Ozzie,
Now I understand. I said the DRIVE is pushing 350K steps, not MACH. Many drives have step multipliers on the input side. If I set the multiplier to 10, for example, MACH puts out 1 step and the drive treats that as if it were 10 steps. You may see this referred to as 'electronic gearing'. Where the trouble come is when the drive itself has a bandwidth that will not allow it to pump OUT enough steps. In this case, you are limited in RPM.
I'm not cutting with the wrong side of the carbide. The threads are calculated by a macro that I wrote and can go either direction. You just enter + or -. Also the tool can be bolted to the mill facing 'in' or 'out', and the 4th axis can run either direction. The big inset obsures the cutting action, so the setup for the video is 'backward' so that you can see the cutting action. I didn't change the program, I just flipped the tool and reversed the rotation for making the video.