I think (hope…?) I have solved this problem! TL;DR? Bed has to be physically leveled to far better than 0.5mm in order to automatic leveling to work.
It became clear that my bed was out-of-level enough physically that the auto-leveling was just plain messing up - it was off by more than 0.5mm. To fix this on the Taz 6, I had to come up with some trick to level the bed physically. I made a very simple one-layer model of soft corner mounts, so I could place them under the pads but above the aluminum bed - thereby raising the heated plate and the washer (on the standoff).
To determine how much to offset each corner, I used the following commands:
M420 S0 ; turn off bed leveling
G28 ; level X/Y/Z
G29 V4 ; level bed, with full diagnostics
This would come back with a report something like this:
...
Recv: G29 Auto Bed Leveling
Recv: Bed X: -9.000 Y: -9.000 Z: 0.209
Recv: Bed X: 288.000 Y: -9.000 Z: 0.174
Recv: Bed X: 288.000 Y: 289.000 Z: 0.026
Recv: Bed X: -9.000 Y: 289.000 Z: 0.541
...
From the above, you can see I was off by more than 0.5mm (comparing the Z values for each corner). The highest corner in my case was the back left corner at 0.541, so I needed to raise the other corners (since it’s much harder to lower a corner).
What I did is design a simple one-layer shim that matched the corner gaskets (that hold the standoff to the washer and the two screws), and printed about 10 of them. I inserted them above the aluminum plate but below the gasket - thereby raising the standard and the heated bed. After each attempt, I would run the g-code sequence again looking at the Z numbers. As I got close, I took a flat blade and scraped some of the shims to get back a tad of space.
Once I did this, I got all corners physically within 0.07mm!
...
Recv: G29 Auto Bed Leveling
Recv: Bed X: -9.000 Y: -9.000 Z: 0.484
Recv: Bed X: 288.000 Y: -9.000 Z: 0.473
Recv: Bed X: 288.000 Y: 289.000 Z: 0.459
Recv: Bed X: -9.000 Y: 289.000 Z: 0.523
...
Now when I run automatic leveling - looking at the Z distance at each washer (and using the M119 command to verify z_probe accuracy down to 0.01mm) - I am now level at all four corners within 0.01mm!
I’m going to run a series of print tests, but it sure feels like I’ve at least nailed the basic leveling problem.
Big shout-out to b-morgan for helping! His pointing out the usage of M119 with G29 V4, and running a test to show his printer had similar (but far less) alignment issues helped big time to figure this out. And … @b-morgan … consider adding a .2 shim to your back left washer! That should nail your alignment…
Chris