Everyone says I should never adjust the bobbin tension on my machine, but I'd like to try heavier threads and different techniques that require some adjustment. Can you help me?
Donna G.,
Lakewood, CO
The bobbin case is a small mystery to most people. Understanding how it works and how to adjust it, reduces the fear surrounding it. The following information is very general; consult your sewing machine manual for specific instructions.
The bobbin tension only makes up half of the machine tension and must work in conjunction with the upper tension. Most machines have a rotary bobbin of some sort, meaning the bobbin turns in a complete circle as a stitch is formed. Some bobbin cases are removable and some are built into the machine, but they basically work the same. The bobbin goes in the bobbin case, which is housed in the race mechanism located underneath the needleplate.
The upper tension consists of two metal disks squeezed together by a coiled spring-the upper thread passes between the two disks. The applied pressure can be changed by an adjustment knob or on the LCD screen-lower numbers are for a looser tension and higher numbers for tighter tension. Raising the presser foot automatically releases the pressure between the disks, and the thread passes through freely. Raise the presser foot when threading the machine and after stitching to pull the work from the machine.
The interlocked stitch forms as the needle moves up and down, pushing the thread through the fabric and into the race mechanism.
On the needle's downward stroke, it pushes the thread through the fabric, down next to the bobbin case (1).
As the needle begins its upstroke, the needle thread forms a loop. A notch on the bobbin case holds the loop back while the bobbin hook catches the front of the loop and pulls it all the way around the bobbin (2). The needle continues moving upward, pulling up the needle thread, sliding it off the bobbin hook and out of the notch.
As the needle moves higher, the upper thread tension pulls the bobbin thread up, forming an interlocked stitch (3). The feed dogs then move the fabric one stitch length preparing for the next stitch.
Be sure the bobbin is wound and inserted in the bobbin case correctly. If the bobbin is loosely wound or the threads crisscross, it will cause uneven tension, resulting in stitch irregularity. When winding the bobbin, feed the thread through the winding tension disks so the thread has proper tension and the threads lie next to each other. If you're hand winding thicker or stretchy threads, wind them evenly with uniform tension.
Insert the bobbin in the case so the thread unwinds in the opposite direction that it feeds out of the case. Draw the thread up under the tension plate and out the opening (4). For thick threads that can't go under the tension plate, simply feed them out the opening, bypassing the tension plate.
Adjust the lower tension by turning the tension screw, which holds the tension plate in place. Turn the screw counterclockwise to loosen and clockwise to tighten. Move the screw only a quarter turn or less at a time. Test the tension by pulling on the thread-it should have a little resistance. Note: Different types of race mechanisms require slightly more or less tension. Test each adjustment-a little turn of the screw can have a big effect on the tension. Note. Embroidery and computerized machine thread tension must be adjusted on the screen.
Once the bobbin tension is set, it rarely needs adjusting. Slight adjustments for different fabrics are made using the upper tension. Some people have a bobbin case for regular sewing and a second, adjusted one for special techniques.
Stitch a sample swatch to determine what adjustments are needed. Use different colors of the same type thread in the needle and bobbin so it's easy to determine where the tension is off. Set the upper tension in the middle of its range and then adjust the bobbin tension as described above.
When the tensions are balanced, the upper and lower threads interlock halfway between the fabric layers (5).
If the upper thread lies on the fabric surface pulling the bobbin thread to the top, the upper tension is too tight (6).
If you can't get the stitch right by loosening the upper tension, tighten the bobbin tension a little.
If the bobbin thread lies beneath the fabric, the upper tension is too loose (7). If tightening the upper tension doesn't fix the stitch, loosen the bobbin tension slightly.
Note: If your adjustments don't seem to be working and your machine is old, the upper tension spring may be worn out and need replacing. Also if the bobbin tension plate gets bent, it needs to be fixed. Both of these adjustments require a sewing machine technician.
When a ball of thread forms on the underside, it's usually because the upper tension is too loose. The race mechanism pulls an extra large thread loop and the loop remains because the upper tension isn't tight enough to pull the thread back up to form a stitch, resulting in the "bird's nest."
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