I found a way to reduce the diameter of a coin with fine precision, using minimal equipment (no lathe).

You need:
- Drill with variable speed, i.e. with rotatory speed increasing proportionally from zero to max when you press on the main operating button
- Strong magnets (neodymium) embedded within a round male threaded axis (see picture below); I found them on Amazon
- Double side adhesive tape to be applied between coin and magnets
- Sandpaper

The drill MUST have variable speed, otherwise your coin will instantly fly across your room and smash on the wall!
The magnets keep the coin firmly attached so that I can connect it to my drill. Centering the magnets requires precision (and several attempts).
I can then rotate the coin (starting very slowly) and with sandpaper reduce the diameter with very fine precision. A very rudimentary lathe!

I prefer to roll the sandpaper (abrasive side inside) and create a cylinder, with a diameter close to the diameter of the coin. That way I am not putting pressure only on one side of the coin, so that I can keep it centered.
With one hand I hold the cylinder and with the other hand I operate the drill.
Securing the drill on a table is also a good idea.

If you are lucky and the coin is well centered, you will notice minimal vibration and you can accelerate a bit. The sandpaper will get warm pretty quickly: use gloves! Go step by step and in a few minutes you will see results, but precision requires time so don't rush.

A digital caliper helped, in particular to notice small differences: the coin ends up slightly oval (we are talking about a fraction of millimeter) but I was able to fix that manually using sandpaper on the larger sides.

I never found a way to re-mill the edge: I'd love to do that with normal (home) equipment!

I managed to fit several coins in my (silver morgan dollar) shell with zero wobbling.
I started with chinese coins: those were super easy! No magnets, because the hole allowed me to connect the coin to the drill with a wall plug and a screw.
Then I moved to a copper liberty bullion, which was the real challenge that required the magnets.

I hope you will find my method useful and if you decide to give it a try... good luck!

Click here to view attached image.

Ah, neat. So you basically made a makeshift lathe. Probably the best way.
Thanks for the instructions. For future readers, this was the other post referenced.
https://www.themagiccafe.com/forums/view......start=20

Can you post a link to the magnets on Amazon?

Sure.
It's the Italian Amazon though:
https://www.amazon.it/dp/B081NL5ZH2
But I hope you will find enough details and references to get the product in other online shops

Thanks.
Couldn't find it on the US site but found something similar from my favorite magnet dealer.
I also need something with a smaller diameter so I can work on half dollar size coins. Will report back with details if it works out.

This looks like maybe the same thing in the US. https://tinyurl.com/2p8vtj4p

Thank you

I bought these magnets (links below) to be able to mill down half dollar size coins.
It worked great. Slow and steady and some patience and I got a perfect fit to an unexpanded shell - see the pic below, that's the dirty side. Rather than holding the sandpaper I put it on a sanding block. Had to hold the phone to take the pic, but one hand holds the block while the other holds the drill, keeping the coin as perpendicular to the sanding surface as possible. I would occasionally angle the coin to keep from developing an overly sharp edge.
Low speed and steady but not overly firm pressure.

Unlike the OP I didn't use any tape. The force of the magnets held everything together. Perfect centering wasn't necessary as the steady pressure sort of self-centered the coin as it rotated.
Play with it and you'll get a sense of how much pressure to apply and a good but not super fast rotation speed.
I used 400 grit sandpaper. Something like 200 might go faster but if you want that perfect almost vacuum seal fit, it's better that the material comes off slowly. You could also start coarser and switch to the finer grit when the fit starts to get close. Reduce a little, test/measure, lather, rinse, repeat.

I will be ordering similar but smaller ones to use with quarters.

Caveats:
Do this at your own risk.
Wear eye protection.
You end up with a fine powder of metal particles. Wear a mask.
Copper isn't ferro-magnetic.
Filing down a steel or similar coin is going to have those fine particles sticking all over the magnets - that would be a pain to clean off.
These magnets are STRONG. Very easy for them to pinch fingers or have them go flying at each other and shattering.
Handle carefully and wear eye protection.

Have fun!

https://supermagnetman.com/products/cap_......48763971

https://supermagnetman.com/products/d108......10277251

One other note - when testing the fit, just before it's perfect the coin will likely get stuck in the shell. Have a bang ring handy to separate them, have a glass to shake them apart, etc.

Wow, is that final picture the filings left over?

I'm personally a big fan of the method using a silk - put it in the shell first, then if it gets stuck, you can just pull on the silk to pop it out. Use an old/crappy silk you don't care about, as there will probably be some sharp edges from all the filing that might damage it a little.

Thanks for sharing!

Yes - pretty much copper powder.

The silk thing is fine if a coin used to fit but has become finicky, but you do risk tearing the rim of the shell if you overdo it.

This could be used to shave off a tiny bit of metal for a coin that sticks in a shell, but my situation is I have unexpanded shells and want to make coins that will fit them.
They start out the exact same diameter as the shell, so using a silk is out of the question initially. For the super snug (dare I say "Lassen-like") near vacuum fit, the silk won't work. You'll get something that's a super snug fit only when a silk is in place. It'll be good/fine but not at that perfect sweet spot.

Hey inigmntoya! I'm glad it worked out well!
I am trying to figure out your "sanding block" technique but I am not sure I fully understood how you used it: in my attempts, when I tried applying pressure on one side only, rather than "self-centering" I only managed to "un-center" the coin pretty quickly.
But either way the final result is great: achieving such a precision without a lathe is what I was aiming for.
Combined with my C/S technique Copper Silver Coins - How They're Made I managed to manufacture an insert that turned my shell into a perfect Sun&Moon.
Many hours of work, but the result is priceless!

Oh, last thing: I agree adhesive tape is not needed to keep the coin in position.
I used it to avoid scratching the face of the coin though: as I also worked with "plated" coins, sometimes I ended up damaging the surface.
But of course "self-centering" is no longer possible: I guess in that case you may try with normal tape (not double sided).

Re: self centering
It may have been a combination of the particular strength of my magnets and that the coin is thinner than a dollar leading to better grip?

I only applied enough pressure to see that material was coming off the coin. If I can figure out how to record myself actually doing it I'll post something.

What a fantastic idea! I don't think I will do it, but I love the ingenuity.

More recently I've started using some "grippy" shelf liner between the coins and the magnets. It helps reduce slipping and protects the face of the coins from any wear if they do slip relative to the magnets.

Interesting... can you share an example (available online) of such a "grippy shelf liner", so I get a better idea of the product?

I'd imagine he means something like one of these:
https://www.amazon.com/DAZZ-Classic-Shel......P5&psc=1

Yes - like those.

Another little idea to help centering the coin.
I made, with thick cardboard, a disc with a hole.
The external diameter is identical to the coin diameter.
The diameter of the internal hole is identical to the diameter of the magnet attached to the drill.
The "paper ring" helps as a guide: I slip it around the magnet and then set the coin using all fingers to center it.
Obviously the ring can be removed before attaching the magnet to the drill.