YouTuber Primitive Technology says, “I made a cord drill and then upgraded it to a pump drill. A cord drill is basically a spindle with a fly wheel attached so it looks like a spinning top. the middle of a piece of cord is then put into a notch at the top of the spindle. The ends of the cord are then wrapped around the spindle and then pulled quickly outwards causing the drill to spin. The momentum of the fly wheel causes the cord to wrap back around the spindle in the other direction. When it stops the cords are pulled outwards again and the drill spins in the other direction.
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