Re: Avid Z stage, Clearpath motor and a 22 pound Spindle. Z drops when powered off.
There are four ways I have seen this done, you mentioned two, coil spring and air cylinder, the others are gas spring and counter weight. Of those choices, I prefer the air cylinder because you get a constant spring rate with very little resistance to movement. Gas springs act like a shock absorber and have resistance to movement, however there some available that have very little shock absorbing action built in.
When using an air cylinder, I tee off the cylinder to a large air reservoir rather than rely on just the regulator to react quick enough to maintain constant pressure. On my machine I use a 5 gallon portable air tank as the air spring reservoir. Some say just the regulator is enough, but we all have our opinions.
The electric brake works OK, and you can set up the timing for no movement in normal operation, but that timing fails in an E-stop condition where it's really needed the most.
Jim Dawson
Sandy, Oregon, USA