When you buy factory-made ductwork, it comes with one end crimped to slide easily into the connecting piece. To make use of ductwork cutoffs you can use duct crimpers to simulate that same connection. Duct crimpers work like a pair of pliers, squeeze the handles together and the staggered jaws deform the metal into a corrugated pattern.
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cool tool, Justin. I have a poor man's way of making the crimped end. I take a pair of 4" needle nose pliers, and grab about 1 1/2" of the edge of the duct with the jaws. I then rotate the pliers about 45 deg clockwise, then the same counter-clock wise. This forms one crimp.
Then reposition the pliers just to the right (or left) of the newly made crimp, and repeat. Do this around the entire circumference of the duct and voila. Slow, but works in a pinch (!!)