Trimming control method

Abstract

The invention relates to the technical field of sewing machines, in particular to a thread trimming control method which is used for a sewing machine, the sewing machine keeps cloth feeding in the thread trimming process, the thread trimming needle pitch enabling the thread end length of an upper thread after thread trimming to be equal to or close to the thread end length of a bottom thread is obtained based on the needle pitch of the last needle before the thread trimming action starts, and when the thread trimming action starts, the thread trimming needle pitch is adjusted. The stitch length of the sewing machine is adjusted from the stitch length of the last stitch before the thread trimming action is started to the thread trimming stitch length, and the thread trimming stitch length is kept till the thread trimming action is finished. The thread cutting stitch length is calculated through equal length or approximate equal length of thread ends of the upper thread and the bottom thread, and the stitch length during thread cutting is adjusted and controlled to be the thread cutting stitch length, so that equal length or approximate equal length control over the thread ends of the upper thread and the bottom thread after thread cutting can be achieved, thread ends are more attractive, a good sewing effect is achieved, meanwhile, the length of the thread ends of cut threads can be effectively controlled, and the thread cutting efficiency is improved. And the thread end is shorter.

Description

Technical Field

[0001] The invention relates to the technical field of sewing machines, and in particular to a thread trimming control method. Background Art

[0002] At present, most of the lockstitch sewing machines on the market are computerized sewing machines, which basically have automatic thread trimming functions. Among them, the electromagnetic thread trimming technology has been used for a long time and is relatively stable. However, in recent years, with the development of stepping technology in sewing machines, stepping thread trimming technology has also been continuously updated. However, whether it is the electromagnetic thread trimming or the existing stepping thread trimming technology, it is impossible to better control the length of the thread ends left on the fabric after thread trimming. Generally, the thread ends are long and unsightly. Summary of the invention

[0003] In view of the above-mentioned defects in the prior art, the technical problem to be solved by the present invention is to provide a thread trimming control method, which can effectively control the length of the thread end after thread trimming.

[0004] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0005] The present invention provides a thread trimming control method for a sewing machine, wherein the sewing machine keeps feeding cloth during the thread trimming process, and based on the stitch length of the last stitch before the thread trimming action starts, a thread trimming stitch length is obtained so that the length of the upper thread end after thread trimming is equal to or close to the length of the lower thread end. When the thread trimming action starts, the stitch length of the sewing machine is adjusted from the stitch length of the last stitch before the thread trimming action starts to the thread trimming stitch length, and the thread trimming stitch length is maintained until the thread trimming action ends.

[0006] Preferably, the sewing machine includes a drive shaft and a feed dog. The drive shaft is driven to rotate and drives the feed dog to feed cloth. From the start of the thread trimming action to the end of the thread trimming action, the angle of the drive shaft rotates from the angle θ1 where the feed dog is below the needle plate to the angle θ3 where the feed dog is above the needle plate. When the feed dog passes through the needle plate and is flush with the needle plate surface, the angle of the drive shaft is θ2. When the thread trimming action starts, the stitch length adjustment action of adjusting the stitch length of the sewing machine from the stitch length of the last stitch before the start of the thread trimming action to the thread trimming stitch length is completed before the angle of the drive shaft rotates to θ2, and the thread trimming stitch length is maintained until the angle of the drive shaft rotates to θ3.

[0007] Preferably, the absolute value of the needle distance of the last stitch before the thread trimming action starts is X, the radius of the needle plate hole on the needle plate is r, the thickness of the needle plate is t, the distance that the feed dog is higher than the needle plate surface when the angle of the drive shaft rotates to θ3 is p, and the distance the fabric moves during the thread trimming process is X m , 2X m =X n , according to the length of the upper thread after trimming is equal to the length of the lower thread, obtain X n The calculation formula of

[0008] When the last stitch before the thread trimming action starts is a normal stitch, X n The calculation formula is:

[0009]

[0010] Trimming stitch length is reverse stitch X n ;

[0011] When the last stitch before the thread trimming action is reverse sewing, X n The calculation formula is:

[0012]

[0013] If X n If it is greater than 0, the thread trimming distance is the normal stitch X n If X n Less than 0, the thread trimming stitch length is reverse stitching|X n |.

[0014] Compared with the prior art, the present invention has significant improvements:

[0015] The thread trimming control method of the present invention calculates the thread trimming stitch length by making the ends of the upper thread and the lower thread equal or nearly equal in length, and adjusts and controls the stitch length during thread trimming to be the thread trimming stitch length, thereby achieving control of the ends of the upper thread and the lower thread after thread trimming to be equal or nearly equal in length, making the thread ends more beautiful and obtaining a good sewing effect, and also achieving effective control of the length of the thread trimming thread ends to make the thread ends shorter. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 Schematic diagram of a thread trimming control method according to an embodiment of the present invention.

[0017] Figure 2 It is a schematic diagram of the coordination of various thread trimming elements of a sewing machine in the thread trimming control method of an embodiment of the present invention.

[0018] Figure 3 yes Figure 2 Schematic diagram showing the definition of lengths related to the thread trimming elements of a sewing machine.

[0019] Figure 4 4a, 4b and 4c are schematic diagrams of small stitch length thread trimming, zero stitch length thread trimming and large stitch length thread trimming during sewing in the prior art respectively.

[0020] Figure 5 5a and 5b are schematic diagrams of small stitch length thread trimming and large stitch length thread trimming during reverse stitching in the prior art, respectively.

[0021] Figure 6 6a and 6b are schematic diagrams of the thread trimming control method according to an embodiment of the present invention, the last stitch before the thread trimming action starts during sewing, and the thread trimming stitch length.

[0022] Figure 7 The thread trimming control method of the embodiment of the present invention is as follows: n Schematic diagram of the relationship with X size.

[0023] Figure 8 8a and 8b are schematic diagrams of the thread trimming control method according to an embodiment of the present invention, the last stitch before the thread trimming action starts when the reverse stitch length is large, and the thread trimming stitch length.

[0024] Fig. 9 9a and 9b are schematic diagrams of the thread trimming control method according to an embodiment of the present invention, the last stitch before the thread trimming action starts when the reverse stitch length is small, and the thread trimming stitch length.

[0025] Fig.10 The thread trimming control method of the embodiment of the present invention, when the reverse stitching is performed n Schematic diagram of the relationship with X size.

[0026] The reference numerals are described as follows:

[0027] 1. Fabric

[0028] 2 Needle plate

[0029] 21 Needle plate hole

[0030] 3 Feed dog

[0031] 4 Upper thread

[0032] 5 Bottom Line

[0033] 6. Cutting

[0034] 7 Fixed knife DETAILED DESCRIPTION

[0035] The specific embodiments of the present invention are further described in detail below in conjunction with the accompanying drawings. These embodiments are only used to illustrate the present invention, but not to limit the present invention.

[0036] In the description of the present invention, it should be noted that the terms "center", "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside" and the like indicate positions or positional relationships based on the positions or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operate in a specific orientation, and therefore cannot be understood as limiting the present invention. In addition, the terms "first" and "second" are used for descriptive purposes only and cannot be understood as indicating or implying relative importance.

[0037] In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0038] Furthermore, in the description of the present invention, unless otherwise specified, “plurality” means two or more.

[0039] like Figures 1 to 10 As shown, an embodiment of the thread trimming control method of the present invention. The thread trimming control method of this embodiment is used for a sewing machine, and the sewing machine keeps feeding cloth during the thread trimming process. The structure of the sewing machine is an existing conventional structure, which includes a thread trimming control module, a stitch control module and an electronic control module. Among them, the thread trimming control module includes a thread trimmer, a thread trimming transmission component and a thread trimming drive component. The driving source of the thread trimming drive component can be a stepper motor or a servo motor or an electromagnet or other driving source; the thread trimming transmission component mainly transmits the power of the thread trimming drive component to the thread trimmer, drives the thread trimmer to move, and realizes the thread trimming action; the thread trimmer can cut the suture (surface thread and bottom thread), and the thread trimmer structure can be a fixed knife + moving knife structure, or a double moving knife structure, or a straight knife structure, etc.; this embodiment is described by taking the structure in which the thread trimming control module uses a stepper motor to control the fixed knife + moving knife to cut the thread as an example. The needle distance control module includes a feed dog and a needle distance control component. The needle distance control component can be any one of the existing step-by-step needle distance adjustment structure, step-by-step feeding structure, step-by-step tooth lifting structure, and track / tug-wheel feeding structure, all of which can realize the control of the needle distance. The electric control module can be an existing conventional controller. The thread trimming drive component of the thread trimming control module and the needle distance control component of the needle distance control module are both connected to the electric control module for communication, and the electric control module monitors and controls the thread trimming and needle distance adjustment actions.

[0040] The thread trimming control method of this embodiment is to obtain the thread trimming stitch length that makes the length of the upper thread end after thread trimming equal to or close to the length of the lower thread end based on the stitch length of the last stitch before the thread trimming action starts, and adjust the stitch length of the sewing machine from the stitch length of the last stitch before the thread trimming action starts to the thread trimming stitch length, and keep the thread trimming stitch length until the thread trimming action ends. The thread trimming stitch length is calculated by making the upper thread and the lower thread ends equal or nearly equal in length, and the stitch length during thread trimming is adjusted and controlled to be the thread trimming stitch length, so that the upper thread and the lower thread ends after thread trimming can be controlled to be equal or nearly equal in length, making the thread ends more beautiful and obtaining a good sewing effect, and at the same time, the length of the thread trimming thread ends can be effectively controlled to make the thread ends shorter.

[0041] Preferably, the sewing machine includes a drive shaft and a feed dog, the drive shaft is driven to rotate, and drives the feed dog to feed cloth. The sewing machine may include a feed mechanism, the feed mechanism is connected between the drive shaft and the feed dog, the drive shaft drives the feed dog to feed cloth through the feed mechanism, or the sewing machine may also be provided with a motor, the output shaft of the motor is used as the drive shaft, and the feed dog is independently driven to feed cloth. The electric control module is connected to the drive source signal of the drive shaft to monitor and control the rotation angle of the drive shaft. This is an existing sewing machine structure and will not be described in detail herein. From the start to the end of the thread trimming action, the angle of the drive shaft rotates from the angle θ1 where the feed dog is located below the needle plate to the angle θ3 where the feed dog is located above the needle plate, that is, the thread trimming signal is triggered when the feed dog is located below the needle plate, θ1 and θ3 are respectively the start and end positions of thread trimming by the drive shaft, when the feed dog passes through the needle plate and is flush with the needle plate surface, the angle of the drive shaft is θ2, when the thread trimming action starts, before the angle of the drive shaft rotates to θ2, the stitch length adjustment action of adjusting the stitch length of the sewing machine from the stitch length of the last stitch before the start of the thread trimming action to the stitch length of the thread trimming action is completed, and the stitch length of the thread trimming action is maintained until the angle of the drive shaft rotates to θ3.

[0042] See also Figure 1 Specifically, the thread trimming control method of this embodiment is that during the sewing process of the sewing machine, the electronic control module monitors the sewing stitch length. When the thread trimming signal is triggered and the electronic control module receives the thread trimming signal, the sewing machine enters the thread trimming state, the electronic control module controls the thread trimming control module to start thread trimming, and the electronic control module controls the driving shaft to continue rotating so that the cloth feeding mechanism and the cloth feeding dog keep feeding cloth. During the thread trimming process, the rotation angle range of the driving shaft is θ1-θ3. Within the range of θ1-θ3, the driving shaft angle when the cloth feeding dog passes through the needle plate and is flush with the needle plate surface is θ2; the electronic control module monitors the stitch length of the last stitch before the thread trimming action starts, and calculates the stitch length according to the upper thread head length after the thread trimming being equal to or close to the lower thread head length. The thread trimming stitch length, or the stitch length of the last stitch before the thread trimming action starts which is known in advance from the sewing process, is calculated based on the upper thread head length after thread trimming being equal to or close to the lower thread head length and the thread trimming stitch length is stored in the electronic control module; based on the obtained thread trimming stitch length which makes the upper thread head length after thread trimming equal to or close to the lower thread head length, the electronic control module controls the stitch length control module to adjust the stitch length when the thread trimming action starts, and adjusts the stitch length of the sewing machine from the stitch length of the last stitch before the thread trimming action starts to the thread trimming stitch length, and completes the stitch length adjustment action before the angle of the drive shaft rotates to θ2, and maintains the thread trimming stitch length until the angle of the drive shaft rotates to θ3, i.e., the thread trimming action is completed.

[0043] Figure 2The figure shows a schematic diagram of the coordination of the thread trimming elements of the sewing machine in this embodiment, wherein the cloth 1 is placed on the needle plate 2, and is lifted and moved by the feed dog 3 for cloth feeding, the sewing thread comprises an upper thread 4 located on the upper surface of the cloth 1 and a lower thread 5 located on the lower surface of the cloth 1, the needle plate 2 has a needle plate hole 21, the upper thread 4 and the lower thread 5 pass through the needle plate hole 21, and the upper thread 4 and the lower thread 5 are cut by the engagement of the movable knife 6 and the fixed knife 7 of the thread trimmer below the needle plate hole 21.

[0044] Combination Figure 3 , define the length of the end of the bottom line 5 after thread trimming as L1. When thread trimming is about to be completed, the bottom line 5 will have a turn at the lower end of the needle plate hole 21. Define the length of the end of the bottom line 5 above the turn at the lower end of the needle plate hole 21 as a, and the length of the end of the bottom line 5 below the turn at the lower end of the needle plate hole 21 as b, then, L1 = a + b. Define the length of the end of the end of the thread 4 after thread trimming as L2. When thread trimming is about to be completed, the upper thread 4 will have a turn at the lower end of the needle plate hole 21. Define the length of the end of the upper thread 4 above the turn at the lower end of the needle plate hole 21 as e, and the length of the end of the upper thread 4 below the turn at the lower end of the needle plate hole 21 as f, then, L2 = e + f. Among them, b = f. Take the center of the plane below the needle plate hole 21 as the reference origin, define the radius of the needle plate hole 21 as r, and the thickness of the needle plate 2 as t. It is defined that the distance that the feed dog 3 is higher than the needle plate surface when the angle of the driving shaft is rotated to θ3 is p, that is, the height of the feed dog 3 is p.

[0045] There are three kinds of thread trimming methods in the prior art: positive stitch length trimming, zero stitch length trimming, and negative stitch length trimming. When the sewing machine is sewing forward, the thread trimming is positive stitch length or zero stitch length, and when the sewing machine is sewing backward, the thread trimming is negative stitch length. The stitch length is fixed and single. For positive stitch length trimming, see Figure 4 In 4a and 4c, during the thread trimming process, the fabric 1 moves forward. Due to the limited range of the needle plate hole 21, when the stitch length is large, the thread lengths of the lower thread 5 and the upper thread 4 above the turning point at the lower end of the needle plate hole 21 are both possible to turn at the upper end of the needle plate hole 21. After the stitch trimming, the length of the lower thread 4 is less than the length of the lower thread 5. Moreover, the larger the stitch length, the longer the lengths of the lower thread 5 and the upper thread 4. Therefore, when the stitch length is closer to zero stitch length, the shorter the thread trimming thread ends are, and the closer the lengths of the lower thread 5 and the upper thread 4 are. For zero stitch length trimming, see Figure 4 In 4b, the length of the upper thread 4 remains unchanged, while the length of the lower thread 5 is related to the stitch length of the last stitch before the thread trimming action begins. The larger the stitch length of the last stitch before the thread trimming action begins, the longer the length of the lower thread 5. For negative stitch length trimming, see Figure 5In 5a and 5b, the fabric 1 moves backward during the thread trimming process. Similarly, due to the limited range of the needle plate hole 21, when the stitch length is large, the thread lengths of the bottom thread 5 and the top thread 4 above the turning point at the lower end of the needle plate hole 21 both have the possibility of turning at the upper end of the needle plate hole 21. During reverse sewing, when the stitch length of the last stitch before the thread trimming action starts is 2r, the thread trimming is performed with zero stitch length, and the lengths of the bottom thread 5 and the top thread 4 are the same; when the stitch length of the last stitch before the thread trimming action starts is between 0-2r, the thread trimming is reverse sewing with small stitch length, such as Figure 5 As shown in 5a, at this time, the length of the upper thread 4 is greater than the length of the lower thread 5, and the larger the stitch length, the shorter the length of the lower thread 5 and the upper thread 4. When the stitch length of the last stitch before the thread trimming action starts is greater than 2r, the thread trimming is reverse stitching with a large stitch length, such as Figure 5 As shown in 5b, at this time, the length of the lower thread 5 is much longer than the length of the upper thread 4, and the larger the stitch length, the longer the length of the lower thread 5 and the upper thread 4. It can be seen that in the prior art, only when the stitch length is zero during forward sewing, the thread ends are the shortest, and the lengths of the lower thread 5 and the upper thread 4 are closest; when the stitch length of the last stitch before the thread cutting action starts is close to 2r, the thread ends can be cut to obtain a shorter length.

[0046] The thread trimming control method of this embodiment calculates the thread trimming stitch length by making the lengths of the upper thread 4 and the lower thread 5 equal or nearly equal, and adjusts and controls the stitch length during thread trimming to the thread trimming stitch length. This can achieve equal or nearly equal length control of the lengths of the upper thread 4 and the lower thread 5 after trimming at any sewing stitch length.

[0047] In the thread trimming control method of this embodiment, the thread trimming stitch length is obtained by the following calculation method. The following is an example of calculating that the length L2 of the upper thread 4 after thread trimming is equal to the length L1 of the lower thread 5. The calculation method of the length L2 of the upper thread 4 after thread trimming is close to (that is, approximately equal to) the length L1 of the lower thread 5 is similar to that of the lower thread 5, and will not be repeated here. In actual application, the difference between the length L2 of the upper thread 4 after thread trimming and the length L1 of the lower thread 5 should be controlled to be as close to zero as possible, so as to achieve equal length as best.

[0048] Define the absolute value of the last stitch before the thread trimming action starts as X, and the distance the fabric moves during the thread trimming process is X m , the distance X that the fabric moves during the thread cutting process m It is half the thread trimming stitch length, so define 2X m =X n , according to the length L2 of the upper thread 4 after thread trimming is equal to the length L1 of the lower thread 5, obtain X n Since L1=a+b, L2=e+f, b=f, when L1=L2, we only need to calculate a=e.

[0049] See also Figure 6 In 6a, when the last stitch before the thread trimming action starts is a straight stitch, the length of the upper thread 4 is less than the length of the lower thread 5 in the last stitch before the thread trimming action starts. In order to make the length of the upper thread 4 and the length of the lower thread 5 after the thread trimming approach the same length, refer to Figure 6 In 6b, it is necessary to reverse feed the fabric with feed dog 3 during thread trimming to move the fabric backwards X m , so that the length of the upper thread 4 ends can be increased and the length of the lower thread 5 ends can be reduced during the thread trimming process, so that the length of the upper thread 4 ends and the length of the lower thread 5 ends can be equal. Figure 6 In 6b, if the length L1 of the lower thread 5 after thread trimming is equal to the length L2 of the upper thread 4, then a = e. Figure 3 ,have:

[0050]

[0051] Solved:

[0052]

[0053] So we get X n The calculation formula is:

[0054]

[0055] According to the X n The calculation formula of X n The relationship with the size of X is as follows Figure 7 shown.

[0056] Therefore, when the last stitch before the thread trimming action starts is a forward stitch, the thread trimming stitch length is a reverse stitch X. n .

[0057] See also Figure 8 In 8a, when the last stitch before the thread trimming action is reverse stitching and the stitch length is large, the length of the lower thread 5 is much longer than the length of the upper thread 4. In order to make the length of the upper thread 4 and the length of the lower thread 5 equal after thread trimming, refer to Figure 8 In 8b, it is necessary to make the feed dog 3 feed the fabric forward during the thread cutting process, so that the fabric moves forward X m , so that the length of the upper thread 4 ends can be increased and the length of the lower thread 5 ends can be reduced during the thread trimming process, so that the length of the upper thread 4 ends and the length of the lower thread 5 ends can be equal. Figure 8 In 8b, if the length L1 of the lower thread 5 after thread trimming is equal to the length L2 of the upper thread 4, then a = e. Figure 3 ,have:

[0058]

[0059] Solved:

[0060]

[0061] So we get X n The calculation formula is:

[0062]

[0063] See also Fig. 9 In 9a, when the last stitch before the thread trimming action is reverse stitching and the stitch length is small, the length of the upper thread 4 is greater than the length of the lower thread 5. In order to make the length of the upper thread 4 and the length of the lower thread 5 equal after thread trimming, refer to Fig. 9 In 9b, it is necessary to reverse the feed dog 3 during the thread trimming process to move the fabric backwards. m , so that the length of the upper thread 4 can be reduced and the length of the lower thread 5 can be increased during the thread trimming process, so that the length of the upper thread 4 and the length of the lower thread 5 can be equal. Fig. 9 In 9b, if the length L1 of the lower thread 5 after thread trimming is equal to the length L2 of the upper thread 4, then a = e. Figure 3 ,have:

[0064]

[0065] Solved:

[0066]

[0067] So we get X n The calculation formula is:

[0068]

[0069] Compare the last stitch before the thread trimming action starts, when it is reverse stitching and has a large stitch length. n The calculation formula is the same as when the last stitch before the thread trimming action is reverse stitching and has a small stitch length. n The calculation formula shows that the X n The calculation results are opposite numbers, so it can be summarized that when the last stitch before the thread trimming action starts is the reverse stitch, X n The calculation formula is:

[0070]

[0071] According to the X n The calculation formula of X n The relationship with the size of X is as follows Fig.10 shown.

[0072] According to the X n The calculation formula is calculated to get X n Value, if X nIf it is greater than 0, the last stitch before the thread trimming action starts is the reverse stitch and the thread trimming stitch length is the forward stitch X n If X n If it is less than 0, the last stitch before the thread trimming action starts is the reverse stitch, and the thread trimming stitch length is the reverse stitch|X n |.

[0073] In the thread trimming control method of this embodiment, due to the wide variety of sutures, different characteristics of sutures, and different stretching amounts of sutures, the actual length of the upper thread 4 and the length of the lower thread 5 deviate from the theoretical calculation, so a suture compensation coefficient J can be added, and the compensation coefficient J can be obtained after a large number of tests. In addition, for sewing materials, there are also a wide variety of sewing materials and different stretching amounts, so a suture compensation coefficient K can also be added, and the compensation coefficient K can also be obtained after a large number of tests.

[0074] It should be noted that any situation in which the relationship between the upper thread and the lower thread conforms to the above calculation method after the fabric moves during the thread cutting process is considered to be within the protection scope of the present invention.

[0075] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and substitutions can be made without departing from the technical principles of the present invention. These improvements and substitutions should also be regarded as the scope of protection of the present invention.

Claims

1. A thread trimming control method for a sewing machine, wherein the sewing machine maintains cloth feeding during thread trimming, characterized in that: Based on the stitch length of the last stitch before the thread trimming action starts, the thread trimming stitch length is obtained so that the length of the upper thread end after thread trimming is equal to or close to the length of the lower thread end. When the thread trimming action starts, the stitch length of the sewing machine is adjusted from the stitch length of the last stitch before the thread trimming action starts to the thread trimming stitch length, and the thread trimming stitch length is maintained until the thread trimming action ends.

2. The thread trimming control method according to claim 1, characterized in that: The sewing machine includes a driving shaft and a feed dog. The driving shaft is driven to rotate and drives the feed dog to feed cloth. From the start of the thread trimming action to the end of the thread trimming action, the angle of the driving shaft rotates from the angle θ1 at which the feed dog is located below the needle plate to the angle θ3 at which the feed dog is located above the needle plate. When the feed dog passes through the needle plate and is flush with the needle plate surface, the angle of the driving shaft is θ2. When the thread trimming action starts, the stitch length adjustment action of adjusting the stitch length of the sewing machine from the stitch length of the last stitch before the start of the thread trimming action to the thread trimming stitch length is completed before the angle of the driving shaft rotates to θ2, and the thread trimming stitch length is maintained until the angle of the driving shaft rotates to θ3.

3. The thread trimming control method according to claim 2, characterized in that: The absolute value of the needle distance of the last stitch before the thread trimming action begins is X, the radius of the needle plate hole on the needle plate is r, the thickness of the needle plate is t, the distance that the feed dog is higher than the needle plate surface when the angle of the drive shaft rotates to θ3 is p, and the distance the fabric moves during the thread trimming process is X m , 2X m =X n , according to the length of the upper thread after trimming is equal to the length of the lower thread, obtain X n The calculation formula of When the last stitch before the thread trimming action starts is a normal stitch, X n The calculation formula is: The thread trimming stitch length is reverse stitching X n ; When the last stitch before the thread trimming action is reverse sewing, X n The calculation formula is: If X n Greater than 0, the thread trimming stitch length is the positive seam X n ; if X n Less than 0, the thread trimming stitch length is reverse stitch|X n |.

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