The ribs (sides) of most stringed instruments are permanently bent into shape using steam. There are a number of standard tools and techniques for doing this. One of the most common combination of tools used in the small production environment is the heated bending iron (hot pipe) and the outside mold (mould). The basic technique for forming the ribs using these tools is to wet the side and then bend it to shape over the hot iron, and then clamp it into the mold to cool and dry. Once cool and dry the side assumes the shape of the mold and will retain that shape once it is removed. The outside mold is also very useful for other aspects of instrument assembly. This page contains some tips for building and using the outside mold. Let me point our here that there is some inconsistency in the naming of molds used in instrument construction. When I use the term outside mold I am referring to a mold or form that goes around the outside of the ribs of the instrument.
Last updated: September 11, 2018
The first step in building the outside mold is making a drawing of it. I make my drawing at the same time as I make the drawing for the instrument. Start with the outline of the vertical profile of the instrument and the centerline. For the most part you can simply add lines on the outside and parallel to the body outline to represent the outside of the mold, but there are some modifications to this outline (discussed below) that you may want to add. Don't make the mold too thick – keep in mind that you'll be clamping the ribs inside the mold when using it, so make the walls of the mold thick enough for adequate structural integrity but thin enough so that you have room to get a clamp and a clamping caul over the walls. If you want to use the mold to hold the rib assembly while the top is glued on, then the walls of the mold can't be any thicker than the depth of your clamps. I shoot for about 3" wall thickness, which works fine with the 4" depth wood cam clamps I use.
As you can see in this drawing of a mold for an acoustic bass guitar the outside edge of the mold does not perfectly parallel the outline of the body. I make three types of modifications to the outside outline – assembly tabs, clamping flats, and curve analogs.
The top and bottom of the mold outline are brought up into square tabs which will be used to assemble the mold itself. I make the mold in two pieces, one for each side of the center line. Doing so offers a couple of advantages. One is that you can make more efficient use of the material you make the mold out of if you lay out the mold in two pieces about the centerline. This is probably not much of an issue for tiny instruments but makes a big difference in the amount of material needed to make a mold for a large instrument like the ABG. The other reason to make the mold in two pieces is that, if the instrument is symmetrical about its centerline, you only need to draw and make a template for one half of the mold. The completed mold will be assembled using bolts and fender washers through the tabs, so the tabs need to be long enough to provide clearance for the washers. But here again, don't make the tabs too long. In use, you'll need to clamp the neck and tail blocks to the ribs, positioning the clamps against the butt ends of the mold. Be sure the tabs are short enough so you can get your clamps around the mold, the ribs, and the block.
I also shape the outside edge of the mold to include flats opposite the acute apexes of the cutaway, and opposite the place where the neck block will butt up against the side wall of the cutaway. It is a good idea to keep in mind that the first purpose of the mold is as a clamping fixture to mold the ribs, and to be sure that it will be possible to place clamps to mold all features of the instrument. Cutaways (and any other areas with sharp curves) are particularly problematic and so it is always wise to provide clamping flats opposite all of the sharp curves. The small vertical flat to the right of where the neck block will be is an important feature for cutaway instruments. This provides for clamping the neck block sideways against the side wall of the cutaway to be sure it is accurately positioned about the centerline of the instrument.
The third modification I add to the outside outline of the mold is to place curve analogs opposite the waist and cutaway curves. As described in my American Lutherie article “An Enhancement to the Outside Mold” (AL #79, p. 58) these make it much easier to use the mold to accurately bend those curves where the mold will be on the inside of the bend – the waist curves and the cutaway curve. When you bend wood over a hot pipe it tends to spring back to approaching its original shape as it cools. You can use this property to your advantage when bending curves where the mold will be on the outside of the curve (the bouts) – the wood can be slightly over bent very approximately to shape, and when it is clamped into the mold it will spring back as it cools into exactly the shape of the mold. This is a great technique but it only works for the bouts. When bending the waist curves, exactly the opposite happens. As the wood cools it tends to spring back and pull away from the mold inside these curves. You can deal with this by making shaped cauls for these curves, but another way to deal with these is to put outside analogs of the curves on the outside of the mold. These analogs are the same radius as the real waist curves plus the thickness of the ribs. To make use of these analogs, the waist curve is roughly over bent to shape and then the rib is clamped into the analog curve on the outside of the mold to cool. Once cool the bend will exactly fit the waist curve inside the mold.
Once the paper plan is made for the mold each side is cut out separately. I build up the final thickness of the mold in layers, starting with one “template” piece for each side. To make the template, the cut out plan for one of the sides is placed on a piece of 3/4" plywood or MDF so that the centerline edge of the plan half is aligned with a straight edge of the material, and then it is traced. If the instrument is asymmetrical about its centerline then the other side is traced in like fashion. These template pieces are cut out on the band saw and then carefully shaped using the spindle sander or a drill press mounted drum sander. The finished template piece(s) is then drilled through with a 1" Forstner bit approximately every 10" along its length. These holes will be used to dowel the template to subsequently made sections of the mold for shaping, and then again when the mold is glued together. Here are the template pieces for a guitar, along with the pile of dowels that will be used to dowel the completed mold together:
Again, each mold half is built up in layers. How deep the mold should be depends on the depth of the sides of the instrument you are building. In general you'd like the mold to be almost as tall as the shallowest part of the ribs of the instrument. If too much rib is exposed over the mold then the rib may bend outward, defeating the purpose of using a mold in the first place. This is particularly a problem with deep instruments such as basses. And if the rib is completely inside the mold at any point then it will not be possible to use the mold when gluing the top on the instrument. So at this point you'll want to figure out how many layers of MDF will be needed to make the mold the required height. Here's a picture of a finished mold with a rib assembly in it. The ribs are 1.5" proud of the mold, which is a little too much. An extra layer of MDF really should have been added to this mold. 1" to 3/4" is just about right.
Once the number of layers is determined, the template mold halves are used to draw additional mold halves on the sheet(s) of MDF. When you trace the outline of the template halves also trace the 1" holes drilled in the template halves. Now all of these pieces can be roughly cut out using a hand held jig saw. After they are cut out, take each piece in turn and clamp it to its original template piece so the holes line up with the drawn hole outlines. Push the 1" Forstner bit through the hole in the template so that its center point center punches the material below. Then un-clamp the template piece and drill the holes through the roughed out piece. Temporarily attach the template to the rough piece using hunks of 1" dowels in the holes. Using a large diameter pattern bit in a router, route the outline of the rough piece flush with that of the template. The above process is repeated for each piece needed for each side of the mold.
After all the pieces are cut out and trimmed the mold halves can be assembled. Cut enough sections of 1" dowel to a bit less than the depth of the mold to fill the holes. Put some glue on one of the mold pieces, position another piece on top, and pin them together with dowels. Just a single stripe of glue down the middle of the piece will do – you don't want to have any squeeze out at all. Continue gluing and driving the dowels through with a mallet until you have completed one mold side. A few clamps can be applied before the glue dries. The other side of the mold is pieced together the same way. After the glue is dry, check to be sure the centerline surfaces are straight and that the inside surfaces of the mold are smooth. Sand as needed.
This last step is really important. Once you have a mold, you will use it to bend the ribs of the instrument and also as an assembly fixture, so it is important that the thing is dimensionally accurate. If you find that the inside surfaces of the end tabs don't meet flush, sand them straight. This is easily accomplished by sticking some 60 grit stick on sandpaper to the top of the table saw or other large flat surface and then scrubbing the inside tab surfaces over it. Keep these surfaces perpendicular to the top and bottom surfaces of the mold, too. Check your progress often. When you are done the two halves of the mold should mate perfectly. Don't worry about the inside surfaces of the body part of the mold too much just yet. These will be sanded after the mold is bolted together.
If the mold is for an instrument with a cutaway I generally saw two slots about 1" long and 3/16" wide to accept a little excess side material around the neck block. This means you don't have to be too precise when bending around the cutaway as it allows you to leave the sides a bit long before bending. Here is a closeup of those slots on a finished (and quite dusty) mold.
Now the two mold halves can be bolted together. Align the halves on a flat surface, drill holes through the tabs at both ends and bolt the mold together with bolts and fender washers. Check the alignment and shape of the inside surface of the mold against a full sized drawing of the outline of the instrument.
As mentioned the mold is used as the assembly reference for the instrument and so you want to be sure it is right. Probably the most important thing is to be sure the inside surface that is in the area of the neck block of the instrument is straight, in line with the centerline, and perpendicular to the top and bottom surfaces of the mold. Use a small square to check. Be sure to check against both top and bottom surfaces of the mold with the square. My instruments generally have a flat in the body outline where the neck joint is, so I straighten things up using a small sanding block:
Again, check your progress often with the square. The flat surface at the inside of the mold here should be perpendicular to the centerline, measured on both top and bottom of the mold. And this surface should also be perpendicular to the top and bottom surface of the mold as well.
After the neck block area is sanded, the rest of the inside of the mold should be sanded so that the inside surface is perpendicular to the top and bottom of the mold and so all the lines of the outline flow smoothly. I do this sanding with a big spindle and sanding sleeve from the spindle sander.
The mold should be well sealed before use. I slop on a few coats of polyurethane floor varnish. The MDF soaks this up like a sponge. Be particularly diligent on the inside surfaces since this is where the wet ribs will be in contact with the mold. The finished mold looks like this.
You can see how careful I am with the varnishing! The stuff that glopped over onto the top surface is due to the fact that I was really troweling it on when doing the insides. Although you can use the mold like this on any flat surface, I like to make a removable back cover for it. This makes it easier to deal with the clamps when using the mold to glue on the top plate. This cover is attached with machine screws into threaded inserts.
The cover gets a coat of varnish too. The last thing I do is to write a lot of useful information on the mold with permanent marker. At the very least the name of the instrument the mold is for and the surface orientation (i.e., whether you are looking at the top or the back) are written on the mold.
This one is for my Tinozza (Italian for bathtub) ABG. Since I use the apexes of the waists as reference points when profiling and bending the sides I also mark those apexes and write how far it is from there to the centerline of the instrument in both directions. This means I don't have to take these measurements each time I use the mold.
The outside mold can be used when bending the sides; to aid in directly profiling the sides of instruments with domed plates; to aid in making profile templates for the sides of instruments with domed plates; as an assembly fixture when gluing on the blocks and linings; and as an assembly fixture when gluing on the plates.
Probably the most important use is as a mold for keeping the bent sides in shape. The basic bending process is pretty straight forward – bend the side over a hot pipe and clamp it into the mold while hot. I use short hunks of dowel with a flat sanded on one side as cauls.
As mentioned in the construction section, if you include analogs of the waist curves on the outside of the mold it makes it really easy to bend sides, even for first timers. See my American Lutherie article “An Enhancement to the Outside Mold” (AL #79, p. 58) for complete instructions.
The mold is also useful for gluing on the blocks. Once the ribs are bent and the extra lengths accurately trimmed off you can glue the blocks on right in the mold. It is a good idea to either wax the areas of the mold where the glue will squeeze out or put a piece of wax paper behind and under the ribs. For cutaway instruments the positioning of the neck block with respect to the cutaway side is critical. If the mold has a flat on the side then the block can be clamped from the side, like this:
Most so called flat top guitars and similar instruments do not really have flat plates. The tops and back of such instruments are usually forced into shallow spherical dome shapes. This means that the depth of the ribs will vary at different points along the rib. Generally the procedure for constructing such ribs is to make a flat template that indicates the depth variation of the finished ribs, and then copy the template onto the flat rib stock and trim the rib before the rib is bent. See my American Lutherie article “Rib Depth of Guitars with Spherically Domed Plates” (AL #84, p. 22) for more about this.
I make a lot of one up instruments and don't often want to take the time to make the side templates. In these cases I keep the rib stock rectangular and bend the ribs, glue in the blocks, and then scribe the varying rib depth right onto the inside of the ribs. To do this the block/rib assembly is placed in the mold and the mold is placed on top of the dished form. If the instrument will have a taper when viewed from the side a block of wood as thick as that taper is placed between the dished board and the mold at the tail block. Then the shape of the dome is scribed right onto the inside of the ribs.
The ribs and blocks can be quickly trimmed down to the scribed lines with a block plane.
Colin Symonds posted a message to the Official Luthiers' Forum describing a great way to use the mold to help make a side profiling template. The inside edges of the mold are lined with non-stretchy tape. Then the mold is set on the dished board as above and the profile of the dome is scribed onto the tape.
Here I'm using the thick brown paper tape used to tape down binding when gluing. Colin uses a different method of doing the scribing but the end result is the same. After the tape is scribed it is removed from the mold and taped onto the edge of a piece of template stock. The stock is band sawed to the scribed line and then the cut is cleaned up. Search out his posting on the OLF for complete details.
After the rib and block assembly is done, the mold can be used to prepare the linings for gluing on the top and then to support the ribs while the top is glued on. Here a dished work board (also called a hollow form) covered with stick on 50 grit sandpaper will be used to shape the ribs and linings to fit the domed top.
The work board is made of a 0.75" thick piece of MDF and it has another piece of MDF glued onto the back. So the assembly is pretty heavy. It is positioned on top of the ribs and linings and scrubbed back and forth to sand the ribs and linings to fit the top.
Since the ribs had been roughly trimmed to mate to the top in a previous assembly step it only takes a minute to do this sanding.