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NewPerfection
I run my Z axis motors in parallel, with a Vref of 0.4V, giving about 1A total current, or 0.5A per motor. For most machines this should give plenty of power to drive the Z axis.
I was basing my calculations on the "reference forumla" to get the full torque and holding power. As I mentioned above, if I go lower than the 1.2v (.6v/motor) both motors squeal and don't move. That's why I haven't tried a lower reference voltage.
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Have you calculated the steps/mm for the Z axis? Have you set a reasonable max speed and acceleration for the Z axis?
Both settings are the default that nophead distributes as part of his Mendel and it's built to his specs with respect to axis movement. steps/mm = 200 * 16 for 1/16th microsteps. I do remember that one for sure. I'd have to double check once I get home but feed rate should be 4mm/sec for Z and acceleration of 150.