Take the appropriate size threaded rod. Jam two nuts together, with wrenches, at the end of the rod. Grind a bevel on the outside nut so that it resembles a cone shape like a lug nut. Make sure that you grind the cone end small enough to fit into the hole where you want the threads. Now assemble the cone shaped nut on a threaded rod or bolt with the cone near the end of the rod. Pass the threaded rod through the parent metal. On the opposite side secure the nut that you will weld in place. If done correctly you will have a sandwich consisting of a cone shaped nut, (Small end into the hole to provide centering), the parent metal, and the nut to be welded. All thread or a bolt with enough thread to pass through all three elements of the set up is required. Do not tighten more than hand tight. Remove threaded stock and cone nut as soon as good tacks are made. This will aid in easy removal.
If this isn't clear email me and I will attempt to send you a drawing as a JPEG.
Dennis