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I sand a radius into the bridge, on top of the guitar it is going on.
This guitar that I have been working on for well over a year- the top has flattened.
And before you say humidify- my shop is manually humidity regulated @ 45% with dehumidifier and humidifier (its almost never too dry). Pondering a solution, and I have one (sand the bridge radius on another like-box in process), I have some concerns:

1. When clamping a radiused bridge, the caul under the bridge ideally would have a matching radius.

2. My clamping caul, which spans the braces, is not radius-able in the area of the bridge plate; the caul must compensate for the thickness of the bridge plate (and it does), hence it is not in range of the sanding surface of the radius dish.

3. As stated above, the bridge plate area of the caul is not radiused, only the extended footprint, beyond the X-braces, is radiused.

WWOLTD (what would Original Luther's Forum do)?

I have pondered this radius-ing/ clamping bridge thing in the past but didn't have a top-gone-flat. What are ya'll's thoughts?

And thanks in advance for not starting a discussion on humidity, it seems irrelevant at this point.

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Measure Twice,

Karl Borum

Glue up a caul from pre-radiused parts? Or, use separate cauls and clamps for each section. I use a vacuum clamp to glue bridges so that I don’t have to use a caul inside. I make enough instruments that the vacuum clamp has been well worth the expense.

I’m not clear on whether you plan to fit the bridge to the actual shape of the top or force the top to conform to the pre-radiused bridge. My vote is to fit the bridge to the top before gluing.

I haven't decided, so I don't have a clear plan, but those are the two options (clamp bridge flat or radius it then clamp it). THe ship has sailed on making a caul from radiused parts, although I could press a styrofoam imprint and cobble one up- I probably wont do that, but it is another option.

I suppose a .110" thk hard maple bridge plate can conform to a radius. My other bridge plates were rosewood, and those tops have a nice radius.

I'm leaning towards doing a slight bridge radius on another boxbthat s on the bench, with same radius. I could radius the wings and leave the area over the bridge plate flat- that is sounding like a good plan.

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Measure Twice,

Karl Borum

I sand my bridge plates to radius in my dish before I glue them into the top. I radius my braces. With both radiused, my top is uniformly radiused. I sand the bottom of the bridge to the same radius. The bridge usually fits perfectly. I seldom have to adjust the fit and when I do, it’s pretty easy.

Ya know, this may seem irresponsible to some folks, but you can go without the caul if your bridge fits the top well and you are careful with the clamps so that they don’t crush anything inside. I use three of the StewMac bridge clamps with the stabilizer leg, with no caul, and it all works fine. Of course, my bridges are radiused to fit the tops; no daylight. So, there is no need to crunch down too hard on the clamps. You only need extra ooomph if something needs to be persuaded into position, which I don’t like to do in this operation.

Perhaps lay some sandpaper on the bridge area of the guitar you’re working on and sand the bridge to match the guitar, and just make a new caul?

Karl I always when building used a radiused caul in the box and I even radiused my bridge plates.

In the repair world we frequently clamp a radiused bridge on a radiused top but use a flat caul inside. Never had a bridge lift yet knock on Adi that we reglued.

So as you can see there is a case here for it may not matter all that much.

Keep in mind that for a 25’ radiused top the height of the radius for a 6” bridge is only 0.015”. I’ve always used flat cauls to glue the bridge with no issues. I do radius the bottom of the bridge on a small convex radius dish, unfortunately I don’t remember where I got it but they’re probably still available somewhere.

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Steve Smith
"Music is what feelings sound like"

Thanks for the replies. I'll radius the bridge on another box and clamp it with the caul I have, which is radiused on the far ends but flat on the bridge plate. A radiused bridge plate might prevent the top from flattening in subsequent builds. Perhaps a flat plate is what pulls a top-in-white flat over time. 

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Measure Twice,

Karl Borum

FWIW…

I made a process change a bit ago. I now sand a radius into the bridge plate and use a flat caul, rather than using a radiused caul, and it made some problems go away.

Save yourselves a heap of trouble. Put a piece of MDF in your radius dish (mine has sandpaper in it, I'm assuming yours does too) and put a radius on it. Then glue some sandpaper to the mdf.
Clamp it in your vice and use that to sand the radius into your bridge. Like a surrogate top.
It is much safer than getting sandpaper near your finished top once you're ready to put a radius on it.
And easier too.

That is one of the process changes I made as well…

Domed radius dishes, you specify the radius from 20' to 42'. This is what I use. Quick/easy and don't have to worry about scratching the top.

https://www.luthiersuppliers.com/lsproducts/domed-radius-dishes

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Steve Smith
"Music is what feelings sound like"

Thanks for that link Steve, FREE SHIPPING, I ordered one.

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Measure Twice,

Karl Borum

I do the same thing--I glue up with vacuum, but the fit before gluing is always spot on.

Dave

And, to radius the bottom of my bridges, I sanded a block to radius in my dish and then fixed between the belt and the platten on my belt sander. I sand the bridge bottom before I contour the top so I have something to hold onto while I sand it. Yes, vacuum is worth the investment.