[Prev][Next][Index][Thread]

Primary Combs

I have seen a couple of others write on making the combs to hold a tubing primary out of acrylic 
for spacing out the tubing in the primary.  Most of the methods seem to have required a vertical 
mill.  I have a method I have used that only required a small drill press and a belt sander. I 
used this method to make a primary with a 20 degree pitch and .25" tubing with .25" spacing.  
This method also requires no cord or glue to affix the tubing.

First, draw out the shape of the combs on a piece of acrylic (I use 6 combs).  Make the combs a 
little taller than you want them, by at 1/16" more than 1/2 the diameter of the tubing to be 
used.  Cut them out with a saw of your choice. A jig saw works nicely.  C-clamp all the combs 
together.  Using an coarse belt, like 60-80 grit, belt sand all of the edges down untill they are 
even and straight.  Keeping them clamped together, draw a line down the edge that will have the 
grooves to accept the tubing on it.  Make sure the line is in from the edge at least 1/16" more 
than the radius of the tubing.  Center punch along this line for the center point of where you 
want the holes for the tubing to be drilled.  Be very careful to make sure you punch exactly 
where you want to hole to be.  There is little room for error here.  Using a drill of the same 
diameter as your tubing, drill your holes at these punch marks.  Note, it is important to use a 
drill bit made for plastic if you are using 3/8" tubing or larger.  A metal cutting bit will 
split the plastic as it bites through the backside of the acrylic.  If you have done it 
correctly, you should now have a straight line of holes down one edge of the combs that is very 
close to one edge.  Now, draw a line down both side of the plastic that is parallel to the edge 
with the holes and crosses the holes.  The line shoud be off center by 1/16"-1/32" or so.  It 
should be off center in the direction closest to the near edge.  Now CAREFULLY and SLOWLY begin 
using the belt sander to remove plastic from the edge with the holes.  Remove the plastic, 
checking both sides as you go, until you are at your line.  If you have done it right to this 
point, you will have a little more than 1/2 of the holes left.  Now change to a fine belt, at 
least 150 grit and sand the edges smooth.  Don't take too much off the side with the grooves.  
You want to have more than 1/2 of the hole left.  Remove the paper from the acrylic.  Now, using 
a propane torch, flame seal the edges of the acrylic.  DO NOT flame seal the edge you intend to 
glue down to your base.  If you are not familiar with flame sealing acrylic, it returns the edge 
to its original clear appearance.  You should practice this on the sanded edge of a scrap piece 
until you are comfortabe doing it.  You don't want to ruin the combs by melting them.  If you 
have done this correctly, you will now have combs that the tubing will simply "snap" into, with 
no glue or cord needed.  Minimal training of the tubing is also needed, as the combs will hold 
the tubing very securely.

The method I have shown above requires that each comb be glued down so the next one will have to 
be moved back (2 x tubing diameter)/number of combs.  This is to allow for a gentle tubing 
spiral.  If you don't wish to index the combs out, you can remove the C-clamps after sanding them 
to the same size.  You then simply index the combs by the above formula  and clamp them back 
together indexed before center punching and drilling.  This will allow you to simply glue them 
down on the same ID and the tubing will still have a gentle spiral, without a jump between 2 
combs.

Once you have guled the combs down you are ready for the tubing.  Bend the tubing so it has 
roughly the same ID as the inner turn in the combs.  Bend the inside end of the tubing down so it 
will go below the base.  I had to drill a hole in my bases to allow for this.  Before bending the 
end down, make sure you have the tubing spiraling in the same direction as your combs.  I made 
this mistake myself.  I had to cut off the bent end and then bend the new end the other way.  A 
small point, but it could save you a little cursing.  Now, starting with the bent end at the ID, 
and start snapping the tubing into place.  It should take you less than 15 minutes.  You may have 
to add or remove a little of the bend in the tubing as you go.  But you will end up with a 
professional looking primary that can be fired immediately.  You may have to adjust the primary a 
little over the next few weeks, but if the combs are tight, the adjustments will be very minor.

I have a photo of a finished primary made in this manner.  I will get it scanned and make it 
available to anyone who wants to see the finished product using this method.

Scott Myers