A rotary broach is a very interesting tool for making non-round holes in the ends of things. So, for example, you could use it to create a hex socket in the end of something you want to be able to turn with an Allen key. The way they work is pretty crazy and hard to describe in words, so I won't try. You can google it if you want to.
This is my attempt at making one. Frankly, it did not work very well, and I will almost assuredly try again. I actually have the Hemingway Kits version of this sitting on a shelf waiting for me to tackle.
