Sewing Machine tension is the most important element in sewing to achieve a good experience while making your project. A sewing machine has two tension that must be balanced. There is a upper tenion, that you can adjust at will, and a lower tension located on your bobbin case. The lower tension should not need to be adjusted very often. When your machine is being service by a techincian they will use this adjustment to balance your tension sewing. So, how does one know if their machine is in balance? The test you can use is simple. Set your machine to do a zig-zag setting. Then set the lenght to 4 and a wide of 4, then sew a couple inches on a good woven fabric. The top should be a perfect zig-zag. The bottom will look like the top with the exception of the upper thread will be slighty pulled down to the bottom corners of the edge where the needle perice the fabric.
1) So your zig-zag stitch is set.
2) Your top tension is set to is mid-point on the dial (the factor has a indicator on the dial)
3) Sew your test stitch.
If your bottom thread tension is 'To loose" it will be PULL UP to the top of the fabric. If your bottom thread tension is "To Tight" it will PULL DOWN the top thread beyond the corners on the bottom of the fabric.
4) When adjusting the BOTTOM Tension. Turn the straight slot screw only, not the phillips or plus slot screw. Only turn it in small increments. Think like a clock. if the screw is at 1:00 oclock position then to tightin it turn it to the the right to 2:00 to 3:00 oclock position. To loosen turn if to the left to the 12:00 to 11:00 oclock position. Then resew to see the results of your your adjustment. You make have to do this several times to achieve the right results. NOTE: It may be easier to have two different color threads to see the results. Also use a 40 weight thread on both top and bottom with size 90 needle. Manufactures use this as the standerd to set machines with. If you dont have a 90 then a 80 is ok, but do not use any smaller needle than that.