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In a couple of previous posts, we have discussed different causes and effects of tension problems. We discussed loops caused by upper tension issues and we have discussed issues with bobbin tension and spring pressure. I think if I had it to do all over again I would have started differently and we could have discussed the purpose of the top tension and its relationship to the purpose of the bottom tension and the formation of the standard lock-stitch seam.
|Tension diagram from Singer 66 manual|
Although the exact duties of the lower thread is vastly different than the top thread the tension is just as important in controlling the stitch quality. If the lower tension is too loose then the loops will be pulled through the fabric by the top thread and will show up on the top side of the fabric,also it will cause excess thread to be hanging hap haphazardly under the deck in the bobbin area just waiting to snarl around the hook and tie the mechanism into a creative knot that will stop even a freight train of a machine and can do seriously ugly things to the timing of the machines. If the lower tension is too tight then the thread will stretch tight and not allow the top thread to pull back up with the needle so the stitch has a ridge on the underside of the fabric at every intersection of the two threads. The resulting stitch will then lose its ability to flex and move with the fabric causing premature wear and thread breakage at the seams.
The solution is to balance the stitch, basically, it means we match the tension of the top thread with the bottom thread so our locking part of the stitch meets in the middle of the penetration point of the fabric. Pushing our top thread through the fabric with the needle so it intersects with the lower thread.then as the needle rises it pulls the lower thread half way through the fabric as the feed dogs advance for the next stitch and the takeup spring lifts the top thread to follow the travel of the fabric and slows down the bounce created by the take-up lever travel. The lower tension keeps the bobbin from spinning off too much thread as the needle pulls the lower thread up. This would then result in the perfectly balanced stitch. Sounds easy enough,until we start adding all the variables which can change the delicate balance.Starting with thread, for example, We need to keep our top thread and bottom thread the same diameter, size, and weight. Also mixing types of thread can lead to different tensions top or bottom which would require the balance to be re-adjusted. Matching our needle size to our thread size will also help as well as choosing the proper needle for the desired fabric being sewn. These are things which must be accounted for before we try to balance tension issues.Once these are accredited to the type sewing we desire to do we can then adjust the tension. As you can see tension is not a cut and dry type of situation so instead of easy black and white solutions, we find ourselves dealing with a lot of grey solutions. Because there is so much grey involved it would be impossible for me to tell you exactly what to do in every situation. So instead I am going to share with you, my routine in the hopes you will be able to adapt my technique to accommodate your continually changing needs and specific applications.
Whenever I set the tension on a new to us machine I always clean the machine thoroughly and oil or lubricate all of the appropriate places. If the upper tension is numbered or has graduated increments then I adjust the upper tension so zero tension just touches the thread and pulls through with no drag. I then use a full bobbin,properly installed and threaded in the bobbin case. I hold the thread and do a drop-bounce test I adjust the tension screw until the drop-bounce test allows the bobbin and case to fall 1-5 inches before it stops releasing thread and holds in place. The heavier the bobbin case the farther I let it slide. Most bobbin cases are pretty well matched in weight so I expect them to slide about 3 inches for the drop-bounce test. I install the bobbin case into the machine and move to the upper tension. With the machine threaded properly by the diagram or as close as I can guess as to what is proper I set the tension knob on about 2-3. If it doesn't have numbers or increments the I turn the tension knob 1 full revolution for the rough adjustment. In the Quilting Room with Mel we generally sew a lot of cotton fabric so I use the fabric we sew to set the standard for the tension. Same thing with the choice of needles,for me it is generally a #12 and for Mel it would be #14. This gives me a pretty decent starting place and from there it is all about balance. I adjust the top thread first then the bottom. If for some reason my starting parameters do not allow me to adjust to a balanced seam I start over by setting my top tension back at a 2 and adjust the bobbin tension until a tight balanced seam appears. Keep in mind the grey variables and don't be discouraged if a balanced seam doesn't appear lightning fast or laser accurate. Persistence will pay off greater dividends.
When the machine uses a drop in bobbin the process slightly differs because the bobbin tension cannot be preset with a drop-bounce test. So I adjust the lower tension for a slight drag approximately double the tension or drag as the top tension has when it is at its starting preset and from there the routine is exactly the same. As with any other learned skill the more you do the easier and better it becomes.
I hope this helps some of you and of course if you get a good laugh from my futile attempts to balance a seam it is still all about enjoying the love of sewing and the machine. Until next time may you all be blessed with perfect seams and unused seam rippers.
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