Thursday, October 15, 2015

New Folded Bass Horn - More Progress On Second Horn

The rain stopped the next day, so I set up outside again.  First, I set up the clamp for a cross cut to separate the matching pair of trapezoids.

Cross Cut To Separate The Matching Trapezoids
Cross Cut To Separate The Matching Trapezoids
 This is just after making the cut.

Cross Cut To Separate The Matching Trapezoids
Cross Cut To Separate The Matching Trapezoids
 Here, I am preparing to attach two pairs of matched trapezoids to the central panel.  Earlier, I had made an extra one of these central panels as a pattern while I was cutting out panels for the first folded bass horn.

Attaching Trapezoids To Central Panel
Attaching Trapezoids To Central Panel
 To make sure that the countersunk holes are centered in the center plywood lamination, I use an awl to mark hole centers before boring the holes into the center panel edge.  I pre-drill all of the countersunk holes in the side panels on a drill press to make sure they are perpendicular.  I use the pre-drilled sides as a pattern to center the awl while marking the center lamination.

Marking Hole Centers In Center Lamination With Awl
Marking Hole Centers In Center Lamination With Awl
It is important to center the trapezoids on the center panel and maintain symmetry as correctly as possible to avoid starting with a slightly warped assembly.  This assembly is similar to a dustpan that, instead of collecting anything, is instead designed to distribute sound from the square shape of the horn throat to the rectangular shape at the opposite end.  In addition to the shape change, the cross-sectional area increases to follow a linear interpolation of an exponential expansion.

Dustpan Assembly - Both Sides Glued And Screwed
Dustpan Assembly - Both Sides Glued And Screwed


Dustpan Assembly - Both Sides Glued And Screwed
Dustpan Assembly - Both Sides Glued And Screwed
As you can see, the trapezoids are in the same plane so it would appear to be possible to cut each side from a single strip of plywood and avoid a butt seam.  I chose the simpler cutting process of separate trapezoids because it allows side cuts to be shared thereby saving plywood.  Tomorrow, I will attach four more panels that will enclose the dustpan passage and provide the sides for the returning passage.



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