The sewing machine’s broken

“The sewing machine’s broken. Do something already!”

Alright, alright. Let’s see what’s up.

I hit the foot pedal — the motor budges a bit, makes this huge buzzing noise, then just… stops.

“Maybe the carbon rod is the problem?”

Wait. Carbon rod? Why does it need one? Isn’t an electric motor supposed to be simple? I mean, I still remember my old Tamiya ̶d̶y̶n̶a̶m̶o̶ from back in the day — that thing never needed a carbon rod.

Curiosity piqued, I cracked open the carbon rod cover. There’s this little screw that pushes the carbon rod in with a spring. The moment I loosened it, it shot across the room — thanks, spring mechanism. And… I lost the screw. Nice.

“That’s it. You ruined it! You definitely ruined it!”

So then I rummaged through an old, dusty, unused sewing machine — pulling parts like a mad scientist. I salvaged a screw, a carbon rod — the whole kit.

Secured. Back to the operation.

The carbon rod looked fine. Still plenty left. Not worn out yet.

“There are two carbon rods. Maybe check the other one?”

Wait, TWO?! One’s already weird. Now you’re telling me there’s another? Why does this motor need two carbon rods while my tiny Tamiya ̶d̶y̶n̶a̶m̶o̶ didn’t need any?

Anyway. I hunted down the second carbon rod — hidden way behind the motor. Couldn’t reach it while it was still mounted, so I had to pull the whole thing out.

There it was. Gotcha. The second carbon rod was busted — the rod had popped out of its spring, so it wasn’t even touching the deeper part of the motor properly.

I clipped the spring back in place, reassembled everything, and boom — the motor ran like new.

But I couldn’t help it. I wondered, why does it need two carbon rods?

One YouTube deep dive later, it finally clicked:

The electric motor used in a sewing machine (right) requires at least two carbon rods, which are housed in screw-type holders (right, red circle). In the motor schematic (left), these carbon rods function as brushes that deliver electrical current to the spinning commutator.

Turns out this motor is actually a universal motor — it needs two brushes (that’s what the carbon rods really are) to complete the circuit. One brush feeds current in, the other lets it flow out — while the spinning commutator switches the direction so the rotor keeps turning smoothly.

The genius bit? They use carbon because:

• It’s conductive, but not too hard, so it won’t wreck the commutator.
• Its graphite structure has these layers that slide easily — friction stays low.
• It can handle the heat. Graphite doesn’t melt; it sublimes at high temps.
• Its slight resistance helps keep sparks under control.
• And it’s chemically stable when hot.

Pretty clever. But yeah — that’s also why these rods wear out over time. The constant friction slowly sands them down. So you’ve got to replace or reload the brushes every now and then.

So, back to my Tamiya ̶d̶y̶n̶a̶m̶o̶.

Did it magically work without brushes? Nope. Turns out, it has them too — but they’re tiny. Those hobby motors (Mabuchi-type) run on low voltage and current, so they don’t need big, beefy carbon rods.

“As far as I know, Mabuchi is no longer manufacturing motors for Tamiya,” said Laperen one year ago. But the last time I tinkered with my Tamiya dynamo was around more than a decade ago, so back then I was probably still using a Mabuchi-type motor. (Using the term dynamo is actually a bit misleading, since it technically means an electrical generator — but back then, my kid self and all my childhood friends still called it a dynamo instead of a motor. I suspect this was due to a bit of semantic drift, because in our mother tongue, the word motor already had a very different meaning: a gasoline-powered motorcycle.)

https://www.mabuchi-motor.com/product/knowledge/structure/explanation.html#tab05

They use built-in metal brushes or tiny graphite strips, and once they wear out, you toss the motor and get a new one.

Case closed.

Sewing machine : fixed.
Missing screw : cannibalized.
My curiosity : satisfied.