I have a few questions for you:
1. How do you have the motor wired? Parallel or series?
2. What kernal speed do you have Mach3 set at and what is your computer speed?
3. 1800 oz.= 112.5 LB. 112.5 LB x 8TPI= 900 pounds of lifting force if the motor is geared 1:1. The head cannot weigh 900 pounds, so 1:1 gearing should lift it easily. Why are you gearing this big slow stepper motor at 3.5 to one? Are you REALLY expecting to be lifting 3,150 pounds of mill head?