Re: Motor suddenly stops
Classic mid band resonance and classic gravity bias on the z axis.
Google "Stepper motor mid band resonance" for a host of ideas. Here are a few:
1. Use a higher microstepping mode on your driver. If it supports that setting... actually... just get a real driver.
2. Try /reducing/ the current on the driver (doesn't always work, but there have been cases...)
3. Add a mechanical resonance damper (e.g. a hockey puck with a hole in the middle) to the shaft.
On the gravity bias, think about this: Only one axis is fighting gravity. Only one axis is stalling. To solve the problem, apply anti-gravity technology:
1. Add a spring pulling or pushing the Z axis up.
2. Add a pulley at the top, and a line over it from the Z axis to a weight.
You could also try better lubrication (e.g. silicone or slick 50 or something like that) but I haven't had good luck with that in the past.
James hosts the single best wiki page about steppers for CNC hobbyists on the net:
http://www.piclist.com/techref/io/steppers.htm Disagree? Tell him what's missing! ,o)