construction photos

Here are photos of the beginning of fabrication of the acrylic bobbin inner core, and the steel backing plate.

The bobbin is formed from two strips of clear acrylic (0.08" thick each), epoxied together, with a paper strip sandwiched in the middle.

The backing plate I cut from the steel enclosure of a broken VHS player.

I used the same screws which will ultimately be the polepieces, to clamp the acrylic strips together during the epoxy process.  However, the screws here are going from the top; in the final arrangement, the screws will come up from the bottom (through the metal backing plate), and only the threaded ends will protrude slightly above the top surface of the bobbin.

 

The main trick with the epoxy, is getting the paper to stay centered as the pieces are clamped.  This clamping, intentionally, causes much squeeze-out of the epoxy, leaving only a nice, thin, uniform layer, free of air bubbles; however, the flow pattern tends to drag the paper sideways.  I found this out the first time, when I thought I'd epoxy the pieces together first, and then drill the holes.  Nope, needs the holes for indexing, so second try I drilled first and then epoxied.  Having the screws in the holes in order to clamp the work, also tends to keep the holes free of epoxy, but I expect to have to re-drill them anyway at least twice more, as I attach the top and bottom flange pieces.

 

J-B Clear Weld.  One of the three glues I use for everything (the others being Titebond II, and cyanoacrylate).

 

Here's the position of the backing plate under the strings of the harp.


 

The VHS player's steel enclosure was made of such thin and mild steel, that it was no problem to cut with my jigsaw, and I was able to drill the holes readily without any oil.  A little work with #80 sandpaper to clean up the edges and the holes.


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