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Hello friends
i have checked the saggita of an arc from following link. its really easy to use . i hope all of u see

http://www.handymath.com/cgi-bin/arc18.cgi?submit=Entry

my radius dish has been warped. for 15 foor radius it has warped to 12' and 25 has warped to 20.5
can we make the dish flat . it is not sealed by laquer or shellac.
or shall i use the 12 and 20 radius for back and top respectively?

or can i use 20 for back and make new 25' for soundboard?
waiting for ur advices
thanks in advance

I would suggest securing the dishes to a flat sub-straight which is always a good idea for any dish.

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Ken Cierp

http://www.kennethmichaelguitars.com/

Warping is pretty common.

My first dishes were 1/2" thick and warped pretty quickly. I laminated them to 3/4" birch ply, and that took care of it.

The next set was made from 3/4" MDF, and they've been fine.

My third set warped, but they seem to have flattened themselves back out.

A friend slices them every inch or so with his tablesaw (just enough to give them a little flex) before gluing them to ply.

What James and Ken said, a backer board of some kind is a must. Mine have 3/4" plywood discs laminated to the back side and they have been stable for years. Wish I could say the same....

When I made mine, I glued two pieces of 3/4" MDF together, before I routered the radius, and then brushed on three coats of oil based poly. They have stayed flat so far.

Alex

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"Indecision is the key to flexibility" .... Bumper sticker

Thanks friends for ur advice.

can i make a new 25' dish for top and use the warped 20 foot for back and and laminate them together?

I believe scooping out material from the center of a plate (even metal) disrupts the surface tension and makes the plate a little unstable. So in my opinion gluing two squirrelly dishes together is not a good idea. A flat sub-straight helps to combat the forces of nature.

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Ken Cierp

http://www.kennethmichaelguitars.com/

If you laminate them to some flat ply, they should go back to their original radius. If you have trouble flattening them back out, you can cut or rout some shallow kerfs in them like I described to help. They should be so deep that they go through the dish in the center, but just deep enough to help with the flex.

After attaching to a sub base I also seal mine with Zinnser's seal coat shellac.

Sent from my HTC Desire 626s using Tapatalk

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Another day, another dollar.