After doing the work that I have done with my plasma table, I've been trying to take it to the next level by shaping the cut plates as well. I managed to roll this 1/8" fire ring simply with a ring roller, and hammer; but how is it exactly that hemispheres are rolled? I'm feeling determined to learn how to properly shape fractions of a hemisphere so I can weld them together into a full sphere to use for a fire sphere or other purposes.
I know it's been a while since this post Chris, but for some reason it's been bugging me. I've been up all night with insomnia anyways, and I started thinking about this again. I guess because in the absence of any of us here having a 200 ton stamping press and 24" die, there must be other options.
The planishing hammer
@Tom O suggests is a great way. In fact it seems everyone who learns how to use one has as his first project a dome that is smaller, but very similar to the hemisphere you want. I've made lots of them for artistic projects, but never that big. It's a fun and easy project, often finished off at an English Wheel...although you can do this all on a planishing hammer and it will look great. BTW, lots of fabricators who buy an English Wheel for the nostalgic and flex factor find that even though it looks great on the shop floor, they can do almost everything on the hammer faster and almost as nice. Most "affordable" hammers tap out at 18 gauge steel. The issue I find is that as the dome size increases in diameter, it gets harder to control the symmetry of the dome. If you've ever hand hammered a dome on a shot bag, you'll know what I mean. Below is a video that shows Ron Cavell in a planishing class making a dome.
A second method I thought of is using a bead roller to run lines towards the center of the disk (see pic) like spokes in a wheel. This will stretch the disk along the lines, doming the circle towards the shape you want. Personally, I would stop the spokes before they intersect in the centre; this will leave a flat spot ideal for attaching a handle. This method can be done with a hammer and section of round rod like coppersmiths used to do. You hammer the round rod on top; underneath you use a piece of angle iron welded to some flat bar. You weld the angle iron so its opening is facing upright on the flat bar and it forms a "V" shape. You'll achieve the same result, but it will be a hand hammered version of a rolled bead...if that makes sense. Using this method will put a series of lines in your dome, which you may or may not like depending on how artistic you prefer your projects. Using a shrinker/stretcher tool on the edges will help bend the dome even more. Lastly, although I haven't tried it, a tipping die on the edge will add a nice lip and strengthen the dome considerably if you have access to one.
Two final considerations for either the planishing or bead rolling method:
1) Order of operations. Something I learned years ago the hard way was that with hand fabrication it's
a journey to a result rather than a process to the product. By that I mean unless you are very lucky, you'll probably not get the exact dimensions and look you originally intended. For that reason I make lids and domes first, so I can tweak the diameter of the cylinder it mates with on the slip roll or ring roller last. That's because it is easier to get it right on the slip roll than predict exactly how the metal lid will stretch on the bead roller or planishing hammer. When slip rolling you can creep up on the exact shape and can hand form small mistakes; but it's hard to go back when planishing to an exact size.
2) There are limits to the radius you can get. With planishing, if you tilt the sheet too far on a curved anvil, you'll get creases that need more and more work to planish out. A planishing hammer really only stretches—not shrinks—so you'll be constantly swapping from a radius anvil to just planishing (flat hammer and anvil) to make it look good. On the other hand, a bead roller can easily stretch so aggressively that you'll tear the metal trying to achieve the strong radius. In other words, with either of these methods you will get a nice dome, but it may not be as dramatic and uniform as you'll get with the 200 ton industrial press and die before to tear the metal.
