SINGER 1725 - Manual (Page 36)

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Thread tension
Upper thread tension
Basic thread tension setting: "4"
To increase the tension, turn the dial to the next number up. To
reduce the tension, turn the dial to the next number down.
A. Normal thread tension for straight stitch sewing.
B. Thread tension too loose for straight stitch sewing. Turn dial to
higher number.
C. Thread tension too tight for straight stitch sewing. Turn dial to
lower number.
D. Normal thread tension for zig zag and decorative sewing.
Correct thread tension is when a small amount of the upper
thread appears on the bottom side of fabric.
Lower thread tension
To test the bobbin thread tension, remove the bobbin case and
bobbin and hold it by suspending it by the thread. Jerk it once or
twice. If the tension is correct, the thread will unwind by about an
inch or two. If the tension is too tight, it does not unwind at all. If
the tension is too loose, it will drop too much. To adjust, turn the
small screw on the side of the bobbin case.
Please note:
- Proper setting of tension is important to good sewing.
- There is no single tension setting appropriate for all stitch
functions, thread or fabric.
- A balanced tension (identical stitches both top and bottom) is
usually only desirable for straight stitch construction sewing.
- 90% of all sewing will be between "3" and "5".
- For zig zag and decorative sewing stitch functions, thread
tension should generally be less than for straight stitch sewing.
- For all decorative sewing you will always obtain a nicer stitch
and less fabric puckering when the upper thread appears on the
bottom side of your fabric.