I had an epic failure on my CNC earlier today. I was cutting out a project using my AirWeights vacuum table and a 1/4" up-cut end mill bit in hard maple. Unfortunately there was a screw hole on the back side of my project that lined up with the perimeter cut I was making. When the cut got down to the screw hole I lost vacuum and my piece of maple started moving around and chattering. Ruining my project was bad enough, but the chattering apparently caused my heavy duty Z slider to drop down approximately 1/4" resulting in the bit machining a huge gouge in my vacuum table before I could get it shut off. Something similar had happened a couple of times when I had a Makita router and an MDF spoil board. I assumed the router bit slipped in the collet, which is why I decided to upgrade to the PWNCNC spindle. After examination of this most recent failure I’m convinced the bit didn’t slip in the collet. Rather, the spindle was sucked down and the Z location was lost. Not sure if my vacuum table is ruined or not. Thinking I should never use an up-cut bit when cutting hard material because it wants to draw the bit into the wood. Just a little disappointed that my heavy duty Z slider didn’t have the strength to keep it from getting sucked down and losing its reference.
I would be pretty skeptical of the Z axis not holding position personally. About 2 years ago, I had the gnarliest bit slip of my life. It completely ripped out my bit, and completely destroyed the collet. The machine itself kept on humming along like nothing happened. The bit pulled itself down past even the spoilboard before it ripped itself free.
When you overdrive the stepper controller it shuts off the current for a moment to protect itself. When that current shuts off your Z axis will let gravity take over. The other option is to burn up the diver. I added a brake to my BB Z-20. It’s controlled by a simple circuit that monitors the steppers current. If it shuts off it trips the board and triggers the brake. It’s not perfect because there’s no way for the board to tell the controller to pause. But it does prevent the Z axis from dropping.