What happens to z-offset with autoleveling?

i'm so confused.

I do

G28

to home. Then I do a paper test and get a z-offset of say -2.4. So I run

M851 Z-2.4

To set the offset.
Then I run

G29

To run the ABL routine, which gives the printer a grid to adjust to.

Now I start my print job and it is printing in the air. How much in the air? I'm guessing 2.4mm. OK, so the offset should have been zero? So I do

M851 Z0

and run the job again. Now it slams into the print bed and I have to back it off by... yep... 2.4mm. What the hell?

What is the PROPER sequence to run a job with ABL? Now, same question but instead of G29, recalling previous with M420 S1.

Thank you.

I am using linear and Bi-linear bed leveling with Marlin 1.1.5, this is what I do.

I use a manual G28, then G29 as I do not need to do the M851. Then I do a M500 and save the bed leveling data, and restore the Data in the normal startup G-Code.

I use the Startup G-Code below

G21 ;metric values
G90 ;absolute positioning
M82 ;set extruder to absolute mode
M107 ;start with the fan off
G28 X0 Y0 ;move X/Y to min endstops
G28 Z0 ;move Z to min endstops
G1 Z15.0 F{travel_speed} ;move the platform down 15mm
;Put printing message on LCD screen
M117 My Print...
G28 X0 Y0
G1 E4.0 ;prime extruder
G92 E0 ;reset extruder length
M420 S1 ; Bed Leveling On
M220 S32.000000 ;Set start Print Speed for Only Layer One

in the start up G-Code after the last G29 I add a M420 S1 ; this turns the bed leveling data back on.

At his point and after the M420 S1 in your case you could most likely enter your M851 Z-2.4 to be at your Nozzle height.
Now most of the bed leveling functions do not work in Linear or bi-linear as they were designed as Unified Bed Leveling functions, but that is OK as I only use the Bed Level Data anyway.
This was just a thought, give it a try don't give it a try. Just wanted to put information out there for you to examine.

More Information I have on my desktop post-it
M420: Enable/Disable Bed Leveling and/or set the Z fade height.
....S[bool] Turns leveling on or off
....Z[height] Sets the Z fade height (0 or none to disable)
....V[bool] Verbose - Print the leveling grid
Send M501 ; Load data
Send a M420 V ; Print the leveling grid

Z-Offset " M851 Z-0.44 " Sets the Z-offset the input -00.44

Edited 1 time(s). Last edit at 02/26/2018 06:17PM by Roberts_Clif.

Quote
bcsteeve
Then I do a paper test and get a z-offset of say -2.4.

Does your G28 homing use a Z min endstop, or does it use a probe?

Assuming you have a Z min endstop, to establish the probe distance, you can do the following procedure. Of course, make sure your Z raises for probing settings are all reasonable before starting.

• Use G28 to home XYZ. The nozzle should end up on the bed and Z should be 0.0.
• Do M851 Z0 to reset the probe Z offset.
• Do a G1 X100 Y100 (or whatever) to move to the center of the bed.
• Next, use G30 to do a single probe at the current position and get a value back. (The height of the nozzle when the probe triggered.)
• Negate the value returned by G30. If it returned "2.4" then do M851 Z-2.4.
• M500 to save the new probe offset to EEPROM.
• G29 to probe the bed using your established Z probe offset.
• M500 to save the mesh to EEPROM.

If you have no Z endstop and instead use a probe to home Z, then you need to have the probe offset correct before you even do G28, otherwise everything will be off.

To establish the Z probe offset in the case of no Z endstop, you must:

• Use M851 Z0 and then G28 to establish a baseline. Your Z (0.0) will be wrong at this point, but that's ok.
• Use M211 S0 to turn off so-called "software endstops." This allows you to move the nozzle below Z0.
• Do your paper test. With the nozzle near the bed, the current Z (as reported by M114) is your new probe offset, a negative number.
• Use M851 Z-4.6 (or whatever) to set the new probe offset, and M500 to save it to EEPROM.
• Do a new G28 using the corrected probe offset. Without this G28 your height will be off.
• G29 to probe the bed.
• M500 to save the mesh to EEPROM.