Water-Rocket Cap Holing Jig

It's easy to make, and allows much better-centered nozzle holes.
Plus it is much quicker to use - I can crank out 2 or 3 nozzle-caps per minute.

Warnings:
1.  This is best done outside - one should not inhale melted-plastic fumes.
2.  Also, there is a chance of fire.  
      Do not do this where a little fire would create a hazard.
      A fire extinguisher should be available.
3.  If your "fire extinguisher" is a hose or bucket of water, make sure you        
     unplug the drill press before using it!
4.  And try not to get the motor wet...

To make the jig, a PET bottle neck is cut off just below the flange.
Since the hacksaw invariably angles, the base is sanded flush to the flange.
Using a better saw might eliminate the need for sanding.
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A PVC pipe fitting is used to complete the jig.  It is a 1/2 inch male threaded adapter.  The other end is 1/2 inch female slip-fit, but that doesn't matter.  The 1/2 inch male threaded portion happens to be a snug fit into the bottle neck, providing good centering.  Notice that there are two PVC adapters.  One has hexagonal flanges, the other a knurled exterior.  The one with flat hex flanges is much better for this purpose because it will clamp more securely in drill-press vise.
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The bottle-neck is fastened to the PVC fitting with 5-minute epoxy.  Yes, I know....epoxy is not the best glue for either of these plastics.  But this joint has a large surface area and will not be highly stressed unless I do something dreadfully wrong. Epoxy is quick, easy, cheap, adequate, and I happen to have some.  Thus it meets all my criteria.
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Here is the first of these jigs (left) and today's project (right.)  Notice the sloppy glue-job on the first one.  I got in a hurry.  Serves me right.  Works OK anyway.
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Mounting the jig.  Here I have cut the head off a 3/8 inch steel bolt and mounted it in the drill-press chuck.  The plastic jig is clamped in the vise, centered under the bolt, and tightened down.  Precise centering is difficult.  This is where a good indexing vise would be helpful.  Guess I ought to buy one.
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Since semi-retiring my 4x4 airframes, they have found many uses.  This one the torch bearer.  It serves most proudly.
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Once the iron is hot, glowing dull red, the torch is removed, the drill press turned on (340 rpm) and the hot bolt lowered into the cap.  It melts a hole readily.  Upon removal of the hot iron, the cap solidifies quickly, is removed and another installed.  With a few seconds reheating, the iron is ready for another one.

I made 8 of these today.  They are tested by mounting in a small PET bottle, connecting to the electric pump, and filling with air until the valve-stem pops.  I watch the gage, and note how high the pressure went before it popped.

There is considerable variation.  Some pop the valve-stem plug out as low as 50 psi, some as high as 220 psi.  The highest of these is a bit scary, but I haven't blown up a bottle yet.  I have marked each with its tested pressure-rating, and will experiment further.

Jimmy Yawn
1/13/03
rev 11/20/04
jyawn@sfcc.net