[TheForge] 8" hemispheres

[Peter Fels And Phoebe Palmer]

Hi Don:
There are a number of different approaches to making them, but a through 
exploration of metal forming,metal spinning, hydro forming and press 
forming companies in the Thomas Register is a good place to begin.
You havn't detailed the parameters...finish? Precision? volume of 
pieces? acceptable variation? budget?  All these will dictate the 
possibilities.
What do you have for tooling? What are you willing to build/buy to do it?
Do the hemispheres and weld them together is probably the easiest 
general approach.
A simple lathe would allow you to spin them up, though 14 ga is a bit 
heavy ...pure iron would probably help.
They could be pressed to hemispheres in stages, preferably hot, with a 
press of  modest tonnage. A similar approach with a power hammer is 
possible too.
How bout a cannon ball dropped down a length of 8" pipe?
Explosive forming might be a lot of fun.
I built a largeish C frame and installed a small anvil block with a 
hardy hole ( for stakes) at the top of the lower arm of the C. I mounted 
a 1 1/2" shaft vertical with clamps, to the top arm of the C . At the 
bottom of the shaft there's welded a slotted pipe that accepts the 
handle and butt of an air powered  chipping hammer. The bit of the 
hammer hits the top of a stake in the hardy hole. A foot valve controls 
the chipping hammer which has the trigger wired open. There is a 
mushroom head on the bit and the 14 ga slides between it and the stake. 
It makes a helluva racket but it will move that steel into a hemisphere 
in  3 annealings or so, maybe 2 with practice. Working with my pop-up 
torch for spot heat, 14 ga bulges under this rig a heck of a lot faster 
than freehand. Using interchangable bits and stakes of different 
contours will aid in roughing and planishing.
Some years ago, Carl Jennings did a demo with a home made  stake holder, 
complete with seat, and what looked like a small forge burner built on 
an old fashoned hair dryer that was mounted on a stand. Carl sat beside 
the stake and used the burner to heat the spot just ahead of the spot he 
was hammering. When the one was cooled, the adjacent area was hot and he 
rotated it under his hammer progressively. It didn't take him all that 
long to pull that disk down well past a hemisphere and a vase shape to 
end up with a hollow standing figure! That old man was hot!
When raising, it's probably fastest to sink the middle on the first 
annealing and them raise ( crowd) the edges in to complete the shape.
I could go on...but that's probably more than anybody wanted to 
hear....at least it distracts from the election................Pete F

PlumDon at aol.com wrote:

>I have been trying without any success, to get some 8" hemispheres in  
>14-16g. Many of the architectural ironworks suppliers offer an 8" hemi in 11g.  I'll 
>bet they all get them from the same place and prices range from $15-26. But  
>having no luck finding anything in the lighter gauges. None of these have  
>responded to my request for lighter gauge hemis. 
> 
>Anyone have any ideas on where to get some or a quick way to make them?  
>Banging them out of a 10" disc with big wooden and leather mallets is too much  
>beating of an arm already in bad shape. Plus they come out a bit lumpy. 
> 
>Much appreciate any ideas. 
> 
>Don Plummer
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