+1 to all that.johnnparchem wrote:A radius dish is very nice but if you want to make a tools with the correct arcs go to http://liutaiomottola.com/formulae/sag.htm.
It shows you how to draw and arc of a big radius. From there you can cut out the arc from a board and use it as a sanding surface for the radius.
I hate sanding so for a 15" radius I also have found I can first get really close with a plane by taking a brace and marking it mid point. I mentally and loosely divide each half in quarters do four strokes on the end quarters 2 strokes half way from the mid point of each half to the end and a stroke from the 3/4 point on each half to the end. I check that against a guide to see how I am doing matching the curve and adjust the strokes accordingly. It takes me a few rounds of this and I am loose enough from where I start that I smooth out any facets I may be making. You can see you are progressively taking less off as you approach the center. For the 25 ft radius I start with fewer and longer strokes making less of a curve.
I clean it up on a radius disk, but a sanding bar with the correct radius will work as well if not better.
a plane will do this pretty quickly:
Also - There are plenty of radius spreadsheets and pdfs on the interweb. see attached...
(edit - credits to Mario Proulx, Charles Tatton, and Steve Senseney for the video and pdfs)