Embroidery bobbin tension: how to test it and when to adjust it

Should you adjust bobbin tension at all?

Most of the time, no. The bobbin case spring is set at the factory and rarely drifts on its own. When bobbin thread shows on the face of a design, the cause is far more often on the top side: a mis-seated top thread, top tension set high, a worn needle or lint. Change the top first, because it is quick and easy to reverse.

Adjust the bobbin only when all of these are true: the needle is new and correct, the top is freshly threaded with the presser foot up, the bobbin area is clean, the bobbin is the right type and wound evenly, and an I-test still shows the bobbin stripe badly out of proportion even after sensible top tension changes. The other legitimate reason is a deliberate change of bobbin thread, for example moving from a thick sewing thread to a fine embroidery bobbin thread.

How bobbin tension works

The bobbin sits in a bobbin case (or a fixed holder on drop-in machines). Its thread passes under a flat leaf spring held down by a small screw. The pressure of that spring is the bobbin tension. In embroidery the bobbin drag needs to be high enough that the top thread loop is pulled down around it to the back of the fabric, but not so high that it drags the knot back up.

Because the bobbin thread is usually finer (60 wt or 90 wt) and smoother than 40 wt embroidery thread, the bobbin tension that suits sewing may not suit embroidery. That is why some machine makers recommend a different bobbin case, or a different setting, for embroidery bobbin thread.

The drop test (front-loading and rotary hook cases)

Removable bobbin cases, used on front-loading home machines and on nearly all multi-needle commercial machines, can be checked by hand. This is often called the drop test or yo-yo test.

1. Load a full, evenly wound bobbin into the clean case and pull the thread through the tension slot and under the spring, exactly as you would for stitching.
2. Hold the thread end and let the case hang in the air.
3. Watch it. If it slides down on its own weight, the tension is too loose.
4. Give a light upward jerk of the wrist. A correctly set case drops roughly an inch or two (a few centimeters) and stops. If it will not move at all even with a firm jerk, it is too tight. If it runs freely to the floor, it is too loose.

The drop test is a coarse check, useful for spotting a case that is far off, not for fine tuning. The I-test remains the judge. Commercial shops that need repeatable settings across many heads often use a bobbin tension gauge instead; if you do, set to the value your machine or thread maker specifies.

Drop-in bobbin machines

On drop-in (top-loading) machines, common on Brother, Baby Lock, many Janome and Singer models, the bobbin sits in a plastic case under a clear cover, and you cannot hang the case. Check it like this instead:

• With the bobbin inserted and threaded through the guide slot, pull the bobbin thread up through the needle plate. You should feel light, smooth resistance, clearly less than the top thread with the foot down. No resistance at all means the thread has missed the spring slot.
• Make sure the bobbin turns in the direction your manual shows. On many drop-in machines the thread should unwind counterclockwise as you look down on it; inserting it backwards changes the drag.
• Check for lint packed under the spring. A small brush and a gentle flossing with a piece of thread clear it.

Many drop-in cases do have an adjustment screw, but manuals often discourage turning it. If your machine maker offers a separate embroidery bobbin case, buying one and setting it for your bobbin thread keeps your sewing case at factory settings.

How to adjust the bobbin screw safely

1. Mark the start position. Draw a line with a fine permanent marker across the screw slot and onto the case, or photograph it. This is your way back.
2. Use the right screwdriver. A small flat blade that fills the slot. A loose fit chews the slot.
3. Turn very little. Think in clock-face minutes: about the distance from 12 to 1 is already a meaningful change. Clockwise tightens, counterclockwise loosens.
4. Hold the case over a cloth or tray. The screw is tiny and, if it backs all the way out, the spring and screw are easy to lose.
5. Retest with an I-test on your usual fabric and stabilizer, and record the new position.

Reading bobbin problems from the stitch-out

Bobbin thread and the bobbin itself

The bobbin thread you choose changes the tension you need. Most embroiderers use a white or black 60 wt or 90 wt bobbin thread, which is thinner than 40 wt top thread and so builds less bulk on the back. Prewound bobbins hold more thread and wind very evenly, but some come with cardboard sides or a different core that sits differently in a given case. See bobbin thread and prewound bobbins and the comparison prewound vs self-wound bobbins.

Bobbins wound at high speed with a thread that stretched, or wound unevenly to one side, feed with a jerky drag that no screw position can fix. If you wind your own, wind at a moderate speed and check that the bobbin fills evenly and not past the rim.

Multi-needle and commercial machines

Commercial heads use a removable bobbin case on a rotary hook, so the drop test and a gauge both apply. Shops typically set the bobbin once to a standard and then balance each needle's top tension to it, rather than the other way around. Keep a spare, pre-set case for each head: swapping a suspect case for a known-good one is the fastest way to tell a bobbin problem from a top problem.

On these machines also check the anti-backlash spring inside the case (if fitted) and the bobbin type. Commercial machines commonly use the larger L-style bobbins, or other sizes listed by the maker; the wrong size will not sit or feed properly.

When the bobbin is not the problem

If the drop test and the I-test both look right and you still have trouble, look elsewhere. Nesting under the fabric is almost always a top threading fault; see bird nesting. Bobbin sensor warnings on screen are covered in bobbin errors, and lint or burrs in the hook area are covered in cleaning the bobbin area.