I’ve had a couple of people write and ask me what Chris (another sewing machine mechanic) and I were talking about in the comments. So here is some more information to help clarify that discussion for the non-mechanics 🙂
The first question was about the top thread looping or “bubbling” on a wide zig-zag. A typical stitch for a mechanic to sew is a 4X4 zig-zag to check tension balance. This should result in a nice top stitch and a dot of top thread on each side of the bottom of the stitch.
As you can see from the two sewing samples above, the top stitch on the right is loose… “bubble” looking. Yet you can see from the bottom stitch on the left that the stitch is in balance on tension. So the question is, what causes this “bubble”? The answer is, the wrong presser foot. We generally use a “regular” foot when working on sewing machines. But most sewing machine manufacturers have a variety of feet for their machines. Each foot has a specific purpose such as a cording foot, a button hole foot, a zipper foot, and the one to correct this “bubble”… the open toe embroidery foot.
As you can see from the image on the left, the stitch sewn with the “regular” foot has a loose bubble on the top of the fabric. That is because as the fabric is being fed under the presser foot, the thread is pinched between the foot and the fabric. The stitch on the right was sewn with an “open toe” foot. So those stitches lay flat on the fabric, because the next stitch goes down before the thread reaches the presser foot.
The second thing we were discussing (and I get this question from a lot of you) is why there is a thread tail on the underside of a wide (5mm – 9mm) zig-zag, or blanket, or hemming stitch. The answer is, because until the last few years, sewing machines were never intended to sew such a stitch.
When you look at the two zig-zag stitches on the right, notice that the tail from the top thread is longer on the left than on the right. The machine left the factory that way. It has a wide stitch width, not so it can sew a wide zig-zag, but so it can sew a wide decorative stitch. That decorative stitch will have several fabric penetrations before it makes it from the left side of the 9mm to the right side of the 9mm stitch width. This will minimize the amount of pull on the left side of the stitch. The closer the two stitches are on that side, the less pull there will be on the top thread. The straight stitch on the right is with the needle in the full left position, and it is balanced. So it is not where the needle penetrates the fabric, but where the previous penetration and next penetration will be in the fabric. Some new high end machines can adjust the top tension for every needle penetration in a stitch, thus virtually eliminating that thread tail on the bottom of a wide stitch.
I hope that all made sense. 🙂
May Your Thread Never Break!