Most of the pieces of the guitar are glued together, so it might not seem at first that lengthy and detailed instructions would be needed for gluing on the bridge of the flattop acoustic guitar. But placement of the bridge is crucial if the guitar is to play in tune, and considerable stress is exerted on the glue joint of the guitar bridge by the tension of the strings. Detailed instructions are presented for locating the bridge. And more detailed instructions are included for removing the finish from the top in preparation for gluing the bridge down. Finally, instructions for gluing the bridge on a typical acoustic guitar are presented. Also included on this page is a discussion of design considerations related to the gluing of the bridge, for those designing non-conventional instruments.
Initially appeared: June 15, 2009
Last updated: September 11, 2018
For conventional guitars (that is, for guitars of conventional dimensions that have a round normal size soundhole located on the centerline of the top, at the end of the fingerboard) there is not much that needs to be considered in terms of locating and gluing the bridge when you are in the process of designing the guitar. For these instruments the bridge can be glued on using three cam clamps (bottom left in the picture) or three or four special purpose bridge clamps (top left in the picture) inserted through the soundhole. The instructions on this page detail how to go about doing that. But if you are designing an instrument that has the hole in another location or that is of a shape that does not permit easy insertion of clamps, or if your instrument is a bass where the bridge is a considerable distance from the soundhole, it is a very wise idea to consider how you are going to clamp the bridge for gluing well before you get to that step in the assembly process. It is possible in a lot of cases to clamp using standard clamps through the soundhole. The instrument I am using to demonstrate bridge gluing is my Ipomoea steel string guitar, which has an offset soundhole. But the guitar designer should be aware that configurations do exist that do not encourage through the hole clamping. Be aware of this possibility and plan accordingly for an alternate method for clamping.
Fortunately there is a type of clamp that can be used to clamp down the bridge for gluing in most any design, the vacuum bridge clamp frame, on the right side of the picture above. This is a device which clamps by vacuum from the top of the instrument only, so you don't need to get a clamp inside. But the device is pretty expensive and you'll need a vacuum pump and some fittings to use it. Again, this page does not detail using the vacuum clamping frame, but for designers of unconventional instruments it is good to know about the existence of this device.
The bridge should be completely built and sanded smooth before it is glued to the top. Making the bridge is not covered on this page, but details of the final fitting are provided here.
The strings of a steel string guitar are under approximately 150 lb. of tension. Although the ball ends of the strings in conventional guitar bridges are wedged up under the bridge plate inside the guitar, the path the strings take out of the bridge pin holes and up to the saddle causes them to exert considerable force on the bridge, pulling it toward the neck. There are also pinless bridges like the one I am using on this page, and on these all the force of the strings is pulling directly on the bridge. The glue that attaches the bridge to the top keeps it in place, but the glue joint must be strong enough to do this job and also to withstand impacts which can exert many times the static force exerted by string tension. The glue used in guitar construction is more than strong enough for this job, assuming a good glue joint. In order to get a good glue joint we must be sure the gluing surfaces, the place on the top where the bridge will be attached and the underside of the bridge, are well mated to each other. During the bridge gluing process we'll do a number of things to help make sure the glue joint will be a strong one.
To this end, the first thing we do is to sand the bottom of the bridge so in conforms to the doming of the top. The tops (and backs) of most modern instruments are domed, having been assembled on a dished form. To be sure the bottom surface of the bridge is dished to mate to the doming of the top, the bottom of the bridge is sanded on a domed sanding board that has the same radius of doming as the top of the guitar has. This sanding board is a shop made tool, made by running one surface of a piece of scrap wood about 2" thick and 10" long back and forth across the same sandpapered dished board used to build the top. After the board is domed in this manner, stick on sandpaper of 80 grit is applied to the domed surface. Of course, if the top of your instrument is flat and not domed, then you'll have no need for the domed sanding board and can just use a flat sanding board instead.
The bottom of the bridge is marked up with pencil marks so you can check sanding progress. Then the bridge is rubbed back and forth over the sanding board, until the pencil marks are all gone.
This is all I do to prepare the bottom surface of the bridge for gluing, but some folks will refine this surface even more, sanding it to finer grits or scraping it with a scraper. Other folks will actually fit the bridge to the top, using chalk and a scraper. This process involves dusting some chalk on the top where the bridge will go and then placing the bridge gently into position. When the bridge is lifted, chalk will mark the high points. These are scraped down with the scraper, and the process is repeated. Each time this is done the area covered by the high point(s) widens, until finally one time the whole bottom of the bridge is covered in chalk, indicating that the two surfaces are perfectly mated. By the way, chalk fitting is a definite necessity for instruments where the top is domed in an ad hoc fashion. A number of instruments like early guitars, lutes, and ouds are made like this. But for modern guitars with tops built in a dished board I have never found anything more than sanding to 80 grit to shape the bridge bottom to the dome of the top to be necessary. Everyone has a different take on such things so I do want to mention what other folks do here.
After the bottom of the bridge is surfaced, the bottom edges should be lightly chamfered or filleted (rounded over), using either a scraper or a bit of 100 grit sandpaper. A fillet of about 1/64" radius is ideal. This is done to help the bridge make better contact with the top after the finish is removed from the top where the bridge will be glued. The finish has thickness and we want the bridge to be able to sit down onto the bare wood, instead of being supported by a ridge of finish around the edges. More on this later.
The location of the bridge is absolutely critical. If the distance from the nut, down the centerline of the fingerboard, to the center of the bridge saddle slot is not within about 1/32" of where it should be the guitar will not play in tune, and it will take some extreme measures to rework it so it does. So bridge placement is an operation which should be taken very seriously. You'll want to get this right the first time. A variation of the old carpenter's rule of thumb about measuring twice and cutting once is appropriate here. As you will see, I am going to recommend measuring three times before actually committing the bridge to glue.
Anyone who is making more than one instrument on the same plan will want to consider making a bridge location jig to make this process a little more mindless. A simple piece of plastic or MDF in the shape of the fingerboard but extending in length to past the bridge, with a couple of pins that fit into the saddle slot or a few small pieces of wood that engage the front and sides of the bridge is worth making if you plan to make more than one instance of an instrument. Folks that make instruments on a production basis will also usually have other jigs and tools that aid in the bridge location and gluing process.
The first thing to do in the process of locating the bridge on the top is to mark the centerline of the bridge on its underside. This is easy to do by measurement if the bridge is basically symmetrical about its centerline. Since all the surfaces of the bridge which will show in the completed instrument are at this point sanded and polished, I don't like to mar those surfaces with marks. So to make the centerline apparent on the front (the surface toward the end of the fingerboard) and the back (the surface toward the tail end of the guitar) I'll cut two little arrow heads of masking tape that point to the centerline and stick them on. The front of the bridge looks like this:
and the back of the bridge looks like this:
With good construction technique and some luck, the centerline of the fingerboard should be aligned with the centerline of the top of the body. This is easily checked with a long straight edge. In this case the bridge can be laterally oriented by placing it on the centerline of the top so that the two tape arrow heads point at the centerline. But in those cases where the center of the fingerboard is not aligned with the top centerline, you'll need to extend and transfer the centerline of the fingerboard onto the top, and use that line as the center of the bridge. I like to make that mark when necessary using masking tape as well, since the top is already completely finished at this point in assembly.
Once the bridge is located on the fingerboard centerline it needs to be slid into position at the appropriate distance from the nut. For this and some subsequent measurements the roughed out nut should be inserted in the nut slot.
If you haven't made the nut yet, just wedge a small scrap of wood into the nut slot to serve as a stop for the end of the ruler. The long ruler is butt up against the nut and aligned with the centerline of the fingerboard. Slide the bridge into position so that the front-to-back center of the bridge saddle slot is located at the distance from the nut that is the sum of the nominal scale length of your instrument plus the nominal compensation value.
If you are working from a full sized plan, the compensated saddle position should be indicated on the plan, and you can just take the distance from the nut to the compensated saddle position directly from the plan. On my plans there is a red reference line perpendicular to the instrument centerline that is the nominal bridge position. The angled red line beneath that is the compensated saddle position.
Other plans will have something similar. If you are working instead from dimensions, look for something like "distance from nut to bridge saddle at centerline" or some such. The instrument in the pictures has a 25.5" scale length and an additional 0.125" of compensation at the centerline. So the front-to-back center of the saddle slot is located 25.625" from the nut.
In theory, and assuming the saddle slot was accurately cut when the bridge was made, the bridge is now located at the proper position. But I like to double check the locations of the ends of the saddle slot to be sure that there is the correct compensation there. Here, two more measurements are made. From the full size plan the distance from the point of intersection between the nut and the left side edge of the fingerboard, and the intersection of the extension of that edge of the fingerboard and the front-to-back center of the saddle slot is taken. I know, that description is difficult to parse, so let me describe how to take the measurement on the instrument. Lay the long ruler flat on the fingerboard so that it is butt up against the nut and one edge is aligned with the left side edge of the fingerboard. Take the measurement to the front-to-back center of the saddle slot, as shown. When you do this, lay the ruler down so you do not disturb the location of the bridge.
That measurement should be the same as appears on the full size plan. With the ruler still in place, take a short ruler and measure from the edge of the long ruler to the end of the saddle slot, like this:
Make a note of that distance. Now, do the same measurements along the right side edge of the fingerboard, again being careful not to disturb the bridge. The long measurement should be the same as that on the full sized plan, and the distance to the end of the saddle slot on this side should be the same (or at least close) to that on the other side. If everything checks out OK, great. But if not, take a look at the differences between the measured values and what they are supposed to be and consider how the bridge must be moved in order to get things to where they are supposed to be. Sometimes a small rotation of the bridge is all that's needed.
Moving the bridge around at this point is tricky, and it is real easy to move it too much. About the best technique I've found for moving it in small increments is to lay a finger on the top, next to the bridge and then roll that finger into the bridge, nudging it a bit. Needless to say (but I'll say it anyway) since bridge location is so critical, if you do need to fine tune the location of the bridge on the top you should then go back and take all of the location measurements, starting with the alignment along the fingerboard centerline. By repeated small movement and repeated checking you should be able to get things in the right place. If you can't it is a good idea to step back and assess the problem. As mentioned, if the measured distances from the nut are off by much more than 1/32" it will be difficult or impossible to get the instrument to play in tune.
If you just don't have the data to check the nut to saddle lengths along both edges of the fingerboard, and if you are sure the saddle slot was angled and cut correctly, and if the bridge itself is symmetrical about the centerline, there is another way you can check lateral placement of the bridge. Lay the long ruler on the fingerboard so it butts up to the nut at one edge of the fingerboard. Measure from here to the tip of the bridge wing on that side, like this:
Now take the same measurement on the other side of the fingerboard to the other tip of the bridge wing. It should be the same. If not, reposition the bridge until these measurements are the same. Once again, each time you move the bridge, go back and do all the positioning checks.
When the bridge is finally correctly positioned it is a good idea to tape it down with two pieces of tape, one at the end of each of the wings. Lay the heel of your hand on the top of the guitar near the bridge, then gently drop your fingers down onto the top of the bridge to hold it in position. Then apply the tape with the other hand. Be aware that, being carved thin, the bridge wings tend to bend up a little. If you push down hard on the end of one of the wings while taping it may pop the bridge up, messing up its positioning. If this happens, repeat the bridge location process.
At this point the bridge should be correctly positioned and lightly taped in place. Since the following steps will scar the top, now is the time to re-check all the bridge positioning measurements.
With the fingers of one hand holding the bridge in position, the finish on the top can be scribed with the outline of the bridge. The knife is held at a 45 degree angle and because of the fillet on the bottom edge of the bridge, the score line will actually be a tiny bit under the bridge.
You want to scribe the finish ever so lightly, enough to leave a line you can see in raking light, but not so deep that is cuts anywhere near all the way through the finish. We take this very light cut for two reasons. Heavily scoring the finish here may cause it to chip, especially under bridge clamping pressure. And, if we screw up and don't have the bridge position correct we can still buff out the lightly scribed lines, something we could not do if they were deeply scored.
Be very careful when turning the corners around the tips of the wings. The knife tends to skid out into the top here. Scribe all the way around the bridge, except where the tape is on the ends of the wings. Now, with the fingers of one hand holding the bridge in position, lift the tape on one end with the other hand, and scribe around that end of the wing with your third hand. Replace the tape, and do the other wing end in like manner.
In a subsequent step we'll glue the bridge to the top, clamping it down in the process. Anytime you glue and clamp there is a danger of the piece shifting around and this is one place where even a little shifting can be a big problem. The most common method for trapping the bridge in place while gluing it down, at least for guitars built in a non-production environment, is to surround it with retaining walls made of masking tape. Production manufacturers will have jigs to position the bridge, or will pin it down to the top and bridge plate using small wood or plastic pins. These are good techniques if you plan on making more than one of an instrument but for small production or one up instruments, building the masking tape retaining walls works very well.
Start out by laying down some strips of masking tape on the top right up to the sides of the bridge. You will eventually build up walls of four to six layers of tape. The first strips you put down should be longer than the subsequent layers. That way, when it comes time to take the tape off you can just pull up the bottom layer and those on top of it will come off too.
Start putting down the tape next to the straight sections of the bridge at the front, and at the front and back of the wings. the bridge I am using in the pictures here has no straight line sections at all so I need to put the tape down in small pieces to follow the curves. Do not let the tape fold over the edges of the bridge. You want to build up walls that keep it in position, but we need to remove it for some other steps and if the tape rides up over the edges of the bridge it will be difficult to put the bridge back after those steps are complete. Likewise, don't put down the edge of the tape far away from the edge of the bridge.
After a couple of layers of tape are down around most of the bridge you can remove the piece of tape holding down one of the wings and build up a couple of layers of tape wall there. Then do the other wing. The wings tend to bend up a little from the top because they are so thin. If this happens, try to build up the tape walls high enough on the ends of the wings to trap them. Put down about four to six layers of tape all around the rest of the bridge. You'll find that it is not critical to tape around the tight outside corners of the wings or into the tight inside corners if any exist on your bridge, at least as far as trapping the bridge in position goes. But I highly recommend taking the time to do this tedious work, because the following step will be scraping the finish off the top inside the tape boundary, and it is very easy to scratch the top which doing this wherever it is exposed. So my recommendation is to build the tape walls completely around the perimeter of the bridge.
After you are done, try to slide the bridge around to check how tightly the tape walls retain its position. A little slop is inevitable, but you don't want it to be able to move around so much that it can be positioned so it uncovers one of the scribed lines on the top. If you find this to be the case, add more tape layers to whatever part of the wall that needs it. When the fit is satisfactory, remove the bridge and replace it again, to check to be sure that when you put it back in that it is completely in contact with the top, that is, that it is not sitting on top of tape anywhere.
You can remove the little pieces of tape that marked the centerline of the bridge at this point.
Since the bridge will be glued to the top, the finish on the top where the bridge will go has to be scraped off so we can get a nice wood-to-wood glue bond. The process is straight forward enough, but please be aware that it is very easy to mar the surface of the top while you do this operation, so take your time and do this carefully.
This is a point of no return. Everything we've done so far is reversible, but once the finish is scraped off we are committed to this location for the bridge. So it is time to take a third set of measurements to check to be sure the bridge is actually in the right location.
I like to scrape the finish off using a single edge razor blade with the edge turned to make it a little scraper. Run the edge of the blade along a scraper burnisher or a metal vise handle a few times to turn the edge.
You'll eventually need to scrape very close to those masking tape walls, and you want to avoid messing those walls up by pushing or pulling them with the scraper. I like to do the scraping in three steps. The first step is scraping from the front of the bridge into the middle of the bridge. When I'm done with this step I've scraped down to wood about 0.125" from the front, and into the middle of the bridge, along the whole length.
After this is done I scrap from the back of the bridge toward the middle in the same manner. The razor blade scrapers wear out quickly because the edge is so thin, so you may need more than one to do the job. This is one of those lutherie operations where different senses come into play. You want to scrape down to the wood, but you don't want to actually remove any wood if you can help it, and you certainly don't want to gouge up the top. As you scrape, watch the scrapings. The instrument in the pictures has been colored amber. As I scrape through the top clear layer of the finish the scrapings are white, but they turn to an amber tint as I get down to the color layers. As scraping proceeds and I get down to the sealer coats the scrapings lose their amber tint and start to pick up the color of raw spruce. And I know I have scraped just deep enough when the shavings both look like wood and smell like wood. It is the smell that hits me first, and I know I've scraped deep enough in that place.
At this point the finish should be scraped off, save for a border of about 0.125" all around. The third step is to reduce the size of that border to about 0.033". This requires careful work and much attention to be sure you do not scrape outside the scribed lines. As you get real close to the scribed lines I find it helps to use a plain single edge razor blade, one that doesn't have its edge turned.
Try to make the transition between the scraped and unscraped finish a smooth one. That is, you don't want to chisel straight down to the wood so the edge of the finish is square. Go for a nice smooth transition.
Run your fingers along the scraped surface to be sure it is not lumpy from uneven scraping. If it is, sand the surface a bit with 150 grit paper to make it smooth.
The subject of choosing the clamping method to use to glue the bridge on was mentioned in the section about design considerations. If you will be using clamps that go inside the sound hole, you'll need two or three for the main, thick part of the bridge. The wings of the typical bridge are so thin and flexible that they can be clamped with clamps that also go into the sound hole, or with clamps that simply clamp against the back of the body. You have to be careful when using clamps this way as you can easily crack the back with over zealous tightening of the clamps. In the example here I am using two special purpose bridge clamps for the main part of the bridge and two "standard" luthiers cam clamps with 8" depth to clamp down the wings.
The first thing to do is to get cauls ready. With the clamping arrangement I'm demonstrating, four cauls are used - one inside the guitar under the bridge plate (directly under the bridge), one on top of the thick central part of the bridge, and two small cauls for the tops of the wings. The bridge plate caul is shaped just like the bridge plate but a little smaller all around. It is thick enough so the clamp pads can bear on it and clear the big braces. For the guitar in the pictures it only has to be 0.25" thick to do that, but for a typical X braced acoustic it would have to be about 0.75" thick. The caul for the top of the central part of the bridge should be the same size or a bit smaller than that part, and about 0.25" thick. The cauls for the tops of the wings should be smaller in foot print all around than the flat part of the wings and about 0.75" to 1" high. It is important to make these small enough so they don't put a sharp dent in the wing where it slopes up to the central part of the bridge. The caul should only touch the flat part of the wing. If the bridge has pyramids or other embellishments on the wings, the cauls should be padded with cork or rubber so they don't crush these features. If you use the type of special bridge clamps that I am using in the pictures, you'll also need two small cauls to go under the leveling screws on the clamps.
The reason the foot print of the wing cauls must be smaller than the foot print of the bridge wings is that, after the bridge is glued we'll need to be able to get at the inside edge between the top and the bridge to scrape out glue squeeze out. If the caul overhangs the wings we won't be able to get in there with a little scraper. this is the same reason these cauls need to be so high - to keep the pad of the clamp from preventing us from getting into the inside edge between the top and the sides of the wings.
As with most gluing operations, it is best to do a dry run first to make sure everything will go smoothly once glue is applied. Line the sound hole with masking tape to protect its edges from being banged up with the insertion and removal of the clamps. You can also put down some protective cardboard between the sound hole and the bridge area to protect the top from the clamps too. Being prone to screw ups of this sort I usually do this, but did not for these pictures so it is more apparent what is going on. Double sided tape is applied to one side of the bridge plate caul and the adhesive is exposed. If you are using the polyester double sided tape used to stick down router templates, stick this on your shirt a few times to pick up some lint and reduce the tack of the adhesive. If you don't do this, it may be really hard to remove the caul after you take the clamps out, and you may have to chisel it off through the sound hole (ask me how I know this). Here's the inside caul ready to be inserted through the sound hole and stuck onto the bridge plate:
Now the bridge is dropped into place inside its retaining walls of masking tape. The caul for the central section of the bridge is placed on top, and the clamps are inserted through the sound hole one at a time.
The pads on the special bridge clamps are metal and can easily scratch the finish on the top. Notice I am holding the clamp with one finger under the pad to protect the top. Position the clamp so the pad is on the caul on top of the bridge near one end of the central part of the bridge. The opposite pad should be resting on the caul inside the guitar, but of course you can't see it. Tighten just enough to keep the clamp in place. if the clamp tries to tip up or down while you are tightening it, it is likely that the inside pad is not resting on the caul. This is usually due to a brace getting in the way and bearing on the arm of the clamp. If this happens, remove the clamp, fix the problem and try again. The other inside clamp is inserted in like manner, with its pad on the other end of the central part of the bridge. When both clamps are in place you can tighten them up. Again, be aware of any racking that would indicate the clamps pads are not resting squarely on the inside caul.
Now the remaining cauls are placed on the bridge wings and the wings are clamped with very light pressure, using the cam clamps. The bottom pads of the cam clamps are just resting on the back of the instrument. Here's the entire clamping setup:
Note that the special bridge clamps have screws along their length to support the weight of the clamps themselves. Once the clamps are in place these should be screwed down to cauls placed on the top, until they just touch the cauls.
I can't repeat enough that if you are clamping the wings down using cam clamps clamping right on the body as I am demonstrating here, that you don't want any more clamping pressure than is needed to get the wings in contact with the top. Any more pressure risks cracking the top or back, so if you can't control yourself, clamp the wings down by clamping through the soundhole too, with cauls to back them up.
If the dry run went smoothly it is time to commit to glue. Remove all the clamps and remove the bridge from its well. Leave the inside bridge plate caul in place. Take a scrap piece of spruce or other soft wood about 0.125" thick and carve a nice chisel edge on one or both ends. This will be used as a disposable non-marring glue clearing chisel to clean up the glue squeeze out.
Also have a damp rag on hand for the same purpose.
Before we apply the glue, let me take this opportunity to say a few words about optimum glue joints. The strength of a glue joint depends on a lot of factors, including the area of the joint and the thickness of the glue in the joint. Too little glue in the joint and the joint is said to be starved and won't be optimally strong. Too much glue can also lead to a weakened joint, but this is less of a problem, because we clamp the joints, squeezing out any excess glue. When we are doing rough work this squeeze out is not much of an issue - we wipe it up to no ill effect. But in a number of lutherie applications excessive glue squeeze out can make for a real mess that, at the very least, points out the builder's amateur status. This is one of those times and places where we want a minimum of squeeze out, because we are going to have to clean up any there is with this little stick and we risk getting glue on the sides of our nice bare wood bridge. We need enough glue for a strong joint, but we would like the minimum of squeeze out.
OK, apply a thin uniform layer of glue to the top, inside the masking tape retaining walls. Apply a thin uniform layer of glue to the bottom of the bridge. Put the bridge in place, and repeat all the caul and clamp placements of the dry run. In an ideal world there will be the tiniest bit of squeeze out all around the perimeter of the bridge once all the clamps are tightened.
Now wait 15 to 20 minutes for the glue to dry to the leathery stage. Remove all the tape retaining walls without disturbing the clamps and the bridge, and start removing glue squeeze out with the little wood chisel.
If the squeeze out is ideally leathery, it should chisel off very cleanly. If it begins to smear in any place, that indicates that the glue is still liquid there. So skip that spot and go on to another one. You'll soon see the correlation between the size of the squeeze out blobs and their state of leathery-ness - small blobs will be more leathery than the bigger ones. Don't bother with the ones that you now know are too big to chisel off without smearing.
If you have some long areas that have been chiseled off cleanly, you can go over those areas with the damp cloth to finish removing the last tiniest bit of glue in the inside edge. If not, wait until you've removed more of the squeeze out.
Wait for about five minutes, and then attempt to chisel up the smallest of the remaining glue blobs with the little wood chisel. Again, if they smear, wait another five minutes before trying again. Eventually all the glue blobs will be leathery enough to chisel off. After chiseling, wipe the joint line with the damp cloth.
Wait an hour before removing the clamps. Remember to remove the inside caul! All of the protective masking tape and cardboard can be removed from the top now too, as the bridge gluing job is now done.
As mentioned earlier, the glue joint is under a lot of stress in the finished instrument, so wait at least a day before stringing the guitar up, to allow time for the glue to thoroughly cure.
Did you remember to remove the bridge plate caul inside the guitar? Did I ever tell you about the single best improvement in tone I ever made to a guitar, which involved taking out the caul that I had inadvertently left in for a couple of months? If, like me, your memory is not what it used to be, you can always leave yourself a little note where you are sure to see it.
Yes, I do this more often than I care to admit!