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The first article I saw on clamping bridges with a vacuum clamp showed a somewhat elaborate process of casting foam plastic to the shape of each bridge for a caul. Is that really necessary?

I use this one for guitar and a homemade smaller one for ukes. No caul is used on top and none needed inside .

https://youtu.be/YCGncLvgxp4


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No caul is necessary. You just need to make sure with vacuum clamping that no air pockets are created -- you want the vacuum to spread all throughout equally. And that should n't be an issue at all with a bridge clamp. The beauty of vacuum clamping is that you don't need any cauls at all and it is also extremely fast - you can get you pieces clamped in seconds.

Only thing, for me anyway, the maximum clamping pressure is less than 15 pounds/square inch - just sounds a bit on the low side to me, quite a bit less pressure than clamps.
The fit of bridge to top would have to be pretty perfect.

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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.

I had some issues with the diaphragm on my LMI vacuum caul staining lacquer. It penetrated and had to be sanded out. Now I use a piece of thin mylar cut to fit inside the gasket as an overlay to keep the diaphragm off of the finish. LMI was aware of the issue and said that their new material would not cause such problems, but I use the mylar anyway.

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Good judgement comes from experience. Experience comes from poor judgement.

The difference is in the way the clamping force is distributed across the part. A clamp presents a point load on part and pressure radiates out from that, so as you move away from the clamp, pressure falls off quickly, so we us lots of clamps to make up for that. A vacuum clamp applies the pressure evenly across the entire surface, it's like having an infinite number of clamps, so in reality you don't need as much clamping pressure as you're not relying on it to spread from a single point of a clamp.

15 psi may not sound like a lot, but consider this; a 10" x 10" area has 1500 pounds setting on top of it (10 x 10" x 15 psi) . That's equivalent to approximately a 10" x 10" x 4 ft piece of steel.

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Jim Watts
http://jameswattsguitars.com

Yeah, I remember the first systems. They said you had to mix up a batch of a special foam to cast a mold of your bridge, and the casting was used as a caul over the bridge and then the vacuum clamp went over that. AND you had to cast a new caul for every different bridge you used.

And a 1.25 x 6.25" bridge would only have 117lbs total on it. A single cam clamp can exert a force of 330 lb.

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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.

What's the force provided by a go-bar

About 8 lb per

Pat

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Pat

I made a caul similar to LMI, and I couldn’t get it leak free, so I went REALLY simple and cheap.

Ditto to Toonces above!

Personally, vacuum clamping with hhg seems TOO natural. The clamping pressure is enough to dent the bridge (I mention this in the vid after 2:12), and the vacuum dries the glue so fast! If you have a compressor, no reason for clamping, and every reason to trash the go bar. Which I did. I have a jig for top and back braces as well.

Go to 2:12 to see the caul in action.
https://youtu.be/i_yE4EbHORk.

The bridge on my personal guitar was clamped under vacuum, and it’s holding up well after five years. A lot of people like Simon, who build full time, also use vacuum to clamp bridges routinely. Based on that and my own first hand experience, I’m perfectly comfortable using vacuum.

My experience with the LMI clamp was similar to Casey’s though. The membrane left a dark silhouette around the bridge, which (thankfully) went away on its own over time. They sent me some new membrane material to replace it with for the future, but I haven’t had the opportunity to use it yet.


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"A vacuum clamp applies the pressure evenly across the entire surface, it's like having an infinite number of clamps, so in reality you don't need as much clamping pressure as you're not relying on it to spread from a single point of a clamp. "
Over time, I've found that I do not need as much clamping force as I used to think was nessesary.