The problem was that when Estop was enabled, the Z axis would come crashing down to the mechanical stop.

After looking into how rather expensive CNCs are designed, I found that some use a nitrogen charged counterbalance system.  So I decided to mimic that with compressed air.

I used a double acting air cylinder from McMaster-Carr (#6498K861).  The air is pressurized on the lower portion of the cylinder (pushing the cylinder closed, essentially).  The upper part of the cylinder has breather vent (#4456K11).

There is a 5 gallon air tank under the machine with a regulator on it.  I tuned the regulator to offer just enough air to the valve to keep the Z suspended.

I can turn the ballscrew with my fingers in either direction with very little effort.  The machine has no problem working against the air pressure during Z- moves.  I've been using it for over a year like this and it came out way better than I could have hoped for.  So now when the Estop is hit, Z doesnt go anywhere, it just stays where it is (encoder DRO proves that)  

Hope this helps someone.

-Nick

Very good system Nick but I dont know why the Z will crash down on E-Stop

Never had this problem with mine

Must depend on the weight of the spindle or whatever is mounted on Z.

Not a particularly safe system if Z drops at all. How is it moved and held in normal use?

You may want to set the air pressure a bit high so that in the event of estop or losing the axis for any reason it rises out and clears the cut. Just my thoughts

Not a particularly safe system if Z drops at all. How is it moved and held in normal use?

You may want to set the air pressure a bit high so that in the event of estop or losing the axis for any reason it rises out and clears the cut. Just my thoughts

Huh?  I think you missed the point.  The Z does not fall because of the air cylinder holding it up.

Does your E-Stop cut controller power and this is why Z can drop if unsupported?

Estop cuts power to the motor, which puts it in free fall. The air cylinder keeps it suspended. It's balanced just right too. I know that because estop does not kill power to the encoder board and the position continues to be monitored even during estop.

Which motor? The spindle motor or the Z axis motor?
I can't see why cutting power to the spindle motor would affect Z position. Equally confusing is why power to the Z axis would be cut on E-Stop as this would cause an unsafe drop and potential crash of the spindle.
My mill E-Stop uses one contact set for VFD spindle E-Stop and one contact set for Mach3 E-Stop, neither of these would allow Z to go out of control, it sounds wierd!

neither of these would allow Z to go out of control, it sounds wierd!

What kind of "mill" are you talking about that you have?  This is a gantry router I have.  Doesnt sound like you are familiar with the concept.

I had a similar problem on my home built mill. With the new spindle and the Y axis mounted to the Z the total weight is about 50 lbs and the Z started slowly drifting down. I just installed 2 50 lb gas springs hose clamped together to give an 18" stroke. About $40 total from Mcmaster Carr.

Gas springs have about 2000 psi of nitrogen in them. It takes the rated load to start compressing them. Then the load increases by only about 10% over the full stroke. So the 50 lb cylinders put out 55 lbs of force at the bottom of my Z stroke. They are also filled with oil so provide damping too.