Bobbin thread
Bobbin thread is the finer thread wound on the small spool under the needle plate. It forms the underside of each stitch and locks with the top thread inside the fabric.
Bobbin thread is the thread wound onto the small spool (the bobbin) that sits under the needle plate. It forms the underside of every stitch. The top thread comes down through the needle and the two threads lock together inside the fabric and stabilizer. In machine embroidery the bobbin thread is usually a different, finer thread from the one on top, so it is a separate choice you make for each job.
It matters because the weight sets the balance of the stitch. Most designs are digitized for 40 wt top thread, so bobbin thread is almost always 60 wt or finer. Some machines call for 90 wt, so check your manual. Using top-weight thread in the bobbin makes the back heavy and can lead to thread breaks and puckers. On a satin stitch, a balanced back shows top thread across about two-thirds of the width, with the bobbin thread as a band down the middle.
You can wind your own bobbins or buy pre-wound ones. Pre-wound bobbins are wound evenly and tightly and hold more thread, so you refill less often on long jobs. Self-wound bobbins let you pick any color or fiber, but uneven winding can cause skipped stitches. You rarely need to match bobbin color to the top thread, since it should not show on the front. Match it only when the back will be seen, as on free-standing lace or in-the-hoop projects. Always use the bobbin style your machine takes, because the wrong shape or size can jam the thread.
If bobbin thread shows on the front, work from the simple fixes to the fiddly ones. Rethread the bobbin first. Brother's manual says a wrongly threaded bobbin is the usual cause, and tells you to reset the upper tension to its standard setting before you rethread. If it still shows, lower the upper tension number a little and stitch a sample on scrap fabric. Adjusting the screw on the bobbin case comes last, in small turns, and only after you have read your manual.
Common mistakes: loading a heavy bobbin thread without checking the machine's weight, letting lint build up in the bobbin area, changing tension before checking the threading, and mixing up bobbin styles.
Example: A design digitized for 40 wt polyester on top usually pairs with 60 wt polyester in the bobbin. On a satin column, the back should show a band of bobbin thread down the middle, with top thread on both sides covering roughly two-thirds of the width. If the bobbin band is much wider, or dots of bobbin thread appear on the front, rethread the bobbin and stitch another sample before you touch the tension. These proportions are a rule of thumb, not a measurement.
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Frequently asked questions
Can I use the same thread in the bobbin as on top?
You can, but a top-weight thread in the bobbin makes the back heavy and can cause thread breaks and puckers. Most embroiderers use a finer bobbin thread and match colors only when the back will show, as on free-standing lace.
What weight bobbin thread should I use?
Designs are usually digitized for 40 wt top thread, so bobbin thread is almost always 60 wt or higher. Some machines use 90 wt, so your manual has the final say.
Are pre-wound bobbins worth using?
Many embroiderers like them because they are wound evenly and hold more thread than a home-wound bobbin. One embroiderer notes that some machines cannot detect low thread on them, so keep an eye on the supply and make sure the bobbin style fits your machine.