counterbalancing the spindle

[ramsbury]

I have been asked by one of my friends how I intend to counterbalance the spindle.
as when another Axis is travelling it may drop and leave ribs showing.

Has anyone done this Or do you rely on the drive to hold the spindle in position.

[JAZZCNC]

I have been asked by one of my friends how I intend to counterbalance the spindle.
as when another Axis is travelling it may drop and leave ribs showing.

Has anyone done this Or do you rely on the drive to hold the spindle in position.

No doesn't happen and if you have the correct size motors the holding torque is enough.

If you have a very heavy Spindle then having a counter balance will help the motor but it's still down to the motor to hold it unless you have a break on the motor.

[magicniner]

Having a counterbalance can reduce the required holding and lifting torque & current but increases the mass which the motor must accelerate for downward motion and so increases lowering torque & current.
Ideally any counterbalance should be about tuning the system, as Jazz said the motor should be up to the job of holding if correctly rated.
Counterbalancing is a widely used technique in industry to allow systems to deal mainly with motive forces, to minimise static forces and to improve safety by ensuring a system doesn't move out of control in the event of power loss,

- Nick

[johnsattuk]

No doesn't happen and if you have the correct size motors the holding torque is enough.

If you have a very heavy Spindle then having a counter balance will help the motor but it's still down to the motor to hold it unless you have a break on the motor.

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 and was c/balanced with a large iron casting via a rack and pinion system. Apprentice left in charge wound the spindle up past the end of the rack. Felt the earth move a quarter of a mile away.

[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.