- Centering rings
I made centering rings from the 20mm tube. The method is to
mount a disk sander on a bench or vice, and with the disk
running, rotate the end of the 20mm pipe against it so as to
get a steep cone shape, say about 1mm of depth over 10mm of
length. It's essential to use gentle pressure to avoid melting,
but *continuous* movement to avoid flat spots. It takes a bit
of practise, I threw away about 4 before I had one I was happy
with.
Cut the cone off the end of the pipe, and push it thin end first
onto a piece of 16mm pipe, and sand the thick end to a cone
also, so you have essentially a piece of PVC 10-15mm long, thick
in the middle and thin at the ends. You can sand it until the
thick spot *just* fits into the 22mm tube, or until it *almost*
fits (which is more useful).
- Launch tube assembly
Take a 350mm length of 16mm pipe, and two pieces of 22mm pipe
(50mm and 37mm long). Sand the ends of the 22mm pipes until
they are smooth, flat and at right angles - most important to
get a good seal. Make three centering rings, one for the top
(tapered both ways to center the 50mm length and to also center
a 22mm launch tube extension), one for the O-ring (tapered both
ways, and such that the 22mm pipes push tight with a 3mm gap
between them), and one for the bottom of the 37mm pipe (tapered
one way only). Glue them using epoxy or hot-melt as shown in
the diagram, ensuring that water won't leak between the two
pipes. The 16mm tube will poke about 100mm out the top to allow
you to fit a launch tube extension. You might want to plug the
end and drill a filling vent, which will help if you want to use
a T-nozzle later.
Adjust the O-ring slot depth by fitting a 3mm drill bit in a
mounted drill and rotating the slot against the bit to cut
the O-ring slot deeper into the centering ring. Adjust the
slot depth until a bottle neck slips over a lightly greased
O-ring with slight difficulty and a satisfying "pop". Try a few
different bottle types until you strike a happy medium.
The bottom centering ring will be under the centre of where the
cable ties are glued in, and the shape will help stop slippage.
Finally a 350mm length of 22mm pipe is centred and glued around
the bottom end of the 16mm launch tube, against the bottom
centering ring. This joint must also be airtight, and you
probably want to use another pair of centering rings (ones you
sanded too thin while experimenting before :-).
- Cable-tie release
I used a full circle of cable-ties (23 small ones to fill a
40mm circle). This was because it seemed that using fewer made
it more difficult to get the bottle to clip in due to the ties
moving sideways. Also when using 23 of them there's no doubt
about whether it's strong enough.
The cable ties will be hot-melt glued between the 22mm launch
tube and a 40mm length of 28mm PVC tube.
I arranged the cable ties by holding them down with a rubber
band while I adjusted the length and position. Adjusting them
was only possible by using a bottle neck and a 1cm length of the
40mm release tube, so cut those before you start. The O-ring
should be between the bottle main flange and the hump between
the flange and the thread. When you have the ties in position,
lash them tightly using linen thread at both ends. The upper
lashing must finish almost exactly 20mm below the centre of the
O-ring, as that's where the 28mm tie-retainer tube will come up
to. The bottom lashing is around the last centimetre of the tie
where it's thin, so I could lash it and still slide the 28mm
pipe over).
Apply hot melt glue between the lashings over a 40mm length,
squeezing the glue between the ties so it contacted the launch
tube. By the time I'd glued the whole area, some of the glue
had set, but undaunted I melted it again by rotating it over a
BBQ, before sliding the 25mm retainer tube over the top. The
result is very solid and strong and I don't anticipate any
problems with it.
- Release tube
This is a 40mm (nominal) tube with an end cap, held up over the
cable ties with a spring (I used stainless steel wire to wind
my own). The release tube is about 300mm long, but this will
depend on the length of your launch tube. I used a 40mm end cap
with a 25mm hole in it, to allow it to slide over the 22mm pipe
but stop it sliding too far back. Set the length so that you
can pull it back 30mm behind the heads of the cable ties before
it hits the screw adapter you will fit to the base. This makes
it easier to change the O-ring. You won't be able to maintain
the ties - if they break, salvage the launcher and make another.
Before gluing the release tube to its end cap, glue a retainer
on the 22mm tube to stop the 40mm end-cap from sliding up too
far. You want the top of the release tube to be level with the
top of the cable ties. Then glue the end cap on, and finally
glue on the screw fitting for the bottom of the 22mm launch tube
and you're done! Make a spring to hold the release tube up, fit
it to your fill adapter/base, attach a trigger pull-string, and
you can fire away!
