The Magnetapult is an Open Hardware Project on Kickstarter: http://kck.st/xQap2h. You can see a video of it in action there or at youtube. In this instructable, I've put the original version with parts that are easier to get.
How'd I come up with it?
Most of us have played with Neodymium supermagnets, and marvelled at how strongly they attract and repel each other. After the success of my Magnetic Levitation Sculpture, I had a ton of magnets kicking around, and one day while playing with them, I thought, how can I make this strong force launch a projectile for me? After a bunch of prototypes, each using bigger and bigger magnets, I came up with The Magnetapult. and launched it on Kickstarter. It has since been successful, so now I'm sharing the instructions on how to build it here. Total cost is between $20, if you have a laser cutter and use the weaker magnets, and $50+ if you don't. Could be more if you live outside of the US due to shipping costs.
If you don't cut the parts right, assemble it right, etc, etc, you may get hurt by the supermagets, including pinching, shattering, taking an eye out, etc. Be super careful and follow directions exactly!
Step 1: Cutting Pattern and Parts List
2 - 3/4 x 3/8" thick disc magnets, either the N42, or for more power, the N52. These are EXTREMELY strong so follow directions exactly to avoid pinching/smashing/shattering/injury. This will cost you from $15 to $20 and up with shipping, depending on where you live.
1 6-32 x 1" bolt. I use brass bolts because I had a bunch from my levitator sculpture project and they won't interfere with the magnets.
3 - 6-32 x 1.5" bolts.
4 - 6-32" nuts
2 - small spacers with 6-32 holes
1 - 1 square foot of 1/8" MDF material
Access to a laser cutter. You can probably get an online laser cutting place to do it for $30 or so. (Or save a bunch of money by backing the kickstarter project or buying it retail after the kickstarter project is over.)
You can also try cutting this by hand but most likely you won't be impressed with the results. The pieces fit together very nicely when laser cut. The holes especially should be an exact tight fit or you'll end up with shattered magnets.
Below you'll see the complete parts after you get the mdf back from the laser cutters.
Step 2: Assemble the Throwing Arm.
Insert the narrower long board into one side of the arm, as well as the magnet stopper, and fit the other side of the arm on. They should fit snuggly. Fit the 1" bolt through the holes at the ammo holder end and tighten with a nut.
Step 3: Partially Assemble One Side
Insert the four parts into one side of the Magnetapult base as shown. They should have a snug fit as well.
Step 4: Insert Leg and Bolt
Now insert the stabalizing leg and the arm pivot bolt into the side, and slip one of the spacers onto the bolt. Keep one finger over the bolt in the back to make sure it doesn't fall out in the next steps.
Step 5: Add Arm and Second Spacer
Thread the throwing arm through the arm pivot bolt as shown, and slide another spacer over that. Hold the arm down with your thumb for the next steps.
Step 6: Add the Opposite Side
Now we can add the opposite side by inserting the opposite side through the leg, and nudging the various parts into place so they fit into the slots correctly. You'll have to play with this a bit to get the sides right, since its a tight fit. Once its on, put the nut on the bolt to keep it all in one piece.
Step 7: Insert and Tighten the Remaining Bolts.
Insert the two remaining bolts at the top and on the left, being sure to thread the bolt through the trigger on the left side. Once you're done, the arm should swing freely, and the trigger hook should fit nicely over the arm's bolt.
Step 8: Insert Bottom Magnet
Time for the magnet magic! Insert one of the magnets into the hole on the Magnetapult base. It should be a tight fit. If its not, GO NO FURTHER! The pattern was made to work with my laser, but if the laser you used leaves a loose fit, edit the file and recut the parts for a tighter fit.
Step 9: Insert Top Magnet
Here's the most difficult part. This was hard to photograph, so please follow these instructions exactly. Holding down the bottom magnet, and gripping the other magnet firmly in your other hand, slowly bring the top magnet closer to the bottom one. If you feel an attraction, move the magnet away, turn it over, and try again. Once you can feel the repulsion, very carefully insert the top magnet in the arm. THIS MUST BE TIGHT AS WELL! Adjust and re-cut if not.
Step 10: Launch!
Now you can launch things with your Magnetapult! Launch dog treats, marshmallows, bouncy balls, or anything else that'll fit in the tray! The sky's the limit!
This is one of the Magnetapult designs I've made during the evolution of the Magnetapult, and it won't be the last. If you're not getting as good results as I show in the video, its probably because I've optimized it further or used stronger magnets. If you have access to a laser cutter, feel free to modify the design for your own personal use. (And send me pics of your success! :)
If the kickstarter project deadline hasn't been reached yet, and you want to get the best possible results, feel free to back that project, or wait a month or so until online resellers pick it up. Just a warning that the aluminum version will probably not be available after the kickstarter project, but the regular and mini version will be going to retailers, at a higher price than on kickstarter.
Robot Boy made it!