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.