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Hi guys

I'm making an 'OM' style guitar with a 17.5' back radius.

I do not have a radius dish and was wondering if you could give me any tips on how to get the correct side profile.

Thanks for your help.

Mat

You need to make a radius board. Take a 2x4 and arch it using a 17.5 radius, then glue sandpaper to it. My first guitar was done this way and worked great. It takes longer is less than perfect but works smashingly.

A quick way of establishing the arc is a pencil and a tethered tape measure, one swipe across the lower edge of the board and you have your template to cut on the bandsaw. Cut it out, sand on a tabletop belt sander and glue on your sandpaper. An fresh sanding belt cut up from the hardware store and some contact glue and you'll have it made in a short short.

With the rim set in the mold, start sanding with your radius board. Pencil witness lines will tell you when you've arched the whole rim. Sand back and forth in line with the board, then side to side in a circular motion. You can even hold one end still on the rim and sand the opposite side in pendulum fashion. Just keep it moving until all your witness lines fade away. I used this method until getting some dishes, it works.

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http://www.dickeyguitars.com

Adding to Bruce's post, you could also place a 1/2" pipe inside the mold where you want the peak of the radius. Then drill a hole in the center of the 2x4 radius board. Keeping a constant location for the apex of the radius will make a more consistent bowl shape I think.

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Formerly known as Adaboy.......

I think I might be able to use a combination of both methods; Cumpiano for the shape and the suggested sanding stick to radius the edges for a tight back to side join.

Many thanks for your help

Mat

Cut a big piece of styrofoam to the inside contour of the guitar, shape that to the radius (very easy because its so soft), stick it inside the guitar, trace the contour to the guitar sides, then shape. Refine with a sanding platen like mentioned.

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Expectation is the source of all misery; comparison the thief of joy.
http://redrivercanoe.ca/

douglas what a great idea....why the hell have i not thought of that.........maybe i should not be making guitars when i cant think of somthing that simple....lol.......does that styrofoam glue together with ca or tite bond????...usually i do this job by using a spokeshave till it looks close then sanding with my dish.....but this foamy stuff could save some time....nice one.....

Having a problem visualizing this... was good up to tracing the contour to the guitar sides. Can you clarify? I presume the styrofoam is sticking up proud of the rims like a little dome? how are you tracing to the sides? thanks!

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"Everyone wants to BE something, but nobody wants to BECOME something" - William Cumpiano

You could use contour maps of a sphere shape.....available here:

http://www.liutaiomottola.com/construction/CMap.htm

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Formerly known as Adaboy.......

Draw a plan view of the top of the guitar body. Mark around it every inch from tail to heel.

Below it (lined up vertically)draw a crossectional view from heel to tail, use a radius template to connect heel and tail with the desired radius

project from the marks around the body on the plan down to the crossection view

Make a template from thin card for the side using the heights from the crossection view(minus top and back thicknesses) and the lengths from the plan view .
This so far just gives a shape radiused from tail to heel and flat across.

Now for each point, measure the distance across the guitar, set up a ruler with this distance across the radius template and measure the offset at the centre. Deduct this amount from the side template height. This gives you the side height with the domed back.

Much easier to do than it sounds