Official Luthiers Forum!

Hi,

Is a clamping caul really needed for inside the guitar when gluing the bridge back on? I'm using deep throat clamps and I've put a thick layer of cork on them where they'll be contacting the bridge pad. Do people usually make a caul and if so, why is it important? I'm guessing it's about spreading the pressure but it doesn't seem like very much clamping pressure is neccessary anyway? If I do decide to make a caul, what's the best way to do it? It needs to span the x brace? does it need to be notched for the x brace or can it just sit on top?

This is for an Ibanez x-braced acoustic guitar.

Thanks!

Yes the caul is necessary and exactly for the reason you anticipated to spread things out. It need only be a very simple shape that fits between the X-brace and has a modified side for the first tone bar. StewMac used to sell one that was made out of material that glue does not stick to. I've used this one hundreds of times and I don't have to cover it with waxed paper (so the glue does not stick).

We set-up our clamps to hold the caul in place when using HHG and time is limited. You can used Titebond original if you are not tooled-up and knowledgable about HHG it's unforgiving in terms of open time and bridges are one of the most difficult things to glue because of the time required to get the clamps in place. I don't recommend HHG unless someone is comfortable and knowledgeable about it's use.

Frank Ford RIP once posted on this forum that he finds the biggest clamp he can find and clamps the hell out of the bridge. Now after reading this you tell me if this caul is necessary... It is you do not want to crack the bridge plate and I will remind you and everyone else that a bridge plate has six or twelve holes in it all in a line making it just asking for the perfect uneven pressure to crack the sucker along the pin holes....

You can tape with masking tape the caul in place and then when the clamps are in position they will hold it. Be sure to remove it when done too

* Just checked StewMac they no longer sell the non-stick caul and I'm home so I can't take a pic of mine that is in our shop in Ann Arbor. What I like the most about my caul besides it's non stick for Titebond and HHG is it's about 1/2" thick, made of cutting board like material and can slightly flex.

Your guitar has a slight dome on the top and I like a clamping caul to not be rigidly flat but able to slightly conform to the curve and dome of the top. The caul I speak of can do that. If you have an old cutting board of non-stick material it would be perfect to cut up.

But you can make one of our plywood or just plain wood just cover it in waxed paper so it becomes non-stick too. It should cover the entire bridge plate.

The height of the caul is also necessary to clear the X-brace intersection with the clamps. Not all C-type-clamps have an adjustable lower contact point and need to clear the X-intersection.

The bridge patch top side should be expanded and cleared of finish very close to the bridge perimeter. The bridge patch needs to be cleaned up of all old glue and finish and so too does the bridge bottom. I scrape my bridge bottoms to conform to the top with only minimal finger pressure required so the bridge is sitting all the way down everywhere especially the wings.

You may need two or three clamps too to get it to sit down everywhere and this is normal although again a well fit bridge can be done with one clamp with a topside caul with adjustable wing hold downs.

If using Titebond original or HHG wait 24 hours before putting strings on. Keep clamps in place at least 4 hours I usually leave them for 12 - 24 hours.

You can make a two layer masking tape well around three sides of the bridge making positioning it for gluing a snap AND it removes with the removal of the tape 5 minutes after the clamps are set it removes 3/4 of the squeeze out in one easy operation.

Bridge reglues are not overly difficult but they are unforgiving in positioning things, clamping and glue open time. Because of string tension this is one of the most critical glue joints on the instrument.

Lastly so why did it happen? This is the time of year when acoustic guitars that are not humidified shed bridges. If a dry guitar is the culprit I would remedy that before I reglued the bridge so that the top is not a moving target for the new bridge glue joint.

But to your original question is the caul necessary - yes.

_________________
Ann Arbor Guitars

I agree with Hesh, you need a caul to distribute the clamping pressure and as he says it keeps you from clamping against the X brace. I do a fair number of bridge reglues so I too the time to build a caul out of UHMW that will adjust to various hole spacings. There are two slotted holes that can put 3/16 bolts thru to locate the bridge relative to the pin holes. The bolts apply some clamping pressure, but I also use three deep clamps on the center and wings.

You will possibly get some glue squeeze out in the pin holes, the UHMW keeps the caul from being glued to the inside of the guitar (embarrassing). If the bolts get glued into the holes I just reach inside with a box end wrench and unscrew them. Drill out the holes and ream to fit your pins

Since reading about the stuff here, I've been making all my cauls from HDPE. Nothing sticks to it, the greatest stuff. Here's a link to what I bought from some small shop online.

HDPE (High Density Polyethylene) Sheet, Opaque Off-White, Standard Tolerance, ASTM D4976-245, 0.750" Thickness, 12" Width, 12" Length https://a.co/d/inTNxNB


Pierre
Guitares Torvisse

_________________
Pierre Castonguay

Finally remembered to take a pic of my most used bridge regluing caul.

We have two drawers are bridge regluing cauls of all shapes and sizes for all manner of guitars but this one, this simple caul gets used by me most of the time.

I don't remember if I bought this from LMI RIP or SM but I did buy it. But you can make your own from cutting board material.

The two things that make this caul a winner for me is it's ever so slightly flexible and can conform to a top dome and glue, the glues we use HHG and Titebond Original won't stick to it.

Now if you can remember to not let it go out the door on the inside of a guitar you're all set.

_________________
Ann Arbor Guitars

And don't nobody try that bright idea about using double-sided tape to hold a wood caul in place against the bridgeplate. It's serious work to pry the caul off after that tape has been clamped down hard, it'll do wonders for one's vocabulary.

_________________
Peter Havriluk

Thanks y'all, I went ahead and made a very basic caul for the inside of the guitar. I wanna make a better topside caul for next time. And wanna get better clamps as well!

Thank you!

Peter's comment reminded me of the crack I made in a top trying to pry the caul off.

Ha! Been there...once. I now put two 1/4" x 1/4" or so pieces of doublestick to hold it in place until I can clamp. Works for me and comes off with little effort.