youd want power to it. the orginal crossbows had a partial metal construction. go to a scrapyard and buy a car spring. the frame should be wooden, since its easier. the much more complicated part is getting the mechanisms to crank the arrow into place. again at the scrapyard, youd want to buy the part of an older car that youd use to open the window, that crank lever thingy, not just the lever itself, but the mechanism inside of the door that opens the window. thats all ive found out so far, other than that, youd want to get a trigger piece from an old gun, bbgun, or even an airsoft gun might work. im trying to get the materials, but since i cant drive for another 2 months, i dont think thats gonna happen. theres no way your gonna make an effective crossbow completely out of wood. the way i look at it, building a crossbow out of old car parts and broken guns might be the only kind of survival we can do in the future, when theres no wilderness left to run to.