Looking for some help repairing a guitar

[Guitar man Jr.]

As of currently, we've been tasked with repairing an acoustic guitar, a Guild JF 55-12 which came with a horribly bent top due to the stain from its strings. We managed to remove the top, but its beyond repair so we have to make a new one. This is where I need your help: Apparently the original top had a curve to accommodate for the force of the strings. Is this true (and if yes, what is the curve's radius) or have we been misinformed? Any feedback is greatly appreciated.
Edit: I am aware of the fact that guitars have a bit of a curve on their top, I'm asking if the specific guitar model has a unique curve, because if It does we'll have to make a custom radius dish for it. Sorry for not being more specific.

[YamahaGuy]

Many "flat top" guitars' tops are not flat. The average guitar is supposed to have a slight belly up in the lower bout (from the bottom edge to the bridge). Taylor laminate guitars 100 and 200 series have a rounded back to them for added strength. I don't know how rounded the guitar you mentioned is supposed to be. My friend just fixed an older Harmony from the 70s with humidity and a bridge doctor.

[cyclistbrian]

The top almost certainly had a slight radius to it. The back will as well which should give you the sense of it. If you kept the top see if you can cleanly remove the braces. They'll conform to the top shape.

There are a lot of things that need to be right on a guitar top. You have to consider not just the strength and quality of the work but also the tone of the final product and that it can play in tune. This gets you into consideration of top thickness, the number size shape and placement of the braces, and where to place the bridge. Hopefully you kept what was left of the top as a reference.

I'm guessing you got roped into this because you do wood working. If that's correct and you have confidence in you skills in that regard the rest is research and math. There are a lot of resources out there and people on the forum have built guitars. They may chime in.

[sinistral]

The belly caused by the stress of the strings is different from the dish or radius of the top of the guitar. As noted, flat-top guitars aren’t actually flat. The tops have a slight dish or radius to them. Stew-Mac and other luthier supply houses sell radius dishes for bracing acoustic guitars. The common way for luthiers to glue braces to the top with enough tension to maintain the radius is with go-bars, and go-bar deck (Martin and some luthiers use a termerature-controlled, high-pressure vacuum). The braces of course need to be radiused to conform to the radius of the underside of the top. Check out the radius sander Goodall uses at this point in the video (and subsequent gluing of the top):

https://youtu.be/gS78naDiB4k?si=ea2cnhleK2XeBf8W&t=781

If this is a customer’s guitar, and you’ve never done this type of repair before, I would think twice about undertaking it without a lot of prep work, proper tools and some practice.

[Fathand]

You do not need a radius dish (they didnt exist 50 years ago) or a go bar deck (saves time though)

Make a radiused sanding stick to desired radius, sand the bottom of your braces to it. Clamp your braces one or 2 at a time with cauls to protect the top. The top will take the radius from the braces.

https://www.flickr.com/photos/194462...7720296629122/

Now use the sanding stick to clean up the edges and make them conform to the top radius.

https://www.flickr.com/photos/194462...7720296629122/

[Earl49]

Many flat top guitars' tops are not truly flat. Numbers like a 40 foot or 50 foot radius seem to come up regularly. A curved surface is much stronger than a flat one in any material. I cannot speak to whether there is something unique about that Guild model. The bellying at the bridge is another matter due to local stress -- torque at the bridge from string tension.