Recently when channeling my Ford Model A I wanted to use Grade 8 fasteners for all of the body mounts instead of just tapping threads into the frame or inserting rivnuts that could fail over time. First of all the 1/4" wall of the tubing wasn't really thick enough to give sufficient threads to hold the weight and twist of the body from normal driving. We came up with a slick solution and figured we'd share.
I started by threading a batch of Grade 8 nuts onto a carriage bolt and locking them all together.
I then mounted the bolt into the lathe and cut off the hex portion of each nut leaving us with perfectly round grade 8 threaded inserts. The nuts were cut down just a hair bigger than 1/2" so they would be a press fit into a 1/2" drilled hole.
I then counter sunk each hole and threaded a bolt into each insert so I could adjust them so they were straight in the holes. I used the TIG 200 to carefully lay a weld puddle on the edge of the threaded insert melting it to the frame. You must take your time here and be very precise because a rogue dab of filler rod could go over the edge of the threaded insert and make your life hell when it comes time to thread a bolt back into the insert!
Hopefully you can use this method to put some clean, strong threaded inserts in your next project.