Marble shooting crossbow
My marble shooting crossbow
The first crossbow was just an experiment, and didn't shoot that far. The second one used two bands of spring steel, and it could shoot some of the toy arrows we used to play with as a kid extremely hard. I always had to pick my targets carefully, to avoid smashing up the arrow completely.
The third, and final crossbow, was built with the remaining narrower bands of steel. I ganged up four strips, more in the middle, less at the ends to produce a very taught, but light bow. The projectiles this crossbow uses are marbles. It took a bit of development effort to get something that actually shoots marbles, as a marble offers no convenient place to hook the bow string onto. My solution to this design problem was to add a nother guide at the top, which prevented the string from jumping 'over' the marble. The two guides are only 3 mm apart, just enough for the string, while a channel in both guides provides room for the marble.
Getting a string that can withstand that sort of abuse was also a challenge. The most durable solution was to braid three strands of baler twine together. Baler twine is amazingly tough, although it would eventually fray where it rubs against the wooden guides. Braiding it made it slightly less prone to fraying.
The piece that holds the string has a gap in the middle. This design allows
hooking an arrow onto the string before its released. For shooting marbles, its not
quite as useful. Note the two inset pieces either side left of the marble. These
are not by design - just that I originally cut the hole too big.
On releasing the trigger, the block holding the string flips down and out of the
way. That way, the string is not pushed off some edge for release, causing a bit
less wear on the string than it might otherwise.
I computed the marble muzzle velocity at 67 meters per second by shooting a marble into a ball of plasticine, and measuring how high this ball of plasticine was thrown. The marbles also go fast enough to penetrate the 1/8" plywood that orange crates are made of, or smash any glass object. Lots of fun! But since then, I have built a marble shooting airgun, which propels the marbles more than twice as fast, and more accurately.
Although designed for marbles, this thing also shoots anything with the same cross sections. AA batteries work particularly well, although they don't travel nearly as fast as the marbles. Some pens also work.
If you want to build your own, you first need to find a suitable source of suitably dimensioned spring steel. This is hard to find, and generally expensive. What might work though is the fiberglass strips used to anchor chainlink fencing to the post at the end of a fence - cheap and maybe worth a try.
Please don't ask me for plans or how to build your own. For liability reasons it I wouldn't give you plans even if I drew plans - sorry. If you can't design one on your own, its probably better if you don't build one.
I have since written a more elaborate article on this marble shooting crossbow