Creating a hemi-sphere - Woodworking Talk

cliff1959 · 05-11-2008, 06:09 PM

Can anyone come up with a quicker and more accurate way to create a hemi-sphere in wood four inches diameter than by careful use of a chisel?

I need to create a hemi-sphere in copper and being able to heat and beat into a 'wooden mould' would be a solution.

Any ideas welcome.

Gerry KIERNAN · 05-12-2008, 11:55 AM

Basically, you need the services of a wood lathe. Failing that, you could try mounting the wood blank onto a face plate and attaching that on the shaft of a motor. [A poor man's wood lathe]

Gerry

frankp · 05-12-2008, 04:45 PM

As Gerry said, the best way I could think of would be a lathe. You could do it with a saw and some time, but a lathe would be much faster. If you use a saw you can basically make it quarters of the "hemisphere" that are effectively just blocks and then shaping them with lots of small cuts, like turning a square into a circle by cutting corners off a little at a time, but in 3 dimensions instead of two. The math is a little more tricky, but you can eyeball it well enough to start, and then sand it the rest of the way. It won't need to be perfect as a "pounding mold" either.

I'd also suggest just going and getting a good anvil to work with, as they generally have a rounded or mostly rounded end specifically for purposes of hammer things into bends.

TS3660 · 05-13-2008, 01:23 PM

Ok, thinking out of the box here, how about a mold made from concrete lined with leather? The mold would be easy to make. Just fill a box with cement and press a ball into it.

cliff1959 · 05-14-2008, 04:32 AM

Hi Guys,

Thanks for your suggestions so far.

I'm making inquiries at a local turning club.

Also TS3660 I've tried what you suggested but it fell apart. Basically I used cement instead of concrete, I'm thinking concrete might just have worked. I especially like the leather idea.

Thanks again for everyone's input.

I'll let you know the outcome.

PS I bet you can't guess why I need a hemi-sphere in copper, here's a clue for you: I need 56 of them!

TS3660 · 05-14-2008, 08:45 AM

Hats for your birthday party?

Capt Crutch · 05-14-2008, 09:24 AM

Why not use the lost wax process?

1. Make (or cast) a sphere out of wax.

2. Submerge the sphere in liquid plaster of paris, with a "string" of wax leading to the outside of the plaster mold. This will be your "sprue hole."

3. After the plaster hardens, head the whole thing in an oven or something to melt the wax, then drain the wax from the sprue hole.

4. Pour your molten copper into the same sprue hole into the (now empty) spherical mold.

Seems to me this could be done a lot faster and easier than what you're talking about...

TS3660 · 05-14-2008, 10:13 AM

Quote:

4. Pour your molten copper into the same sprue hole into the (now empty) spherical mold.

"Molten" copper??? Either I misunderstood or you did. I thought he was talking about forming a piece of sheet copper into a type of half dome?

Cdat · 05-14-2008, 10:49 AM

How big is a softball? I'd just find something the right size and use it.

frankp · 05-14-2008, 12:51 PM

CaptCrutch, if you followed your method you'd end up with a solid (and very heavy) sphere of copper, not a bowl, and certainly not a hemisphere. You'd also only be able to use your mold once. Pretty cool technique, though.

Capt Crutch · 05-14-2008, 11:35 PM

I guess you boys are right, he only wanted a hollow sphere, duh. I sure did wonder about that idea of pouring copper into wood...

Better switch my tinfile helmet for COPRfile I guess...

cliff1959 · 05-15-2008, 06:44 AM

The lost wax method is ingenious but as was said all I want is a hemi-sphere in copper (x56).

I bought a bag of concrete mix yesterday so I'll try a Mk2 version that should stand up to a bit more heating and beating.

My local wood turning club doesn't meet for another four weeks so I'll have time to try it out.

'Hats for a party'

, I like it, but no, keep guessing. Here's a clue for you, it's a very British thing.