Sphere starting point

[tkeen]

If i am going to make half of a sphere, is a circle equal to half the circumference a good starting point or should it be larger? I wondered if it would stretch or shrink more.

Tom

[Fe-Wood]

Here is a formula I use for cutting the blank for spinning parts the first time.

The sheet metal blank should be a disc approximately equal in
radius to the desired part's length plus radius times 80%[D =.8(l+r)].

Its not exact but it gets you in the ball park. Raising half a sphere by hand is not much different than doing it on a lathe. Hope this helps

[Kerry Pinkerton]

Tom, it depends on how you choose to make it. The more you shrink the edges, the smaller the diameter will be. I once made a bowl with ALL stretch. The diameter of the blank was about 10" and the bowl was still about 10" across.

If I was doing the contest I'd probably start with a 12" diameter blank. You'll probably do some stretching and shrinking and end up with something about 10" across and 5" deep. When you put two of them together you'll have a 10" sphere.

[tkeen]

Thanks for the replies. I was thinking of using a bowling ball for a pattern I think they are about 8 inches. Is the larger diameter easier?

Tom

[Jacob]

The diameter itself does not make a big difference. Something that is easy to handle is good. Your hammer or mallet has to reach to the bottom of the hemisphere that's all.