Rick's b.log - entry 2025/12/06

How a single-thread sewing machine works

Most people sew better than this.

Clearly this is impossible to do using mechanical methods because it involves pushing the needle back and forth from either side.

So a traditional sewing machine solves this by using two threads. One is attached to the needle, and the other comes from a spool inside within something called the shuttle.
Every time the needle passes into the fabric, a hook on the shuttle picks up the thread and whizzes it around the shuttle itself, so when the needle pulls back out, it forms a nice tight loop around the thread from inside the shuttle. This is called lockstitch. In very old machines, before the rotating shuttle was invented, the shuttle would be a thing that physically whizzed back and forth to perform the same sort of action (looping the upper thread around the lower).

With correct tension, the threads meet in the fabric.

This is exactly how old machines such as a pedal operated Singer worked. More modern machines, by virtue of moving the needle and shuttle relative to each other, can create quite complex patterns. But, hang on, there's another type of sewing machine. These ones, pretty cheap ones, work using only one thread. How?

Well, let's take one apart and discover its mysteries. ☺

Singer ZDML-2 portable sewing machine teardown

A cheap sewing machine

That being said, for small jobs that need to be done quickly on fairly thin material, these machines are capable.

Note that you can get versions/clones of this machine on Amazon for around €25. They come with safety pins and some extra thread spools; but that doesn't really justify why it is so much more expensive than the one I bought from Lidl...

This box has a pleasing amount of weight.

Looking inside the box reveals more surprises.

The machine and its accoutrements.

You get four spools of different coloured thread, an adaptor for using regular thread spools, a replacement needle, and one of those dinky little needle-threader things.
All of this for less than the price of a fast food burger. That's kind of nuts, isn't it?

Taking the machine out, you can see that some poor underpaid worker has threaded, tensioned, and tested the machine.

Set up and tested.

A first look at the stitching

How the stitching looks on top.

While underneath it has a very distinctive loop pattern that is central to how this machine does what it does. This is called chain stitch, and it is often used in cross stitch to give extra volume.

How the stitching looks underneath.

What's actually inside?

This is what is inside.

There is a motor, that turns quite quickly, that is geared down (for the torque) to a large gear wheel that has an offset connection to the long piece of metal that the needle is connected to.

Looking at the gears and drive system.

Essentially all this does is push the metal bar up and down, which, in turn, causes the needle to go up and down.

Above is a picture of the machine with the needle up. This is it with the needle down.

The needle in the down position.

Now if we look at the front, where the needle goes, we can see that there's a second piece of metal that can rock like a seesaw.

This thing powers the part where the magic happens.

This is driven, at the back (left in the picture) end by being connected to the needle head that moves up and down. And what does it do? Well, it appears to simply move a little metal rod up and down. But since it goes through that rocking mechanism, it inverts the movement, so the rod moves in the opposite sense to the needle, but perfectly synchronised with it.

The picture above is of the rod in the down position when the needle is all the way up. Here is the rod in the up position when the needle is all the way down.

Up and down and up and down and...

Where the magic happens

So let's take a step by step look at what is actually going on as the machine places the needle into the fabric.

Here we are underneath the needle looking almost directly up at it. This is where we need to be to best see what is going on.

The starting point.

You can see, to the left of the copper hook, the needle starting to enter into the machine.

The needle enters the chat.

And at this point, the copper hook is moving itself out of the way almost with a snapping action, and that screw that was annoyingly in the way is moving in the opposite direction to be out of the way as well.

Everything getting out of the way.

Here the needle is all the way into the machine, and the copper hook has moved clear of the needle.

All clear.

The needle is starting to move up now, and you can see that this releases tension on the thread, which forms a little loop to the right of the needle.

There's a loop!

The hook moves into place, catching that loop of thread.

Gotcha!

The hook holds on to the loop as the needle comes completely out of the machine.

Hang on tight now.

The platen now shifts (look at the position of the piece of white thread in the slit to the right). This advances the fabric ready for the next stitch, and you'll see the screw is now back in the way.

A little shuffle ready for the next stitch.

The needle penetrates into the machine once again, this time neatly sliding down in between the copper hook and the loop of thread.

We're coming around again.

The copper hook snaps out of the way, and now we have the needle into the fabric, with a loop of thread from before around the needle.

This is making me a bit loopy.

The needle moves back up, making a loop.

Another loop...

The hook grabs onto the loop.

...grabbed.

The needle comes down again, in between the hook and the loop.

The needle returns.

Which wraps the loop around the needle.

Loop around the needle.

Then as the needle goes up, another loop is grabbed. If you look at this picture, you can see the previous stitch loops to the left of the copper hook.

And another loop grabbed.

This process repeats. Every time the needle enters, it does so into a loop created from the previous time. Which, in turn, leaves a loop for the next time.

In terms of my rubbish diagrams, it's something vaguely like this.

I'll fess up: I draw a small segment and copied it lots.

This is why I have not further disassembled the machine to show what the piece of plastic actually looks like, or what is going on in order to have the hook and platen move in opposite directions. It's because, even for something that is barely more than a toy, the precision here is quite remarkable. A little bit off, there's a risk the needle could slam into the hook, or that the hook would miss catching the thread.

You'll know, most likely from my previous investigation into a washing machine controller (here and here), that I am quite fascinated with clever design of seemingly simple mechanical things that can do complex tasks.
So even though this is a cheap little mechanical sewing machine, there is something rather pleasing and absolutely satisfying about seeing and understanding how it actually does what it does.

Why does it unravel so easily?

Why the stitching comes undone.

To solve this, you need to gently tug on both sides of the loop from the loop side to see which one is connected to the final piece of thread passing through the fabric.
When you know which it is (it'll be the one you can pull on), then gently pull it through until the last part of the thread is on the loop side.

Tying off the stitching, step 1.

Now pass it under the loop and around to tie it off. If your eyes and/or fingers aren't up to this sort of finicky nonsense, you can cheat. Just use that needle threader gizmo. That's what I do.

Tying off the stitching, step 2.

Pull it nice and tight.

Tying off the stitching, step 3.

Now do that once again in order to stop it unravelling (step 4), and then you can cut the excess thread (step 5).

My machine won't release the thread from underneath!

At any rate, the way to finish sewing is this:

• Bring the machine to a stop.
• Turn the red knob clockwise to bring the needle to the top position.
• Carefully tease out the thread from between the needle and the fabric. Pull it out until around seven or eight centimetres of thread is present.
• Cut the thread in the middle (this critical part was missing from my instructions!)
• Turn the red knob clockwise to lower the needle into the fabric, and keep on going until the needle returns to the topmost position.
• Carefully lift the tensioned arm holding the fabric in place, and then gently remove the fabric to the left (with the battery end facing you and the needle facing away). There may be a slight resistance due to the hook, but since you cut the thread it'll give. Just be gentle.
• Then sort out the final loop and tie off as shown above. You should also tie off the start, but that one won't cause it all to unravel so it's less critical to do it immediately.

If you don't cut the thread, leaving about 4cm either side of the cut, then you will find that the hook holds on to the thread and the only way to release it is to either cut it really short, or pull until the thread breaks (which, obviously, risks damaging the machine).
Yes, it was a rather important thing to miss out of the instructions! ☺

At any rate, I hope today's blog article has given you an understanding of how these things work, and an appreciation for the design that allows Lidl to offer a functional little sewing machine for... say it with me... less than the price of... ☺

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