Mechanical sewing machines with zig zag use cam gears to create stitches. The needle is suppose to drop straight down and rise straight up. At the top of the needle bar movement, the needle bar arcs or snaps to the other side of the zig zag stitch. This movement from one side to the other is called the swing or parabolic movement of the needle bar.
What happens when the needle bar fails to swing as it is suppose to do?
It may be a little problem distorting the stitches, but it can be so messed up that the needle breaks, stitches may not form, or the machine jams. In worst case, the needle may even try to snap from one side to the other when it is inside the needle plate.
What do you do?
Open the machine and find the cam stack. Look carefully at the connection between the cam gear and the upper shaft. You will see a worm gear on the upper shaft with two or more set screws on the worm gear collar. Loosen the set screws. Leave one slightly snugged, but you need to be able to turn the shaft without moving the cam gear. Rotate the hand-wheel slowly toward you. Observe the needle bar. Adjust until the movement is correct. Tighten set screws. Test to make sure the swing is swinging properly.