Sewing Machine Spool Cap Guide

Technician Tips

Spool Cap Tips from a Technician

There are two main purposes to the spool cap on your sewing machine.

The first is fairly straightforward: a spool cap holds the thread spool onto the spindle and keeps it from rattling around or falling off.

The second is less obvious, but just as important: spool caps are designed to help the thread flow from the spool as smoothly as possible.

If you examine your spool cap, you’ll notice it’s made of an extremely soft, smooth plastic. We want the thread to slide across that ultra-smooth plastic rather than the relatively rough (and potentially jagged) plastic the spool itself is made of. Thread passing over rough plastic is susceptible to snagging and breaking. Thread passing over the smooth spool cap avoids that problem.

Whether the thread slides off smooth plastic or jagged plastic depends on one thing: using the right size spool cap.

How to Pick the Right Spool Cap

Slightly larger than the spool

You may have noticed your machine came with several spool caps in various sizes.

Sewing machine spool caps in several sizes — Most machines come with a range of spool cap sizes.

Which size should you use? The general rule is this:

The spool cap ought to be slightly larger than the plastic end of the spool itself.

By “the plastic end of the spool,” I mean the left end of the thread spool, closest to the needle. A spool with a larger plastic end needs a large spool cap; a spool with a smaller end needs a smaller cap.

Correctly sized spool cap on a spool of Aurifil thread — Right size on an Aurifil spool.

Correctly sized spool cap on a spool of Isacord thread — Right size on an Isacord spool.

Too small

What we don’t want is a spool cap that’s smaller than the plastic end of the spool, so the thread passes over the rough spool instead of the smooth cap. This is no good.

Too big

While we want the spool cap larger than the end of the spool, we also want the path of least resistance. An unnecessarily large spool cap can become an obstruction that snags the thread. In other words, don’t make the thread pass over a mountain if it doesn’t have to. A cap that’s too large can cause the same problems as one that’s too small.

What a Wrong Spool Cap Does

The un-threading needle

An incorrectly sized spool cap can cause a number of problems. I’ve seen it cause intermittent thread loops on the top side of the fabric, from intermittent snagging at the spool.

The more common thing I see is the needle becoming un-threaded after using the machine’s automatic thread trimmer. If the trimmer cuts the thread while it’s snagged on something like the jagged edge of the spool, the thread recoils and slingshots out of the needle eye. The most common culprit is the thread snagging on the spool’s rough plastic because of an incorrectly sized cap, but I’ve also seen sharp nicks in the machine’s plastic cover (or in the spool cap itself) cause the snag.

The bigger principle

Widening our scope beyond spool caps, the deeper principle here is the importance of an unobstructed thread path, whether it’s the upper thread going through the needle or the bobbin thread inside the bobbin case. It’s a useful concept to keep in mind whenever you troubleshoot your sewing machine.

Read the full articleThe Importance of an Unobstructed Thread Path

Summary

Spool caps are designed to help the thread flow from the spool as smoothly as possible.
The spool cap ought to be slightly larger than the plastic end of the spool.
Avoid a spool cap that’s smaller than the end of the spool.
Avoid an unnecessarily large spool cap, too.
The general principle: keep the thread path unobstructed. The right spool cap helps you do that.

Replacement spool caps for Baby Lock & Brother