If the nut threads conform to the spindle thread, you have a tool to repair the damaged thread. Lock the spindle by putting a couple loops of rope around the pulley, tying that off and then winding a bunch between the hand wheel and headstock. Oil the spindle thread and put the halves of the nut on the spindle, grab the half nuts with big arc joint pliers and squeeze hard and unscrew. Depending on the relative hardness of the nut and spindle you might scrape off a little of the bumps. If so, repeat until the nut easily unscrews. Turn the half nuts around so you get fresh sharpish edges. If this does not remove any metal from the spindle, you should at least get the high places shiny, which you can carefully file or sand off.
Originally Posted by 88rob88
Once the half nuts, or better a intact nut, being sure you have the right pitch and thread shape, goes all the way on and off, you may use a die to carefully clean up the threads. If you do not have access to a die, wet the spindle threads and put a paste of abrasive household cleaner, valve grinding compound, or abrasive hand cleaner on the threads and run the nut back and forth a few times. Clean off the grit from the spindle and nut, oil up and try. If it runs on and off without being catchy, you are done. METICULOUSLY clean all the grit off the spindle and lightly oil.
So much timber, so little time.