Imagine walking into your house one day and in the middle of your living room is a brand new 60-inch plasma screen TV. Your husband just spent $3,000 on the biggest and best. And then he says, "This is the best ever! Great picture, stereo surround sound, and it gets only one channel all the time . . . ESPN!! Sports, all day every day." $3,000 for a one-channel TV sounds crazy. But, that is what you are doing when you use only the preset automatic tension settings on your expensive sewing machine. If you never change the auto tension settings, you are stuck on only the channels your machine likes. You are prohibiting yourself from enjoying the other channels available, such as the heavy cottons, the fine holograms, the sensitive Metallics, and other specialty threads. By overriding the auto tension settings, you can use different threads in the top and bottom. Many decorative threads require a looser upper tension setting than the auto setting provides.
By adding to or taking away from the top tension, you can equalize the strength of the two tug of war teams and create a perfect stitch. The fiber content does not need to match. The weights or sizes of the threads do not need to match, but they should be reasonably close (for example, don't use an ultra fine 60 wt. bobbin thread with a heavy 17 wt. top thread). Some educators teach that you should always use the same thread on the top and bottom. That's OK, but you're limiting your channels. It is much more fun to get all the channels and be able to use all those fancy threads. 90% of all sewing frustrations can be eliminated by using quality thread, choosing the right needle (the Topstitch 90/14 is the needle of choice for many professionals and educators) and by adjusting the upper tension (usually loosening it). Tension adjustments allow threads of different textures or weights to freely pass through the tension disks. And don't forget to clean out the tension disk area. Lint can accumulate and affect your stitches.
Learning to override your machine's automatic tension system will allow you freedom of choice in selecting those different thread channels. Don't worry about getting so far into the computer settings that you will really mess up the computer. The factory preset settings will reset themselves every time you turn off and on the machine.
Thread Tug of War: How Tension Works
Sewing machines are factory preset to have the top and bottom thread form even stitches when sewing with a 50 or 60 wt. thread. If the top and bottom threads are identical in fiber and weight, adjustments may not be necessary. However, if we use cotton on top and poly underneath, or metallic on top and poly underneath, or a heavy thread on top and a fine thread underneath, it is necessary to adjust the tension settings. It is perfectly OK to use different thread types and weights on the top and bottom. Relying on a machine's automatic tension system is not enough.
Think of the top and bottom thread as having a tug of war. If the threads are identical and you are sewing on a single layer of fabric, both sides have equal strength and the result will be a draw. The sewing should therefore produce perfectly even stitches with no top thread showing underneath and no bobbin thread showing on top. However, in the real world, the teams are rarely equal. One team will be stronger or bigger or faster than the other. We sometimes use decorative or sensitive threads on top. We often use different fibers for the top and bottom threads. We also add stabilizer or batting. Sometimes we might use a cotton bobbin thread and other times we use a polyester bobbin thread. All these factors make it necessary to adjust the tension for each project. By adjusting the top tension either up or down, we are able to add or take away strength on the top thread team to equalize the tug of war battle.
Following is a list of things that affect stitch results:
1. Batting. This adds drag on top thread. Cotton batting tends to grab the thread more than poly batting, adding more friction on the thread.
2. Fabric type. Dense fabric puts more stress on the thread.
3. Top thread thickness and type. Metallic is less flexible than cotton or poly. Poly is usually stronger than cotton or rayon.
4. Bobbin thread type. Cotton bobbin thread tends to grab more than a smooth filament polyester. Sometimes grabbing is preferred and sometimes it causes problems. A smooth filament poly thread (not spun poly) in the bobbin will work better with metallic and other sensitve threads because its smooth finish acts almost like a lubricant, sliding nicely with the thread.
by Bob Purcell - Superior Threads (Chief Threadologist of Superior Threads)