Full-Auto Crossbow Rocks and Rolls on Rubber Bands and Electric Drill

You’ve got to enjoy any project where the hacker clearly loves what he or she is doing. And when the project is as cool as a motor-driven, rubber band powered, fully automatic crossbow, it’s hard not to laugh along.

A full-auto crossbow is no mean feat, and it took a man with a love for rubber-powered firearms to get it right. [JoergSprave]’s design is based on a rack-and-pinion system and executed mainly in plywood. The main pinion gear is a composite of aluminum and wood, in a bid to increase the life of the mechanism and to properly deal with the forces involved. The pinion, turned by a powerful electric drill, drives the rack back and locks the carrier under the 30-bolt magazine. A rubber-powered follower forces a bolt down and a cam on the pinion trips the sear, the bolt is fired and the cycle continues.

We slowed the video down a bit and it looked to us like the cyclical rate of fire was about 7 rounds per second, or a respectable 420 rounds per minute. Pretty powerful, too, and the accuracy isn’t bad either.

We’ve seen [Joerg]’s inventions before, like this soda bottle Gatling arrow launcher, or his ridiculous machete launcher. We hope he keeps having fun and letting us watch.

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Maglev Drummer Needs to Be Seen and Heard

Sometimes Hackaday runs in closed-loop mode: one hacker makes something, we post it, another hacker sees it and makes something else, and we post it, spiraling upward to cooler and cooler hacks. This is one of those times.

One of our favorite junk-sound-artists and musical magicians, [Gijs Gieskes], made this magnetic-levitation, rubber-band, percussive zither thing after seeing our coverage of another magnetic levitation trick. Both of them simply have a Hall sensor controlling a coil, which suspends a magnet in mid-air. It’s a dead-simple circuit that we’ll probably try out as soon as we stop typing.

But [Gijs] took the idea and ran with it. What looks like a paperclip dangles off the magnets, and flails wildly around with its tiny steel arms. These hit a zither made of rubber bands with a bamboo skewer as a bridge, pressing down on a piezo. The rest is cardboard, copper-clad, and some ingenuity. Watch it work in the video embedded below.

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A Refrigerator Cooled by Rubber Bands

Ever noticed that a rubber band gets warmer when it’s stretched? The bands also get cooler when allowed to snap back to relaxed length? [Ben Krasnow] noticed, and he built a rubber band cooled refrigerator to demonstrate the concept. The idea of stretching a rubber band to make it hotter, then releasing it to make it cooler seems a bit counter intuitive. Normally when things get smaller (like a gas being compressed) they get hotter. When pressure is released the gas gets cooler. Rubber bands do the exact opposite. Stretching a rubber band makes it hot. Releasing the stretched band causes it to get cooler.

No, the second law of thermodynamics isn’t in jeopardy. The secret is in the molecular structure of rubber bands. The bands are made of long polymer chains. A relaxed rubber band’s chains are a tangled mess. Stretching the band causes the chains to untangle and line up in an orderly fashion. By stretching the band you are decreasing its entropy. The energy of the molecules in the band don’t change, but entropy does. All the work one does to stheatwheelretch the band has to go somewhere, and that somewhere is heat. This is all an example of entropic force. For a physics model of what’s going on, check out ideal chains. If you’re confused, watch the video. [Ben] does a better job of explaining entropic force visually than we can with text.

To test this phenomenon out, [Ben] first built a wheel with rubber bands as spokes. Placing the wheel in front of a heater caused it to slowly rotate. [Ben] then reversed the process by building a refrigerator. He modeled his parts in solidworks, then cut parts with his Shaper handheld CNC. The fridge itself consists of an offset wheel of rubber bands. The bands are stretched outside the fridge, and released inside. Two fans help transfer the thermal energy from the bands to the air. The whole thing is hand cranked, so this would make a perfect museum or educational demonstration. Cranking the fridge for 5 minutes did get the air inside a couple of degrees cooler. Rubber is never going to displace standard refrigerants, but this is a great demo of the principles of entropic force.

For more thermodynamic fun, check out [Al Williams] recent article about building a DIY heat pipe.

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Flat Pack Elastic Band Lamp is a Thing of Beauty

[Matt] was looking for a project for his senior industrial design studio at Wentworth Institute of Technology. He ended up designing a clever lamp that can be flat packed. [Matt] started by drawing out designs on paper. He really liked the idea of combining curves with straight lines, but he wanted to translate his two-dimensional drawings into a three-dimensional shape.

zOz0ys6 - ImgurHaving access to a laser cutter made the job much easier than it could have been and allowed [Matt] to go through many designs for the lamp frame. The two main pieces were cut from acrylic and include mounting pegs for the elastic bands. The two plastic pieces are designed to slot together, forming a sort of diamond shape.

The final version of the lamp required that the elastic bands had holes punched in them for mounting. The holes were placed over the small pegs to keep the bands in place. [Matt] used 3/4″ industrial elastic bands for this project. He then used a 120V 15W candelabra light bulb to illuminate the lamp. The final design is not only beautiful, but it can be flat packed and manufactured inexpensively.

If you want more inspiration for artistically designed lamps check out this one that uses the corrugation in cardboards as a shade pattern.

[via reddit]

Amazing flight of a 3D printed rubber band powered ornithopter

We’re actually going to link to an old post from back in February because we think it’s equally as impressive as the most recent work. This is a 3D printed ornithopter powered by a rubber band (translated). The frame is much like a traditional rubber band plane. The difference is that after winding it up it doesn’t spin a propeller. The flapping of the four plastic membrane wings makes it fly like magic. Seriously, check out the demo below… we almost posted this as “Real or Fake?” feature if we hadn’t seen similar offerings a couple of years back.

The flight lasts a relatively long time when considering the quick winding before launch is all that powered it. But the most recent offerings (translated) from the site include the motorized ornithopter design seen above. It carries a small Lithium cell for continuous flight. These designs have a 3D printed gear system which makes them a bit more complicated, but brings steering and remote control to the party. If you want one of your own they’re working on a small run of kits. We figure it’d be a lot more fun to prototype and print your own. Sure, it’s reinventing the wheel. But it’s a really cool wheel!

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Office warfare

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a Ruler, rubber band, and a pen make a bow and arrow? How about tape, a ping pong ball, and a lighter coming together to make a ‘Zooka. We didn’t think such destructive weapons could possibly be made from office supplies, but the famous [John Austin] is here to prove us wrong. He’s been miniaturizing toys and their munitions including Transformers, Star Wars, Jurassic Park for years. With the resent release of his new book, he’s left us the grace of a few teasers.

[Thanks Chris]