The following is an example exercise from the book How To Start Sewing. In this exercise, we look at how to adjust the thread tension and presser foot pressure on your sewing machine to achieve the best possible stitch quality.
Each time that you sit down at your sewing machine, you should sew some rows of stitching to check that everything is working correctly. One way that you can improve the stitch quality is to adjust the thread tension and the presser foot pressure. Note that not all machines allow you to adjust these features so you should refer to your manual. Also, ensure that your machine is threaded correctly before making any adjustments.
To begin, we need to check how the machine is sewing straight stitches. Cut some squares of fabric about 20 cm wide (approx. 8 in) for test samples. Fold the fabric in half to create a double layer; this better represents the way that the fabric will be when sewing garments. Sew a couple of rows and then take the fabric out of the machine. Look closely at the stitches, and run your fingers along the top of the stitches to see how flat they are.
If these points describe your stitches then you don’t need to make adjustments:
- Stitches sit flat and are slightly embedded in the surface of the fabric.
- Stitches look the same from the top and bottom of the fabric.
- Fabric sits flat and has no puckers.
If these points describe your stitches then you will need to adjust your machine:
- Stitch loops are loose and lift above the surface of the fabric.
- Stitches look good on only one side of the fabric, but the thread creates loops on the other side.
- Machine is skipping stitches or stitches are uneven.
- Fabric is puckering or becomes marked from the teeth of the feed dogs.
Thread tension is essentially a tug of war between the upper and lower threads. Ideally, you want the threads to be balanced. When balanced, equal tension pulls on both the upper and lower threads so that they lock together right in the middle of the fabric layers. If stitches are loose on both sides of the fabric, then you may need to increase the tension on both threads. If both threads are pulling tightly against the fabric and causing puckers, then you may need to loosen both threads. Aside from tension, you can also alter the amount of pressure applied by the presser foot. This pressure will affect how the fabric feeds through the machine, which may also change the look of the stitches. Essentially, you just need to sew rows of stitches and tweak each of these settings one at a time until the stitch quality improves.
Thread Tension Controls
Tension on Upper Thread
In many cases, you can improve the appearance of your stitches just by adjusting the tension placed on the upper thread. This thread tension is controlled by the tension dial, which is usually located on the front or top of your machine.
Be aware that on some machines, especially on industrial machines, you may have more than one point where the tension is affected. If you are unsure what is used on your machine, then you should refer to your manual. Secondary tension discs are sometimes called a pretension device. These additional discs and guides may not be marked with numbers, but may still affect the tension on the upper thread.
Tension on Bobbin Case
At times, you will not be able to fix tension issues by adjusting only the upper thread. You may need to slightly adjust the bobbin thread tension as well, by turning the small screw on the side of the bobbin case. Refer to your manual if your bobbin case has two screws, but it is usually the larger of the two that you will need to turn.
If your machine has a drop-in bobbin, then you will need to refer to your manual to locate the screw position. Remember that the lower tension has to be adjusted along the thread path of the bobbin thread. So any tension control screws for the lower thread will probably be close to one of the guides in the bobbin chamber.
Adjusting Upper Thread Tension
Sew a row of stitches, check the result and take note of your original tension settings before making any changes. It can help to fold your fabric to create a double layer because most of the time you will sew through two layers at once to sew seams. Try turning the tension dial a small amount to the left or right, by about half a number, and then check the result again to see if you have made the stitches better or worse.
If the upper thread is pulled too far towards the underside of the fabric, then this can mean that the upper thread is too loose, or that the bobbin thread is too tight. Try turning the tension dial to the right, or clockwise, to increase the number on the tension dial. Increasing the number will increase the tension on the upper thread. This change will pull the upper thread tighter and straighter towards the surface of the fabric.
If the upper thread sits too straight on the surface of the fabric, then this can mean that the upper thread is too tight, or that the bobbin thread is far too loose. Try turning the tension dial to the left, or anticlockwise, to a lower number. This change will decrease the tension and will loosen the grip on the upper thread.
This trial and error process will take some practice; you just need to change the tension setting a little at a time. If you are having trouble seeing the difference between the upper and bobbin threads, then you can also try using different thread colours as we’ve shown in the diagram. The contrasting colours will help you to see if one thread is being pulled too far towards the top or bottom of the fabric.
Remember that the aim is to balance the tension between the upper and bobbin threads. This means that there is a limit to what you can achieve by only adjusting the upper thread tension and if you cannot fix the stitches, you can move on to tightening or loosening the bobbin thread tension.
Adjusting Bobbin Thread Tension
As a guide, your starting point for bobbin thread tension can be worked out by using a simple drop test. This test will show you how tightly the bobbin case is gripping the thread. Hold the thread so that the bobbin case hangs down. Ideally, if you move your hand up and down slightly, then the bobbin case will slowly slide down the thread from its own weight. If the tension is too loose, then the bobbin case will quickly drop straight down the thread and you will need to slightly tighten the screw. If the tension is too tight, then the bobbin case will not drop at all, and you will need to loosen the screw. Use a small screwdriver to change the position. As shown in the diagrams, you will only need to make very tiny adjustments to this screw to change the tension. Recheck the tension by sewing test stitches after each adjustment. Once you have made a change to the bobbin case then you may need to adjust the upper thread tension as well to achieve balanced stitches.
Presser Foot Pressure Controls
As we mentioned in Chapters 10 and 11, the presser foot pressure will usually be located on the top of your machine directly above the needle. On some machines, you will simply turn the knob to increase or decrease the pressure. On other machines, usually industrial machines, you will need to undo the bottom nut first before you can turn the screw that affects the pressure. If you are unsure, then you should refer to your manual.
Adjusting Presser Foot Pressure
When adjusting the foot pressure, it is again a case of trial and error to see what will work best for your fabric. If your machine is skipping stitches, or creating uneven stitches, then you may need to increase the pressure by turning clockwise or towards the right. If the fabric is puckered, or the teeth of the feed dog are marking the fabric, then you will need to release the pressure by turning anticlockwise or to the left.
This is an example exercise from the book How To Start Sewing.