Making a 2" half sphere from sheet metal

[Futterama]

Hi,

I'm brand new to this forum and came here for help on a little project I have.

I need to shape two hollow half spheres, a little less than 2" in diameter (45mm), to form a hollow sphere. The two halves will be silver soldered together.

I'm planning on using some 0.5mm (~gauge 25) sheet metal, I have regular cold formed steel sheet and austenitic stainless steel sheet.

I'm thinking about either try to spin the two halves, I have a small lathe that should be able to do the job, or maybe make a die and tap (is that the correct terms?) for pressing the shape using my shop hydraulic press.

I have seen some spinning videos, and it looks "easy" to do if you have everything set up right. But I can't loose the idea of using the hydraulic press to form the sheet. My idea was to make the "tap and die" on the lathe from hard wood (see the drawing), but I have no idea whether it will hold up, or how much pressure it would take, the hydraulic press I have is a table model and is rated 10T or 20,000 pounds. I guess there is also the question about friction between the wood and sheet metal.

Any suggestions are welcome

tap-die.jpg

[AllyBill]

You could always buy a 45mm dia ball bearing.

Will

[Futterama]

Hi Will, I see I forgot to mention that I need them to be hollow, I'll make an update.

I found this website where they do something similar to my hydraulic press idea, but with copper:

http://users.hubwest.com/hubert/proj...lls/balls.html

So copper will work harden, I know stainless steel will too, but can that also be annealed by heating?

[AllyBill]

Hi Martin,

I'd use copper. You can anneal it very easily by just heating it red hot and allowing it to cool naturally. That will make it soft as butter and very easy to work but it will harden quite quickly so you'll have to repeat as necessary. I used to make spherical sleigh bells using copper for Christmas giveaways for the local children. They don't ring too well but they look lovely when polished. Just make some simple tooling from a piece of hardwood and tap your copper over it. Cut a disc and shrink the edges first so they go down flat onto the tooling as you get towards the final shape.

Will

[Futterama]

I actually do have some copper too.

Tap the copper over the tooling, do you mean tapping with a hammer?

Can you elaborate on shrinking the edges?

Sorry, I still need to learn the tech-words

[AllyBill]

Tap it with something lightweight. Copper goes soft enough to work it with another piece of hardwood. It depends on what sort of finish you want.
And if you just turn the edge of your disc a little all the way around the circumference before you start making the main shape it will ensure the edge goes down neatly onto your tooling.

Will

[galooph]

I'll be interested to see how you get on with this as I had a job involving steel hemispheres a few months ago. It was a tight deadline, so I ended up buying them in.

The job was making a film prop using some concept artwork as the design. Client was happy, so that was the main thing!

[billfunk29]

Mcmaster has Hollow Stainless 2" balls, 24 ga. 2832K11, $16.

[Charlie Myres]

Originally Posted by AllyBill

Hi Martin,

I'd use copper. You can anneal it very easily by just heating it red hot and allowing it to cool naturally. ...

Will

Good advice from Bill!

Copper can also be quenched when it is still red and remain annealed. If you use a quenching bath with, 1 part sulphuric acid, to 10 parts water, all of the black scale will be stripped off; then wash in clean water,

Cheers Charlie

[Ken Hosford]

what is the tolerance on dia? on sphereicty ? finish ?
To get a full half sphere you will need to go deeper and trim to size. You might consider clamping sheet between two pieces of plate with a hole in one that a ball fit nicely in and 1 with hole ball dia +2 material thickness that is radiused.
with plates sandwiching the sheet press the ball through hole over 1/2 the diameter to allow for lead in radius