5/8th coarse thread is 11 tpi, commonly available.
buy a 5/8-11 coupling nut and some 17mm threaded rod. drill out one side of the 5/8th coupling nut and press the 17mm thread in (you might be able to just hammer it in). and then weld it.
problem with set screws of course is you can't get it off, several ways to solve that problem, grind a flat on the shaft, or drill a hole through it, ream it for a taper pin, safety wire the tapered pin in, etc