I’ve watched 4 animations and slow motion videos, and no one addresses this. I know I must be the problem, so to speak, in that obviously the machines work, but the explanations made by the videos are poor. One of the animations literally has the thread go through the axle, as far as I could tell. Does anyone know of a video that shows the whole thing so that it solves this “contradiction”?
Watch this video:
Tim Hunkin is one of my favourite people on the planet and he does his usual amazing job of showing how common mechanisms work. If you want to go to the exact spot he answers your question go to about 8 minutes into the video, but it’s worth watching the whole thing.
Underneath the needle plate is a mechanism that catches the loop of thread emerging from the needle, enlarges that loop and draws it around a small spool from which a different thread is emerging.
That spool is not on an axle, it spins inside a cavity, thus the thread loop from the needle is able to be pulled all the way around/over it.
The machine then tightens the top thread, pulling the stitch tight.
There is a hook thingy that grabs a loop of thread from the needle, but the bobbin thread does not go around anything, it just gradually unspools. So perhaps I did not understand the question?
I think the thing that’s confounding the OP is how the loop of thread from the bobbin, via the needle, gets around the spool (the small reel of thread under the needle plate), if the spool is running on an axle. The answer is: it’s not. The spool is running free inside a hole, with enough clearance all around it that the loop of thread can pass right over it and be pulled tight again.
So if I understand correctly, the upper thread (needle) gets passed completely over the bobbin (lower thread source)? OK, that explains how the stitching works as it does. I’ve never really been able to figure that out, just kind of took it on faith that people who actually understand mechanical things figured out how to make it work. 
That’s right - the loop of thread from the needle is enlarged and the bobbin, trailing its own separate thread, jumps through that loop.
Thank you for explaining that so well. I mean that.
You have those turned around - the bobbin is the reel under the needle plate, the spool is the reel whose thread goes through the needle.
So what makes the bobbin spin, if it’s not on an axle?
The thread gets caught by a hook and yanked.
That’s the terminology taught to me by my mother, a seamstress. It may be UK-specific or perhaps specific to the factory where she learned the trade. Googling it a bit, I don’t think they are actually hard and fast definitions; some people seem to call the small thing under the plate a bobbin, or a spool, or a bobbin spool.
Ah, now I get it. I had to watch the video several times. So the bobbin just lays there and is completely free, not attached to anything, and there is a separate rotating part next to the bobbin. That one has a hook that stretches past the bobbin and makes sure the hoop goes all the way around the bobbin.
So it’s like the completely free bobbin falls through the hoop, only that it’s the hoop that moves and not the bobbin.
And that’s the trick. You need one unattached part, or it’s impossible to make it work.
Probably lots of variation in time and region and manufacturer. I was going by what my Singer labels them (brand new Heavy Duty machine FTW) and what I was taught.
Hear, hear!
There’s also a Veritasium video that tells you more than you ever wanted to know about how sewing machines work.
Beat me to it.
In that video, I learned some archeologists found a sewing needle made 50,000 years ago, by a now extinct species of early humans called the Denisovans. Amazing.
After learning more about how this works, I’m even more impressed that people like Howe & Singer dreamed this up, and made it work reliable – 200 years ago.
We’ve been clever at complicated gizmos for hundreds if not thousands of years - but we kept making them smaller and now they’re too tiny to see or understand…
The Veritasium video is what made me ask this. I watched the key part repeatedly and all I can see is thread going through where the axle must be.