15' Radius

[ken cierp]

How do you intend to contour the rim?

[NY_Keith]

Ken,

That's what really brought up the question.

I'm making a radius sanding bar, and as I was laying it out, I realized that I may have made an error in my back bracing. Basically using your rim construction part 3 article.

[John Parchem]

So, since I just finished my linings and I don't have my rims contoured, I guess I could just carve off my bracing and start again correctly. Best option?

I would consider taking off the 7.5 ft braces, given that you have not profiled the rims. 7.5' is a big difference from the plan you are building to. Basically from the sound of it, this was mistaking radius and diameter. It is really easy to carve off the braces and replace them with the originally planned radius, I think much easier than to work around this mistake through the rest of the build process.

I have used 10' radius on a guitar and am using a 10' radius on my current classical guitar build. Amazingly most do not notice the smaller radius. Before I bend the sides I profile the sides to very close to the correct shape so that I have enough height where the waist is. Before bending, the profile I cut for a 15' radius vs a 10' is different.

Another way to make a 15' arc is as follows. You can use a rod a bit longer than a 2' and place a couple of nails in a board such that they are close to the ends of the rod. In my following example the nails are 2' apart. Draw a straight line from one nail to the other. Up from the center of that line mark the Sagitta of a 15' radius arc formed between the two nails. You can use the following calculator to get the sagitta. http://liutaiomottola.com/formulae/sag.htm. Now to form the arc bend the rod against the nails on the ends up to the marked point. That is a curve you want. To make life easy you could have placed a nail at the marked point as well.

For example if the nails were 2' apart into the calculator I would enter 180 (15*12) for a 15' radius and 12 ((2*12)/2) for length. The sagitta in that case is .4 inches. So if you bend a rod supported by nails 2' apart from the center .4 inches the rod would form a guide to draw the 15 radius arc.

[NY_Keith]

Thanks for that link John.

I made my 15' radius bar and contoured the back rim. I'll rebrace the back this weekend.