how to measure a dome...

Yesun Tege · Post by **Yesun Tege** » Mon Jun 06, 2005 5:27 pm

ok so i got some steel for my helm. 14ga mild steel. as you may have seen from my other thread im going to be piecing it together via curved cone like shapes. i know how WIDE i will need each cone to be but i have no idea how to figure out how long each peice will need to be in order to have them bend into a nice bowl shape when i peice them together. any one know how to do this?

Halberds · Post by **Halberds** » Mon Jun 06, 2005 7:46 pm

Yes, now is the time to make your cardboard patterns.
Lots of duct tape and cutting ahead for you little pilgrim.

Best of luck on your quest,

Hal

Yesun Tege · Post by **Yesun Tege** » Mon Jun 06, 2005 9:40 pm

im 98 percent sure theres a geometric formula that tells me exactly how long it needs to be. it woul dbe finding the measure (not angle) of the arc using the radius. the radius i get from the circumfrance around my head, picture it as a hemisphere....

god forbid.. those damned math teachers were right

Halberds · Post by **Halberds** » Mon Jun 06, 2005 9:51 pm

Yes their is. Apprentice # 3 wrote a program that uses the ellipse of the circumference.
You input the size and how tall and the program figures the pattern.

Great little spangen cal. program. It was based on my "Norman Spangen" pattern and band sizes.

Best of luck on your quest pilgrim.

Hal

jodi · Post by **jodi** » Tue Jun 07, 2005 6:46 am

I'm not so sure on your construction techniques using these 'cones' but in silversmithing where if you wanted to create a dome (or helmet) you would cut a circle with a diameter equal to the widest point of the desired dome, plus the total height. You would then dome (dish) the circle then raise it using various raising steaks. i don't know, but someone may be able to tell me, if this is the way they did it back in the good old days before fancy welding equipment and the likes. This technique is fairly 'rule of thumb' but i think it works.

This doesn't realy answer your question - and sorry if you know this already!

x

RalphS · Post by **RalphS** » Wed Jun 08, 2005 4:42 am

If you just want to stick to a perfect hemisphere, the math is simple.

I'll assume you'll use 4 panels. For a diameter D, each panel should be pi*D/4 wide at the bottom, and pi*D/4 long at each standing side, and pi*D/4 high in the middle.

This is going to be quite difficult in 2 dimensions, which means that you'll have to dish the pieces to 3 dimensions (surprise!).

But the important thing is that the sides of the cone-triangles will have to be curved, so that the length along the curved side is correct. The height will have to be shorter in the undished piece, and should be stretched during dishing to get the desired length.

For a different shape, things get more difficult.

How well this works depends on your dishing skill/style, and material thickness. Do as Hal says. Make an accurate pattern for the pieces, cut out in cardboard, and see where things go wrong. Adjust till you're happy with the pattern. Now do the same in plate, and once again, adjust the patterns till you're happy. Expect to lose a panel or two if you're not familiar. It's better to cut or grind off a bit than to weld a strip in, so add a moderate amount of margin for your first panels.