Let's look at bobbin cases

I want to shed some light on a sewing machine part that's rarely discussed though critical to successful sewing: bobbin cases.

A common lock stitch sewing machine (like the one you probably use at home) uses two threads that "lock" in the fabric to produce a stitch. One thread is fed over the top of the machine, through tension disks, thread guides, and pickup levers, before reaching the eye of the needle.

It's this top thread most of us pay the most attention to since it's what's visible on the outside of our fabric and it's also right in front of us as we sew. But the top thread is only half the stitch. The bobbin or bottom thread makes the stitch complete.

The bobbin thread is wound on a metal or plastic bobbin, placed into a bobbin case and inserted into the bottom of our machine. Most home machines use what's called a class 15 bobbin and unless you have a drop-in bobbin system, the bobbin is inserted into a bobbin case before being inserted into the lower thread hook area.

Most of us hardly ever think about this bobbin area until it's empty of thread or there's a problem with our stitch quality. If the problem is the bobbin thread tension, this is adjusted by tightening or loosening the little screw on the outside of the bobbin case.

While there is definitely a manufacturing standard when it comes to a class 15 bobbin cases (or they wouldn't be as universal as they are), one clear difference is the tension spring design. This is the area where the class 15 standard allows a manufacture some leeway in design -- in fact, it seems each one is different. I think of thiis little tension spring as the business end of the whole bobbin case: it regulates the feed of the lower bobbin thread -- the other half of the perfect stitch equation.

When you pull the thread from the bobbin case you can feel how evenly the thread feeds. Remember that the bobbin thread is fed straight up at a 90 degree angle from the tension spring. At that angle the thread rubs aggressively on the edge of the tension spring. How does it feel to pull the thread at this angle from your case? Ideally it should offer smooth resistance without much catching.

Some tension springs have a rough edge that seems to catch the thread as it pulls, creating uneven catching and drag. This causes uneven stitch quality, as some stitches are formed when the bobbin thread gets caught and others not. It's difficult to compensate for this.

Other case designs start out with proper tension but the faster you pull the thread the lighter the tension becomes. While sewing slowly all seems fine but the faster you go the more your stitches are pulled to the top of your fabric -- not good.

Some cases also have a larger or smaller hole in the center for fitting over the hook shaft. If the hole is too big it can cause the bobbin case to shift around excessively, causing a racket but not affecting stitch quality. If the hole is too small it may not even fit on your machine's hook or just be difficult to slide on and off.

In the video below I explore a range of bobbin case designs and point out some of their differences. If you're doing everything right and your stitches still are not coming out perfect, examine your bobbin case and consider trying another one.

The top thread is only half the story.