Introduction: Simple 50lb Ballista

Before i say anything about this crossbow, you should know that a 50lb crossbow is dangerous. Anyone that wishes to make one should realize the dangers associated with it. Do some research before even thinking about this project. I dont want any injuries resulting from this particular project. So BE WARNED!

Now that that is out of the way, let me explain. This is a 50lb draw crossbow that I made in about 2 days of light work. It requires very little supplies and is relatively simple to fabricate.

I'm not going to dissassemble it to explain, for reasons of being too lazy, but if I get enough feedback MAYBE I will.

Step 1: Materials

As i said before, you don't need very many materials to make this project. In fact, most of these parts can be modified to your own preferences.

-First, you are going to need a 2x4(the lenghth of the 2x4 depends on the length of your track) and a four short 1x2 peices of plywood.
-Two hinges that will fit cross ways on the plywood.
-Four 2" wood screws, eight 1" wood screws and screws for your hinges(depending on the hinges, preferably 2 screws on each side of them).
-Tennis racket string or 1/8" laminated steel cable, both work very well.
-A track, can be any material as long as it has some way to be mounted and there are two rails.
-Two pieces of band iron about 3/4 of the lenghth of the 2x4 you use for the bow arms, and two more pieces of band iron around 6" long for the support pieces.
-A 3" nail.
-Two lost head nails to hold track
- Duct tape or electrical tape.

Step 2: Bow Arms

OK... lets just start with the bow arms. You take the band iron that is 3/4 of the lenghth of the 2x4 and drill 3 holes, one quite close to the end of each piece for the spring, and 2 at 3" and 4" from the end with the hole for the securing screws.

On the other end, loop some welding rod and weld it on as i've shown. If you're really lazy, you can even just drill another hole (just remember to get rid of any sharp edges that might cut the bowstring.)

Step 3: Brackets

Drill a hole in each end of your 2 6" band iron pieces(close to the ends). Grab a 2x4 your willing to scrap. Secure it in a vise with the 6" band iron 2" in with the rest sticking up... hammer it around the board so it is halfway across the board on each side. These are your securing pieces that hold the crossbow together under such stress.

Step 4: Preparing Board and Trigger

Take an inch wide by 2 long out of the middle of the back of the board. Then drive a nail through from the side. Twist about 6" of welding rod onto the nail. Cut a slot out for the trigger into the track. 
Secure the track, centered all the way down the board. I just put some lost head nails through some holes i drilled(small holes) in the track to secure it.
Bend the end of the welding rod into a slight hook, this will require tweeking later so it's not too hard to fire as well as not able to miss fire... ere on the side of caution. The pulling do-hickey is easy enough, if you can do better, then do it.

On the other end of the board, attach the 2 1x2 pieces of plywood using the 4 2" screws near the middle of the plywood pieces. Make sure the pieces are flush with the end of the wood.

Step 5: Putting on the Securing Pieces and Bow Arms

Push the securing pieces on over the 2" wood screws on either side. If too tight just hammer them on. Then use 4 1" wood screws to secure them in place.

Secure hinges as shown(note the little bit of an overhang). Secure the last 2 pieces of plywood to the arms(tightly!!!) using the 4 remaining 1" wood srews. Attach the arms to the hinges as shown, but also leave enough room for the spring to fit.

For a guide/sight, bend a piece of welding rod into a kind of long staple and then just hammer it in. Remember that your bolt will have to travel through this space when shot.

Step 6: Put on Spring and Bow String

Easy enough, put on the spring. It should be a close if not tight fit.

Grab the tennis racket string or other equivalent and thread through each side. There should be enough to go from one arm end to the other and back with extra string left over. knot it close to one of the arm ends as soon as the spring causes a little tension on the line(I used some random knot, but I reccommend that you go and find the apropriate knot on the internet).

I added a small trigger piece (which consists of a triangle of welding rod) so as to reduce wear and tear on the bow string. I just used duct tape, but electrical tape would work as well.

Step 7: Bolt

Bolts are quite simple to make, just make sure there is a little drag at the back and the weight is at the front. Make sure that it slides along the track without catching.

I dont recommend this particular bolt because it tends to spiral, even though its accurate to within 2m from 15m back.

Step 8: Ready to Go

Again, i can't stress this enough, don't goof around with this. People used these in the medieval ages TO KILL and some still do, so don't aim it anywhere near other people. Use away from people and animals.

As I said before, adjust the trigger mechanism so that there is almost no chance at a missfire(more hook means less slip, less hook means more slip). MISSFIRE=BAD.

To set to fire, pull back string until the trigger can hook the line and wait until you are absolutely sure that it wont slip before loading the bolt.

NOW, to fire, press butt end against hip firmly and hold the underneath with your stronger hand/arm. Use free hand to pull up on the trigger mechanism and CLANK!!! Bolt goes in excess of 150Km/h.

If you see any spelling errors, or parts that need clarifying, tell me and i'll fix them as soon as possible. This is my first instructable, so go easy.