OK, thanks for your reply.

First post and wish i had found this forum a few months ago

They say stepper motor suppliers love newbies

I have a Warco wm18 mill and have added a power z axis stepper but it stalls at high speed

This is the driver i am using - https://www.oyostepper.com/goods-111-Digital-Stepper-Driver-10-42A-20-50VDC-for-Nema-17-23-24-Stepper-Motor.html

This is the stepper motor - https://www.oyostepper.com/goods-88-Nema-23-Stepper-Motor-Bipolar-3Nm-425ozin-42A-57x57x114mm-4-Wires-CNC-Stepper-Motor.html

This is the power supply -https://www.oyostepper.com/goods-375-150W-24V-65A-115230V-Switching-Power-Supply-Stepper-Motor-CNC-Router-Kits.html

I have set the driver to 4.2a and i think, can't remember without opening enclosure, to 1600 steps/rev

It works quite well but tends to stall at max speed especially raising the head. For info the head does have a gas strut helper

so where have i gone wrong?