INTRODUCTION
Testimony to my condition
which I call "chronic, untreatable youthfulness" (CUY) is the reaction
I have when
talking a group of my
neighbor's about hometown politics at the annual block party, and then
seeing an 11 year
old out of the corner
of my eye with a backpack mounted, high pressure "Super Soaker" squirt
gun. One minute
I'm engaged in a discussion
on some local building inspector's policies, next, I'm distant thinking,
"Cooool! I bet I
could make one of those!!!!"
But I don't think you
have to really have this condition to be intrigued by simply the name "potato
gun". The very
term invokes curiosity;
with two words not ordinarily seen next to each other. They surfaced on
the VW
newsgroup for some strange
reason, many people chiming in about how fun and potentially dangerous
they were.
And speaking of, there
are two more phrases that grab the attention of my condition: "fun" and
"potentially
dangerous". Perhaps
it is that they always intrigued you, but suddenly in life you find that
there are no more
parents around to tell
you can't do something, and you can now just hop in your car and drive
to Home Depot
and buy stuff.
This page chronicles
how I come about building my potato gun, without all the technical details.
If you are
familiar with my writing,
it is more of a story. And it is long.
In an idle moment one
day, I did an Infoseek search on "Potato Gun" and got plenty of hits; proving
to me once
again, that there is
nothing that isn't on the internet. I ended up on page called "Backyard
Ballistics". Pretty cool
stuff. I started considering
building one of the "hair spray" guns, but then I saw a link to the page
for the
"Pneumatic spud gun".
I had to read the text and stare at the diagram for many minutes before
it all made sense.
Then it seemed waaay
to cool not to try. I have a compressor and any use that I can find for
it only serves to
justify its purchase.
Any use. I forwarded the URL to my partner-in-waste-of-time-projects, Bill,
and called him.
He suffers from CUY
as well, but not as severely as mine. He closed the conversation saying
"When you build
that thing, give me
call. I want to come over".
A WEAK FIRST ATTEMPT
I made a mental note
of the main parts and a few days later even wrote "PVC" on my shopping
list prior to one
of my Saturday pilgrimages
to Home Depot. But I intentionally passed up the "plumbing" isle. "Too
much stuff
going on right now,
too many other projects. Maybe another day". But a few weeks later, may
brother-in-law and
his large family came
to stay with us for a weekend. Their arrival was considerably earlier than
expected and I
had planned to do yard
work that Saturday. I spent a few hours working in the yard but began to
feel a bit guilty
at not spending time
with my wife's young nieces and nephews, and our own, curious 6 year old
son. For some
reason I went into the
basement to get something. Out of the corner of my eye, I spotted high
above on a rack, a
long length of 1 1/2"
PVC. The exact diameter specified for the barrel of the potato gun and
about a foot and a
half longer. I had no
intention at that point of building the full-fledged gun, but thought that
just applying
compressed air to the
end of the pipe with a potato shoved in there might make for some amusement.
I rifled the
"plumbing" box in the
basement and came up with a cap and some glue. I glued on the cap, then
drilled a hole in
it and put a compressor
fitting. I cut about a 3" piece of the PVC off the end and with the bench
grinder ground
a sloping bevel on the
circumference of one end, producing a sharp edge.
My 10 year old nephew
strolled into the garage looking inquisitively at my hands all covered
with white PVC
"dust". "Drew, go tell
Ryan I'm making the potato gun" I said, Drew strolled back out. A few minutes
later I
rounded the corner of
the garage with the seven and a half foot spud delivery prototype, with
a blow gun affixed
to the capped end and
compressor line attached . I met Ryan running up the yard. "Daddy, is that
the potato
gun??" I was distracted
by the sound of a car door closing on my driveway. It was Bill, coming
over to pick up a
tool he had left at
my house. "Boy Bill, your timing is perfect. Ryan, run in the house and
grab the bag of
potatoes in the hall
closet..!" Bill walked up the driveway with a big grin on his face.
It was reminiscent of
a Disney movie about the mad professor stepping out into the sunshine with
some wild
contraption, kids scurried
everywhere, I spotted the neighbor's girls peering out of their open bedroom
window. I
felt like "Doc Brown"
from Back to the Future. You could feel the excitement building. Ryan returned
with the
potatoes, "There's only
three left daddy!?". "Ok", I thought, "we have to make the best use of
them". Placing the
beveled 3" piece on
the top of one of the spuds its end, I forced it down over the veggie until
it touched the
bench. Then I pushed
the "core" of the spud out of the PVC. A perfect potato cylinder. I quickly
filled a small
bucket with some warm
water and dropped the 3 cut potato cores into it. "Which way you gonna
shoot it?"
someone asked. "I'll
aim it up over those trees, nothing but woods that way" I replied. I had
conservatively dialed
in 20 psi of pressure
on the compressor regulator, not knowing what might happen. I honored a
neighbor kid by
letting him use the
broomstick to push the potato core down into the barrel as far as the stick
could push it. "Here
we go" I said and squeezed
the inline blowgun trigger...
There was a hiss and
the harmonic sound of air rising in a tube. In what seemed like about a
minute and a half
later, the potato made
an appearance at the top of the barrel, rolled over the lip and fell to
the ground. One of the
kids started giggling.
"Ok, we just need more
pressure", I dropped the gun on the ground, ran into the garage and dialed
the regulator
up to 120 psi. We picked
up the unscathed potato plug and re-inserted it into the barrel. "NOW this
puppy is
going to fly" I shouted....
This time the hiss was
more urgent, the harmonic a bit quicker and with a slow "wa-THOOoooo" the
little spud
cleared the barrel and
continued skyward for maybe another 10 feet before falling to the ground.
"I thought this
thing was supposed to
go 200 yards?" Bill asked. We reloaded and shot the core up a few more
times, the young
kids ran to try and
catch it, but quickly we began to loose our audience. When most of the
kids had gone I
noticed Bill staring
at the tube with that bent gaze that he has when he is trying to solve
some totally insignificant
problem. I know this
look. He is hooked and there is no stopping him now.
"Ya know" he said, [when
he starts with "ya know", it usually means that he has some really far
fetched idea up
his sleeve] "if you
want to try something dangerous, I could try to hold my hand over the end
while we build up
pressure behind the
potato, then let go." Naturally, with the apparent lure of danger, we couldn't
resist (and the
fact that Bill had volunteered
to place his hand over the barrel of the gun). Most of the kids had dispersed
and we
longed for some real
action. We held the tube at each end, almost completely horizontal, Bill
pressed the palm of
his hand hard over the
end of the barrel while standing to the side. "Ok, go." he said. I squeezed
the trigger and
the sound of air pressure
building started instantly. A micro second later, with a grimaced face,
Bill moved his
hand from the barrel.
"FOOOP!" The potato core flew from the end of the barrel with yet-unseen
velocity, hit
Bill's hand and broke
into pieces. Bill dropped the end of the barrel and strode quickly across
the yard raising his
hand up and down with
an even more grimaced look on his face, just like I remember my dad doing
when he
smacked his thumb with
a hammer.
I think I have some idea
how Edison felt when he saw that glow of light from the first light bulb
in his lab. This
was a huge milestone.
[Oh, about Bill. His hand was Ok. It continued to sting for some time,
he came in the
house and put an ice
cube on it. I got kind of red. "It felt like someone whacked me across
the hand with a
broomstick" he said
the next day. "It's still a bit sore"] Suffice to say if I
had not seen that potato go (and the
look on Bill's face)
I would have never pursued the "real" gun any further. But I saw the potential
for something
enormous, and so did
Bill. "So what exactly do you need to make the real gun?" Bill asked, still
swinging his
hand.
THE REAL DEAL
Bill is one of those
people who always is busy, seldom seems to have time to do anything remotely
fun, always
has something "scheduled";
until something pops up like this. Then, time is a non-issue. This is a
symptom of the
advanced stages of CUY.
I ran in and got the
cutaway diagram that I printed from the web page and went about explaining
to Bill how it
worked. Like me, it
took a few minutes staring at the diagram, then the comes on. "oOOoohhhhhh,
I see!" he said.
He volunteered his wheels
to run down to the hardware store to buy the stuff. We always have the
same dilemma
where we live. Home
Depot is 25 minutes away and is guaranteed to have what you need. But there
a few
"hometown" hardware
stores within minutes who might have it. We opted the try Aubuchons in
Hudson, a local,
friendly place that
for some reason I remembered having a fairly extensive plumbing isle. The
real gamble, we
thought, was finding
the 3" PVC. I didn't remember every seeing any PVC at Aubuchons.
We got there and headed
straight for the plumbing isle, they had lots of stuff. We found the 3"
x 1 1/2" reducer
pretty quickly, but
the "cleanout" thing stumped us. We spent a good 25 minutes there rifling
through all sorts of
adapters, boots, and
threaded gizmos; fitting part on top of part. Eventually my "cheap" side
kicked in and we
decided that a removable
plug cleanout was overkill and that a simple 3" cap, if placed correctly
would do the
job. As it turned out,
we were wrong about that. Just as we went to ask an employee about the
3" pipe, I noticed
a rack in the back near
the loading dock with PVC pipe stored upright. We walked quickly over and
picked up a
10' piece.
We loaded it through
the open hatch of Bill's GTI and headed to my house. On the way home, we
discussed the
big mystery and only
unprocured part, the "diaphragm". The web page wasn't real clear on what
this thing was
made of. All it said
was: "Cut a 3 3/16 diameter disk of 3/32 thick polypropylene or polyethylene.
A cheap
dishpan from Kmart may
be a good source of diaphragm material." What's a "dishpan"?
A plate? A food
storage container? That
tray you put dishes on to dry? I had no idea.
Once we got home, Bill
went about cutting a 3 foot piece of the 3" pipe with a hacksaw. The perpendicularity
of
the cuts on that pipe
were not critical. I sliced the end off the 1 1/2" piece, just next to
the cap I had glued on for
the earlier, anti-climactic
experiment. The web page said that the perpendicularity of this cut was
critical. For this
I used my 10" miter
saw. It cuts PVC very nicely and smoothly if you go slow. Then I ground
the little stop-ridge
out of the inside of
the narrow end of bell adapter and glued it on the 3' piece of 3" PCV.
"Sliding" the bell
reducer over the 1 1/2"
piece was much harder than it sounded, in spite of the fact that I thought
I did a good job
grinding out the ridge.
It was a very snug fit. We ended up laying the whole thing on the floor,
with the 3" piece
butt up against the
garage wall, and placed a wooden block on the snout end of the barrel and
pounded it with
heavy rubber mallet.
We stopped when the end of the barrel was about 1/4" shy of being flush
with the back end
of the 3" pipe section.
We thought this placement was going to be pretty critical, so we did not
glue the bell
adapter to the 1 /1/2"
PVC at this time so we could adjust it later.
"Ok, what are we using
for a diaphragm?" Bill asked. I shrugged. We knew that it had to be fairly
flexible, but that
it must also contain
an enormous amount of pressure behind the barrel end. Within an hour, my
work bench
was littered with all
sorts of plastic circles; and the tupperware cabinet in the kitchen was
now void of at least a
half dozen lids, sacrificed
in the interest of science. We cut and flexed and examined, and realized
the first
mistake in buying a
cap for the end instead of the cleanout with unscrewable plug. "We have
to get it right the
first time" I said,
"once it is glued, it is in there". "Hmmmmmm" Bill replied, "well, if we
really have to, we can
just slide the whole
chamber off the end to get access. Remember we didn't glue it yet."
Yeah right. "Just slide
the whole chamber off" We had to pound the damn thing on there with a 3lb
mallet, now
we would have to pound
it back off with some kind of special "Spud Gun tool #23456" fork, then
pound it back
on again. I'm not sure
which diaphragm we eventually chose, it is insignificant as as it never
worked. We drilled a
small hole in the back
of the cap and squirted air in using the blow gun, but it just hissed out
of the barrel.
Dismayed, Bill agreed
that we had to get the cleanout. I tried to convince him to run to Home
Depot with me, but
his CUY had waned and
he said he had to get home.
After dinner, I told
my wife that I needed to run to Home Depot. As expected, I got the "We've
got company,
what could you possibly
need now, that stupid gun, NO WAY!" reply. But a few hours later, I got
"permission"
to run out after all
the kids were in bed if they were still open. I called at around 9:30 and
found out they were
open until 11:00. I
ran out and got a 3" cleanout.
Upon my return, I placed
the gun up on the bench with the back end under the 10" miter saw. I carefully
brought
the saw down just in
front of the glued on cap, cutting the 3" pipe but stopping short of the
barrel pipe inside. By
rotating the gun, and
cutting a few more times, I cut the 3" pipe all the way around just in
front of the glued on
cap and removed the
cap. I glued the new cleanout on the end of the 3" pipe. Now I had the
task of cutting a bit
off the end of the barrel
off so that it was positioned properly when the plug was installed. As
I write this, it has
been many weeks since
this experimentation and I don't remember the exact sequence of events.
But for some
reason at this point,
I had glued the narrow end of the bell adapter to the barrel and was now
stuck "guessing" the
proper offset. I took
some rough measurements, including the thickness of the plug, and using
a thin carbide disk
on the Dremel tool,
cut the barrel from the inside as the diameter of the cutting disk was
only about 1". I knew
that I needed the barrel
end to be perfectly perpendicular and that it was going to be all but impossible
to get it
square. It was.
I could look at the barrel
end and see that it wasn't square. So I put worn down 3" disk on my air
die grinder and
inserted it into the
cleanout opening. I was hoping to use the face, not the edge, of the disk
so smooth and square
the barrel end. This
work OK, but now there was another problem. The barrel end was now too
far inside the
cleanout. I used a cheater
pipe and a pipe wrench to get the plug in far enough to hold the diaphragm
against the
end of the barrel. Eventually,
I ended up buying another cleanout, and cutting the old one off in the
miter saw and
cutting all the way
through the barrel too, making it square.
But back to the diaphragms.
I still had no idea what the "right" diaphragm was, but I had a few more
candidates
lined up. But with the
cleanout plug I could now swap out the diaphragms easily. A started with
a thin one, cut out
of a Folgers coffee
can lid. I had the regulator set on about 40psi. With the gun lying on
the garage floor, no
potato in the barrel,
I tried it. Pressure built up in the chamber, but the air just hissed back
out of the hole in the
cleanout plug when I
removed the blow gun. I cranked up the air to around 60 psi and tried again..
When I released the gun
this time, there was very loud and fast "FOP!" and something hit the recyclable
bin
under the stairs. With
serious force.
This was the second big
milestone (the first was Bill's hand getting whacked, if you don't remember).
That sound
of air suddenly decompressing
out of the barrel was very encouraging. And it sounded dangerous. What
had
happened was the Folgers
lid had collapsed into the end of the barrel and was expelled out. Thus
I knew I needed
a more rigid diaphragm.
Much more rigid.
ALMOST THERE....
One of my "proto" diaphragms
was cut from the bottom of a hard plastic food storage container, was by
far the
stiffest material that
I had cut and was my next choice given the demise of the flimsy coffee
can lid. I examined
the end of the barrel
to insure it was square and smooth, it looked fine, and inserted the thicker
diaphragm and
installed the plug behind
it. At this point I wasn't really sure how far in the plug should go. Should
the diaphragm
have some "play" between
the plug and the barrel? Should it be snug or should it pressed to make
it "bow" a bit?.
I eventually concluded
(and have demonstrated) that they should have about 1/32" of play.
But at this time it still
didn't work. When I squirted the air in, the diaphragm made a good seal
on the end of the
barrel and I could hear
pressure build in the chamber. But when I released the blow gun, the air
just hissed back
out of the hole. The
diaphragm was not being forced back off the end of the barrel for some
reason. I didn't know
why. It was past midnight
on a Saturday night, I quit for the night.
The next day, after the
usual Sunday morning rituals, was my mother-in-laws surprise 70th birthday
party.
Needless to say, the
opportunities to mess with the gun in the middle of the day were not realized.
But my mind,
all day at that party,
was trying to solve that mystery. It was drawing diagrams on an easel with
color coded
shaded areas showing
pressure differentials. It was doing state diagrams at blow gun release
t-minus-zero and
doppler color flow to
identify areas of turbulence in the chamber. The program never stopped
running. Sometime
after the 17th party
guest took the stage to laud my MIL, the program stopped and returned a
result.
The seal that the
outer, backside face of the diaphragm makes with the front face of the
plug is just as important
as the seal that the front of the diaphragm makes with the back of the
barrel.
Funny how you can be
so certain that you have solved something without even verifying it. But
it HAD to be.
The backward movement
of the diaphragm to allow the air to escape up the barrel was dependent
on complete
depressurization of
the tiny cavity in the plug, behind the diaphragm. If the diaphragm didn't
seal well against the
plug face, all of the
air in the chamber would just leak past and out of the hole (that is what
was happening). I had
been focusing only on
the diaphram-to-barrel seal. The plug was an injection molded part and
was probably far
from smooth. I couldn't
wait to get home....
THE FINAL ELEMENT
As soon as I got out
of the car, I grabbed the plug and looked at it. Yep, it had burrs and
rises as you would
expect any injection
molded part to have. Later that night, after the kids were in bed, I returned
to the garage. I
placed a full sheet
of sandpaper on the bench, grabbed the plug by the square on the back,
and rubbed its face
back and forth on the
sandpaper until it was smooth. I did the same with the backside of the
diaphragm. I quickly
re-assembled it, laid
it on the floor and grabbed the blow gun. This was gonna be it. I squirted
the air in the back,
the diaphragm "whined"
bit, there was some hissing, then all noise stopped. I let go of the gun
and yanked it back
....
FOPP!!!.
All I could think about
was those guys standing in the desert watching the first atomic bomb being
detonated. The
awe, the magic,... the
danger. I pumped it a few more times. Predictably, and instantly each time
FOPP!, FOPP!.
Yes, this was the third
milestone. I still had the bucket with the potato cores that we had experimented
with
earlier. It was around
11:00 PM. I turned on the flood light in the front yard, shoved a spud
core down the barrel
and laid the gun on
the floor with the business end of the barrel on a piece of 2x4 laying
flat, pointing out of the
garage door. Thus, the
gun was inclined by 1.5" over is 7.5 foot length.
Now my house sits 150ft.
off the road and is almost completely surrounded by woods. The gun was
aimed
diagonally out of the
garage door, pointing over the front yard which sloped down just a bit.
At the yards edge, is
a bark mulch covered
area with some very tall mature trees, behind that a stone wall, then about
100 ft of woods
before you hit the street.
I aimed the gun at an oak tree about 1.5 feet in diameter and pumped the
gun.
FOPP! The little spud
rocketed across the lawn, missed the tree, just cleared the stone wall
and vanished into the
woods with the sound
of something tearing through the light foliage, ripping leaves as it went.
Most stunning to
me was that its trajectory
was a perfect straight line as best I could tell, it didn't "fall" at all.
Second most stunning
to me was that I missed
the tree by a good foot at only about a 60 ft range. I reloaded
and fired again. I missed
the tree again. This
time the spud hit the upper edge of the wall and appeared to splatter into
a hundred
fragments.
Just then, my wife came
out of the house into the garage. "Linda from next door is coming over,
she wants to
know if you could look
at Kevin's bike, something is broken". "Put your shoes on and come out
here. You've
gotta see this!!" I
said excitedly, ignoring what she had just said. Just then Linda came into
the garage. I got a
piece of paper and drew
a quick sketch of the gun and explained how it worked (they didn't seem
interested nor
like they understood).
I showed them the "core cutter" (they seemed a bit more interested, something
that cuts
vegetables was closer
to home) and then loaded the gun. "Watch this." I said sharply, "I'm gonna
try to hit that
tree". FOPP!! This time
the shot was a bit lower (although the gun was in the same position) but
I missed the tree
again. Instead, the
little 'tater hit one of my landscape lights. It went out. I looked up.
Linda, the next door
neighbor had a stunned
look on her face. My wife exclaimed "Great. You broke a light. Great."
TO THE SKIES
I had no more potatoes
and it was dark. I was dying to "see" how far this thing would propel a
veggie. I
contemplated asking
Linda if she had any Idaho's I could borrow and timing the interval between
when I fired one
straight up and when
I heard it hit the ground. I could get out my old Physics book from school
and figure out the
height traveled. But
I didn't, I went inside. The next day after work, I went right to
my son's soccer practice. I
rushed home afterward
in the last few minutes of daylight to fire a spud upward and see how high
it would go.
My wife had actually
gone grocery shopping and bought a new bag of brown Idaho potatoes.
I cut 4 cores and dropped
them into a bucket of warm water. My philosophy was that the warm water
would
soften the outsides
of them a bit and make for a better seal. I had no mount for the gun, so
I brought a stepladder
outside and propped
the end of the chamber on one of the top steps. I pointed the gun as perfectly
straight
upward as I could, squeezed
the blow gun and released fffFFFOOPPPP!. I couldn't believe what I saw.
This little
core went skyward faster
than anything I had ever seen before. It violently wiggled around as the
air stream
worked on it. It continued
upward for about 5 seconds, then started an accelerating plummet back to
earth. It
landed with a thud on
the neighbor's roof.
The "Backyard Ballistics"
web page cites that this gun can propel a spud some 700+ yards. That is
almost 1/2
mile. I have since fired
this gun at 120+ psi straight up, and while I don't know for sure, my estimate
is that it
travels straight upward
at least 1/4 mi. It is absolutely mind boggling. Since my house is completely
surrounded by
very tall woods, I can't
really fire it any distance and actually see where it lands. Yes, there
is another
neighborhood over the
trees and up a hillside, the thought did occur to me. But since I would
never see where it
landed, I didn't see
the point. One day I'll by one of those portable tanks and take the rig
to the park.
I have fired this gun
at big boulder in the backyard at about a 30 yard range. I literally never
saw the spud. I just
heard a loud WHAP! on
the rock, and there was a wet spot there. No sign of potato. Anywhere.
And recently, I
fired it at my wood
pile at 120 psi. But I missed high, and it traveled into the deep woods
with the sound of a
huge bullet ripping
through foliage. But firing it straight up on a clear blue sky sunny day
is still awe inspiring.
I know that I have not
divulged much technical information here, refer to the "Construction"
and "Theory of
Operation" pages for more detail.
If you are into to big
boy toys, this gun is hard to resist. It is cheap and easy to build, and
the way it works is
fascinating to me. I
was told that the diaphragm type release is used in truck air brakes as
a fast dump valve. But
let me say again that
this gun has the potential to do some real damage. It is for responsible,
intelligent people. If
you make one, the first
time you fire it, you will see what I mean. I know that that warning will
make many
people build one, but
be careful. I really think this thing could KILL someone under the right
circumstances.
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