Singer Professional 5 Manual page 53

Turn Auto Tension Dial to "A".
Turn Stitch Length Dial to "N".
Engage Upper Knife.
The knife will not trim the fabric when it is disengaged.
1. Open Looper Cover.
2. Pull the movable knife holder (silver screw) to the right as far as it will go.
3. Turn the holder clockwise until the knife clicks into a lock position.
Upper Knife Disengaged
Five-Thread Safety Stitch Narrow page 3
Upper Knife Engaged