Hi,
I've done the same sort of thing on my mill.
The stepper/ planetary gearbox/ ballscrew resolution is 1um. The manufacturer of the planetary gearbox claims less than 2 arc min backlash equivalent
to 0.7um lash.
When I did the same measurement that Roger describes I got 4um lost motion? As it turns out I'm using some cheap aluminium shaft couplers with a spiral
slot cut into them. They wind and unwind a bit like a spring and there is my lost motion. I have retained those things because they are a bit weak, I've had
the occasional crash and the coupler twists itself up and shears rather than some other component breaking. While I didn't intend it they serve as a mechanical
fuse.
With the gearboxes and mechanical advantage of the screws the stall thrust of my axes is over 1500 N or 150kg thrust, enuf to crush things like hands and so
on. This machine is tiny, machining volume of 200mmx200mmx200mm, and yet its till got the grunt to injure badly. Was quite a surprise when I realised that.
As I've said before 'some surprises are surprisingly more surprising than others'.
Craig