Quote:
Originally Posted by Steeveedee
If you make a bend every inch on the perimeter of an 8 inch circle, that will be about 25 bends. 360º/25~14.4º. Strike 25 (maybe 24) rays out from the 8" circle onto the 18" circle, divided into 25 (maybe 24) pieces. That perimeter is about 57", so the marks should be 57"/25~2.8". You will have a 25sided cone. Not too shabby, I guess. Sort of round.
If your die isn't tapered (pretty likely) the bends on the 18" circle will be more noticeable, though.

Another train of thought...
1.5" die = .75 between bends on the small end
8*pi = 25.132
25.132/.75 = 33.5 bends
So call it 32 bends
360/32 = 11.25 degrees/bend
.75 * tan 11.25 = .1492 depth of bend (punch)
A template for both ends and one for the middle since your punch/die combo is not full length. For air bending, start with a depth of .135.140 and then see how the template matches in the first four bends, adjust from there.Might want to cut a strip 11/2" wide and experiment.
What kind of press are you working with? 10/20/30 ton HF hydraulic press or a press brake with air bending dies?
On edit,...
18 * pi = 56.5487
56.5487/32 = 1.7671
For quickie calculating, I'm assuming 8" and 18" are the neutral axis. If your shooting for an outside of 8" and 18", the neutral axis is the OD minus one plate thickness. For an ID, add one plate thickness for the neutral axis.
And, yes, start from the ends and work toward the middle.