I think it might be a damaged or loose Z-axis coupler. The spindle is quite heavy and the coupler would normally hold the spindle in place, but if it is broken or loose the weight could pull the spindle down until it bottoms out (hopefully not into the middle of your project!).|
I HIGHLY recommend buying a few (5?) couplers since they have a habit of breaking when you least want them to, and it's not like you can run out to a 24hour big box store to buy one.
ok...so...per the admin...add this to your checklist of things to do:
When turning the machine off to quit for the night (or whatever), if you leave the bit in the spindle, place something (like a chunk of balsa or a piece of hard rubber) beneath it (on top of your "still-jigged" workpiece or the spoilboard) to "catch" the spindle when it sinks down. I suppose you could forego this if you move the spindle to a place where it's not over your work and remove the bit.
other handy things to keep in mind:
(a) If you live in Florida (the lightning capital of the continent) or (b) A thunderstorm is predicted, unplug the machine from the wall when you pack it in for the day.
I only know about the lightning in Florida because I'm (also) a sailor...and I've read that the chance of a (moored, docked, or under sail) sailboat mast being struck by lightning at some point in time during the year is THREE percent. Those are staggering odds.
I would like to note one thing from my experiences: I am leery of turning off the machine (ie doing a breakpoint) because I still have a lingering fear that the machine will "forget" where it is (I started on a DIY machine, so you learn to have doubts about these types of things), so I will sometimes pause the operation (turning off the spindle, naturally) and start it anew whenever (a few minutes later if I have to leave for a short period; or for the night) and I have noticed that there can be a distinctive line where the machine was cutting and then when it starts up again (whether it's the few minutes, or the several hours). I have always thought that it was due to the wood (removing all that material can cause the wood to relax or shift), or the difference in the "load" (going from full speed and then stopping; when it starts up again it would be under different load, causing the line that I mentioned), but I suspect now that it is indeed the spindle dropping down that slight amount (even a fraction of a millimetre can be obvious on highly detailed 3D carvings) and when it starts up again, it is that slight amount lower.
I am going to force myself to start trusting the machine/hardware/software to remember where I stopped and created a breakpoint, and then shut down the machine for the night, and then do as Gerry said: re-home after restarting and the machine SHOULD go back to exactly where it stopped, and that tiny amount of drop (gravity pulling down the spindle) will be removed. Hopefully. Remember, I still have my lingering doubts about ghosts in the machine, so trusting this machine is a bit of a leap of faith
The machine must be HOMED every time it is powered up. Unless the limit switches have moved, you're good to go. If not, calibrate and tighten the limit switches. You can check this by setting the origin @ an arbitrary point. Drill a hole. then power off the machine. Power up the machine and go to your origin. The bit will drop right in the hole. Z zero should still be the exact surface.