I'm not sure that Simpson is stiff enough for milling.
Generally, mills use threaded rod to guarantee that they can exert a lot of pressure, and the toolhead is mounted to a huge lump of iron to reduce vibration and keep it rigid. In 5-axis milling, they move the toolhead, but again, everything is built around stiffness and maximum force. In a cartesian mill (x,y,z, no funny business), ever bearing is mounted to be orthogonal from any other bearing that's attached to the same piece. That means that if you put force on the Y direction, X will be completely unaffected.
With Simpson, we have plastic arms, which are going to have some flex in them, friction-fit drive string, and all the axes are tied together. These factors combine to offer a mill which is less than stiff.
None of this stops you from building a metal Simpson. You could replace the strings with a rack&worm drive, which would be far stiffer, but you'd end up dealing with backlash.
So, no, as designed, Simpson is good for printing and light milling, but I wouldn't try steel.
Generally, mills use threaded rod to guarantee that they can exert a lot of pressure, and the toolhead is mounted to a huge lump of iron to reduce vibration and keep it rigid. In 5-axis milling, they move the toolhead, but again, everything is built around stiffness and maximum force. In a cartesian mill (x,y,z, no funny business), ever bearing is mounted to be orthogonal from any other bearing that's attached to the same piece. That means that if you put force on the Y direction, X will be completely unaffected.
With Simpson, we have plastic arms, which are going to have some flex in them, friction-fit drive string, and all the axes are tied together. These factors combine to offer a mill which is less than stiff.
None of this stops you from building a metal Simpson. You could replace the strings with a rack&worm drive, which would be far stiffer, but you'd end up dealing with backlash.
So, no, as designed, Simpson is good for printing and light milling, but I wouldn't try steel.