The subject matter of the invention is a sewing machine and also a piercer for a sewing machine.
Hemstitching machines and two-needle sewing machines have been known for over one hundred years and still operate using the same principle today. According to this principle, two needles, which are each associated with one hook, create two zigzag stitching sections that are parallel to each other. In order to separate the two stitches from each other, the woven threads are pushed somewhat apart with an auxiliary piercer and an opening or a hole is formed in the fabric. In order to be able to loop the woven threads forced apart with the needles at the edges of the holes, deep grooves at the side running in the axial direction are embedded in the piercer. Due to the tension of the woven threads, there is still free passage for the needles in the grooves for a piercer that has already cut into the fabric. In order to be able to lead the needles through these two opposing passages, the latter are mounted so that they can rotate on the lower end of the common needle-and-auxiliary piercer bar oscillating vertical to the needle plate and are moved in an oscillating manner. That is, a first stitch is realized with inwards pivoted needles, so that their tips are guided into the side grooves of the piercer through the opening created in the free space of the grooves in the fabric reaching downwards to the hooks. During the piercing, the needles contact the grooves with such a strong force that they exhibit bending. The subsequent stitch is then realized with needles that have been pivoted back, i.e., that are not bent and that penetrate essentially vertically into the fabric. For the next stitch, the needles are pivoted inwards again and are bent when they cut into the fabric and contact the grooves of the piercer. They are also bent somewhat to the side by the hooks, so that the tip of the hook can grip the needle thread loop.
This method, which has been used unchanged for a century, has the disadvantage that for every second stitch, lateral forces act on the needle and bend the needle. In addition, it is not guaranteed that all of the woven threads will be pressed outwards during the inner stitch, i.e., the stitch, for which the needles are guided bent into the grooves of the piercer. Often there are still individual woven threads within the opening, which degrades the visual image of the hemstitch, because they cross through the formed hole. Another disadvantage consists in the fact that the tips of the needles lie very close to each other for complete engagement in the fabric and consequently the tips of the two hooks under the needle plate must also lie very close to each other. Due to the close arrangement of the two hooks, the piercer cannot penetrate very deeply into the fabric or its tip must have a large tapering angle in order to form sufficiently large holes. This has the consequence that when the woven threads are spread by the quick penetration of the “blunt” tip of the piercer, the woven threads can be ripped. Such a hemstitching machine is disclosed in U.S. Pat. No. 2,093,558.