Top Thread And Bobbin Thread
The top thread and bobbin thread are the sewing threads used in a lockstitch sewing machine: the thread that passes through the needle and the thread wound onto the bobbin and inserted into the hook. They are balanced in tension to form the stitch.
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Definition and Overview
Top thread is the thread that passes through the needle eye of a sewing machine and is supplied from the top side of the textile. Bottom thread is the thread wound onto the bobbin and set in the hook, and it crosses with the top thread carried by the needle to form a lockstitch. Usually, the thread visible on the top side is called the top thread, and the thread visible on the underside is called the bottom thread.
In lockstitch sewing, the knot formed by the top thread and bottom thread is ideally positioned near the center of the textile thickness. If the top thread tension or bottom thread tension is too strong, the thread on the opposite side is pulled to the surface, affecting the appearance and strength of the stitch.
Materials, Twist, and Structure
Top thread and bottom thread are made from materials such as polyester, cotton, nylon, and silk. In general apparel sewing, polyester thread is often used because of its strength, abrasion resistance, and range of colors, but it is selected according to the texture of the textile, dyeability, and the intended use of the final product.
Thread is divided into spun thread made by twisting short fibers together and filament thread made from continuous long fibers. The direction and amount of twist, and thread fuzz, affect ease of sewing, luster, and the appearance of the stitch, so it is also necessary to check compatibility with the sewing machine and needle being used.
The bottom thread may sometimes be finer than the top thread, but it is not necessarily limited to the same material or same thickness. In embroidery, a finer bottom thread may be used in relation to the top thread that creates the design. In contrast, for lockstitch sewing, the combination of thickness and strength is considered according to the purpose.
How Thickness Is Expressed
The thickness of sewing thread is shown by count or nominal count. The relationship between the count number and the actual thickness differs depending on the labeling system, so numbers alone cannot be simply compared across different systems of thread. Check the material, construction, and count shown on the thread label together.
Even with the same nominal count, thickness, tensile strength, elongation, and bulk will differ if the material or structure differs. In the specification sheet, it is important to state the distinction between top thread and bottom thread, the material, color, and count or nominal count, and to perform test sewing with the actual textile and sewing conditions before mass production.
How to Choose and Sewing Precautions
Thread is selected according to the thickness and weave of the textile, stretchability, stitch type, and the forces applied during wear. For lightweight fabrics, thread that is less likely to damage the textile and less conspicuous is considered, while for heavy fabrics or areas under stress, thread with the necessary strength is considered. When the stitch is intended to be seen as a design element, thickness and luster are also selection factors.
Thread tension changes depending on the textile, needle, sewing speed, and thread thickness. If shrinkage, puckering, fabric damage, skipped stitches, or thread breakage occurs, check not only the top and bottom thread tension but also the needle type and count, threading, bobbin winding condition, and cleaning around the hook.
In sergers and chainstitch machines, looper thread is used in addition to needle thread. These looper threads are not usually referred to as the bottom thread in a lockstitch sewing machine, but in sales settings they may be introduced together as thread for the bottom thread side, so confirm the machine model being used.
- For lockstitch top thread and bottom thread, confirm compatibility through the same sewing test in principle.
- Use a bobbin type and size that fit the sewing machine model.
- Check color matching not only on the top side of the textile but also on the underside and with the seam opened.
Differences from Similar Terms
Sewing thread is a term that refers to thread used for machine sewing in general. Within that, the thread passed through the needle in a lockstitch machine is the top thread, and the thread wound onto the bobbin is the bottom thread. Hand-sewing thread is designed for hand work, and when used in a sewing machine, its sewability should be checked in advance.
A bobbin is not the bottom thread itself, but a small spool onto which the bottom thread is wound. Also, serger thread refers mainly to thread used for serger needle thread and looper thread. It differs from lockstitch bottom thread in the way the stitch is formed and the mechanism used.
Frequently Asked Questions
Should the top thread and bottom thread be the same thread?
For lockstitch seam construction, it is often common to combine threads of the same material, same color, and similar thickness. However, different threads may also be used when you want different colors on the front and back of the textile, for embroidery, or when matching the thickness and sewing specifications. Confirm the finish and strength through test sewing.
What causes the bottom thread to show on the top side?
In general, the cause is that the top thread tension is too weak or the bottom thread tension is too strong. However, incorrect threading of the top thread, improper bobbin insertion or winding, needle-thread incompatibility, and lint around the hook also have an effect. Check the basic condition before making large adjustments to tension.
Does a serger also have a bottom thread?
A serger differs from lockstitch sewing, which uses a bobbin, and forms stitches with needle thread and looper thread. Therefore, the thread passed through the looper is not strictly a bottom thread. The types and number of threads required vary depending on the sewing method and machine specifications.
