How to Build a Compound Crossbow

Build a version of a compound crossbow.

Compound bows use complicated designs to provide more power than the traditional longbow. When the string is pulled back, it "loads" the spring as it is drawn over a lopsided wheel known as an "eccentric." When the archer releases the string, the wheel uncoils, releasing the power in the spring. A crossbow is designed to lock the string in place after the eccentric wheel is loaded, taking the load off the archer and placing it on the crossbow trigger. The best, and safest, way to build a compound crossbow is to combine the two designs into one.

Things You'll Need

• 3/4-inch dado blade
• Table saw
• Wood blank, 2-by-3-by-32-inch
• Band saw
• 1/2-inch drill bit
• Drill press
• Jig saw
• Plywood, 1/2-by-1 1/2-by-4-inch
• 1/2-by-3-inch dowel
• Compound longbow
• 1/2-by-4-inch bolt, with nut and washer
• 1/2-inch open end wrench
• 2 pieces oak lumber,1/2-by-1-by-16-inch
• 1/4-inch drill bit
• 2 bolts, 1/4-by-4-inches with washers and nuts
• 1/4-inch open end wrench

Instructions

1. Install a 3/4-inch dado blade on a table saw. Raise the blade to 3/4-inch in height. Place the wood blank on the table saw on its edge. Slide the fence over to 1 1/8-inch from the left side of the blade and lock it down.

2. Make a pencil mark on top of the wood blank at 16 inches from the end. Turn on the saw and push the blank over the dado blade until the mark is directly above the center of the blade. Turn off the saw and let the blade slow down and stop before removing the blank. You should have a 3/4-inch channel running down the center of the blank half way.

3. Cut a 2-by-2-inch notch out of the end of the wood blank using a band saw. This is where the compound bow grip will fit.

4. Drill all the way through the stock with a 1/2-inch drill bit and drill press at the terminating point of the 3/4-inch dado. Insert a jig saw blade into the hole and cut a 1/2-by-3-inch slot into the wood blank toward the end of the stock where your shoulder will be.

5. Cut a 1/2-by-1/2-inch notch, 3/4-inch from the end of the 1/2-by-1 1/2-by-4-inch piece of plywood with the band saw; this is the trigger. The notch is where the string locks onto the end of the trigger. Insert the trigger up through the hole at a 30-degree angle so that the notch is extended up through the slot 2 inches.

6. Drill a 1/2-inch hole through the stock. Drill the hole 1 inch up from the bottom of the stock, penetrating through the end of the trigger and both sides of the stock. Insert a 1/2-by-3-inch dowel through the hole. The trigger should still be movable, as it can tilt up or down slightly using the dowel as an axle.

7. Remove the grip from the compound bow. Slide the end of the stock over the place where the grip was before. The bow will be vertical, not horizontal. Using a cordless drill, drill all the way through both sides of the wood blank and the compound bow with a 1/2-inch bit. Insert a 4-inch bolt into the hole. Put a washer and nut on the end and tighten with a wrench to secure the stock to the bow.

8. Place the two 1/2-by-1-by-16-inch pieces of oak on both sides of the wood blank, angling up at a 30-degree angle to the bow. These are for diagonal bracing. Drill through both ends, penetrating through the stock with a 1/4-inch bit. Drill through both sides of the other ends, penetrating through both pieces and through the compound bow. Insert a 1/4-by-4-inch bolt through the hole in the bow, and the hole in the stock. Place a nut and washer on the bolts and tighten with a wrench.

Tips & Warnings

• You can use a rasp and file to smooth and round the edges of the stock to make it look more like a traditional stock, or leave it square for a medieval look.

• Fire the compound crossbow by first pulling the string back all the way and locking it into the notch in the plywood. Place an arrow in the 3/4-inch dado directly behind the string. To fire the bow, press down on the back of the trigger with your thumb. The plywood trigger will descend into the bow, releasing the trigger, hooking on the the arrow.

• The instructions here are for a weapon, not a toy. Some states prohibit the use of crossbows; check your state regulations.

References

• Mother Earth News: The Hybrid Compound Bow