MRRF: Mostly Harmless 3D Printed Arms

The Midwest RepRap Festival isn’t just people hanging out with their 3D printers all weekend; There are also people bringing all the things they made with their 3D printers. There was an R2D2 and half of a B1 Battle Droid, a 3D printed quadcopter and of course 3D printed weaponry. [Ryan] and [Kane] from Mostly Harmless Arms brought a collection of their totally not trademark infringing not-Nerf guns.

The guys have a few designs for guns that shoot silicone-tipped extruded foam darts much further than a Nerf gun. There’s a bow, a more traditional spring-powered blaster, and a crossbow. All the designs with the exception of a few pipes and tubes and springs are 3D printed, and all the parts are small enough to fit on an 8″ bed. The darts are made with a dome mold for silicon and insulation foam that’s normally wedged in window and door frames. They’re dusted with cornstarch to prevent sticking, although in the video below there were a few jams. That’s to be expected; there was a camera around.

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Cannon Sucks Balls at 600 mph

Every day we humans hang out and think nothing of the air that is all around us. It is easy to forget that the air has mass and is pulled down to the earth by gravity creating an ambient pressure of about 14.7 psi. This ambient pressure is the force that crushes a plastic bottle when you lower the internal pressure by sucking out the air. [Prof Stokes] from Brigham Young University has used this powerful ambient air pressure as the power source of his ping pong ball cannon.

Instead of filling a reservoir tank with compressed air and using that to fire a projectile, this canon has the air removed from the barrel to create the pressure differential that propels the ping pong ball. The ball is put in one end of a 10 ft long tube. That end of the tube is then covered by a sheet of Mylar. The other end is covered with the bottom of a disposable plastic cup. A vacuum pump is then used to remove the air inside the tube and it is this pressure differential that keeps the plastic cup secured to the end of the tube. When it’s firing time, a knife is used to cut the Mylar at the ping-pong-ball-end of the tube. Air rushes in to fill the vacuum and in doing so accelerates the ping pong ball towards the other end. There is a large jar at the business-end of the cannon that catches the ping pong ball and contains the shrapnel created during the ball’s rapid deceleration!

Since this was a science experiment at a university, some math was in order. Based on the atmospheric pressure and ball cross sectional area, the calculated speed was 570 meters/second or about 1300 mph. The calculations didn’t take into account leakage between the ball and the tube or viscosity of the air so a couple of lasers were set up at the end of the cannon to measure the actual speed – 600 mph. Not too bad for just sucking the air out of a tube!

Making A Paintball Gun from Scratch

[Ben’s] big brother [Brian] has been slowly building up a respectable mini-machine shop in his garage over the past few years, collecting odds and ends off of Craigslist for cheap. Looking for a fun project to do together, they decided to try their hand at building a paintball gun — completely from scratch.

They have a Spyder paintball gun that they have taken apart many times — but it uses a stacked tube configuration for the firing mechanism — a bit too complex for a first project. After discovering (which is an awesome site that has animations of all the different styles of paintball guns) they settled on making a Tippman clone.

Trying to keep the budget as small as possible, [Brian] found a free 3D CAD program from the makers of Pro/E — it’s called Creo Elements/Direct Modeling Express 6.0, and with that they began designing the gun…

Once they had the mechanism down pat they just had to start machining. Here’s the highly anticipated first test fire — can you hear the joy in the success?

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A Staple Gun, Caulking Gun, And Four-Barrel Shotgun

In its native form, [Clint]’s K-441 is a caulking gun, able to apply silicones, resins, and liquid rubber from a reservoir with compressed air. It’s accurate, powerful, has a huge capacity, and looks strangely steampunk, even for caulking gun standards. This isn’t any normal caulking gun; this device was made from a staple gun. Oh, it also fires shotgun shells with the help of four rifled barrels.

This device that shoots lead, steel, and glue started off its life as an ordinary staple gun, with the usual 23lb pull you’ll find on these guns. By adding a few plates, hand-winding a spring, and milling a few parts, [Clint Westwood] turned this staple gun into a device that would shoot a single .410 bore shell. A practice round as far as shotguns go, but still a serious amount of punch.

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[Mehdi’s] Shocking Stun Gun Tutorial

[Mehdi Sadaghdar] never lets little things like fire, shocks, or singed fingers get in the way of his projects.  His latest is a tutorial on making a simple electroshock device. A stun weapon creates a very high voltage, and is used in law enforcement to temporarily disable a person. [Mehdi] stresses repeatedly to not use this on anyone. If you do, he won’t like you anymore. Of course, if you’ve seen any of his previous videos, you know he’ll shock himself and set something on fire before the project is complete.

To create his stunner, [Mehdi] used a car ignition to produce a high voltage. The igniton coil, which is a specialized transformer, allowed him to generate the >10000V output needed for the stunner. The coil has a 60:1 ratio and is powered by a 12V DC supply. Since a coil is a short at DC, the system only creates a high voltage pulse when power is disconnected. However, the pulse was too short to create a satisfying arc. [Mehdi] added a capacitor, creating an LC circuit that oscillates as the charge decays, creating a nicer spark. He then used an RC circuit and a relay to create a simple oscillating switch. For the finishing touch, he created a spark gap on the secondary of the transformer with two nails. In typical [Mehdi] fashion, he nearly fried his digital caliper in the process.

The end result is a nice spark that warms the cockles of [Mehdi’s] fibrillating heart. We commend him for being such a brave masochist in the name of science.  Check out his tutorial after the break!

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Shocking Idea: Prank Stun Baton

[Christopher] has put together a Prank Stun Baton to annoy his friends. It delivers a slight shock to the person on the business end of the device. Oddly, it’s powered solely by static electricity, there is no battery here and the resulting injury is no worse than touching a door knob after scooting your socks around on some shag carpet.

The design is super simple and is effectively just a rudimentary capacitor. The main housing is a PVC pipe that acts as a dielectric in the ‘cap’ system. Two separate pieces of tin foil are wrapped around the inside and outside of the PVC pipe. These layers of tin foil provide a conductive path up to the a couple of screws stuck in the end of the baton. A ping-pong ball and some foam act as an insulator between the PVC and the screws.

To charge the baton it only has to be brought close to a source of static electricity, a tube TV will do the trick. Rubbing it with a piece of wool will also work. When this is done an electrostatic field is stored in the PVC between the two pieces of tin foil, one side takes on a positive charge and the other a negative charge creating an electric potential between the two screws at the end of the baton. When something (with a low-enough resistance) shorts the screws, the stored energy on the positive screw tries to go to the negative screw, shocking the unsuspecting victim.

Need something a little more powerful? You may want to check out this other stun baton.

Halo-style Paintball

paintball buggy

It seems as though [Nathan] has taken some serious inspiration from the Warthog. The iconic armored buggy from Halo video games has a turret mounted to the roof. Although [Nathan]’s buggy only shoots paintballs from its turret.

Mounting paintball markers (guns) to various objects such as vehicles, robots, or other machines isn’t quite as straightforward as it seems. Vibrations from anything can transfer through a clamping system and cause paintballs to break. This, of course, inhibits the functionality of the marker and is a messy cleanup to boot. Then there has to be a way to fire the paintballs, which is usually handled by soldering to the electrical connections in the marker. And the entire rig has to stand up to the normal jostling and sudden turns from the buggy.

[Nathan] has solved these problems first by creating a custom fast-change mount that allows any malfunctioning markers to be changed rapidly. The electronic firing mechanism is handled by an ATtiny microcontroller and there is a custom electrical connection that is automatically made when the marker is bolted to the mount.

The new system allows markers to be changed in about 30 seconds, much better than any other system. Maybe in the future [Nathan] can upgrade the buggy’s turret to accommodate a paintball minigun.