Making the Main Arch
The function of the tools I use and how they work are explained here. Specific tools used are noted in bold.
The main arch tapers in thickness from the middle to the ends. It is constructed by laminating together five plies of pending ply, which is a type of plywood that easily bends in one direction. While each ply is very flexible, once they’re glued together they form a rigid entity. The two outer plies are the full ⅜” thickness the length of the ply. The three inner plies are tapered on the ends. Each ply’s taper begins at the end of its mating ply’s taper. Therefore the each inner ply gets shorter than its mate. The tapers are formed with the widebelt sander. Typically the widebelt sander is used to sand boards flat and to an even thickness. To make the tapers the plies are fed through the sander on a tapered carrier (the top bed of the carrier is at a slight angle to the bottom). Multiple passes through the sander are required to sand the ply down to a wedge point.
Once the plies are constructed, glue is rolled on all mating surfaces. The packet of plies is then placed on the bending form inside the vacuum press’s large polymer bag. The vacuum pump extracts all the air, with the resulting atmospheric pressure pressing the plies together over the form while the glue dries. Atmospheric pressure is ~14.7 pounds per square inch. The total pressure on the arch therefore calculates to 9,260 pounds of force.
After vacuum forming, the ends are trimmed to length on the sliding table saw. Small imperfections in the curve are removed by hand sanding. Pencil lines are draw on the arch, and a flexible sandpaper lined block of wood is used to sand the curve until all the pencil lines are removed. This indicates that all the high spots have be flattened, and an even fair curve results.