I have heard that millers don't want to saw trees that came from a residentual lots. Too much chance there are nails burried somewhere in the trunk. Sometimes there can also be wire burried inside the trunk if it was used for an amature radio antenna. I have also heard that "Metal Detectors" can't read very deep inside wood. At the speed those blades move it doesn't take very long before your have an expensive problem to deal with.