One of the issues of milling with urban timber is that many pieces will also contain urban material. As soon as you hit these objects, they typically ruin the blade and are a safety hazard to the operator. Unfortunately, we just put a brand new blade on and hit a nail within six inches on the first cut.
This culprit looks to be a nail, and they are the most common item to see. But I have seen examples of wire, entire fence posts and even bullets. These objects cause issues with milling the tree, but they also can cause issues with the health of the tree. A small nail in a large heathy tree probably won’t cause any issues, but try to avoid putting metal objects into trees when possible, please. Usually this occurs when someone hangs a sign on a tree, builds a tree house, hangs bird feeder, uses a tree as a fence post, etc. Then the tree grows around the foreign object (urban material).