I have the 65mm VFD spindle on my 5.1 pro 4x4. Today after use when I finished a project and parked the machine and shutdown the Z axis dropped to the hybrid table. Powered it back on and initialized the machine all was good. Parked and shutdown same thing happened, Z axis droppped to hybrid table. Machine is only 1.5 months old. 1st time this has happened.
This is a known problem. It happens on the 80MM spindles more. I think C3D sells a hydraulic piston that keeps the spindle from dropping during power off. The cheaper way is to get a block of foam and place it under the spindle before powering down.
I have the older 65 mm spindle. I put blocks under the spindle in case it wants to drop.
I would say you maintain the Z axis pretty well. !
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Yeppo, I had the same issue with the older 65mm spindle, gas strut was the fix!
Another suggestion would be to remove the endmill when shutting down. That way you wont cause possible dammage to it if/when the spindle drops
You can put a wood block under the end mill to protect it.
The bigger concern is what happens when you hit your emergency stop and spindle stops with the spindle still running.
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Haven’t tried nor would I want to FAFO!
This issue usually indicates that the 65mm VFD spindle’s weight is overcoming the motor’s holding torque or a loose lead screw coupler once power is removed.
My 65mm spindle drops on shutdown too, so I installed the C3d gas strut kit on my 4x2 5 pro. While it did hold up the spindle on shutdown, would also overpower the Z axis stepper and would cause shallow cuts and a nasty sound when the Z axis was raised up to top. Support wanted a video showing the issues, but I already uninstalled it. Now I just block under the spindle on shutdown.
They had some bad struts go through that were causing issues. I got one of them. Got a replacment shipped quickly and never had an issue after.
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I use the strut also with my 65 and it works perfectly.
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In my Tormach days I used a piece of PVC pipe for when the spindle dropped. 
That would accommodate either an empty or loaded spindle…
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I had no clue this was available! Thanks for posting this. Soon to be ordered as my spindle has dropped at every shut down from day 1.
SOLUTION 
great idea. . .and maybe a piece of foam at the bottom just in case you loaded an extra long end mill?
Good idea, but not needed in my case, @bryanf. The piece of PVC I used with the Tormach was at least 9-10 inches long. I liked to keep the head up in the region of the Z homeswitch, and just moved the table to the side so that the vise wasn’t under the spindle. My Nomad is well-behaved in that aspect, the Z axis would need to be driven downwards deliberately.
When I coverted my Sherline mill to CNC, I went through a phase of counterbalancing the head with a pair of linear-force springs. I ended up taking them off, because I realized it was better to have gravity pull the head down against the Z axis backlash. I hope that on the Shapeoko, the Z-axis gas-spring assist is only a partial compensation for the head weight for the same reason. [EDIT] I see @Lyoshimura already commented on this several posts above…[/EDIT]