Changing radius

[Brad Hall]

I have joined sides that I need to radius. I do have a MegaMold that I used for my first KMG Kit. My current scratch build calls for a 25' radius on the top. The profile sanding bar on the MM is at 15'. How would I go about making an accessory to attach to the bar for the different radius. I'm assuming if I knew the height at the center above a 22" straight line I could scribe an are using a piece of flexible binding. I have no clue how to determine that distance. Would one of you mathematical genius ' please help an old guy out? Thanks.

[John Parchem]

What you are looking for is the sagitta of an arc. Here is a lutherie page that describes it and includes a calculator.

For a 25' radius and a 22" chord that would be .202 inch.

http://liutaiomottola.com/formulae/sag.htm

[Brad Hall]

Thank you. Surfing won out over math for me in high school. Mom was right...I'll need it some day.

[Kevin in California]

John, I think the Sagitta of the Arc can be found somewhere in France? Sheesh, I've never even heard that term before. I wasn't surfing, but I wasn't studying my math either! Way to go John!

[ken cierp]

Just a suggestion and simple solution --- build the guitar with a 15' back, the plan you are using is simply an extrapolation, variation and an interpretation of other guitars good and bad of similar size. In other words there is no president for the end product to be some kind exceptional instrument, so you certainly can fearlessly apply personal design and artistic license. In fact that is part of the fun with this great hobby. Perhaps the higher dome will have the effect of improving resonance? Structurally it will certainly make the back a little stronger. Just a thought.