Our objective here is to
cut three equally spaced and perfectly straight fin slots. Of course
they need to be the right length and width too! The photographs below will show
how this was done.
You can click on any of the photos here to see a larger image.
This is a
simple wood fixture for slotting 4" body tubing using a router.
My router is
mounted to a "sled" with three small bolts. (Not shown.) The
sled is made from some 1x2 pieces of wood and a transparent sheet of
acrylic. The acrylic makes it more convenient to see the alignment
of the router bit to a mark on the end of body tube.
Building this fixture took
all evening. Setup was just a few minutes and cutting the perfect fin
slot only took a few seconds!
is now positioned for cutting the first fin slot. I used a 3/16" straight cut
bit. The fins are 1/8" thick so this bit produces a slot that is
slightly wider than the fins to allow for tolerances and alignment errors
between the slots and fins.
A sacrificial bulkhead is
taped in place near the end of the body tube to help support the body tube
as the slot is being cut. This helps maintain a more consistent slot
Top view. Ready to go!
The length of the slot can
be precisely set by putting the router bit up against the end of the tube
and then measuring the desired length from the edge of the sled. The
length is then marked on the top of one of the side pieces using a bit of
blue tape. Then it
is just a matter of stopping the sled at the blue tape.
Top view. First slot
The sled is near the ending
position as marked by a piece of blue tape.
End view. First slot
complete. Rotate and
repeat as needed!
When rotating the
tube, it will likely move lengthwise a little bit, so it is necessary to
double check the slot length each time. The ending mark may have to
be adjusted if the body tube is not positioned lengthwise to exactly the
By the way, an astute
observer will notice that the fin slot cut here is much longer than needed
for the fins on Coldfire. This is because Coldfire went through a
redesign part way through the construction process in order to save
weight. Originally it was longer and had a different fin design with
a much longer root edge. However, it became clear Coldfire would be too
heavy and the design was changed. This tube had already been slotted
by then, but it could still be used by simply cutting the tube off at the
end that was slotted. That left a shorter slot that was perfect for the
new fin design.