Principle of lockstitch formation - How to correct sewing machine stitch?

Currently, industrial sewing machine stitches can be categorized into chain stitch and lock stitch. Among them, the lock stitch is the most common, which consists of two sewing threads, intertwined with each other like a rope, with the intertwining point in the middle of the sewing material. From the cross-section of the stitch, the two threads look like two locks interlocking with each other, hence the name lock stitch.

When the sewing machine is in operation, the needle leads the thread through the sewing material and lifts it upward when it reaches the lower limit position. Due to the friction between the material and the thread, the thread is not lifted in sync with the needle but stays under the material and expands to both sides of the needle under the effect of elasticity, forming a loop. Then, the tip of the rotary hook reaches the needle in the process of movement and passes through the thread loop, and in the continuing rotary movement, the hooked thread loop is constantly expanding. When the bobbin reaches its own radius, it "leaps" over the enlarged loop like a skipping rope. Then the action is to pick the rod to collect the thread and feed the cloth teeth in order to make these actions have enough time to proceed smoothly, a rotary hook is still rotating at the original speed of one week, but no longer hooked thread, just an empty circle. As a result of the thread take-up action of the thread-picking lever, the thread that was originally enlarged by the rotary hook and "jumped" through the thread loop is rapidly narrowed down until the bottom thread drawn from the rotary hook's locking core is pulled into the fabric, forming an interweave. When the needle reaches the upper limit and leads down to the material again, the sewing machine repeats the above action process over and over again, thus forming a continuous and regular lockstitch on the sewing material.
Lockstitch is suitable for sewing materials such as cotton, woolen fabrics, or leather with low shrinkage.

Household sewing machine stitch is a double thread interlocking stitch, the correct stitch, is the face, and bottom line chain good two intersection points just in the middle of the sewing material, the sewing line is not tight, not loose, neat, and beautiful.
In the sewing process, there may be a variety of bad stitches, according to the principle of stitch formation (as mentioned above) and experience to slowly be adjusted, bad stitches are: floating face line, floating bottom line, stitching loose floating, sewing material wrinkles. Must understand the reasons for their generation and master the elimination method, in order to solve the problem of correctly adjusting the correct stitch.

(1) Floating thread: The tension of the thread is too large, or the tension of the bottom thread is too small, the bottom thread is pulled to the top of the sewing material by the thread, and the intersection of the two threads floats on top of the sewing material, and the thread is in a straight line, and there is an obvious bulging phenomenon, which is called a floating thread. Solution: Adjust the tension of the basting thread to a smaller value from the basting thread tension adjusting disk, relax the tension of the basting thread, or adjust the basting thread to a larger value. (As Figure 1, 2)
(2). Floating bottom line: the thread tension is small, or the bottom line tension is too large, the thread is pulled to the bottom of the sewing material, the intersection of the two lines is exposed to the bottom of the sewing material, the bottom line forms a straight line, and there is an obvious bulging phenomenon, which is called floating bottom line. Solution: Adjusting the tension of the top thread or the bottom thread can be solved. (Such as Figure 3, 4)
(3). Stitching loose floating: face, bottom line tension is relatively small, the intersection of the two lines in the sewing material between, but the face, bottom line intersection is not tight. Loose floating in the sewing material on the top and bottom of the two sides, a little force to pull, sewing the two layers of sewing material will have a gap. Solution: Increase the tension of the top and bottom threads appropriately (as shown in Fig. 5 and 6).
(4）. Sewing material wrinkles: The tension of the top and bottom threads is too high, and the intersection point of the two threads is between the sewing materials, but the sewing materials are wrinkled (especially the thin materials) and uneven, and the sewing materials will be pulled with a little force to make the sewing threads pull off. Solution: Reduce the tension of the top and bottom threads. (As shown in Figures 7 and 8).

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