counterbalancing the spindle

[Jonathan]

Once upon a time, many years ago I worked at a place which had a large vertical boring m/c, the spindle weighed 7 tons

Just to illustrate why worrying about the spindle dropping when the motor is powered, on your average CNC router - with a 5mm pitch ballscrew and 1:2 ratio you'd only need 28Nm holding torque to support that 7 ton spindle.

It's fairly typical for people to use a 3Nm motor on their Z-axis, with a 5mm pitch ballscrew and 1:1 ratio, in which case the motor can support about 370kg (T=F*L/2pi). This also demonstrates why setting the stepper motor to 50% when idle is generally acceptable - half of that holding force is still plenty.