Instructions for building the Back plate of a modern flattop guitar. The instruments used in the example photos are classical guitars but the procedure is the same for the steel string guitar as well. Power tools (planer, jointer) are used in the examples but hand tools can be substituted as desired.
Initially appeared: March 15, 2010
Last updated: Tuesday, May 08, 2012
The process of joining the two pieces of the back together is similar to the same operation performed on the top. See Building the Top Plate of the Flattop Guitar for details on this. For the purpose of learning to build the guitar back from this page it is a good idea to have completely read the aforementioned page on building the top as many operations are similar and will not be discussed in as much detail here.
There is a construction decision that you must make before doing any work on the back, and that is to decide whether you intend to inlay the decorative back strip or laminate it between the back halves. Some folks maintain that inlaying the strip is the preferred method, but I have never seen any good reason not to laminate it in. In my own work I will do both, depending on the thickness of the back strip and my back halves. If all three pieces are the same thickness I will usually laminate the strip in. If the strip is either thicker or thinner than the back halves then I'll inlay it. I also laminate the strip in if I don't have a router bit that matches the width of the strip. It is possible to cut the channel for the strip in multiple passes, but I find that I often end up with a channel that is slightly wider than necessary. Another way to put this is that I often screw this up. So if I don't have a router bit that matches the width of the strip, I laminate it in.
In either case, the two halves of the back must be thinned to close to final thickness and then jointed and candled as for the guitar top. If you will inlay the back strip, the two halves are joined using a gluing board, same as for the top. Then the router is fitted with a bit that is as wide as the width of the back strip and the depth is set to cut a channel for the strip. Measurements are taken for placing a fence to one side of the centerline, so the router baseplate can run along the fence and cut the channel right down the centerline. The fence is just a board clamped down on top of the joined back. The channel routing operation looks like this:
If instead of inlaying you want to laminate the back strip in, you do that when the halves of the back are glued together. The back strip is simply glued in between the two halves. About the only thing you need to do differently when laminating in the back strip is to be very careful that the two halves of the back are very flat right at the seams with the strip. I do this by clamping a board across the halves right near the glue joints, like this:
You can see that the rest of the setup of the gluing board is the same as used for gluing the top halves together.
Now the back can be thinned to close to final thickness, which will be somewhere in the vicinity of 0.1" for most instruments. Get this info from the plan you are using. In my shop I use a drum thickness sander to do this. If you use a sander like this be sure to take very light passes when sanding the back, particularly when it is made of oily tropical wood like rosewood. If you take a deep pass you risk burning the plate and clogging up the sandpaper on the drum. Since the sander leaves noticeable grooves in the wood even with 80 grit paper I try to only sand the side of the back that will be inside the instrument. The outside surface will always have some hardened glue at the seam and one side may end up raised a bit at the seam, too. Imperfections like these can be quickly dealt with using a card or cabinet scraper before beginning the thinning process. After the back is thinned if I've obliterated my pencil markings I will write on the outside surface in pencil to indicate which is the outside face.
The next step is to mark the outline of the instrument back onto the blank. Lay the blank outside face up on the bench and then lay the mold on top, aligning the centerlines of the mold and the blank. It is often difficult to see the center seam of the blank, particularly if you've done a good jointing job, so if necessary mark the centerline with pencil now.
Since most of the work on the back will be done on the inside surface I like to mark the outline on that surface as well. On the outside surface, measure down from the top edge of the blank to the top of the outline. Flip the blank over and place a mark on the inside surface at that distance from the top edge. Then position the mold over the blank as before but this time also aligned with that top edge of the outline mark. Now the outline can be traced onto the inside of the blank with pencil.
If the instrument you are building has a cutaway (or if the outline is otherwise asymmetrical about the centerline) be sure the inside and outside drawings are mirror images of each other.
No matter how you installed the back strip, the center seam of the back is fairly delicate, so we generally reinforce it with a strip of cross grain spruce on the inside of the back. The reinforcing material is usually quarter sawn, about 0.5'' wide and about 0.125'' thick. You can prepare a bunch of it by resawing a quartersawn spruce board, thinning the pieces to 0.125'' thick, and then cutting those pieces into 0.5'' cross grain strips on the table saw, using the mitre gauge.
Another good source of center seam reinforcement strips is the top and bottom off cuts from the top. If, when you cut out the top, you took my advice to save those off cuts, you now have two pieces which look like this:
After these are squared off, they can be cut into long 0.5'' wide cross grain strips on the table saw using a thin push stick and a featherboard. I like using these because they end up nice and long. Here are some strips cut from top off cuts next to a bundle of strips cut from a board:
The strips are glued down right over the center seam. In the finished instrument they extend from the tail block right up to the neck block (or in the case of the classical I am building here, the Spanish foot). So the first thing to do is mark the edges of the blocks on the centerline of the back on the inside. You don't have to be exact, and in this case it is a good idea to make the marks so the reinforcing strip will be a bit longer than necessary. During assembly the ends will be trimmed so the fit between the ends of the strip and the blocks is tight.
After the end locations for the strip are marked, you can lay down some pieces of strip material on the center seam end to end to figure out how many pieces will be needed. I glue the reinforcing strip on using the go bar deck but without the dished work board for the back, that is, using a flat board to back up the back plate. First, a guide board is clamped down 0.25'' from the centerline. Then glue is applied to the strips and they are clamped down to the back right up against the guide board.
Then the guide board is removed. When the glue has had 15 or 20 minutes to dry, the glue squeeze out can be chiseled along both edges of the strip.
After the glue is dry the reinforcement strip is sanded to a rounded profile. I put down two layers of heavy brown paper tape on either side of the strip to protect the back from sanding scratches.
And then the profile is sanded using first 80 and then 100 grit sandpaper on a small block.
I like to keep the back blank rectangular as long as possible, but at this point it is convenient to cut out the shape of the plate. The outline can be cut out using the band saw, keeping a good distance (0.25'' or so) outside of the outline.
The bracing of the back of most guitars is done in a simple pattern of braces perpendicular to the centerline. This pattern is called ladder bracing, because the braces are oriented the same way that the rungs of a ladder are. The classical guitars I am building here have back braces like this:
There are three braces on the back of most classical guitars, and four on the back of most steel string guitars.
The blanks for the braces are made next. For this back I intend to do most of the shaping of the braces on the belt sander (the luthier's universal power tool) so I start off with ever-so-roughly quarter sawn spruce brace blanks. These are rectangular, the thickness of the finished braces and roughly the height of the finished braces, too. The blanks are sawn to the approximate length of (on this instrument) the three back braces.
The surfaces of the braces that will be glued to the back plate must be arched to the radius of the dome that the back will have. In general, back doming is more pronounced than top doming. The instrument I am working on here has a back dome radius of 15'. Although it is possible to fit the gluing surface of each brace by simply scrubbing it on the sandpapered dished work board, there is a lot of material to remove and so this would be quite time-consuming. So I'll begin by scribing the contour of the dished form to the brace.
There is a board sitting on top of the dished form, and the brace blank sits on top of that, right in the middle of the dish. The purpose of the board is just to get the blank high enough off the surface of the workboard to do the scribing. The compass is opened up so that the lead is just touching the bottom edge of the brace blank at the middle of the dish. I have a piece of heavy plastic drop cloth on the dish. This prevents my compass from being sanded as I drag it across the form to do the scribing.
After the line is scribed, the excess wood is removed using the belt sander.
Then the brace is scrubbed on the sandpaper covered dished board to finish up the contour.
The process is repeated for each brace. As mentioned, on the instruments pictured I am going to profile the braces on the belt sander. Although it is possible to also ramp the ends on the belt sander, I'll do the ramping with a chisel after the braces are glued to the back. Here is the brace shaping operation:
I'm just rolling each side of the brace blank on the belt sander to create a nice "parabolic" profile.
This process is pretty straight forward. In general, all brace location measurements should be referenced from a single point. The point on the centerline that is the neck end of the body is the one I usually use. As you take measurements from your plan, you can mark the locations of each brace on the inside of the back right on the reinforcement strip in light pencil.
You'l also want to mark the end locations of the braces. A carpenter's framing square is very useful for this process. It is big enough so you can reference the centerline anywhere on the plate, plus it is heavy enough to keep the plate flat on the bench.
The back braces will be glued to the plate along the entire length on each brace. That means we need to cut a slot in the reinforcement strip for each brace. Since the positions of the braces were marked right on the strip, the first step in slotting the strip is to cut through the strip at each brace position.
I use a Japanese razor saw and a block of scrap wood to do this. The block is used as a guide for the saw blade, to keep it perpendicular to the reinforcement strip. It is placed next to the strip at the marked position for one of the braces and held there by hand. Then the saw blade is squeezed to the block with the fingers of that same hand. The reinforcement strip is so thin that once you start the saw cut you are pretty much done with it. One very nice thing about the Japanese razor saw is that there are no sawtooth gullets at the very end of the blade. So by tilting the saw ever so slightly as you cut you can arrange the cut so that the un-toothed end of the blade just rests on the plate material as you are done with the cut. The result is a clean cut through the strip without marring the plate underneath.
I'll cut a single cut like this at each brace position. Then, using these cuts as a reference, marks can be made for the cuts that will bound the other side of the slot through the reinforcement strip. These cuts are made in the same way, using the block of wood to guide the saw blade. I try to make these second cuts so that the resulting slot will be a bit thinner than the thickness of the braces. This way the slots can be trimmed for a tight fit with a chisel.
After saw cuts are made for the slot, the waste wood is easily removed with a chisel.
Then you can attempt to fit the brace in position in the slot. If you made the slot slightly undersized it can now be trimmed to a perfect fit with the chisel.
When all braces fit snugly in their slots it is time to glue them to the back plate.
Note that in this picture I have not yet profiled the braces. A light coat of glue is applied to the bottom of each brace and it is pushed into position. When the glue grabs, the clamps or go bars are applied.
When the clamps are applied you'll get some glue squeeze out around the brace. You can wipe this up, but if you do you will smear it into the wood and it will take some doing to clean it up so it is not visible later. You may not care about this, as it is on the inside of the guitar after all. But the generally accepted modern hand builder's method of dealing with the squeeze out is to wait 20 minutes or so until the glue gets leathery and then chisel it away using a long chisel. I use a special bent glue clearing chisel designed by the late Richard Schneider. It doesn't take a lot of work to clear the glue - you just run the chisel up against the brace on one side, then the other. If the glue is still wet, that is, if it smears when you attempt to chisel it, wait a few more minutes and try again.
As mentioned above and in other pages on this site, you can do as much of the shaping of the braces before they are glued down as you like. On the instrument pictured I profiled the back braces before they were glued down. After gluing I ramped the braces ends using a chisel. Now all brace shaping is done and the inside of the back looks like this:
Cleaning up the inside of the back is the last step before the back plate is finished and ready to attach to the rib garland. This generally involves scraping and sanding the inside surface to whatever level of cleanliness you desire. Any remaining pencil marks or glue squeeze out can be removed at this time. Some folks leave the inside of the back beautifully smooth, while others consider any serious effort on an unseen part of the instrument to be superfluous. Even if you don't want to spend a lot of time on this step, it is probably a wise idea to at least clean up the parts you will be able to see through the soundhole once the instrument is assembled.