1) How well the bobbins are wound.
2) Bobbin case and back-lash spring health
3) Bobbin case tension
4) Type and quality of thread used
5) Type and quality of fabric and batting used
6) Rotary tension assembly health (this included a present and functioning check spring) and proper adjustment
7) Aux. Intermittent Tension assembly health and proper adjustment

This isn’t an exhaustive list, but it covers most.  The most comment mistake I have seen made, is having bobbin tension too tight.  Our instinct is to tighten tensions when we are unhappy with the quality.  This invariable leads to 1) loss of ability to adjust tension and 2) thread breakage.  The KEY to tension is balance, not more tension.  The bobbin tension must be consistent and not real tight.

There are two methods to checking bobbin tension.  1) pull up on the thread from the case while it is laying in your hand.  The bobbin should be able to stand up, but not lift out of your hand.  Then I usually loosen it a little more.  2) The well-talked-about TOWA bobbin gauge.  This gauge holds the case and will show exactly what your tension is.  This nice part is that bobbin case problems show up in the gauge that you can’t see in your hand.  Now… I like the gauge as a tech support person – because it is must easier to work with customers on the phone when they have the gauge.  The other method is subject to too much interprutation.  But I use both daily to fix machines.

Once you are comfortable with you bobbin case health and tension setting, you can then move to the top.  Both tension devices work together.  I will often begin by setting both tension knobs to “flush” or “level” with the threaded shafts.  In most cases, this means unscrewing the knobs.  Check that the “check spring” is functioning on your white knob (Rotary Tension). This looks like a paper-clip. It needs to have a medium firm pull. Also check that the felt washers are still there and not completely black. (they are white when new).  Then check the black knob. The two main things to check are 1) that the discs “pulse” out every stroke about an 1/8″.  Many are set to stroke too far.  2) do not wrap competely around this device.

Now sew.  These setting should be close.  Now you have plenty of room both in the bobbin case and top tensions to make adjustments. BTW. The white knob takes large turns and the black knob takes small turns

Here are a couple of links to addtional information on tension adjustments:

Top Tension

Bottom Tension

I hope this information helps.  You are right in that the many, many varieties of threads and fabrics influence tension.  Some combinations simply have limited ability to produce good quality in the stitch.  Heres a couple of reasons:

1) Higher loft battings and softer fabrics allow the stitch to be more full… which in-turn makes balance easier.  On the other side, flatter battings and stiffer fabrics resist allowing the threat to settle-in.  This often causes the stitch to fall to one side easier.

2) Anytime you don’t use the exact same thread in the bobbin case as in the top, you run the change that one thread is slightly or greatly different in thickness – which makes balance very difficult.

3) Rotary Hook sewing machines used in Free motion create different amounts of bottom tension for each direction and speed you move.  From the front of the stand, going right to left and back seems to create the most bobbin tension.  You will see this if you use contrasting threads and make quarter-sized circles.

In the end, it is maintaining balance and flexability.  If your tensions have been tightend too much, in an effort to achieve balance in one direction, then the quality will diminish in other directions and you will have limited yourself with tight tension.

Well… that’s a lot of information.  If anyone needs me to clarify a point, just say so.

On a related note:

Rick and I are beginning to create a second volume to his well-known maintance DVD.  We would love suggestions on topics to cover that we haven’t gone over in the first volume.

Cheers and happy quilting!
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Todd Fletcher