The rib assembly consists of the ribs (sides) of the instrument, the blocks at the neck end and the butt end, and the linings that provide gluing surfaces for the top and back. This assembly is also called the garland. These pages show construction of the rib assembly for a flat top guitar with a taper, that is, where the body is deeper at the butt end than it is at the neck end. This type of instrument presents the most complicated rib assembly construction of all the common stringed musical instruments. Construction for all other common instruments is generally simpler and can be performed by following these steps and leaving out those that are not needed. The technique outlined here makes use of an outside mold.
Initially appeared: December 26, 2007
Last updated: September 11, 2018
Techniques employed in the construction of guitar rib assemblies vary, and the variability is mostly related to two factors. The first is the fixture and tool combination used to actually form the ribs to shape, and the second is whether the top and bottom surface of the ribs in the finished instrument are parallel to each other or not.
The ribs of most stringed musical instruments are
heat-bent to shape. The three most common fixture and tool
combinations used to do this are:
A simple work board and a hot pipe or bending iron;
An outside mold and a hot pipe or bending iron;
A side bending machine;
On the subject of parallelism of the top and bottom surfaces of the ribs: One way instruments can be classified is whether or not the top and back surfaces of the ribs (the surfaces in contact with the top of the instrument and the back of the instrument respectively in the finished instrument) are parallel. In most instruments with arched plates like archtop guitars, mandolins, and violin family instruments these surfaces are parallel in the finished instrument, and so the ribs can be bent from rectangular pieces of wood. But in most modern so-called flat top guitars, these surfaces are not parallel at all. There are two reasons for this. The first is that in most modern instruments the depth of the body tapers from one end of the body to the other. In these instruments the body is deeper at the butt end than it is at the neck end:
The second reason is that the plates (top and back) of modern guitars are often forced into domed shapes instead of being flat. This means the depth of the edge of the plate is different at different points along the edge.
There are basically two general methods for shaping the ribs to reflect any butt-to-neck taper and any plate doming. The first is to cut the rib material to its ultimate shape before the rib is bent, and the second is to cut the rib material to its final shape after it is bent. I use the latter technique almost exclusively as most of my instruments are one-of-a-kind, and I find it takes less effort to trim the sides after they are bent than it does to construct a template for the sides and use it to mark and trim the material before it is bent. I am going to assume that most people reading this will be making a single instrument and so the techniques described on this page will detail trimming the side after it is bent.
There is a complete article entitled Building and Using the Outside Mold which discusses how to build the mold and includes a general discussion of its use. One thing I want to repeat here is the utility of marking the mold with the distances along the inside surface of the mold from the apex of the waist. In my work I use the apex of the waist as the reference point for side bending. Folks that use side bending machines to bend sides generally use the butt end as the reference point, but that is because the machine firmly locates the butt end of the side before bending. For hand bending I find the apex of the waist works best. I measure the distance from the apex of the waist to the butt seam and also to the neck end of the side. For symmetrical profile instruments this will be at the centerline at the neck block. Here, I am building an instrument with a Florentine (pointy) cutaway. So on the cutaway side a measurement is taken from the apex of the waist to the point of the cutaway, and another is taken from the point of the cutaway to the end of that side.
On the non-cutaway side, the distance from the apex of the waist to the edge of the cutaway at the right side of the fingerboard is measured and written down on the mold. These distances will be used to mark and cut the side material before bending.
Given the choice I would always make blocks out of mahogany. It is strong, hard, stiff, relatively light in weight, and it is very stable. But the price is going up and these days I am making more blocks out of quartersawn poplar.
The first step in making the blocks is to plane the stock to the thickness of the blocks. The instrument I am building here has three blocks, all of different thickness, so that means I need three different pieces of stock. After the stock is the correct thickness it can be ripped to the correct depth. Here again, the instrument I am making has a tapered body, so the tail block is deeper than the neck block.
With the blanks the correct thickness and depth, the shape of the blocks can be transferred from either the plan or the mold to the blocks. The following pictures show transfer from the plan. A copy of the block profiles is made and cut out to make a template:
The block that goes in the pointy cutaway is really small, so I'll mark this on a relatively big piece of stock:
The instrument I am building here has a cutaway, so the neck block has to be shaped to the curve of the cutaway. I use the spindle sander for this:
The spindle sander is also used to shape the small block for the cutaway. After it is shaped, it is sawn off of the blank. It is a lot easier to shape such a small profile while it is still connected to the relatively large blank.
Other tools can be used to shape the block profiles. Tall drill press mounted drum sanders work well. You can cut these with gouges, but if you do I suggest marking the profile on both ends of the block before you start. This helps to keep things aligned.
I mentioned earlier that instead of copying the block profiles from the plan, you can copy them from the mold. This is actually a better idea, since the mold is really the reference surface for the instrument. Here is the tail block profile being marked using the mold. The mold is sitting on the bench with its centerline perpendicular to the front edge of the bench. The block is held against the front of the bench and up against the bottom of the mold. This way I can use the curve of the mold to mark the profile onto the top of the tail block:
The tail block is profiled to the curve at the butt end of the instrument. It is easy to shape this by rolling the block blank back and forth on the horizontal belt sander:
You can use the mold to refine the surface to the exact shape by putting a piece of stick-on sandpaper on the inside of the mold at the tail block location and sanding the block by hand:
After the blocks are profiled, the vertical edges that will not end up being right up against the sides are rounded over or chamfered a bit. This helps keep cracks from starting at those points in the finished instrument. Generally, any shaping of the neck block for attachment of the neck (cutting a mortise for a mortise and tenon joint, drilling holes for a bolt-on neck joint, etc.) will be done after the body is at least partially built.
Here are the finished blocks for this instrument - from left to right, the cutaway corner block, the tail block, and the neck block:
Side material generally comes from the lutherie wood suppliers along with that for the back of the instrument. Backs and sides are generally cut from the same billets or are at least matched in terms of color and grain pattern. Like the pieces that will make up the back, the two side pieces are generally bookmatched, that is, they are made from two successive slices of wood, so that the grain pattern on one piece forms a mirror image of the other.
If the sides have not been planed and sanded to thickness by the supplier, the first step in preparing the side material is to do that now. Side thickness varies considerably from maker to maker and with the species of wood used. When in doubt (a condition I have much experience being in) I thickness the sides to 3/32". Tips for beginners: 1. Highly figured wood should be thickness sanded, rather than planed, as the plane blade(s) tend to catch in the figure and rip large chunks out of the wood; 2. Don't use highly figured wood - it can be a nightmare to bend, and hand bending is one of those skills that takes some practice to get down. For a first project, I'd recommend sticking to poplar, unfigured soft maple, or mahogany.
After planing to thickness, the sides need to be ripped to depth. Before committing them to the saw, it is a good idea to take a good look at them and figure out which surfaces will be on the outside of the instrument and which edges will be oriented toward the top. My general strategy for the first of these decisions is to make the best looking sides be the outsides. As far as which edge should be in contact with the top, I try to keep two things in mind:
The player's view of the instrument is from the left side upper bout, so if one end of the side looks better than the other I try to orient the piece to put the nicest looking end on the neck end of the instrument;
Just looking at the side blank and imagining it is a side of an instrument may make one orientation look better to you than the other;
Here's a set of plain soft maple sides:
Clicking on this and the other pictures of the sides will show the grain detail a lot better. You can see that the sides are bookmatched. When I rip them to depth on the table saw, I'll first fold the sides together and then saw them at the same time. This guarantees that I haven't screwed up my bookmatching, by taking the cut from a different edge of each side.
As mentioned above, the instrument I'm building here is a flat top guitar, and in this instrument the top and bottom edges of the sides are not parallel. As also mentioned, for one-up instruments I cut the sides parallel and bend them that way, then plane them to their final dimensions later. The upshot of all this is that, when making this style of instrument, you should rip the sides to whatever the maximum depth will be on the finished instrument.
Having selected the outsides and the orientation of the sides it is a good idea to mark this information right on the side. I do this on the outside in pencil, so I can read it when the side is in its correct orientation:
I do this on the outsides because the outsides will be sanded in preparation for applying finish to the instrument, sanding off the pencil marks, whereas I rarely (OK, never) sand the insides of an instrument. The arrows point up. You'd think that the arrows would be redundant - if I can read the text then the sides are pointing the right way. But I can't tell you (actually, I won't tell you) how many times I've screwed the side orientation up while I was bending them, so I try to make it as clear as possible.
I've also marked the point of the apex of the waist curve on the side. The mold has previously been marked with the distance from the butt seam to the apex of the waist. I add an inch to that for error and then measure that distance on each side piece. On the instrument I'm building the butt seam to waist apex is 17.5" so I marked the sides at 18.5" from the end that will be the butt end.
This instrument also has a Florentine cutaway and this requires that the cutaway side be cut at that point. The distance from the apex of the waist to the cutaway point is taken from the mold and is measured and marked onto the side piece that will have the cutaway (the right side). I also mark the end of the cutaway piece, giving myself a little extra length just in case:
This piece is also labeled as you can see. Then it is cut off:
Keep in mind that this cutting of the right side is only necessary if the side will have a pointy cutaway. If it has a Venetian (rounded) cutaway or no cutaway at all, then the side does not need to be cut.
It is a good idea to check the total length needed for each side and to be sure the side material is long enough. Wood vendors have been getting more parsimonious in terms of side length these days and it would be a shame to start bending only to find out that the piece is not long enough for the job.
With the pieces planed, ripped and marked it is time to bend the sides. But first, the voice of experience has something to say to you: Never throw away the off cuts from the sides (or the plates, or the neck, or from any other piece of wood that will be visible in the finished instrument). If you have some sort of construction disaster along the way, you may need a piece of matching wood to patch things up. So put the off cuts aside for now.
Rib bending is one of those lutherie operations which first timers tend to find daunting. But it is also a satisfying skill to have under your belt and if you are planning on building more than one instrument in your life it is worth figuring out how to do it. I'll get into the actual bending of the ribs for the instrument under construction in a bit, but first let me offer these tips for those that have never tackled this task before.
Practice first. This is a little tough to suggest because the kind of stock you need to practice on is not readily available. Ideally you could buy slats of inexpensive straight grained domestic hardwood of about 3/32" thick to use for practice bending, but I know of no place to get such things without special order. If you have a thickness sander or a power planer you could certainly cut some for yourself. And some of the lutherie wood suppliers would be happy to sell you some slats for practice, even though they may not list such things in their catalogs. Remember: 3/32" thick! Even just a bit thicker may make it impossible to bend.
Use an electric bending iron. The kind of bending iron in the pictures below is expensive but the combination of good heat control and the ability to orient the thing upright make bending to shape a bit easier. These days you can find bargains on eBay if the prices from the lutherie tools suppliers scare you. If you have such an iron, set the temperature so that it is hot enough so that, when you wrap a soaked piece of wood around it you can hear the hissing of the generated steam and hear the crackle as small drops of water sputter on the hot iron. But don't set it so hot that the wood starts to scorch after just a few seconds of contact. Don't bother setting it to a specific temperature - set it as specified above. Note that a typical electric iron takes about 20 minutes to heat up.
Use easy to bend, straight grained hardwood for your first instrument(s). I recommend plain soft maple, Indian rosewood, birch and poplar as woods that are easy to bend. Definitely avoid highly figured wood - it tends to fracture on the figure grain lines unless it is specially handled.
For your first instrument don't try to bend a cutaway or any shape that has really tight bends. A standard, non-cutaway guitar is a good place to start.
Use an outside mold and take the time to add analogs of the waist curves to the outside of the mold. I discuss how and why to do this on the page entitled Building and Using the Outside Mold. Using the waist analog to bend the waist curves and the inside of the mold to bend the lower and upper bout curves takes advantage of the fact the wood tends to relax back to its original shape as it cools. This is called springback, and taking advantage of it allows you the freedom of over bending the wood slightly, knowing that it will relax into the exact shape of the mold as it cools.
Wear gloves - the iron is hot!
The process of bending over an iron is basically this: you position the wet piece on the iron and apply bending pressure with your hands. Not enough to crack the piece, but enough so that, when the heat finally begins to soften it, it relaxes around the iron. This process is better felt than explained, but let me try anyway. Imagine (or better yet, try this out) bending a dry thin slat of wood around a pole or a screwdriver handle held upright in the vise. If you bend the wood slightly and then let it go it will spring back straight. If you try to make a deep bend in the wood it will suddenly break at the point where it is bending around the pole. But if you don't go so far as to break it, it will spring back straight when you release it. If you bend the piece a bit around the pole and just hold it there, you can feel that it takes a certain amount of force to keep the bend. Remember the feel of the pressure you have to exert to maintain the bend. When you are actually bending wood over an iron you'll have to apply that pressure to maintain the bend. But after a short period of time you will feel the wood start to relax and it will require less pressure to maintain that bend. This is how you know you are making a permanent change in the shape of the piece - you can feel it.
Move the wet wood in contact with the iron to heat it up, and make your bend
as it is leaving contact with the iron. Keep the wood moving, slowly.
This will make the wood bend gracefully rather than simply
folding or cracking. This is one of those cases where a picture is worth 1k words. Here's the picture:
The wood is passing over the iron, then once it is soft, it is bent around the iron. All this is done while the wood is moved continuously but slowly across the iron, to avoid simply folding it at one point. One more thing: constant bending pressure and constant iron temperature and constant speed of the wood over the iron will result in a constant radius bend. In reality, even the picture is not as useful as actually getting your hands on a wet piece of wood and a hot iron.
It is difficult to make a bend near the end of a piece. If your layout requires a piece ending in a bend, make that piece longer than is needed, bend it, then trim it to length, if this is possible. This is another reason to start off with a non-cutaway instrument, since the two halves of the sides end at the relatively flat sections at the neck and tail blocks.
It is difficult to make tight (short radius) bends, and every piece of wood will have some limit to how tight a bend you can make in it. If you need to make tight bends, sometime it is better to make then in multiple passes, bending the wood a little more each time. The thicker the wood is, the harder it is to make a tight bend in it. And some species bend easier than others. Poplar and soft maple bend so easily that there is generally more of a danger that it will fold over and kink than there is that it will snap.
Listen to the wood as you are bending it. What you are listening for are the first telltale sounds of the wood cracking. If you hear one, back off immediately and slide the wood so you are bending in a different place. If you actually crack the wood, immediately glue it back together with cyanoacrylate glue and continue bending. You'll have to assess the crack later to see if you can still use the side, but generally if you are listening carefully while you bend you are not likely to break the piece clean off. If the break is on what will be the inside of the guitar it will be invisible in the finished instrument, and it may very well be invisible even if it is on the outside if it is well repaired.
Bend one curve at a time. After you take the wood off the iron, keep pressure on it and wave it around a bit to cool it for a few seconds. Then compare the curve you just bent to its place on the mold. Not tight enough? Bend it again. Too tight? Move the convex surface back and forth over the iron while applying gentle pressure to un-bend the curve a little.
With these guidelines in mind, it is time to actually bend a side. The instrument I'm making here has a cutaway, but I'm demonstrating side bending on the non-cutaway side. The first step is to collect all the materials you'll need to bend the sides. In addition to the outside mold, bending iron and the pieces for the sides, you'll need a pair of cotton gloves, a pile of clamps, and a pile of side bending clamping cauls. For the latter I use lengths of 1" diameter dowel with a flat sanded on one side. You will also need a spray bottle of water, or a damp rag. This is used to re-wet a side that is dry and needs more bending. Orient the mold so when you are looking down into it, it is as if you are looking at the top, not the bottom, of the guitar.
Have all these materials ready. When you're in the middle of the bending process you don't want to have to go looking for this stuff.
The second step is to heat up the iron. It takes a good half hour to heat this type of electric iron, so plan accordingly.
While the iron is in its last minutes of heating it is time to soak the wood. Use clean warm water is any vessel that will contain the longest piece you have. I work at home and so I use the bathtub:
In an ideal world you'd have a way to keep the sides completely submerged for about five minutes. Soak them for about three minutes on each side in the tub. After the soak, drain the sides so they are not dripping.
Now here I must admit that, as an experienced luthier, I don't actually soak the sides, but rather simply spray them with the spray bottle just before bending. But for beginners it seems better to get the wood a bit wetter, because it won't dry out so fast on the iron.
The side will be bent in three basic steps: the waist bend, the lower bout, and the upper bout. As mentioned, I use an outside mold that has an analog of the waist curve on the outside of the mold so the waist curve can be molded on the outside. Using the mark made on the side for the apex of the waist, I 'll first bend the side at the waist. But first, take a look at the waist curve on the mold. Keep the image of that curve in your mind's eye as a guide to making the bend:
Then bend the side over the iron at the waist apex mark. It is ok to over bend a bit, and in fact doing so will make for a more accurate curve once the side cools. While the side is still hot, clamp the waist bend onto the mold using one caul and a clamp:
Let it cool here for about two minutes. Then un-clamp the piece and start in on the lower bout. At this point the idea is to just get the side bent enough so that it will stay in the mold. We'll go back and work on a good fit later.
Be sure to keep the wood moving over the iron to avoid kinking the side. If you hear any cracking noises as you are bending, stop bending at that point and move the wood along. If the iron completely dries out a section of the side, dampen it again using the spray bottle. The wood will tend to spring back as it cools, so after you've made a bend hold the wood in the bent position while it cools. After the lower bout is roughly bent, check it against the curve of the mold.
Next, the upper bout is bent over the iron in like fashion. Again, we're not aiming for a perfect fit to the mold at this point, only trying to approximate the curves and make the side fit into the mold.
After the upper bout is bent, try fitting the side into the mold. If it is too long, mark and trim off the end as necessary. Then fit the side into the mold.
When the side is in the mold it will be very apparent where the side needs more bending work to make it fit. You can attack each of these areas one at a time, wetting the side as needed, bending, and checking the fit. The side invariably needs more bending around the waist curve, at the transitions to both the upper and lower bouts. Resist the temptation to simply force the side to fit the mold by clamping it in. It will spring back when the clamps are taken off and you'll end up building an instrument that is either malformed or has its sides held in position by the glue attaching the plates. Neither of these is desirable, and the latter will result in an instrument with internal stresses just waiting to crack sometime in the future. You don't need to get an absolutely perfect fit, but you should strive for a fit that is good enough so that you can make it perfect with the application of gentle finger pressure in one or two places.
Here the left side has been well fitted to the mold. Once this is done, the side can be clamped into the mold with cauls and left there overnight to fully cool and dry. Use the neck and tail blocks as cauls in their respective places when you clamp the side in the mold. This will help to form the wood to the shape of the block and make it easier to achieve a good glue joint when it comes time to glue in the blocks.
You may notice that the side material tends to pull away from blocks at the top, where it is taller than the mold and is not supported. If this happens, back up the side with a piece of scrap wood and clamp that in place while the side cools and dries in the mold.
The other side (and, in the case of the instrument I am building here, the cutaway piece) are bent in similar fashion and then are also clamped into the mold for overnight drying. If you overlap the ends of the sides (like at the tail block end) don't clamp so hard there that you kink the overlapping piece. It turns out that for this pointy cutaway instrument that I can't clamp the cutaway piece into the mold at the same time as the other pieces, so I'll have to bend this one separately.
On the subject of bending the cutaway piece, as mentioned it is difficult to bend a short piece of wood over an iron, so it is better to leave a piece like this long while bending it and then cut it down to length. I didn't have the luxury of a long piece here, so I use a bending brake fashioned of two pieces of angle bolted together to help me bend right to the ends of short pieces:
The next day, you can un-clamp the side pieces and take them out of the mold. If the sides noticeably spring away from the mold, some more bending will be required at those points. If you need to do this it is best to simply wet the side with the spray bottle at the place needed rather than wetting the entire side.
Bent sides sitting around for a while tend to un-bend a bit so if it will be some time before you glue on the blocks and the linings you may want to store the sides lightly clamped into the mold.
Here's the cutaway piece clamped into the outside of the mold:
Note that I have a clamp lightly across the top edge of the piece. This is because the mold isn't as tall as the piece and the piece will tend to straighten out at the top, where it is unsupported by the mold. Here's the right side piece clamped into the mold:
The first block to be glued on is the tail block. Before gluing, the ends of the two sides that will meet at the tail block are trimmed. How careful a job you need to do with this trimming depends on what kind of seam you'll have here in the finished instrument. If there will be a trim piece here (called an end graft) then you don't have to be too finicky about trimming the sides for gluing on the tail block, since later you'll route away some of the wood around this seam to fit in the graft. But if your instrument will not have an end graft then you'll want to fashion a very clean and tight butt joint here. (Luthier's secret: I often "decide" to use an end graft on an instrument after realizing I've screwed up this butt joint.)
The instructions that follow assume you'll want a nice tight joint here. The process is the same no matter what, but you won't have to be so careful with the following steps if you know you'll be using the end graft.
The first step is to mark one of the sides at the centerline:
If your mold is constructed in two halves like mine then the seam in the mold indicates the centerline of the instrument. After marking, take the side out of the mold and use a square to continue the line right across the side:
Note that the square can be used because the sides are rectangular - they have not yet been planed with a taper or otherwise shaped to fit the plates. Now the end is clamped to the bench top between two pieces of scrap wood.
The scraps of wood need to be at least a bit longer than the width of the side and they can't be too deep, as the sides are bent and clamping it between them will flatten it out here. The scraps should be sawn square, and the front edge of the top piece should be straight - you'll use this as a reference surface to cut the end off the side. Position the front edge right on the line you drew to mark the cut. Finally, the bottom piece should protrude a bit more than the top piece does, so that the saw cut will be supported. Here's a top view of the whole affair:
The end is cut off using a small back saw with its blade resting against the front face of the upper block of wood. I'm using a Japanese razor saw here. The blade is very thin, so to be sure it makes a straight cut I'm trapping the blade between the top scrap and another straight piece of wood that I'm holding against the other side of the blade.
The end of the other side is marked and sawn in like fashion. Now you can rest the sides on the bench top with the ends butted together to assess the butt joint. If it is not perfectly flat you'll need to do a little work here with a small plane or sanding block. Once the joint looks good on the bench you can set up to glue it. Clamp one of the sides in the mold, using three clamps on the lower bout. Be sure the side is resting firmly on the floor of the mold (or on the bench top if the mold has no floor and is just sitting on the bench). Put a piece of wax paper between the mold and the sides to prevent the sides from being glued to the mold. Align the end with the centerline before tightening the clamps. Locate one of the clamps close to the end but not so close that you won't have room to fit the tail block in there.
Then put the other side in the mold. Push the ends together tightly and then clamp that side in, too. Be sure this side is also sitting in contact with the floor of the mold. If the joint opens up when you clamp it, re-position the side and clamp again. If you can't seem to make the butt joint tight no matter what, make it as tight as you can and clamp the side. Then remove the two clamps closest to the joint and slide a couple of pieces of veneer or cardboard between each side and the mold. This will push the joint together. Reinstall the two clamps and check again.
When all looks good you can do a dry run of clamping the tail block in. Use a couple of clamps to clamp it in place. Be sure it is resting firmly on the floor of the mold.
Since the mold I am using is not as tall as the sides, the sides tend to bend away from the block at the top, where they are unsupported by the mold. You can see that the top of the left side has pulled away:
When this happens a small shaped caul can be used to support the back of the exposed top of the sides. This is clamped right to clamps that are used for the block.
When you are satisfied that all will clamp together well, the tail block clamps are removed, glue is applied to the back of the tail block, and the block is clamped back into place. Let the glue dry, then unclamp. The finished butt joint looks like this:
If you are building a standard, non-cutaway guitar, the ends of the sides are trimmed at the neck block end and the neck block is glued on in similar fashion. You rarely need to implement a really tight butt joint at the neck block, as the heel of the neck covers the joint in most guitars.
Since the sides are currently not tapered but the finished instrument will have a taper when viewed from the side, the neck block is shorter than the tail block. But we need to glue it so its top is flush with top of the sides. This means we need to push a little wedge under it when positioning it for gluing.
An alternative would be to flip the mold over so we are looking down at the bottom of the finished instrument, and glue in the neck block with it sitting right on the floor of the mold.
On the Florentine cutaway guitar I'm building here, there is a similar issue with the height of the cutaway block. I'm using a wedge under it while gluing to keep its top flush with the top edge of the sides, too.
Here is the rib assembly with all the blocks glued in.
If we ignore minor variations due to slipping of the glue when the blocks were glued on, the top and bottom surfaces of the rib and block assembly are now flat and parallel to each other. So, if we were to place a large flat board on top of the rib assembly in the mold the board would contact the entire top surface of the ribs. If the instrument you are building has a top with a flat bottom surface (like an archtop guitar, mandolin, violin, etc.) then you can flatten the top surface of the ribs by stroking it with a large flat board covered with sandpaper. But modern "flat top" guitar tops and backs are spherically domed, not flat like a board. We need to shape the top surface of the rib and block assembly so that it has the same domed shape that the top plate will have.
The tool used to both force the top into a domed shape and to sand the top of the rib assembly so that it fits the top plate is called a dished workboard, also referred to as a hollow form. The dished workboard can be purchased from luthier suppliers with various radii of dishing. I use a 30' radius dome for the tops of my instruments and a 15' dome for the backs. Purchased dished workboards are made of MDF and are generally 2' in diameter. Some folks make their own dished workboards.
As the workboards will be used to sand the top and bottom surfaces of the rib assembly to fit the domes of the plates, the dished surfaces of the workboards are covered with stick on sandpaper, which conveniently comes in 2' diameter disks. I use 40 or 50 grit sandpaper disks for this, which I get from McMaster Carr. In this picture I'm holding a sandpaper covered dished workboard over a completed rib assembly in the mold:
The sandpaper is a teal blue color.
It is apparent what needs to be done to shape the rib assembly to fit the top if you just lay the dished workboard on top of the ribs in the mold. The dished surface of the workboard will contact the ribs at the blocks and for some part of the lower bout, but there will be no contact at the waist. So the fitting job involves removing material from the ribs and blocks so that the entire top surface contacts the dished surface of a workboard laid on top.
For the top of the instrument there is not too much material that needs to be removed, as the doming is so shallow (30' radius for my instruments) that it is close to flat. So for the top it is easy to just use the sandpaper covered workboard as a giant sanding block to simply sand the ribs down to fit.
In preparation for this task, the ribs must be secured top up in the mold using rib jacks. These are simple spreader clamps that are used inside the ribs. Here are the ribs clamped into the mold with the rib jacks:
When the rib assembly has been secured in the mold, the top of the ribs and blocks are marked with a black marker. This is done so you can check your progress while sanding. I usually put a few lines on the top of each block and run a single line across the top of the ribs.
When the rib assembly has been marked, the dished workboard is placed dished, sandpapered side down on top.
The top surface of the ribs and blocks is sanded down by grabbing the workboard with two hands and rotating it back and forth around its axis. The workboard is heavy and so the sanding goes quickly. I like to check my progress after about twenty clockwise/counter clockwise strokes. The blocks and lower bout will sand down first, and the marker lines will disappear there first. Check to be sure that you are not sanding down one side more than the other. This is usually only a problem for instruments with cutaways. If one side is going quicker than the other, press down a bit on the slow going side when you continue sanding.
The waist is the last place to be touched by the sanding board. When the marker only remains at the waist ...
... then you only have a few more strokes to go before the sanding of the top surface of the rib assembly is complete.
A number of folks that build instruments on a production basis make use of a motorized dished disk sander to perform this operation. These shop built sanders mount the dished workboard dished side up on a turntable of some sort, and use a motor to spin the thing around. The rib assembly, clamped into the mold with rib jacks, is lowered down onto the spinning sanding disk. Using a tool like this makes this operation go very fast. Luthier Mike Doolin wrote an article about the construction of his shop built dish sander, which appeared in American Lutherie #74. A reprint of that article appears on Mike's website.
Linings are pieces of wood that are glued at the junction of the ribs and the plate. They increase the gluing surface, and in violin family instruments make it practical to remove the top for repair. Linings also stiffen up the rib assembly quite a bit. There are basically four choices for linings: no linings, kerfed linings, tentellones, and solid linings. Which you choose will probably be what is most customary for the type of instrument you are building. Here is a bit of information on each type.
Lutes, ouds and other early bowl-back instruments generally use no linings at all. There isn't much gluing surface but these instruments are pretty light and delicate all around, so it's not like you lose a lot of strength and robustness if these instruments don't have linings.
Kerfed linings are by far the most popular for steel string guitars and are used as top and back linings in a lot of classical guitars, too. They are made of strips of wood that have regularly spaced kerfs sawn into one of the surfaces. This makes the strips easy to bend. You can make kerfed linings yourself, but they are readily available and cheap from the lutherie suppliers. I will show the use of kerfed linings (also sometimes called kerfing) here. Factory made kerfed lining strips look like this:
Some older instruments and classical guitars often use tentellones (Spanish, pronounced ten-te-YOWN-ess), also called peones (pey-OWN-ess) or glue blocks, for linings for the top. Tentellones are small individual blocks of wood, sort of like if you broke all the little pieces off of a length of kerfed lining. They are a pain to put on as they have to be glued on individually, but if you are building an authentic instrument the original of which used them then you will want to use them, too. Tentellones are always shop made.
Solid linings are solid strips of wood. They are used in all violin family instruments and in a lot of classical guitars, particularly for the back. Since it is not possible to cold bend a solid lining strip while gluing it to the ribs, each strip will have to be wetted and hot bent on the bending iron, in a process which is just like bending the ribs. Solid linings are often made of willow or basswood or some other easy to bend wood. I'll often use off cuts from trimming the ribs as solid linings if they are thick enough.
The strips of wood for solid linings should be quarter sawn, about 0.1" x 0.625" and long enough to cover one whole side of the instrument. Two or more pieces can be tied or taped together and then bent at the same time. It is actually easier to keep from cracking them if you gang them up like this. After they are bent they are clamped into the mold to cool and dry. They are glued to the ribs in the same way as kerfed linings, described below. After they are glued in they are profiled with a long chamfer. I do this with a sanding drum, like this:
OK, back to work. Gluing on kerfed linings or solid linings is easy. The rib assembly should be put top up in the mold and held in place with rib jacks. I'll only describe gluing the top linings here as we have not yet trimmed the ribs for the back. Glue is applied to the back of the lining strip, and it is clamped in place with small spring clamps. Start at the neck block end, with the lining butted right up to the side of the neck block. This way you get a nice neat butt joint here where you can see it through the soundhole.
I have enough spring clamps to do one side at a time. You should only glue up as long a strip of lining as you have clamps for of course. Some folks use clothes pins to glue linings. They work fine for kerfed linings but I find they don't exert enough pressure for solid linings.
Try to position the linings so that the tops of the linings are flush with the tops of the ribs or stick up a wee bit above the ribs. After the clamps are applied, wipe any glue that squeezes out from the bottom of the linings.
When the glue is dry, the clamps are removed and you can go on to glue the linings to another section of the top of the rib assembly. Once all the linings for the top have been glued on, the top side of the rib assembly is sanded again using the sandpaper covered dished workboard, this time to sand the tops of the linings to fit the top of the instrument. Once ribs, blocks, and linings are sanded uniformly the top of the rib assembly is ready to receive the instrument top.
The next step will be to shape the back side of the rib assembly and this will require moving the rib assembly in the mold. But before you do that it is a good idea to lightly pencil a line around the outside of the ribs right where they emerge from the top of the mold. This pencil line will make it easy to align the top square to the mold, a process which will otherwise be difficult once the back side of the rib assembly is cut down to implement the taper of the instrument body.
For instruments where the back is domed to a large radius like the top, and where the back is essentially parallel to the top, it is possible to dome the ribs, blocks and linings using the dished workboard as described in the section on preparing the ribs for the top. But the backs of modern instruments are generally domed to a smaller radius (I use 15' for the backs of my instruments for example) and dressing the ribs down just by sanding could take some time.
Also keep in mind that a number of instruments taper in depth from butt end to neck end. As explained earlier, since I build mostly one of a kind instruments I usually do not taper the sides before bending, but instead taper them at the same time as I do the rough doming in preparation for attaching the back.
The processes involved here are to scribe the desired taper and doming on the inside of the ribs, plane the ribs down to the scribed lines, and then finish up the ribs using the sanding dish. Then the linings are glued on and the whole assembly is worked over once more with the sanding dish. Here's the drawing that shows the taper and doming of the back. I'm including it here again because it is good to have it in mind when considering the steps that will be outlined for scribing the taper and doming on the insides of the ribs.
If your mold has a bottom, take it off. Set it on the bench top side up and put the rib assembly into it so it is sitting squarely on the bench. Clamp it in place using four clamps.
The clamps at the blocks should be offset to one side of the centerline, as you'll need to be able to get to the centerline for some measurements.
The workboard you use for the back should have the center marked right on the sandpaper. Mine has a small hole drilled through the board at the center. When I put a disk of sandpaper on I run an awl through the hole from the back to pierce the sandpaper and mark the center of the disk.
Put the workboard for the back on the bench sandpaper (dished) side up and set the mold with the rib assembly clamped in on top. The ribs are still top side up. Try to roughly center the ribs on the workboard.
Take a scrap stick and position it on top of the rib assembly so one edge is right on the centerline. Mark the butt end and neck end of the instrument on the stick.
Here's an enlargeable picture of the stick with both ends marked:
Measure the distance between the end marks and place a mark on the stick at the center point between the end marks. Put a square on the stick so the beam points down. This indicates the center of the rib assembly. Now skooch the mold around on the workboard until the beam of the square is pointed right at the center point of the workboard. Now the rib assembly is centered on the workboard.
We need to fashion a block of wood to raise up the butt end of the instrument enough to simulate the taper of the body. With a ruler, measure the distance from the bottom of the neck block to the surface of the sandpaper below it on the workboard. This gives you the amount the rib assembly will need to be raised to indicate the taper you will scribe onto the ribs.
Cut a small block of wood to the thickness measured above. In the instrument I am building here, that is 7/16".
Carefully lift up the butt end of the mold so you don't disturb the centering, and place this block under the tail block of the rib assembly. Looking from the side, it looks like this:
Check the centering again, to be sure you didn't mess it up when placing the block.
Now, set your compass to the thickness of the block. You'll be scribing a line onto the inside of the ribs, with the pointed leg of the compass dragging on the sandpapered surface of the workboard. Before actually scribing the line, check to be sure the pencil point will just miss making a line on the ribs at the tail block. Adjust the opening of the compass as needed. Now place it as if you were going to scribe the ribs at the neck block end and be sure that the scribed line there will be at the same level as the bottom of the neck block.
If these two checks don't work out you'll have to reposition the little spacer block to raise or lower the tilt of the mold, reset the opening of the compass, and try again. Once everything checks out, go ahead and scribe the line on the insides of the ribs. As you do this you'll notice that the elevation of the line relative to the bottom edge of the ribs gets higher the closer you come to the neck block, but that it dips down a bit around the waist. This is all as it should be, so don't try to compensate by tilting the compass. Just keep it at the same level and scribe.
Now the ribs have to be planed down to the scribed line. Take the mold off the workboard and put the workboard aside. Take the four clamps off, flip the mold upside down so the rib assembly is bottom side up, and position the ribs in the mold so the tops of the blocks are resting squarely on the bench that the mold is sitting on. Clamp the rib assembly into the mold using the rib jacks. Now the edges you need to plane are sticking out of the mold where you can get to them.
Use any tool you want to plane down the ribs to the scribed lines. I generally use either a paring chisel or a knife. The ribs are thin so the work goes quickly even if you use a small block plane as I am doing in the picture:
Do be careful about the direction you cut. Since the ribs are so thin, if you accidentally cut in to the grain you could easily split a hunk off the ribs well below the scribed line. Also be careful about running into the ribs with the front or back of the plane, if you use a plane. Since the ribs are unsupported above the mold it is very easy to crack a rib by bumping into it. You don't have to be too finicky about the planing, just get the wood off. The actual dome shape will be sanded, using the sandpaper covered workboard.
After planing or chiseling off the waste wood, the preliminary doming of the back side is done using the sandpaper covered dished workboard. Remember, you'll probably want to use a different (smaller) radius dome for the back than you used for the front. Be sure you use the correct workboard.
Then the linings for the back are glued on in the same manner as the linings for the top. After the glue dries the entire back surface is re-sanded with the dished sanding board to shape the linings to the back dome. I like to write which plate will go where, right on the neck block:
As I've mentioned a number of times in these pages, I make a lot of mistakes and so I tend to write all over the instrument so I'm sure which side I'm looking at. Here's the finished rib assembly:
There's a common option which I should point out, and that is the use of vertical reinforcing strips on the insides of the ribs. Here is a picture of a rib assembly that uses these strips:
These strips are common in archtop guitars, but can be found in other instruments as well. I use them in any instrument that has flatsawn sides, sides made of figured wood, or thin sides (or extra deep sides) that might be in danger of buckling. One reason they may be common in archtop guitars is that those instruments often have figured maple sides and also a venetian cutaway. Figured maple is hard to bend - it tends to break on the figure lines. And cutaways are hard to bend too, as the bends involve are usually pretty severe. It is a lot easier to do these with this wood if the sides are very thin, but that makes them a lot less stable. The reinforcing strips help to keep the sides stiff.
I use thin strips of wood from side material for the reinforcing strips. It is convenient to make these as thick as two blocks of kerfed lining. That way you can glue two blocks worth of kerfing onto the strips. Spacing of the strips is not critical, but I try to put them every 4 - 6 inches along the length of the side, but not where they will be in the way of any top or back brace ends that will get pocketed in the linings. You don't need to put the strips anywhere where the side is sharply bent, as these places tend to be pretty stiff and don't need reinforcing.
The procedure for making the sides is a bit different if you will use the strips. The preliminary doming of the ribs for the top is done, but the linings for the top are not put on. Then the planing of the back taper of the ribs is done, followed by preliminary doming of the back. Now the reinforcing strips can be cut accurately to length and glued in place. After the strips are on, the linings for the top and back can be glued on. As you can see from the photo, the pieces of lining material stop at the reinforcing strips. Small two block pieces of kerfed lining are glued onto the strips. When the glue is dry the faces of these pieces that are toward the interior of the instrument are shaved back so these pieces of lining don't protrude farther than the rest of the linings do.