One of the greatest joys of being a child was figuring out that rubber bands make awesome sounds when they are plucked, and that the sound is easily changed by stretching the band to different lengths. For those of us who need firsthand experience to truly understand how the world works, these types of self-discovery are a pretty great way to learn about physics.
If you’re looking to build a physical music lesson or musical physics lesson into your burgeoning home school curriculum, look no further than the junk drawer, the broom closet, and the 3D printer. [Ham-made] used to stretch his bands across an empty tissue box, but came up with a much more professional implementation based on a broom handle. Check out this fat sound!
You don’t even need to find a spare broom handle, because none of this is permanent — the headstock piece with the hooks is meant to slide up and down to create cool sounds, and the tailpiece threads on in place of the broom bristles. Inside the tailpiece is a piezo disk and a 1/4″ jack so you can plug it in to your amp stack and start an impromptu jazz group. Just keep it under 10 people, okay?
Many Hackaday readers may also be familiar with the Discworld series of fantasy novels from [Terry Pratchett], and thus might recognise a weapon referred to as the Piecemaker. A siege crossbow modified to launch a hail of supersonic arrows, it was the favoured sidearm of a troll police officer, and would frequently appear disintegrating large parts of the miscreants’ Evil Lairs to comedic effect.
Just as a non-police-officer walking the streets of Ank-Morpork with a Piecemaker might find swiftly themselves in the Patrician’s scorpion pit, we’re guessing ownership of such a fearsome weapon might earn you a free ride in a police car here on Roundworld. But those of you wishing for just a taste of the arrow-hail action needn’t give up hope, because [Turnah81] has made something close to it on a smaller scale. His array of twelve mousetrap-triggered catapults fires a volley of darts made from wooden kebab skewers in an entertaining fashion, and has enough force to penetrate a sheet of cardboard.
He refers to a previous project with a single dart, and this one is in many respects twelve of that project in an array. But in building it he solves some surprisingly tricky engineering problems, such as matching the power of multiple rubber bands, or creating a linkage capable of triggering twelve mousetraps (almost) in unison. His solution, a system of bent coat-hanger wires actuated by the falling bar of each trap, triggers each successive trap in a near-simultaneous crescendo of arrow firepower.
On one hand this is a project with more than a touch of frivolity about it. But the seriousness with which he approaches it and sorts out its teething troubles makes it an interesting watch, and his testing it as a labour-saving device for common household tasks made us laugh. Take a look, we’ve put the video below the break.
One of humankind’s dreams has always been to fly like a bird. For a hacker, an achievable step along the path to that dream is to make an ornithopter — a machine which flies by flapping its wings. An RC controlled one would be wonderful, controlled flight is what everyone wants. Building a flying machine from scratch is a big enough challenge, and a better jumping-off point is to make a rubber band driven one first.
I experimented with designs which are available on the internet, to learn as much as possible, but I started from scratch in terms of material selection and dimensions. You learn a lot about flight through trial and error, and I’m happy to report that in the end I achieved a great little flyer built with a hobby knife and my own two hands. Since then I’ve been looking back on what made that project work, and it’s turned into a great article for Hackaday. Let’s dig in!
If you’ve ever worked on a small PCB, you know how much of a hassle it can be to hold on to the thing. It’s almost as if they weren’t designed to be held in the grubby mitts of a human. As designs have become miniaturized over time, PCBs are often so fragile and festooned with components that tossing them into the alligator clips of the classic soldering “third hand” can damage them. The proper tool for this job is a dedicated PCB vise, which is like a normal bench vise except it doesn’t crank down very hard and usually has plastic pads on the jaws to protect the board.
Only problem with a PCB vise is, like many cool tools and gadgets out there, not everybody owns one. Unless you’re doing regular PCB fabrication, you might not take the plunge and buy one either. So what’s a hacker on a budget to do when they’ve got fiddly little PCBs that need attention?
Luckily for us, we live in a world where you can press a button and have a magical robot on your desktop build things for you. Online model repositories like Thingiverse and YouMagine are full of designs for printable PCB vises, all you have to do is pick one. After looking through a number of them I eventually decided on a model designed by [Delph27] on Thingiverse, which I think has a couple of compelling features and more than deserves the few meters of filament it will take to add to your bench.
Of course the best part of all of this is that you can customize and improve the designs you download, which is what I’m about to do with this PCB vise!
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.
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.
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.
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 stretch 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.