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I'm getting close to attaching the bridge of my second dread build; 28' radius spruce top.
Do you guys radius the bridge?
And if you do, is your bridge caul radiused as well?
I can see the merits of radiusing both the bridge and the caul, but not the bridge alone.
The bridge plate has already assumed the radius of the top, as the top is curved directly above it.
What are your thoughts on this?

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Measure Twice,

Karl Borum

Well, I'm certainly not the expert, but I radius both my bridge and bridge plate before installation. I don't use a caul (I've used a Fox style clamp and I'm back to using vacuum now), but if I did, I'd radius it as well.

I have a radius dish that I use with a go bar deck to install bracing--I use that with sandpaper to radius the plate. Then I use the top itself with sandpaper to radius the bridge.

Dave

I radius the bridge, I don't radius the bridge plate or the bridge caul. The bridge caul that fits under the bridge plate is usually 1/2" or 3/4" ply with a 1/16" layer of cork on top.

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Steve Smith
"Music is what feelings sound like"

I radius both the bridge plate and the bridge. I use a Fox-style bridge clamp so no caul is used.

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

It seems some people radius the bridge and some don't. Either method gets the job done.
Me, I do the radius. Since I go through the process of putting a radius on the top I might as well
do the bridge as well as the clamping caul, when not using vacuum. To me, this is a guarantee
that the bridge to top joint is perfect. As if... anything is ever perfect. Others choose a different
method and they work fine as well. In the end you'll decide which process works best for you.

I like a perfect fit and I’m lazy. So, I sand the top of the bridgeplate to the top radius in the radius dish (25’ for me). I made a block with a 25’ radius by sanding in the radius dish. I put this under the belt in my belt sander to sand the bottom of the bridge to the radius. Usually, this makes my bridge fit perfectly with no further work. I glue it with a vacuum clamp. But, since the bridgeplate is still flat on the bottom, a curved caul would not be necessary. I do make the bridgeplate thick enough to be fairly rigid such that the sandwich of the thin part of the bridge where the pins go, the top, and the bridgeplate is absolutely stiff. I want no looseness where the strings drive the top—anything lost there will not be heard.

Left field comment: I do sand the bridge bottom to fit the top of the guitar. I do it the way most folks do it, with sandpaper on the top. But I don’t like using an inflexible caul inside the guitar for gluing on the bridge. I use three independent clamps to glue on a bridge: one over the main body of the bridge, and one over each wing. I would rather get my good fit between the top and the bridge by letting those two things conform to each other via direct clamping, with an expanded pad for the bottom leg of each clamp to spread the pressure out a bit. To me, a large, inflexible caul can interfere with a tight fit between the top and the bridge, which is the goal.

One thing I've done a lot for cauls on radiused surfaces is to use pieces of stiff insulation foam glued to a plywood backing, sanded to basically fit the radius as the caul. It's really quick and easy to sand to the radius on the foam, but it still has enough "give" to make up for any little irregularities that inevitably show up in real life.

Dave

Gluing surfaces with differing curves/shapes seems a bit counter-intuitive, so I've always radiused both bridge bottom and top of caul.
Talking steel string here.
I have read some classical builders want to introduce some tension into the soundboard by gluing a curved bridge onto a flatter top.
Also I know one professional (SS) who glued a flat bridge to a radiused top with a flat caul.
Different strokes....

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

"Also I know one professional who glued a flat bridge to a radiused top with a flat caul."

According to Romanillos, Torres sometimes radiused the bottom of the bridge and sometimes glued it on flat. I have also done it both ways. I don't radius tops too much on guitars, so this has not been a problem.

Thanks everyone for the input! It was very informative.
My method of work going forward:

1. Radius the bridge
2. Radius my cauls on radius dish
3. Line cauls with soft cork

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Measure Twice,

Karl Borum

Like the idea of a radius under the belt, I take it was a fairly thin piece of wood

Sent from my SM-A715F using Tapatalk

I missed the original "radius block under the belt" . Thanks for re-pointing that out.

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Measure Twice,

Karl Borum

The other thing you can do for the inside clamping caul is 'kerf' it. Put like 4 saw kerfs in it to make a flexi caul.

Keep in mind that on the top with a 28' radius the vertical change over 3" is only 0.025". So, the difference between the ends of the bridge (typically 6" long) and the center of the bridge is only going to be 0.025". The reason why I just put 1/16" (0.0625") cork on my bridge clamping cauls.

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Steve Smith
"Music is what feelings sound like"

I made the radius block for the belt sander out of a piece of 3/4” plywood. I anchored it with coat hanger wire.

Went to transfer the top radius to the bridge (psa sandpaper to the top) and lo and behold, the radius on my top is gone. If anything, the top is ever so slightly lower ( couple of thousandths) at the middle/ center of the bridge position, indicated by the steel straightedge. One problem solved (radius on bridge). Not sure where my radius went (new prob to resolve). Possibly a new thread. Humidity meter going in-hole tonight. THank you for all your answers to the bridge/ caul radius question.

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Measure Twice,

Karl Borum

Karl—

You live in Missouri, which means you probably have been heating your home here recently. That makes the relative humidity in your home go down. It is no surprise that your guitar would react.

Getting on top of relative humidity was something I wish I had done sooner in my building.

+1 with of piece of packing tape on the caul surface so that glue squeezing through locating holes doesn’t glue the caul to the guitar.


Sent from my iPhone using Tapatalk

yep - don't forget the packing tape

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Steve Smith
"Music is what feelings sound like"