The torque calculation is easy. The drum radius is 0.05m plus 0.009mm for the cable, the cable hanging off it has 12Kg mass worst case, and g is approx. 9.8. So the torque you need is 0.059 * 9.8 * 12 = 6.94Nm. The motor holding torque of 6.4Nm at full rated current is not enough.
Is there a reason you want to use a stepper motor? A geared DC or AC motor may be more appropriate, unless you want precise positioning.
Is there a reason you want to use a stepper motor? A geared DC or AC motor may be more appropriate, unless you want precise positioning.