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I still struggle with trying to get a perfect gap-free match between the bridge and top. I spend a lot of time sanding the bridge on sand paper on the guitar top and I think I get it perfect (both by looking and testing by trying to slide a corner of a piece of paper under all around) but then after gluing it on I always seem to find at least one corner a little off the surface which I can see by looking at a very low angle. I can also slide a piece of paper about 1/4" under the corner. I don't think it matters structurally so I just fill it with glue mixed with the bridge wood dust. The gap I'm talking about is so small that it never gets noticed but the fact that I have to do this bugs me.

The ends of the bridge seem to be the trickiest part to get right down to the surface when sanding against the top. So now I always leave the bridge a little long when sanding and trim off the edges after. When I'm done the sanding, I remove the finish from the top in the bridge area with my rotary tool with router base and I also put a shallow rabbet around the bottom of the bridge since I leave finish going about 1/16" under the bridge. Since it seems perfect after my sanding I think something must be happening in the finish removal, bridge bottom rabbeting steps.

I've read and heard of some guys that don't try to sand the bottom of the bridge to conform with the top, i.e. they start with a perfectly flat bridge. They say they have no problem with gaps. I don't understand how this work out since the top is not flat.

Does anyone else struggle with this? Can anyone offer any suggestions for me to get this perfect?

Thanks,
Pat

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1) Make a sanding block especially for the bridge. I use a perfectly seasoned piece of lumber that has been sanded in my radius dish and then topped with 3M Stickit paper. This is better than using the actual soundboard to sand the bridge.

2) Consider switching to vacuum clamping. Perfectly even pressure everywhere. I use hide glue with a slightly warmed bridge. Glue for 12 minutes, remove the clamp, clean the squeeze out, and then clamp for 1 hour.


I have never struggled with bridge joints and have always done it like this. One other thing, I remove the finish by scoring with a razor blade and removing the finish with a chisel. Make sure you have a freshly sanded surface.

I contour the bridge to fit the top and then scrape/sand the middle of the bridge until there is a slight gap visible. Then when I clamp it down, the wings tend to come down first and there is less chance of a gap at the ends.

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Pat, you're prep work sounds prefect, it has to be your clamping setup.
I've had the same problem occasionally, and I could usually tell as soon as I took the clamps off.
You need to have cauls on the inside and outside that match the curvature of the top, otherwise clamping pressure can distort the top and compromise the fit of the bridge. I've had this problem every time I've tried to use bolts through the outside pin holes to clamp the bridge. I used large washers on the inside instead of a caul, and I believe that's the cause.

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I glue a flat bridge to a 10' radius top. Probably flattens the top just enough to compensate for string tension. Works...

Hi Pat
How you clamp the bridge can have an affect on how it conforms to the top of the guitar. Toonces # 2 comment (above) about getting perfectly even clamping pressure is a good one. I have 2 different bridges that I use. One is very much like a Martin bridge and I use a LMI clamping jig that uses the outside string holes to hold the bridge and jig down. I hand tighten those and then use the outside jig screws to hold the wings down. My other bridge is an SJ type where the front of the bridge is curved. The string holes are also curved in the reverse direction. Using the LMI bridge clamp results in some gaps under the bridge because of the way the clamping pressure is applied. So for this bridge I use 3 bridge clamps to hold the bridge down.
I would suggest doing a dry clamping of the bridge and then checking with your paper to find any gaps. Then try another clamping method and see if or where you have gaps. Find one that works for your bridge. You can also try different clamping pressures on the various screws or clamps used.
I also use a dedicated sanding block for the bridge and then touch it up on the guitar top. I use LMI glue.

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+1. Alternatively, Luthier Suppliers (forum sponsor) sells really cool convex mini-radius dishes if you have trouble making one.

Thanks for the suggestions. The clamp I use is the two bolts through the outside bridge pin holes along with the screws that press down on the wings. I've just been using large washers on the inside. I'll try a caul instead. I'll also do some experimenting with different clamping methods. I will also make a separate radius sanding block as has been suggested. That sounds like a better idea than sanding right on top of the guitar. I may also start scraping/sanding the middle of the bridge a bit to encourage better contact at the ends.

Pat

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There are three kinds of people:

Those that make things happen,
those that watch things happen,
and those that wondered what happened.

I had some wing issues that were from not having the finish squarely removed in the corners. Even though I cut a small ledge on the underside of the bridge it still hung up. Really watch those corners.

The other thing is clamping. With the clamps that bolt through the pin holes or are clamped to the bridge and have screw down wing clamps pressure on the wings can ease pressure on the center of the bridge unless you use an extremely rigid clamp. I went that route for a while but drifted back to what I used at first and use now. It's been the most reliable for me.



I radius the bridge on a dedicated convex sanding block, have a radiused caul, and cut about a 1/32 ledge around the bridge to clear the small area of finish left just inside the footprint. I usually do test routs on scrap until the depth just clears the finish.

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Pat, what I've found (at least with classical bridges) is that the water based glue we use tends to curl the bridge up at the thin ends due to the water swelling the bottom of the bridge. Extra care as to clamping these areas helps, and although I haven't tried it, wetting the top of the bridge at those points when gluing might balance out the swelling,
Mike

Fitting and gluing on a bridge well is not an easy thing by any means. Lots of folks including me struggle with it until we learn a method that works well every time.

Everything that I am going to post here I learned from Dave Collins back when I was an apprentice. The method for clamping is Dave's too and I have never heard or seen anyone else doing this quite this way. It's brilliant I will add and was a mind blower for me the first time that I saw it done and then later did it myself. Now hundreds of bridges later it's how we work day in and day out and other then some set-backs when using Fish glue it's been providing flawless results over and over again.

My first question to the good folks participating in this thread is do you use HHG for bridges? If not it could be why I did not originally use HHG for my creations either... I did not believe that I had the chops and method to have everything in place clamp wise and snugged down in 15 - 20 seconds time. The following method can easily permit even a slow poke to get it done in around 10 seconds effortlessly, no drama, and correctly the first time. It also lets you use HHG with no risk of premature... jelling.

First we like the Ibex clamps and the bridge caul from Stew-Mac. As you can see in the pic this caul has been modified with the addition of a dimple to help locate the clamp in the center or near the rear of the bridge. The white nylon thing pops off and IIRC a 1/4" bit drills a nice recess for the clamp. This caul also has two holes that are no longer used but once were from the days when I pinned my bridges in place with holes though the saddle slot.

Fitting aside, which is uber important to have NO gaps because the bridge is one of the most important braces on a guitar. I'll address fitting later if anyone is interested.

Assuming that your bridge fits well enough to be down everywhere with no gaps with only moderate finger pressure (that's our standard) the bridge is positioned and then a "well" of sorts is made out of masking tape butted up to the bridge on three sides. The purpose for the tape and well is to prevent the bridge from sliding around on the wet glue and to know that you have it exactly where you want it.

Dry runs are your friend, mine too... So step one for us when the bridge fits well is to using a proper caul inside the guitar and the Ibex clamp and SM caul over the bridge to dry clamp it, position it, make your tape well.

Now here is where things get different and the beauty of the method. Once the bridge is clamped in place in a dry run and the well is made, I use two layers of masking tape because I can... (it makes the well easier to locate when there is still lots of glue and squeeze out). Notice the stack of blocks under the middle of the clamp?

They are positioned and built up to be in contact with the underside of the clamp. Then masking tape is applied as shown using the the blocks as a fulcrum of sorts.... Making sense yet?

This permits us to preposition the clamp, the caul inside the guitar which is also held in place by the taped clamp and with the masking tape well slapping the bridge down in the exact position that we want it will be effortless.

I like to put my bridges (be mindful of any exotic flammables used for inlay if you have an inlaid bridge) in the micro wave for 10 - 15 seconds which further extends the open time of the HHG.

Once the bridge is preheated, the well is prepared, the dry runs have been done, the bridge fits well with only moderate finger pressure the HHG is slathered on (be sure to use wax paper over the caul inside the guitar....)(you can skip this step and have an extra thick bridge plate too...). The warm bridge is slapped down, wiggled into the well, the clamping caul is placed over the bridge and the Ibex clamp is tightened down with the wing clamps still up. Once the Ibex is snug the wings are lowered and then the Ibex is snugged one last time and so too are the wings.

Then the tape can be removed from everywhere (it's never a good idea to leave any masking tape on fish very long) and in removing the tape most of the squeeze out comes up too if you wait about two minutes to do this. The guitar is hung and the next day we take off the clamp and set her up.

Again the time to complete once everything is in place can only be about 10 seconds.

I have heard of people making the underside of fingerboards slightly hollow to compensate for the effects of the water in the glue causing swelling.

I use and think the convex sanding block with the planed radius is a WAY better idea than using the top on a new guitar.

A full length stiff internal caul is critical if using wing press screws. If each wing has its own clamp, not so much.

Nice method Hesh and Dave!

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Pat

Here is one of the things that makes me glad I built my CNC, as I can pretty accurately hold to a specific radius on the bottom of the bridge, which will match the radius in the top extremely closely. I also relieve the underside edge of the bridge by about 1/16" or so and let the finish actually go under the bridge by that much. There is always going to be some deviation of the top's curve based on humidity and temperature conditions in your shop. The trick is to control everything as tightly as possible so that any adjustments that are needed are minimal.

Another big help to the bridge glue joint is to keep the top wood as pristine as possible. Some may not like what I'm about to say, but here goes...

The best thing you can do for this joint is to not scrape or route off the finish.
That's right... don't do that.
Instead, make a slightly under-sized pattern of your bridge, and use it to lay out the location of the bridge on the top after the neck is set, and use Friskit film in that location. You can use the undersized bridge as the template to cut the film. Then after you finish, simply use the undersized bridge to score a line through the finish around the edge of the Friskit film, and simply peel it off. The film doesn't leave any residue, so the wood surface is ready for gluing. No sanding, no scraping, no routing.

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I found the bridge wing is actually quite flexible, and often if you don't have pressure there it will pop up (don't rely on the wing's stiffness to keep it glued to the soundboard). So when gluing bridges I would use about 5 clamps, 3 Stewmac mini cam clamps (they are light and reaches further than standard 4" cams, so it doesn't distort the soundboard) in the middle since the mini cam clamps are fairly limited in the amount of pressure it can apply. Then I use those 6" extended C clamps on the wings (it can exert much more pressure than mini cam clamps) on the wings, making a caul to fit the bridge wings perfectly. This usually does the trick.

If you can try to tape the caul to the underside of the bridge plate BEFORE clamping or you'll spend 15 minutes struggling to get that caul onto the underside of the bridge (a job made hard because you can't see what you are doing). By that time the hide glue or even titebond will have set. You really need 2 hands to install the cam clamps because if it doesn't latch well you'll end up pushing things with those cam clamps rather than clamping (meaning you'll struggle trying to keep the bridge from sliding off)

Guide pins is a MUST. Use the existing bridge pin holes if you must but make sure you drill holes onto your caul to make space for the pins.

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Pat,

Using cauls instead of washers is a huge gain.

Not that long ago, I switched from a 'universal' caul, to a model specific caul, and it helped a lot.

Dry fitting as well is very helpful.

You also don't need a perfect clamp free match. If it goes down under moderate pressure, it will stay down. Finger pressure...

Don,
I like the idea of using the Frisket film on new builds, but it is of no use on bridge re-glues on guitars thay have had their bridges previously glued directly to the finish. On these, unfortunately, scraping or routing is the only option unless you decide to repeat the error of re-gluing to the finish(yech).

Regarding fitting a bridge I used to use the radiused form that some are speaking of here and pre-radius my bridges to a set radius, the target radius for the top. But I learned over time that at least my tops are never exactly the same radius as what my dish is or what I intended the radius to be. Once a top is braced, voiced as a free plate the radius may not be the 25' radius that I was shooting for so my bridge needed to be fitted to the exact top anyway.

After that I did what my toot that I wrote said and that was taping 120 grit to the top over the bridge patch and sanding the bridge on the actual top. That worked great but took more time than what I do now.

These days since I am gluing on a bridge around every other day.... and often on anything from a Martin where a flat bridge may be fine to a Taylor GS Mini with a profound top radius requiring the bridge be radiused I simply get close with the belt sander, know your platen... and then use a single edge razor blade to make adjustments as needed for a moderate finger pressure perfect fit.

What I like about what I do now is that the single edge razor blade is also perfect for helping have a superior glue joint in so much as a freshly scraped surface has more joint energy than a sanded surface. It's also pretty fast and one learns to read... the bridge fit just like we learn to read the neck joint. If I have a slight gap I scrape everywhere else and the gap goes away. Most bridge woods respond very well to a single edged razor blade too.

Most of you guys are working on new builds, CraigG and I have to deal with what presents itself in our shops and lives. This can at times be guitars with very distorted tops, wood missing from the bridge patch, curled bridge wings from the removal process, etc. As such in the repair world we have to do a lot of unnatural acts to get things to fit well.

Frank Ford said in a previous thread on this site that he uses the biggest clamp he can fit and clamps the hell out of the things. That works too and needs to be mentioned.

A very related topic is bridge design. Some designs are not very serviceable and may have elements what will be issues in time when the bridge lifts and it likely will someday too - bridges lifting are a pretty common thing. Very shallow wings can be problematic in that they curl with age and what it takes to get that curl out when regluing may require replacing the bridge in that there is not enough meat to sand flat. Lots of inlay on bridges can be an issue too since we use heat to remove a bridge, or a Big Bertha gold driver.... Dave has been known to use a hatchet and I will post that video again here for all to enjoy.

For those who have seen this video before my apologies, for everyone else enjoy!

https://youtu.be/t5Q4iUBXNEM

Hmmmmmmmmmmm Can i buy the " Bridge Tomahawk" from stewmac ?

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On final bridge fitting: use PSA sandpaper cut to fit the bridge footprint. Mark the underside of the bridge with pencil marks right up to the edges. When the pencil marks disappear, you can be prety sure that the fit is exact. I use HHG and a vaccum clamp which I heartily recommend. Preheating the bridge extends the working time. After 5 minutes of clamping, I remove the clamp and clean up the squeezeout, then replace the clamp.

When using a vacuum clamp on reglues, what is your method for sealing bridge pin and UST holes?

Hesh,
I have always used multiple clamps and no bridge plate caul.
With the single Ibex clamp and SM bridge caul do you use radiused bridge plate cauls?

For reglues: Masking tape across the top of the bridge covering all of the holes and the slot.