Friday, July 6, 2012

Weekend Project: PVC Blow Gun

I'm a firm believer that in order to teach respect, the danger of a lack of a respect must be well understood.  In polite society, crassness is the least of the problems that flow from a lack of respect for one another.  With kids, respect is paramount to their well-being, both at home in a peaceful relationship with family members, and outside the home when dealing with other kids and adults.  Respectful children are almost universally recognized and praised as worthy companions.  We would all do well to be respectful to the people around us, and of the things around us... especially the dangerous things.

Dangerous things come in all forms.  Potential kinetic energy is all around us.  A rock perched on a deck railing has the potential energy of its mass and the height at which it is perched.  As a rock falls at 9.72 m/s squared, it gains momentum and speed.  When the rock comes to a stop, its kinetic energy is transferred into whatever it lands on, whether it's a siblings head or a toy car or just the good ol' earth.  Damage will be done in some form.  A failure to respect this fact will result in injury.

Putting kinetic energy to practical use is one of man's crowning achievements in the engineering arts.  The humble blow gun is a fantastic opportunity to look into such variables as mass, air pressure, drag, mass and velocity. To make a blow gun, you're only going to need a few thing easily had at Lowe's or any other reputable hard ware store.  I made ours from 3/4 inch pipe adapters (for the mouth piece) and ordinary 1/2 inch PVC schedule 40 pipe.  I used standard two-part epoxy to weld the pieces together to ensure a good seal that would hold however much pressure we could blow through it.  We used 30" pipe sections and made four blow guns so everyone can have fun.  Maybe we'll decorate and personalize them later.

1.  Use a disposable surface to mix your epoxy, such as a cardboard box.  This stuff is pretty much permanent and you don't want to apply any of it to yourself or your fellow humans, so beware.  Also, the fumes are icky.
Here we made a good puddle, mixed for 2 minutes, and then dipped the end of our pipe in it and
swirled it to coat thoroughly.  Twist together to coat the adapter and then press firmly for 30 seconds to set up.
Once assembly is complete, the mouth piece I chose needed a little finish work for comfortable use owing to a lip in the plastic casting.

2. File or sand down lip, if any, for comfort
When blowing through the gun, the pressure tends to move your lips out a bit which can become uncomfortable after a few shots.  I used 200 grit sand paper to smooth the inner surface down and all was well.

For our projectiles, we experimented with a few things.  I first bought a bag of 3" wooden golf tees.  As we found, they lacked sufficient mass to penetrate our test target, a cardboard box.  Next I tried sling-shot marbles with some cloth wadding in front of it to keep it from falling out the other end of the pipe.  The marbles did pretty good damage owing to their mass.  The weight of the shot and it's near perfect fit in the pipe allowed most of the force of air being blown through the pipe to go into accelerating the shot.  The end result was some pretty grievous dents in the box, but no penetration.  For the purposes of our experiments, we're equating penetration of a layer of card board with "kills a rabbit" though I doubt we'd be able to achieve that in real life.  We're just killing imaginary cardboard rabbits.

With our 30" gun, the range is pretty limited with most projectiles we came up with.  The best was a 16 penny nail pushed through an inch square of cloth.

3.  Cut a 1 inch square of scrap cloth and press a 16 penny nail through the center.
The nail has sufficient mass to carry through and penetrate target (hence "kills a rabbit").  The cloth provided two functions.  One, as shown below, it provided wadding to accept the air we were blowing down the pipe.  Without it, one's breath would simply pass around the nail and it would only slide out of the pipe rather than fly from the gun.

4.  Wrap wadding around nail head to form a plug.
Carefully inserting it into the back of the pipe keeps the nail from pulling the wadding all the way down the pipe.

5.  Our "dart" is loaded into the "breech" of our "gun".  
Rabbits everywhere tremble in fear and run away to hide in their holes.
Then, just aim and BLOW!!!

6.  Keeeled a rabbit!
The gun lacks any useful accuracy, but is NONE-THE-LESS dangerous.  A 16 penny nail flying through the air for 15 feet with enough kinetic energy to poke through a single sheet of cardboard will MOST DEFINITELY BLIND someone so unfortunate as to be in the way.  This makes this a very dangerous toy and ADULT supervision should be observed at all times.

Other than that, have fun cramming stuff into the back of the pipe and seeing how far a good hard blow will send it.  It took me about 25 shots with various projectiles to grow tired of it.  Good clean and dangerous fun.  Perfect for a father on any given weekend.

Taking it further

A longer bore would give more time on acceleration before the wadding cleared the end of the pipe and your blow was no longer providing thrust. Maybe 2 meters of pipe would be better?  This would improve the accuracy, and lethality of the gun.  One could also epoxy a small brad to the end of the pipe as a front sight so that more practiced aim could be taken over time.

As for the darts, I'm sure a hat pin and cotton ball would be a better choice.  The hat pin has less mass than the nail, but greater mass relative to the size of the impact area which contributes to the resultant penetration on target.  With our nail, the surface area to mass ratio is sufficient for the nail to carry through.  With the marble, the surface area was much higher relative to the mass, so all we got were some gnarly dents.

You can probably do better than an unmarked cardboard box as your target.  My eldest minion made some nice circles with a pen, but my vintage vision couldn't see them clearly at firing range!