Big BenD Bass Horn: The Mouth Bend
The third part of the horn air path is the mouth bend. This is the most complex part of the build, because all four sides of the bend are curved. In addition, the sides twist as they go around the bend. The best way to go about this is to do a similar approach to what we did for the midrange horn: laminating several layers of plywood on a jig.
Then bend is 90 degrees,and the end surfaces are well defined. The inner bend radius is constant, but the outer radius varies. Additionally comes the twisted side walls.
As a start, I set up two boards at right angles. To these boards, two other boards were fixed, these serve to hold the inner dimensions of the horn.
Two arms will hold spacers for shaping the side walls. These are fixed to the main jig with T-nuts.
Here the side wall spacers are in place. They are important since the side wall twists as it goes around the bend.
The flanges are used as part of the jig, to help push the plywood towards the inner panels.
The inner bend is made from bendable plywood. The bracing is fixed to the flanges (the little rectangular 3mm plywood piece is used as a guide for the wooden dowel in each brace), and then works as the jig for the bend.
Three layers of bendable plywood are laminated to make the inner wall.
The sides are cut using a jig. The jig is a piece of plywood marked from an "on-the-flat" drawing of the side panel. The on-the-flat dimensions are calculated from the computer model by a series of vector rotations etc. These pieces (cut a little oversize, as it's hard to get perfect accuracy) are laminated on the jig, and clamped tight against the jig spacers.
Here you can see how the side walls twist around the bend. The same phenomenon can be seen in the Western Electric horns, and is very clearly visible in the 12A horn.
The outer wall is made from a combination of 3mm hardwood plywood (inner and outer layers) and bendable plywood. I found it easiest to fix the layers with small nails as I put them on, rather than using clamps. Pieces of plywood are used to hold the layers in place against the flanges.
Or not really. The final thouches will of course be bracing, sanding and painting. But when laminating this many layers, and also pushing them against a flat surface while at the same time trying to align them correctly, put on clamps etc, it's hard to get a clean edge at the flanges. To improve this, I used the router to remove 3mm of the inner, outer and side walls, and fill these spaces with strips of 3mm plywood. This removes all the unevenness between the layers, and creates a flat, smooth surface for the flange.
The finished bend (except for bracing) looks like this.