Why is everyone using a 8mm lead screw i.e. 8mm movement per rotation? The advantage 8mm lead is faster Z movement. But the huge drawback of a 8mm lead screw is that if the steppers are not powered, the z axis drops down. The mill bits can then break or damage the work piece. I am seeing a lot of parking frameworks just to get around this issue.
I am using a 2mm lead screw (2mm movement per rotation) for my lowrider2, and that works very well. The Z axis stays in place, even with unpowered steppers. And the Z movements are still fast enough.
So why use a 8mm lead?