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#1 Langley

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Posted 18 January 2008 - 02:26 AM

This post can now be found on LGLF Online

Here on Nerfhaven, people tend not to take you seriously until you have something concrete to show them. So after sitting on an idea for about two years, I finally went ahead and tried to implement it. What follows is a thorough report on how not to build a homemade out of a pneumatic screen door closer. Hopefully someone will be able to improve upon my work and come up with something usable.

A couple of years ago someone on the forums made a comment about the possibility of using a screen door closer (that metal thing that makes the hissing sound and keeps your screen/storm door from smacking you in the ass) as an air reservoir to be used with a semi-auto zero valve. It dawned on me that a door closer was a spring loaded pressure chamber just waiting for a barrel and a trigger mechanism. I immediately ran out and bought one, cut off the air regulator (a loose screw), and slapped a 1/2" coupler on it.
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The trigger mechanism was basically the little metal bit that keeps the door open, a flathead screwdriver, and some luck. On testing, it got about the same range as my pvc'ed crossbow (70-80 feet with a slight arc from chest level), but without a workable trigger or a handle, it was useless as anything other than a conversation piece.

-Time passed, I got distracted by other projects, boltsniper appeared on the forums, I stopped nerfing for the most part, etc.

This week I just happened to have some extra time, a new workshop, most of the raw materials on hand, and an excuse to go to home depot for the rest. I took a look at the SNAP and had a couple of chats with Carbon, and worked out a system where I could basically build a SNAP around a door closer and expect it to work.

Two days, a dremeled finger, and a valuable life lesson about beer and powertools later, I had this monstrosity.
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From top to bottom, my crossbow, the barrel I use with the crossbow, the doorfucker3000: maimer of hands, the comically huge barrel that it requires (more on that later), and what's left of the cocking arm. The cocking arm--a length of #10 threaded rod--decided to bail on me when I tried to cock the gun one handed by pulling back on only the part of the rod which extended out of the left side of the gun. Right after I riveted the back end of the thing on. Permanently. I can no longer retract the plunger from outside the gun.

Valuable lessons learned from this experience:
-Keep your finger out of the way of the fucking dremel. Your tungsten carbide bit does not merely grind plastic and metal, it is a demoniacally possessed nightmare which spews itchy bits of aluminum at your skin, howls like a banshee, and hungers for human flesh. You know how it rips curly bits out of whatever you're cutting? Yeah. Just think on that a while.
-Don't drink a six-pack of heineken and expect to remember where you set down any of your tools for more than ten seconds, or even weather or not you left them on. Ironically, the injuries stopped after the beer. I just experienced a huge surge in wtf moments and really close calls before I decided to call it a day.
-Pneumatic door closers have a really really powerful spring. So powerfull in fact, that if you put the thickest possible threaded rod through the hole that you're supposed to use to mount the damn thing, it will snap in half. Bracing the gun against my chest and using both hands on either side of the cocking arm was the only way to prevent this.
-Your gun should never have so much rough unfinished metal on it that rapidly cocking it is tantamount to shoving your hand into a blender.
-Some pneumatic door closers work just fine (like the first one pictured). Others inexplicably dump a bunch of under pressured air into the barrel, popping the dart out before it can build up any velocity. Cx's telescoping barrel would probably fix this. Stock darts of the right size in 1/2" pvc might do the trick too, what with the rubber suction cups increasing barrel friction. Using a ludicrously long barrel will work too, but it's probably hurting the range.
-The heavy duty trigger I built for this project could be used as an alternate design to the one Carbon uses for the snap, if you live in a parallel universe where clothespins are made out of jam or if you want to use a four million newton-meter spring in your homemade.
-If Dremel doesn't already make a model with reversible rotation, they ought to. Being left handed, I have to hold mine in such a way that it flings chunks of whatever I'm cutting at my face. By the time I was done grinding the channels into the sides of the guns shell, I had enough white powder down the front of my shirt to make Tony Montana blush.



Conclusion:
The point of the project, now that so many other scratch-built homemades are coming onto the scene, was to come up with a design where most of the hard work was already done for you. Making a plunger and matching a spring and a pressure chamber to it would already be taken care of if you use a pneumatic door closer. Unfortunately, the door closer created more problems then it solved. My recommendation is to leave this door closer design alone, but don't let that stop you from finding a better way to use it. Like the perfect whore, it's cheap, available, and won't give you herpes.


Here are some more pictures of the beast

The door closer after disassembly. To attach a barrel, I pretty much just hacked off the mounting hardware and air regulator and glued on a 1" coupler with a 1/2" reducer brushing.
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Closeup of the trigger. It was made out of different sizes of aluminum U-Channel. This stuff is meant to be used to frame the edges of different thicknesses of plywood, and can be found by the threaded rod and metal tubing in the hardware section of your local lowes/home depot. I had to grind some of it down with a file to make it all fit. It's held in with a machine screw and the catch it attached with pop rivets.
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The end of the door closer. Don't drink and mod, kids.
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Closeup of the back end.
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Edited by Langley, 04 November 2008 - 03:59 PM.

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#2 jwasko

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Posted 18 January 2008 - 10:52 AM

Well, now that you've given me all sorts of ideas...

My first question is about that #10 threaded rod you bought: how big is the diameter of that stuff? In addition, is that the biggest diameter that you could find, or the biggest diameter that would fit through the hole in the door closer?

How did you keep the chamber portion of the door closer from moving (or, flying) backwards after it had been cocked?

Also, what does the catch catch on? And, good idea with the u-channel.

Did you test the range of the completed homemade? Was it any better than the 70'-80' you got when it was unfinished?

And finally:

If Dremel doesn't already make a model with reversible rotation, they ought to. Being left handed, I have to hold mine in such a way that it flings chunks of whatever I'm cutting at my face. By the time I was done grinding the channels into the sides of the guns shell, I had enough white powder down the front of my shirt to make Tony Montana blush.


I feel ya, brother.
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#3 Langley

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Posted 18 January 2008 - 11:08 AM

My first question is about that #10 threaded rod you bought: how big is the diameter of that stuff? In addition, is that the biggest diameter that you could find, or the biggest diameter that would fit through the hole in the door closer?

The second one. Actually, it was slightly loose in the hole, but the next biggest size of threaded rod was way too big. You could probably find something that's a little thicker that you could attach. Oh, and #10 refers to the diameter. Smaller machine screws are numbered, and I never bothered to actually measure the diameter manually.

How did you keep the chamber portion of the door closer from moving (or, flying) backwards after it had been cocked?

I drilled a hole in the shell right behind where I wanted the closer to sit, and I jammed in a pin that came with the closer that's used for mounting it on your door. The back of the closer rests against this.

Also, what does the catch catch on?

Just like in Carbon's SNAP, the back of the plunger rod is bolted to a 3/4" coupling, which fits snugly in the 1 1/4" shell. So my trigger works pretty much the same way as the clothespin trigger, but it's a little easier to implement so that it catches when you cock the gun.

Did you test the range of the completed homemade? Was it any better than the 70'-80' you got when it was unfinished?

I would estimate it was about the same. I only got to test in in the rain at night once, and I only found the dart because I stepped on in while searching for it.
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#4 Carbon

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Posted 18 January 2008 - 12:14 PM

While your experiences aren't funny, your descriptions of them made me shoot bits of my lunch into my keyboard. And I feel your pain about left-handed Dremel usage as well.

I wonder about the nature of lube used in the plunger. These things aren't made to go fast, so maybe the lube is a bit more viscous than usual... That, and maybe it'd be more usable using less than a full draw. I don't think this experiment is a failure...more an example of what doesn't necessarily work. Prebuilt plungers still have potential, it's just a matter of how to implement them.

By the way, that trigger is pretty.
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#5 Langley

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Posted 18 January 2008 - 01:01 PM

Thanks for reminding me, we don't know what's under the hood. Today I'll hack the whole thing open and take a look. I suspect it uses graphite or some kind of dry lube, or no lube, from the raspy noise it makes when it fires.

Oh, another tip I learned while working on this one: if you press a piece of U-channel against a pipe, it will stay parallel to pipe. This is useful if you need a straight edge to mark or cut a line lengthwise down the pipe.
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#6 l337n3rf3r

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Posted 18 January 2008 - 02:11 PM

I was wondering when you would post the results on this thing. Leftnut talked to me about it a while ago. It's sad to see that it didn't work. When I get less poor, I'll invest in this and eventually post my results.

~l337
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#7 sn1per

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Posted 19 January 2008 - 12:43 PM

Why is everyone but me in this post left-handed?
QUOTE(Scotch @ Feb 3 2008, 05:47 PM) View Post

sn1per I appreciate your humor, that made me laugh literally out loud.

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