023] In the above-mentioned thread trimming method of the sewing machine, the distance from each point on the connected convex arc segment ca to the axis center of the driving shaft is smaller than the distance from the needle thread trimming peach tip a to the axis center of the driving shaft, and the step B is specifically as follows: the thread trimming power mechanism starts working, the driving motor drives the driving shaft to start rotating, the connected convex arc segment ca of the auxiliary knife cam slides against the abutment portion 2, and the working circumference of the moving knife cam slides against the abutment portion 1 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.

[0024] When the working surface of the movable knife cam slides against the abutment part 1 from the concave turning point of the first concave arc segment AB to the starting point B of the first convex arc segment BC, it pushes the abutment part 1 away from the axis of the drive shaft, so that the movable knife can swing downward until the starting point B of the first convex arc segment BC abuts against the abutment part 1, and the movable knife swings to the lower dead center. Since the distance from each point on the connecting convex arc segment ca to the axis of the drive shaft is smaller than the distance from the needle thread cutting peach tip a to the axis of the drive shaft, when the connecting convex arc segment ca of the auxiliary knife cam slides against the abutment part 2, the auxiliary knife is lower than the thread cutting position and is located on the same side of the rotary shuttle with the movable knife to avoid the movable knife from biting with the movable knife and cutting the upper thread in advance during the downward swing of the movable knife. After that, the needle can perform the needle insertion action.

[0025] Compared with the prior art, this sewing machine thread trimming method has the following advantages: 1. In this thread trimming method, since a double-cam thread trimming power mechanism is used to achieve thread trimming, and the moving knife cam and the auxiliary knife cam are fixedly connected to the driving shaft, the moving knife cam and the auxiliary knife cam can be driven to rotate by one motor, so that the error accumulation that may occur between the two motors when two motors are used to drive them separately can be eliminated, and the control algorithm can be simplified, so that the movement of the moving knife and the auxiliary knife is more coordinated. Moreover, since the geometric shapes of the moving knife cam and the auxiliary knife cam are pre-designed, and the two are driven by the same driving shaft, the respective positions of the moving knife and the auxiliary knife and the matching relationship between the two depend on the rotation angle of the driving shaft, that is, when the motor drives the driving shaft to rotate to a specific angle, the moving knife and the auxiliary knife will reach the corresponding position and form a corresponding matching relationship. This design enables the moving knife and the auxiliary knife to perform actions such as thread trimming and resetting after thread trimming more accurately, thereby improving the stability and reliability of the thread trimming work.

[0026] 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 set on the moving knife cam, even if the motor that drives the drive shaft to rotate has a slight error in the rotation angle, it will not affect the final position of the moving knife, ensuring that the moving knife is always located directly below the needle hole waiting for thread cutting. Relatively speaking, if a motor is used to directly drive 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 the position of the moving knife to change, and it is impossible to ensure that the engagement position of the moving knife and the auxiliary knife is always directly below the needle hole. Therefore, the thread cutting method of the present invention can tolerate a certain error in the motor without affecting the precise position of the thread cutting, so that the thread cutting position can be more accurately controlled below the needle hole of the needle plate, achieving a better anti-bird nest effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a structural diagram of the thread trimming mechanism of a sewing machine.

[0028] Figure 2 It is a structural diagram of the thread trimming mechanism of a sewing machine.

[0029] Figure 3 It is a schematic diagram of the transmission structure of the moving knife.

[0030] Figure 4 It is a three-dimensional structural diagram of the moving knife cam.

[0031] Figure 5 It is a schematic diagram of the corresponding parts of the working surface of the moving knife cam.

[0032] Figure 6 It is a schematic diagram of the auxiliary knife cam.

[0033] Figure 7 It is a front view of the thread trimming mechanism in its initial state.

[0034] Figure 8 It is a partial schematic diagram of the thread trimming mechanism in the initial state.

[0035] Fig. 9 It is a schematic diagram of the motion relationship between the moving knife cam and the auxiliary knife cam when the thread trimming mechanism is in the initial state.

[0036] Fig.10 It is a front view of the thread trimming mechanism when it is in the thread pressing state.

[0037] Fig.11 It is a schematic diagram of the motion relationship between the moving knife cam and the auxiliary knife cam when the thread trimming mechanism is in the thread pressing state.

[0038] Fig.12 It is a front view of the process of the thread trimming mechanism cutting the upper thread end.

[0039] Fig.13 It is a partial schematic diagram of the process of the thread trimming mechanism cutting off the upper thread end.

[0040] Fig.14 It is a schematic diagram of the movement relationship between the moving knife cam and the auxiliary knife cam in the process of the thread trimming mechanism cutting the upper thread end.

[0041] Fig.15 It is a front view of the thread trimming mechanism in the thread hooking state process.

[0042] Fig.16 It is a partial schematic diagram of the thread trimming mechanism's thread hooking state process.

[0043] Fig.17 It is a schematic diagram of the motion relationship between the moving knife cam and the auxiliary knife cam during the thread-hooking state of the thread trimming mechanism.

[0044] Fig.18This is a front view of the process in which the thread trimming mechanism cuts the upper thread and the lower thread.

[0045] Fig.19 It is a partial schematic diagram of the process of the thread trimming mechanism cutting the upper thread and the lower thread.

[0046] Fig. 20 It is a schematic diagram of the motion relationship between the moving knife cam and the auxiliary knife cam during the process of the thread trimming mechanism cutting the upper thread and the lower thread.

[0047] In the figure, 1, machine base; 2, needle plate; 21, needle drop hole; 3, moving knife; 31, moving knife edge; 32, lower needle hole; 33, thread hook groove; 4, auxiliary knife; 41, auxiliary knife edge; 5, driving shaft; 6, moving knife connecting rod assembly; 61, abutment part 1; 7, auxiliary knife connecting rod assembly; 71, abutment part 2; 8, moving knife cam; 81, thread cutting arc surface; 82, acceleration concave arc surface; 9, auxiliary knife cam; 10, rotary shuttle; 11, thread pressing plate; 12, driving motor; 13, knife holder; 14, lower connecting rod; 15, upper connecting rod; 16, hinge shaft; 17, suction pipe. DETAILED DESCRIPTION

[0048] The following are specific embodiments of the present invention and the accompanying drawings to further describe the technical solution of the present invention, but the present invention is not limited to these embodiments.

[0049] Embodiment 1

[0050] In the thread trimming method of this sewing machine, Figure 1 and Figure 2 As shown, the sewing machine includes a movable knife 3, an auxiliary knife 4, a rotary hook 10, a thread pressing plate 11, an air suction pipe 17, a machine base 1 with a work surface and a needle plate 2 with a needle drop hole 21, the movable knife 3 has a movable knife edge 31, a thread hook groove 33 and a lower needle hole 32, the lower edge of the end surface of the auxiliary knife 4 close to one end of the machine needle forms an auxiliary knife edge 41, and the machine base 1 is provided with a thread cutting 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.

[0051] 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 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. Figure 5 and Figure 6 From the perspective, the driving motor 12 drives the movable knife cam 8 and the auxiliary knife cam 9 to rotate in a clockwise unidirectional cycle through the driving shaft 5.

[0052] like Figure 3As shown, the movable knife connecting rod assembly 6 and the auxiliary knife connecting rod assembly 7 both include an annular knife holder 13, a lower connecting rod 14 and an upper connecting rod 15. The movable knife 3 and the auxiliary knife 4 are respectively mounted on the corresponding knife holders 13. The annular knife holder 13 is rotatably arranged on the machine base 1, and the rotation center line coincides with the center line of the sewing machine hook 10. The movable knife connecting rod assembly 6 and the auxiliary knife connecting rod assembly 7 have similar structures, in which one end of the upper connecting rod 15 is hinged to the knife holder 13 corresponding to the movable 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.

[0053] The middle part of the lower connecting rod 14 of the movable knife connecting rod assembly 6 has a contact portion 61 that contacts the working circumference of the movable knife cam 8. When the driving motor 12 drives the driving shaft 5 to rotate, the movable knife cam 8 and the movable knife connecting rod assembly 6 transmit power, and the movable knife 3 can be driven to swing up and down between the upper part and the side part of the rotating hook 10 around the outer circumference of the rotating hook 10. When swinging upward, the movable knife 3 is close to the needle drop hole 21 of the needle plate 2, and vice versa, it is away from the needle drop hole 21 of the needle plate 2.

[0054] like Figure 2 As shown, the middle part of the lower connecting rod 14 of the auxiliary knife connecting rod assembly 7 has a contact portion 71 that contacts the working circumference of the auxiliary knife cam 9. When the driving shaft 5 drives the auxiliary knife cam 9 to rotate, the auxiliary knife cam 9 and the auxiliary knife connecting rod assembly 7 transmit power, and the auxiliary knife 4 can be driven to swing up and down between the upper part and the side part of the rotating hook 10 around the outer circumference of the rotating hook 10. When swinging upward, the auxiliary knife 4 is close to the needle drop hole 21 of the needle plate 2, and vice versa, it is away from the needle drop hole 21 of the needle plate 2.

[0055] Further, such as Figure 4 and Figure 5 As shown, the working circumference of the movable 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 which are connected end to end in sequence. The first convex arc segment BC has a trimming 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 trimming arc surface 81 to the axis of the drive shaft 5. The second convex arc segment DA is an arc surface which is concentric with the drive shaft 5. When the working circumference of the movable knife cam 8 pushes the abutting portion 61 to move in a direction away from the axis of the drive shaft 5, the movable knife 3 swings downward.

[0056] like Figure 6As shown, the working circumference of the auxiliary knife cam 9 has a needle thread cutting peach tip a, a waiting thread cutting peach tip b and a tail needle thread cutting peach tip c arranged in sequence along the circumferential direction, the needle thread cutting peach tip a and the tail needle thread cutting peach tip c are at the same distance from the axis of the driving shaft 5, the tail needle thread cutting peach tip c is connected to the needle thread cutting peach tip a by 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 smaller than the distance from the needle thread cutting peach tip a to the axis of the driving shaft 5, and the needle thread cutting peach tip a and the waiting thread cutting peach tip b are connected by a connecting concave arc segment ab. When the working circumference of the auxiliary knife cam 9 pushes the abutting portion 2 71 to move in a direction away from the axis of the driving shaft 5, the auxiliary knife 4 swings upward.

[0057] The thread trimming power mechanism adopts a cam-connecting rod structure, so that the position and speed of the swing of the moving knife 3 and the auxiliary knife 4 at a certain time can be determined by the design of the working surface shape of the moving knife cam 8 and the auxiliary knife cam 9. Therefore, the shape design of the first convex arc segment BC and the connecting convex arc segment ca can be used to achieve that during the upward swing of the moving knife 3 and the auxiliary knife 4, the speed of the auxiliary knife 4 is lower than that of the moving knife 3, thereby avoiding the situation in which the moving knife edge 31 engages with the auxiliary knife 4 and cuts the surface thread prematurely before reaching the bottom of the needle drop hole 21 during rapid movement. Specifically, Figure 5 As shown, on the movable knife cam 8, there is a concave accelerating concave arc surface 82 on the working circumference between the starting point B of the first convex arc segment BC and the cutting arc surface 81, so that the accelerating concave arc surface 82 can accelerate the upward swinging speed of the movable knife 3 when sliding over the abutment portion 61.

[0058] The thread trimming method comprises the following steps: 1. Initial state: Figure 7 , Figure 8 and Fig. 9 As shown, the thread trimming power mechanism works in advance, and the driving motor 12 drives the driving shaft 5 to start rotating, so that the concave turning point of the first concave arc segment AB on the movable knife cam 8 abuts against the abutment portion 61, and the movable knife 3 is stopped at a position where the lower needle hole 32 is vertically opposite to the needle drop hole 21 on the needle plate 2, and then the thread trimming power mechanism stops working.

[0059] 2. Start and stop sewing: Place the sewing material 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 the top of the needle plate 2, and the upper thread end remains in the lower needle hole 32; 3. Pressing line: Fig.10 and Fig.11As shown, the thread trimming power mechanism starts to work, driving the movable knife 3 with the upper thread end around the outer circumference of the rotary hook 10 to swing from the upper part of the rotary hook 10 to the side of the rotary hook 10, and after the swing, the movable knife 3 and the auxiliary knife 4 are located on the same side of the rotary hook 10, and the driving motor 12 drives the driving shaft 5 to start rotating, and 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 turning point of the first concave arc segment AB to the starting point B of the first convex arc segment BC, and the movable knife 3 and the auxiliary knife 4 both swing downward to move away from the needle drop hole 21, and the movable knife 3 swings downward to the lower dead center, and is located on the same side of the rotary hook 10 with the auxiliary knife 4, and the lower side surface of the movable knife 3 abuts against the thread pressing piece 11 to clamp the upper thread end passing through the lower needle hole 32, and then the thread trimming power mechanism stops working; 4. The needle bar power mechanism drives the needle to sew the set number of stitches, and the upper thread on the needle is locked with the lower thread in the rotary hook 10, and the needle sews the set number of stitches, such as two, three or four stitches; 5. Cutting the upper thread end: the driving motor 12 drives the driving 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 and the speed is less than the upward swing speed of the movable knife 3. like Fig.12 , Fig.13 and Fig.14 As shown, the driving shaft 5 rotates continuously to make the thread cutting arc surface 81 on the movable knife cam 8 slide over the abutment part 1 61, and the movable knife 3 stops at a position where the movable knife edge 31 is located directly below the needle 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 part 2 71 to make the auxiliary knife 4 engage with the movable knife edge 31 on the movable knife 3 to cut the upper thread end. In addition, in this process, before the upper thread end is cut, the suction pipe 17 is already in the suction state and the thread pressing piece 11 has released the upper thread end, so that the upper thread end that has been cut is directly sucked away by the suction pipe 17.

[0060] 6. Continue sewing: The driving motor 12 continues to rotate, driving the driving shaft 5 to rotate, and the working surface of the movable knife cam 8 slides from the thread trimming arc surface 81 to the end point C of the first convex arc segment BC against the abutment part 1 61. At the same time, the working surface of the auxiliary knife cam 9 slides from the needle-starting thread trimming peach tip a to the middle position connecting the concave arc segment ab against the abutment part 2 71, so that the movable knife 3 and the auxiliary knife 4 move to the same side of the rotary hook 10 again, avoiding the needle lowering position of the machine needle, so that the needle lowering will not hit the movable knife 3 and the auxiliary knife 4, and then the thread trimming power mechanism stops working, and the driving shaft 5 stops rotating; the needle bar power mechanism drives the needle to continue to sew until the sewing action is completed, and the needle bar power mechanism stops working; 7. Cutting the upper thread and the lower thread: The driving motor 12 drives the driving shaft 5 to start rotating, and the working surface of the movable knife cam 8 slides against 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 surface of the auxiliary knife cam 9 slides against the abutment portion 2 71 from the middle position connecting the concave arc segment ab to the peach tip b to be cut, so that the movable knife 3 and the auxiliary knife 4 both swing upward, as shown in FIG. Fig.15 , Fig.16 and Fig.17 As shown, 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, and the upper thread and the lower thread fall into the thread hooking groove 33. The movable knife cam 8 continues to rotate, and its working circumference is close to the abutment portion 1 61 and slides from the concave turning point of the second concave arc segment CD to the second convex arc segment DA. At the same time, the auxiliary knife cam 9 is close to the abutment portion 2 71 and slides from the thread cutting peach tip b to the thread cutting peach tip a, so that the movable knife 3 swings downward and the auxiliary knife 4 swings upward.

[0061] like Fig.18 , Fig.19 and Fig. 20 As shown, the driving shaft 5 rotates continuously, causing the second convex arc segment DA of the movable knife cam 8 to slide over the abutment portion 1 61, and the movable knife 3 is stopped at a position where the movable knife edge 31 is located directly below the needle 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, causing the auxiliary knife 4 to engage with the movable knife edge 31 to cut the upper thread and the lower thread.

[0062] 8. Reset: The driving shaft 5 continues to rotate, so that the moving knife 3 and the auxiliary knife 4 are reset to Figure 7 , Figure 8 and Fig. 9 Of course, in the actual sewing process, the movable knife 3 and the auxiliary knife 4 can be reset to other positions. When the sewing machine is started for the next sewing, the control system controls the driving motor 12 to move, so that the movable knife 3 and the auxiliary knife 4 first swing to the position of the movable knife 3 and the auxiliary knife 4. Figure 7 , Figure 8 and Fig. 9 The initial state is shown.

[0063] Embodiment 2

[0064] The method and steps of this embodiment are basically the same as those of the first embodiment, except that: before the sewing machine is turned on, the inner concave turning point of the first concave arc segment AB on the movable knife cam 8 has already abutted against the abutting portion 1 61, so that the movable knife 3 is stopped at a position where the lower needle hole 32 is vertically opposite to the needle drop hole 21 on the needle plate 2. Therefore, after the sewing machine is turned on, the step of early operation of the thread trimming power mechanism is omitted.

[0065] Embodiment 3

[0066] The method steps of this embodiment are basically the same as those of the first embodiment, except that: the thread trimming power mechanism does not use the movable knife cam 8 and the auxiliary knife cam 9 in conjunction with the connecting rod structure to realize the up and down swinging of the movable knife 3 and the auxiliary knife 4, but adopts a structure similar to the thread trimming method of the double movable knife thread trimming mechanism patented in the background technology (application number: 202411329869.0), including a movable knife motor for driving the movable knife 3 to swing back and forth and an auxiliary knife motor for driving the auxiliary knife 4 to move back and forth. The movable knife motor and the auxiliary knife motor both use servo motors, which are connected to the movable knife 3 and the auxiliary knife 4 through a connecting rod mechanism, and the movable knife motor and the auxiliary knife motor are controlled by the control system to work according to the set rotation time and rotation angle, thereby controlling the movable knife 3 and the auxiliary knife 4 to swing in a corresponding manner.

[0067] The specific embodiments described herein are merely examples of the spirit of the present invention. Those skilled in the art may make various modifications or additions to the specific embodiments described or replace them in similar ways, but they will not deviate from the spirit of the present invention or exceed the scope defined by the appended claims.

[0068] Although the following terms are used more frequently in this article, such as 1, base; 2, needle plate; 21, needle drop hole; 3, moving knife; 31, moving knife edge; 32, lower needle hole; 33, thread hook groove; 4, auxiliary knife; 41, auxiliary knife edge; 5, driving shaft; 6, moving knife connecting rod assembly; 61, abutment part 1; 7, auxiliary knife connecting rod assembly; 71, abutment part 2; 8, moving knife cam; 81, thread cutting arc surface; 82, acceleration concave arc surface; 9, auxiliary knife cam; 10, rotary shuttle; 11, thread pressing piece; 12, driving motor; 13, knife holder; 14, lower connecting rod; 15, upper connecting rod; 16, hinge shaft; 17, air suction pipe, etc., the possibility of using other terms is not excluded. These terms are used only to more conveniently describe and explain the essence of the present invention; interpreting them as any additional restrictions 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 2, 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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