I tilt the rod 60 degrees (almost vertical) and grind with the belt sander while rotating the rod. Works well with the angle grinder too if you secure it.
Cutting hundreds of gang bars a week, doing almost like in the movie, but grind from the bar and turn counterclockwise. Have never changed to a grinding disc, use the same disc as I cut with. Learned it in workshop school 40 years ago, and have never failed.
Bad and dangerous method. Using the bench grinder for this. You should never grind on the side of a grinding wheel that is not intended for this. The wheel can explode.
You should never place an angle grinder on the floor and hold it with one hand. You could slip.
If you don't have a bench grinder, you should screw on a nut from the other side. And then fasten everything in a vice. And then a regular file. Alternatively, an angle grinder with a disc for grinding on the side.
You can do it in two ways, one is to simply cut with an angle grinder and then make a small chamfer on the end all around, then just thread on a nut. Sometimes you need to use a wrench for leverage the first time you thread it on. I've done it this way with stainless threaded rods every day at work for several years, works excellently.
The second option is to thread nuts onto where you are roughly going to cut, then do the same but remove the nut, voilà the thread is fixed.
The easiest way is to put a nut on the side that should remain, when you have cut away what you want, you unscrew it, so you get the threads nice again