The top and back of modern flattop guitars and similar instruments are not flat at all, but are spherically domed. The radius of the doming is very large, typically 12' to 30' so the doming is slight. The plates are forced into the domed shapes by pressing them into dished forms while the braces are glued on. When the glue dries, the plates retain the domed shape. The need to clamp many small braces to a plate while pressing the plate into a form can make for a challenging arrangement of clamps. When instruments are built in large scale production environments this clamping is done using vacuum bags. But smaller production shops tend to use flexible compression rods called Go Bars to do the clamping. These are generally used with a simple clamping fixture called a Go Bar Deck. Such a fixture is simple to make and use. Here's how.
Last updated: Sunday, June 01, 2014
This is definitely one of those cases where a picture is worth a thousand words. Here's the picture:
What you are looking at is a go bar deck, sitting on the extension table of my table saw. The deck consists of two double pieces of plywood, a floor and a ceiling, separated by four pipe nipples and flanges at the corners. In the picture the upper transverse brace is being glued to the top of an acoustic bass guitar. Here's a closeup:
Underneath the guitar top is a dished work board with a 30' radius. The work board is made of two thicknesses of MDF (actually, this one is a commercially available dished work board, also called a hollow form, and it is made of a single thickness of 3/4" MDF, but I've glued another piece of MDF to it to make it stiffer). The dished surface looks blue in color because it has a disk of self stick blue 50 grit sandpaper stuck to it. This is used to sand down the ribs and linings of the guitar so the domed top will fit correctly. See Building and Using the Outside Mold for more on this. To keep the sandpaper from pressing 50 grit dimples in the soft top, a couple of thicknesses of plastic drop cloth are placed on top of the sandpaper. Then the top goes on. The braces are glued on by applying the glue, positioning them, and then clamping them down using flexible go bars, which are bent between the top of the braces to be clamped and the underside of the ceiling of the go bar deck. Here I'm using commercially available fiberglass go bars with a rectangular cross section. You can make go bars yourself by buying a fiberglass panel and ripping it into bars on the table saw, but fiberglass is tough on even carbide saw blades, so if you don't own a diamond blade you may want to figure in the cost of getting your saw blade re-sharpened when trying to figure out if it would be cheaper to saw your own rather than just buying commercially available go bars. Or you can make the go bars out of wood.
There are no critical dimensions involved in the construction of the go bar deck, but if you want to use commercially available dished work boards (2' in diameter) and commercially available go bars (2' long) it is best to keep all dimensions at 2'. So, the floor and ceiling pieces are both 2' square (actually a bit shorter on mine, since I just cut up a 4'x4' half sheet of plywood to make all the pieces) and there is a 2' long threaded 1/2" pipe nipple at each corner, with pipe flanges at both ends.
By the way, in use the pipes are in tension so you may be tempted to build a go bar deck with flimsier corner supports. I would recommend against this. Once you have a go bar deck you'll use it for all sorts of other purposes – a short assembly table, tool stand, etc. and so it is a good idea to make it sturdy enough to be able to support some weight placed on top of it.
I saw a picture once (sorry, don't remember where) of a very well thought out permanent go bar deck arrangement, I think at a lutherie school. There was a line of benches, each butted short end up against a wall. Above the ends of the benches on the wall was a heavy 2' deep shelf. The shelf was used for storage, but the underside could be used to support go bars used to clamp parts placed on the end of the bench. Very clever.
Allen Cary suggests a great enhancement, which is to mount the whole deck on a base with a lazy Susan bearing so you can rotate the deck easily while you are placing the go bars. This makes it easier to get to all the places you need to clamp.