Building a Guitar 3 -- Moulds and Workboard

[Bruce Taylor]

Note: This is a continuation of two earlier threads:

The Rosette

Some Thoughts on Design

* * *

You don't need a mould to build a guitar. The traditional Spanish method, used by most of the great classical and flamenco builders of the 20th century, is to assemble the instrument freeform on a flat workboard, or "patron." The method is very thoroughly described by William Cumpiano in his useful book _Guitarmaking: Tradition and Technology_ . Here's a Cumpiano-style workboard:



The cork and paper "shim" on the top of the patron is designed to allow for the slight arch cut into the bracing of most classical soundboards.

It's a proven method, well suited to small shops with few power tools.

However there are advantages to building the instrument in an outside mould, and in recent years I've switched over to this method of construction. My main reason for adopting the outside mould is that it is better suited to the use of "spherical" doming on the back and soundboard. More on that presently.

Here's a typical outside mould (this one's for a mandolin, but the principle is the same):



I don't have a mould for the body shape I've chosen, so in this thread I'll make a new one.

I start by gluing two sheets of 3/4" ply together. On this, I trace the outline of the guitar and then use a bit of scrap wood to offset the outline by two inches:



I cut out the workboard and set it aside.

[Bruce Taylor]

I glue up some more 3/4" ply, making a stack 3" thick. I clamp the stack together with wood screws, taking care not to place any where the bandsaw blade will need to go.

I mark out the shape of the guitar, as above and cut right to the line.



The inside of the mould isn't as smooth as I'd like -- the new bandsaw blade I used is truly worthless. Vermont American, again...16 bucks, and headed straight for the trash.

Fortunately, the sides are square, and the small ridges in the surface don't matter. Some builders take their bandsawed moulds to the Robo-sander template-sanding drum for further refinement...but I won't need that quality of finish, here.

Now, I rout a quick n' dirty rabbet around the bottom of each mould. This is just to provide a bit of relief in case I want to leave some "overhang" on the soundboard.

I don't have a template bit on hand, so I just round the end of a stick and clamp that to the router table:



I now have an outside mould. I want this to be removeable, so I'll attach it to my workboard surface with these little things (whatever they're called):



I always keep some of those around (along with some T-nuts, wingnuts, washers and a few feet of threaded 1/4"-20 rod). Very handy for making up clamps, and whatnot:



If I had some nice knurled platic knobs I'd use those, but it's a long way to the hardware store, so I make my own with a hole saw, and epoxy some bolts into them:



To sand them, I chuck the bolts into my drill press and hold some sandpaper around the knob (please, no safety lectures...it's a fairly wimpy drill press, geared to the lowest possible speed).



I assemble the mould, and test it with an unused soundboard I have lying around:



I'll refine the mould later, tweaking the shape a bit, sanding and adding a couple of coats of shellac.

Note the turnbuckle and spreader at the waist of the guitar. This will keep the sides flush against the moulds during assembly. To make it, I simply hacked off the eye of the reversed-thread bolt on the turnbuckle and epoxied it into a piece of scrap butternut. I glued a length of threaded rod into another piece of scrap, then shaped the end-pieces and added a bit of cork to them to make a couple of padded cauls:



The workboard is mostly done. I'll put it aside for the moment and do a few other chores.

[Bruce Taylor]

Another mould I use is a simple side-bending form. This will be used, along with a silicone heating blanket, to bend the sides, binding and solid linings of the guitar.

All of this bending can be done manually, using a hot pipe (as in the Rosette thread posted earlier). However, some woods (like ebony, honduras mahogany) can put up a real fight, and it's far more relaxing to bend them over a form like this one:



This form is bulit up from five pieces of 1" MDF. The outside laminations are cut to shape on the bandsaw, creating a "lip" to which spring clamps can be applied during the bending operation.

The last important jig is the "radiused dish."

As you probably know, guitar backs and tops are not usually flat, but slightly domed from end to end and from side to side. Perfectly flat backs and tops are not unheard-of (Arthur Overholtzer used them), but they are rare. The domed plate, which can rise and fall with changes in humidity, is more structurally sound.

In this pic of a zebrawood steelstring, the convexity of the back is slightly exaggerated by the camera angle:



Until about twenty years ago, the dome was established by hand, and the arching was done by rule of thumb. Then, some bright luthier had the notion that it might be practical to think of top and back as sections of a sphere. He reasoned that if a guitar were built in a dished workboard, it would be easier to create -- and endlessly repeat -- exactly the right arch in the bracing. Furthermore, if the dish were then lined with sandpaper it could be used to shape the sides of the guitar to perfectly match the back, so that there was not unnecessary tension built into the back when it was glued on.

The idea worked and, it caught on. These days most builders seem to be using radiused workboards.

You can buy these things ready-made but they're shockingly expensive. Fortunately, they're not to hard to make with a router, a hunk of MDF and a simple CAD program.

For this guitar I'll need a dished workboard with a "radius" of 25 feet. First, I go to TurboCAD and draw a circle with a twenty-five foot radius. Then I snip out a small portion of the circle -- about two feet of its circumference -- and delete the rest. I print it out full size (easiest if you have a plotter and big sheets of paper, but it can be done with an ordinary printer if you're willing to tape sheets together).

I use this as a template to cut a curved router sled, as in the following pic (of a 15-foot radius dish, used in an earlier instrument):



Then, I cut piece of 1" thick MDF into a circular disk with a diameter if about 2'. I drill a hole in the center of this circle, and mount it with a single drywall screw at the center of disk (I countersink the screw so that the router blade won't touch it).

Using a 1/2" bit in a 1/2" shank, I begin running the router back and forth down the sled. As I work, the action of the router causes the disk to spin, and dust flies everywhere! Do this outside, and take a shower afterwards. As the disk spins, new parts of its surface are exposed to the router.

When every inch of the disk has been routed to the same 25' radius as the sled, I remove it and clean up the concave surface with a random orbit sander.

The process doesn't take long, but it makes a hell of a mess.

[Todd Bradshaw]

Get to work Bruce. We want more cool pictures!

[Bruce Taylor]

I have some uninterrupted shop time today, and I'll keep the camera handy.

When I only have a few minutes to spare, I sweep the floor, or make sanding sticks. Carpet tape, again:

[Mrleft8]

I'm not quite sure what I'm looking at when it comes to the top/back mould....Seems that clamping the edges down would introduce the opposite arch than you want (dish as opposed to arch)
And on that Zebrawood guitar, is the edge banding off center, or is that an optical dillusion?

[Bruce Taylor]

I'm not quite sure what I'm looking at when it comes to the top/back mould....Seems that clamping the edges down would introduce the opposite arch than you want (dish as opposed to arch)

I think it'll be clear when I get to the assembly.

Note that the "radiused" dish/workboard is a separate jig, from the mould/workboard. It will be used only for shaping and attaching the back.

I'll build the guitar "face down" with the top on the workboard/mould. The top will have an arch in it, too, but it's much shallower than the arch in the back, and on a classical I won't make this a section of a sphere (it is more like the section of a cylinder). I'll either cut a slight hollow in the workboard to allow for it, or I'll add a little cork shim to the edges, as on the Cumpiano workboard in the first photo, above.

After the top and sides are together, the back (which has been "pre-arched" and glued together in the radiused dish) will be glued on like a lid. Before gluing it down, I'll stick some sandpaper inside the radiused dish (again, with the carpet tape!) and sand a domed profile into the sides, linings, heel and tailblock. That hefty outside mould and turnbuckle spreader will hold everything in place everything while I grind away at the assembly with the big sanding dish.

That's really the main purpose of the outside "mould": to provide support to the sides during this sanding operation. It isn't really needed to provide "shape" to the sides, since these will have been pre-bent to fit the template, anyway.

It might seem like a lot of trouble to go to, just to get a good fit for the back, but I think it's well worth it. I've built instruments with "tension" in the back and they are far more vulnerable to seasonal fluctuations in humidity.

And on that Zebrawood guitar, is the edge banding off center, or is that an optical dillusion?

The banding is on center. It's the white maple binding that is joined off center (with a tiny scarf). It doesn't show much on the actual instrument, but the camera is merciless.

Visible joints are not a problem with dark binding, but maple is less forgiving. I put the joint off center on purpose, thinking it would fool the eye into thinking the binding was continuous (like the ivoroid binding on a factory guitar). It obviously doesn't work! Next time, I'll try something else.

[Bruce Taylor]

I'll either cut a slight hollow in the workboard to allow for it, or I'll add a little cork shim to the edges, as on the Cumpiano workboard in the first photo, above.

Revisited the workboard today and put a slight hollow in the lower bout.



Knotty bits in the ply needed scraping:



I need a dish 1/16" deep.