One might think that fixing sewing machines is all about turning the right screw, and they would be at least partly right.
When you remove the covers of your sewing machine, you might be amazed at all the little parts and all the different screws. It can look pretty complicated.
Yet, if you take a second look and begin to trace the shafts and levers with your eyes, it gradually starts to make sense. Start your gaze at the hand wheel and move across the top of the sewing machine from right to left. You will notice a turning shaft running the length of the machine with some pulleys, belts, gears, and levers connected here and there. You might notice buttons or levers from the front of the machine reaching back to adjust stitch length, stitch width, or even select different stitches.
Relax and allow yourself to process what you see. From the hand wheel, you can see where the belt from the motor drives the whole upper shaft. You will also see a shaft or belt directing the action of the upper shaft down into the bottom of the machine. As your eyes move to the left, you might see a round gismo with bumps or grooves all around it and little fingers that follow along against them. This is the cam or device that controls the movement of the zig zag arm. This enables the machine to make many different stitches just by altering which groove the finger follows.
To the far left you will see the needle bar and presser bar. Check out how they are connected. See how the movement of the upper shaft transfers movement to the needle bar making it move up and down as well as right and left.
All of these parts and those on the bottom must work in harmony and perfect time. Unfortunately, they sometimes get jolted out of position. Then the machine will not perform as expected. Adjustment or repair is needed.
For example, the needlebar must be adjusted properly for height, hook-needle clearance, centering front to back and left to right. Plus the swing of the needlebar known as parabola must be set.
These settings require expert knowledge of the sewing machine repair technician, who is trained to adjust the various settings.