A handy technique for making the coil holders


Just to add my 22c worth to the discussion on making a primary.

I used plywood for the main part of the triangular primary forms, but then
put 1/4" plexiglass "combs" on top of the forms to actually hold the 
tubing.  Since I have a drill press, I was able to use it to produce 
consistently located holes/notches in the combs.  Here is the technique:
Chuck up an end mill (a kind of cutter that you can get from machinery suppliers
such as MSC or Enco) that is the diameter of the tubing that you are using.
(I would bet that a forstner type wood drill bit would work fine though)
Get two pieces of scrap that have straight edges.  Clamp the first one to 
the drill press table so that the cutter takes a nick out of it that is a 
little less than half. 

I figured about 1/32 to 1/16 inch for a 3/8" diameter cutter.

Next take your strip of plastic and place it against the first straight
edge.  Then take the second straight edged scrap and place it against the
other edge of the plastic and clamp the scrap down.

 E  | P |  E
 E  | P |  E
 E  | P |  E

E == straight edged scrap
P == Plastic to be cut

Since you placed the first straight edge correctly, the cutter will cut out
a little more than half a circle from the edge of the plastic.  This notch
will allow you to snap in the tubing without using any type of ties.
The only remaining trick it to advance the plastic the correct amount 
so that your notches are evenly spaced.  I used a spacer block and clamps
to space out to the next stopping point on the scrap and put a clamp there.
Then I moved the plastic that amount.

To get around the problem of alignment, I cut grooves in the wooden 
supports that allow me to slide the combs along the hypotenuse of the
supports.  Then I used nylon screws to go through the grooves and into
the side of the combs.
Completed comb: (sort of)
      o                   o

Completed triangle (only the hypotenuse is shown):

    ______              ______
   (______)            (______)

Note that the grooves line up with the o's in the combs, which
are holes for the screws.