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http://www.janeansbernina.com/_fileCabinet/sewingmachineTLC.pdf

TENSION: As you change projects and start sewing on different
weight materials, you should test stitch on a piece of scrap material
of the same weight before beginning the actual project so you can
adjust your upper tension to that particular fabric. As an example, if
you're changing from a denim type fabric to a silky fabric, you would
definitely want to make sure the tension is correct and the stitching
looks right before you start to sew the garment.



To determine whether the upper tension is too tight or too loose for
the fabric you're wanting to use, try the following test. Take a small
scrap of the fabric, fold it, and stitch a line ON THE BIAS of the
fabric, using different colours of thread in the bobbin and on top.
Grasp the bias line of stitching between the thumb and the index
finger. Space the hands about 3 inches apart and pull with an even,
quick force until one thread breaks. If the broken thread is the colour
of the thread in the needle, it means that the upper tension is too
tight. If the broken thread is the colour of the bobbin thread, the
upper tension is too loose. If both threads break together and take
more force to break, it means that the tensions are balanced.



BOBBIN: The most probable cause of the lower thread breaking is
an improperly wound bobbin. Regardless of where you wind the bobbin,
inside the machine, on the top of the hand wheel or on the front side
near the hand wheel, the basic "bobbin" rules apply.



Always start with an empty bobbin. Never wind one colour over another colour.

Don't wind the bobbin so full that it would be tight and hard to insert
into the bobbin case. Most machines have an automatic "shut off" when
the bobbin gets full, but if yours does not, be careful not to fill it
too full.

Wind the bobbin evenly across and in level layers.

Never mix different sizes of thread in the bobbin and on the spool,
unless you're doing sewing machine embroidery or some specialty type of
sewing. Using different weights of thread on the spool and in the
bobbin for general sewing will cause ragged stitches as well as other
stitching problems.



TENSION: As you change projects and start sewing on different weight materials, you should test stitch on a piece of scrap material of the same weight before beginning the actual project so you can adjust your upper tension to that particular fabric. As an example, if you're changing from a denim type fabric to a silky fabric, you would definitely want to make sure the tension is correct and the stitching looks right before you start to sew the garment.

To determine whether the upper tension is too tight or too loose for the fabric you're wanting to use, try the following test. Take a small scrap of the fabric, fold it, and stitch a line ON THE BIAS of the fabric, using different colours of thread in the bobbin and on top. Grasp the bias line of stitching between the thumb and the index finger. Space the hands about 3 inches apart and pull with an even, quick force until one thread breaks. If the broken thread is the colour of the thread in the needle, it means that the upper tension is too tight. If the broken thread is the colour of the bobbin thread, the upper tension is too loose. If both threads break together and take more force to break, it means that the tensions are balanced.

BOBBIN: The most probable cause of the lower thread breaking is an improperly wound bobbin. Regardless of where you wind the bobbin, inside the machine, on the top of the hand wheel or on the front side near the hand wheel, the basic "bobbin" rules apply.

Always start with an empty bobbin. Never wind one colour over another colour.
Don't wind the bobbin so full that it would be tight and hard to insert into the bobbin case. Most machines have an automatic "shut off" when the bobbin gets full, but if yours does not, be careful not to fill it too full.
Wind the bobbin evenly across and in level layers.
Never mix different sizes of thread in the bobbin and on the spool, unless you're doing sewing machine embroidery or some specialty type of sewing. Using different weights of thread on the spool and in the bobbin for general sewing will cause ragged stitches as well as other stitching problems.