Hollowing a coin?

Use two coins. face to 1/2 thickness, bore out 1/2 of the pocket. Repeat on other coin. assemble and bond the three parts. For holding the coin, I'd suggest softjaws, very wide for a lot of holding area.

Karl

And a very small endmill? Or very shallow cuts? I'm referring more to the "boring" part than the facing.

Why put a magnet *inside*? Why not steel shim? Or get a Canadian coin. Or go to a magic shop.

That's the easy way. Not as much fun, however.

"axolotl" wrote in message news:1210112755 snipped-for-privacy@isp.n...

Hi Kevin. Don't make the mistake of facing the coin down all the way across the face. Instead of that, face a hollow which stops before the rim of the coin, if possible coincident with a ridge or line of denticles. This will make concealing the join very much easier. The large half (with the hollow) can be turned easily in a three-jaw chuck, using a turned timber spacer to keep the coin normal to the axis of the lathe. The second piece, the small plug, is a little trickier as it must be turned accurately to the correct diameter - to wedge fit into the recess you have turned in the other half. I usually do this by wedging the coin bewteen two hardwood centres turned to slightly smaller than the desired final diameter. (Actually, it doesn't matter if they are too big, you simply turn them down along with the plug.) The coin is held in place by (over)-tightening the tailstock screw. To reduce the plug to the desired thickness, turn down a hardwood mandrel, supported in the three-jaw. Face the mandrel and bore a 1/4" hole right through it. Turn an accurate recess in the hardwood face to match the plug, then wedge the plug into it. Face the plug until the dia is OK. (Use your feed graduations to determine this.) Remove the plug by pushing a pencil through the 1/4" hole. (Betch'a glad you didn't forget that hole, yes?) The plug can now be inserted in the recess in the face of the other half, and if you've done it well the join will be utterly invisible. (Bonus: No seam around the rim.) Warning: You won't be able to get it apart, so go easy when trial fitting. No glue will be necessary if you fit them accurately.

This is how two-headed coins are made. It's up to you to organise the recess for the magnet.

This awfully complex-sounding method is actually not that difficult, and it results in an excellent deception.

(So they tell me :-) )

-- Jeff R. (who never defaces the Queen's currency)

Sounds like you've done this (or read Holtzapffel). Do you turn the plug to diameter entirely by measurement? Unless you glue the plug to the mandrel I don't see how to check the fit. Maybe no one notices if a slightly oversized plug expands its edge of the rim by one or two thousandths.

When I make a finger-pressure press fit between steel parts I taper the outer end of the plug very slightly with a file until the ring will barely start onto it, then hone the bit to a knife edge and tweak it to start shaving metal a little inboard of where the ring stopped.

I adjust the final fit by filing and sanding, watching the tool marks to stay parallel.

I've noticed that a surface with tool marks works better than a smooth polish. I think the high spots interlock a little and keep the friction up longer as the two pieces wear together.

The press-fit assembly I'm playing with right now is a 5C collet closer nut with a 1" hole in the back for sharpening large drill bits in an end mill sharpening fixture on a surface grinder. The 1/2" S&D shank adapter plug is a press fit to keep it in place while I tighten the collet. I haven't actually ground a bit yet but a trial assembly looks promising.

Jim Wilkins

Don't know the reference, so... (1+1=...)

Mostly - then I abrade the inside of the recess until a press-fit becomes "likely". Like you, I apply a little taper to the plug. (Thanks - I forgot that bit.)

Oh heck, no. Its got to fit down flush.

Yup. That'll work.

Indeed. Even a bit of chatter :-)

Well I never! A non-frivolous application!

Keep it up, sir!

-- Jeff R.

Hand or Simple Turning, by John Jacob Holtzapffel, London, 1881 Mine is a paperback Dover reprint from Lindsay. He describes shop-made wooden chucks for turning billiard balls etc.

I'd like to make something for fun, such as a radial engine, but so far it's been , special tools, repair parts and prototypes for work.

Hollow coins were used to hide microdots during WW2. I've read how to make the photo but not the coin. I think the cover screwed on with lens threads. Supposedly one or more were lost into circulation in Canada.

Jim Wilkins

But just don't go do'n none o' that magic stuff at Florida schools.