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Hi Guys another newbie question. I've just thicknessed my front and back, and am just about to radius the back. I've got 2 books Kinkhead and Cumpiano using the offsets they give for the braces I've worked out the radius at each point and averaged it whith quite a large difference.

Cumpianos Back radius is 13'6"
Kinkheads Back radius is 17'9"

I've also done a search on this site for the radii you all use and have come up with a range between 12' and 20' averaging 16' overall. The 2 authors are both respected luthiers as well as you all here I wondered why the vast variation and what are the advantages either way for a large radii or small. My guess is the tighter the radius the stronger the back though I don't know if that's true/

I'd appreciate any clues or ideas you have and why you do what you do.

Thanks

John

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Formerly JJH

I learn more from my mistakes than my successes

Don't over think this and the radii that we end up when we brace a back AND tune a back rarely is exactly what we built it to be.

Many builders radius their braces for backs, to 15' and accept what results be it 14' or 16'. I use 15' for the back and 25' for tops. It's not as important in my view what specific radius that you use but that you understand that the tuning process and the spring back of the flat plates will to some degree change the radius that you intended to use. No worries though, no particular radius on earth has been proven to be the magic bullet.

At this stage of your building development I would recommend using a radius that permits you to take advantage of some of the off the shelf templates out there. Again 15' and 25' seem to be pretty common and I am building 4 body styles using these same radii.

I made my radius dishes some years back, but my quick and dirty routing templates were a little too 'dirty'... I was aiming for 15' / 25', but ended up at something like 22' for the top, the back I'm not sure; it is what it is, probably not 15’ anyways. I use these dishes for all my guitars, and I guess it is part of what makes them what they are. I can tweak brace layouts and profiles, soundboard graduations, wood species, but these radii always stay the same. So there is one constant I ‘know’, both in terms of the instrument’s geometry and, to some degree, what it brings to the palette of parameters that influence its sound. As long as you learn to work with it, get to know its properties and you know how to tweak it for different purposes I believe there is some leeway for what the radii can be and still make a ‘normal’ sounding steel string guitar.

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Rian Gitar og Mandolin

Definitely don’t get too bothered by the radii. Once your guitar is built it will change with the relative humidity. Of course the dome shape to the sides will not change but the top and back plates will. Guitars breathe in and out due to seasonal changes and the dome shape allows them the breath more freely than they would if they were left flat. In a way they are almost living beings.

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Rich S

"The inconvenience of poor quality will linger long after the thrill of a bargain has been forgotten"

Hi Guys, Wasn't gonna use dishes to do it in was just gonna shape the braces the clamp to the braces. Hope that'll work.

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Formerly JJH

I learn more from my mistakes than my successes

we usualy use the dish to profile the rims (sides) and the linings to the same profile as the braces , to continue the arc to the edges of the guitar ... Jody

http://bluescreekguitars.com/blog/
this tutorial may help you understand. As mentioned above don't over think this. The reason for this is more structural. If you think of this as jointing an edge you will be more on the right track. Once you glue the braces to the back you in essence created a hygrometer. The radius of the actual back and top will change as humidity does. You do want the braces to match the contour of the sides.
On the top you want to use a dish that matches you neck angle. I have talked to a number of my martin connections and they all have a bit of conflicting answers. The machining dept that guy that made the rig said they are using a 15 on the backs of the 000 and smaller and 20 on the dred and larger.
The top neck angle should match your radius. If you want to figure a 1/2 high action you use about 1 1/2 degree. This is what martin uses on the top. 2 degree ala Gibson you would want a 25. Fact is I used both in the past and I gravitated to the 28 and the process I use that is demonstrated on the blog.
It is good to see you are thinking things out for yourself. There are many good builders here. We all may do things a little differently but that doesn't me we disagree.
Have fun , that is what this is to be about
john
blues creek guitars

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

On my first acoustic I did exactly what you propose (clamped the back & top to radiused braces), but waited several weeks to close the box....to my consternation. Humidity changes had done their work, and it was difficult to get them fit to the rims.

I suggest that you close the box within a day or two if you go this route.

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The member formerly known as erikbojerik....