My experience with the machinist’s trade consists mostly of oohing and aahing over the precision tools they get to use. Fortunately, there are others out there who know all about machining—including the fellow who posted the videos embedded below.
In the first video he shows a tool that I’d never seen before. In the second he explains what the tool is. . If you want, you can go straight to the answer—but really, it’s more interesting if you watch the first video and try to come up with it on your own. It takes him a while to get going; if you want to get straight to business, skip ahead to the 50 second mark in video number one.