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What do you do that's different one from the other for those that build both ways?

All braces for a flat top are flat on the bottom.

Most braces on a radiused top are curved on the bottom to the given radius. Some folks make the UFB's flat or radiused and some folks make some of the finger braces flat or radiused, and some folks make the bridge plate flat or radiused.

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Sweet... except that you can get a dome top with flat bracing....

If you think about it a bit -- the dome adds quite a bit of stiffness....
When you don't have the dome -- you gotta compensate with a bit of thickness....

How much? From what I can tell, it doesn't take much.... Less than 1/32" difference.... (Well... 1/32" is still a bunch... but you get the idea...)

Thanks

John

Right, I should have not thought of the obvious. When gluing flat braces for a flat top, use a flat board to back the plate. I have a 1-1/2" this plate to do this. It's really flat

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"Generosity is always different in the eye of the person who didn't receive anything, but who wanted some." Waddy Thomson

I know of a professional maker who creates a dome with flat braces he gently dries the top out under an anglepoise lamp with a 60 watt bulb for a day regularly turning it for even drying this drives out most residual moisture, and then as the last job of the day he glues up in the go bar deck, then once the top comes back to relative humidity it's domed. I'm not saying this is ideal practice, it seems very imprecise to me as to what radius dome you'd end up with, but that's how he does it and it works well for him.

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I don't scallop braces on flat tops (but don't always on domed tops). I also use a slightly bigger bridge plate, and move the 1st tone bar up until it's even with the corner of the bridge. I use 3/16" tall "soundhole" braces with flat tops, but just thin (.100") strips with domes.

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I am just getting started on my first guitar and have been wondering about this. If you are building with a radius top, does the underside of the bridge need to be radiused as well? If so, how is this done?

Most braces on a radiused top are curved on the bottom to the given radius. Some folks make the UFB's flat or radiused and some folks make some of the finger braces flat or radiused, and some folks make the bridge plate flat or radiused.[/quote]

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Well, some builders don't, they just flex the top to match the bottom of the bridge or visa versa. But this just introduces unnecessary stress on the glue joint.
IMO, you need the bottom of the bridge to be radiused the same as the top. There are a few ways to do this, but the best way would be to make a convex sanding block from the radius dish you used to glue the braces in with (if you did this) then clamp the block in a vise and sand the bottom of the bridge on it. Make the block from a piece of glued up 1-1/2" MDF and after sanding in the radius dish seal it with shellac.

The other way to get the bottom of the bridge matched to the radius of the top would be to tape a full sheet of sand paper (120 grit) to the top in the bridge location and with very little downward pressure twist the bridge back and forth to sand in the curve of the top. You don't want to push down on the bridge as it can distort the top and the bridge will not be matched to the shape of the top well.

You want this glue joint to be perfect and getting the curve on the bottom of the bridge as close as possible to that of the top.

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"There's really no wrong way, as long as the results are what's desired." Charles Fox

"We have to constantly remind ourselves what we're doing....No Luthier is putting a man on the moon!" Harry Fleishman

"Generosity is always different in the eye of the person who didn't receive anything, but who wanted some." Waddy Thomson

Or not.
I glue a flat bridge on a 15' radius top with HHG and it stays put. The bridge ends up assuming the radius. Works for me.
I may be bending a flat top over a 15' radius set of ribs soon, so we'll see what happens.

This is the way Martin did all their tops for a really long time.... like from the beginning until 20 or so years ago........ It works pretty well. Results are more consistent than you may think....

If you haven't already noticed -- there are a whole lot of right ways that work just fine.... In the end, the best thing you can do is to decide on a plan and try it out for yourself....

Thanks

John