Twin Crossbow Introduction

In our engineering class we have to create a medieval siege machine. Some of the rules of this project are that the machine has to fit in an 18 by 18 inch box and must be able to fire a 1 inch ball. We also were not allowed to use any metal springs.  We choose to design a twin crossbow because it would be compact and still shoot a far distance. Please keep in mind that this instructable may not be what you are use to. This is not done retrospectively.  We will be updating this instructable as the project continues.

For this project we needed to write a brief summary of our machine. The following is a summary of all the research that we did for the project. This step, although intresting, is not needed for the final product.

The crossbow is an innovative ancient weapon that was developed sometime in the 4th century. It is debatable which country or region invented it first, but the most likely nation was China. It was later created in Europe, where it quickly spread. Though the crossbow was never designed to be a siege weapon, it has been used extensively on the battlefield and in defense against sieges. The basic design of the crossbow has remained unchanged, but there are many variants created for specific uses. Most crossbows were able to be carried and operated by a single foot soldier, so it was primarily an anti-personnel weapon. There are larger crossbows that have been mounted on fortifications and require a 4 person team to operate, but those are for defense only due to its size and lack of mobility. It is a very powerful and accurate weapon but it was slow to load, often leaving the user vulnerable when reloading. Despite this problem, it was widely used because it only took 1 week to train a soldier to use it affectively. This became a favorite weapon of peasant soldiers because they could shoot farther, more accurately than a skilled archer and have enough power to kill a knight in armor. Crossbows were expensive to buy in those times due to the bow being made of iron, but a skilled woodworker could make it completely out of elm or spruce which would greatly reduce the price. The weapon proved to be so deadly in warfare that the church tried to outlaw its use.

In modern times, the crossbow has not greatly changed even with the invention of the compound bow. The compound bow has allowed for a compact sized bow to produce the same amount of power as a full sized recurve. This benefited the crossbow by making it lighter and less cumbersome to handle. One example of this is the Twinbow 2 crossbow. Built by SCM in Switzerland, it is half the width of a recurve crossbow and 2/3 the width of a regular compound crossbow. It utilizes two small prods linked together facing sideways instead of using one large prod facing forward. Unlike a normal compound bow, the Twinbow 2 uses pulleys instead of cams, reducing the draw length.

Even though crossbows underwent a large change from a longbow with a stock to the modern compound crossbows, the projectile has remained consistent. Crossbows have always been shown firing an 8-12 inch long arrow with a metal tip and fletching and nothing else. There is a crossbow that breaks this rule by firing stones or bullets. The bullet crossbow, or “stone-bow”, was designed in England in the 16th century for small game and bird hunting. By being used for small game, the bow was not as powerful, allowing for it to be reloaded faster and with ease. It operated by using a pouch, similar to that of a slingshot, to seat the projectile until it was ready to be fired. Being a gentleman’s hunting tool, it tended to be smaller and lighter than the crossbows used in war.

Here is a list of all the materials you will need before starting this project.
A Mill
Master CAM
1x2x8 Plywood
2x4x6 Plywood
1/4 Flat End Mill Bit
Hand Saw
Nails
Hand File
String

After all of the tools have been assembled, you will need to create 3D models of your parts in MasterCAM. You can design your bow to any scale but for our project we decided to make the entire apparatus inside a 18 by 18 inch box.

There are 6 main different parts to the crossbow. There is the top of the stock, the bottom of the stock, the extension of the stock, the two clamps, and the bow limbs. All of these pieces need to be created in MasterCAM and have their own tool paths before starting to mill them out. Once the entire design is complete upload the file onto a flash drive in a NCD file format.

Once all of the parts are designed you will need to set up the mill. The first thing that you need to do is place the wood onto the mill and make sure that the zeros line up. Then take take the flash drive to the mill and open up the program. It is always good to preview the program before starting to make sure that you are running the correct program. If it is the wrong program, change it. If it is the correct program then you will need to process the program. This may not work the first time and you may need to hit process again if it registers as incomplete. When processing is complete simply run the program. As the program is make sure that you keep the emergency stop is close by incase something goes wrong.  Repeat this process until all of the pieces are created.

After all the parts are created you will need to assmble them together. Attach the top of stock in the bottem of stock and add the extension onto the back. Then on the top of stock A

The final bow did not look as nice as our intended design. However, it was able extremely well. Although it was not very accurate we were able to fire the ball across the class room (a distance of about 70 feet).

Originally we were going to set up a complex trigger system for the bow. However, we did not have enough time to finish it. Instead, we used a nail as a way to hold the sting in place.
Another change was desiding not to create a pully system. We decided to use nails because it was much simpler design.