Auto Bed Leveling G29 problem (what am I missing?)

[paulpangrazzi]

Well, I'm stumped.

I was auto-bed-leveling like a champ with Clough42's Itty Bitty Dual extruder, but recently I put aside that excellent assembly and installed a single extruder with his ABL servo. I replaced the firmware with beckdac/Marlin from github yesterday, and after changing my z_probe_offset_from_extruder to -4.87, servo angles (0,0 0,0 61,155) and estop = 915, I thought I was golden... but for some reason, I get a different Z height reported with G28 (5.0) than I do in the same spot with G29 (~3.9).

Furthermore, the LCD is reporting nearly the same heights (as expected) on 1-8 probes on the topological probing... but on the 9th/last probe, it goes from ~4 to 7.8! As a result, the hot end tries to dig a trench into the bed on print.

If I simply print with G28, and omit the following G29 command, the first layer print is as expected, but obviously I have no ABL routine.

I've disabled EEPROM settings, and I can't think of anything else to do. I've been up and down the Configuration.h file ... but even more puzzling, when I flashed back to Clough42's firmware (which was working well for several weeks) the printer seems to exhibit the same bad behavior... erroneously high z-height report on the last probe, and a resulting trench run.

What am I missing?

[paulpangrazzi]

Just to close this out, somewhat...

I don't understand why, but I am printing successfully after a G29 command again. I started over from scratch with a new DL of the firmware, and changed the default origin of the beckdac (dacb) settings from "front_left" to "back_right" in Configuration.h. I made a number of little changes here and there to the z-heights, so I'm not sure exactly what 'fixed' it... but I'm just glad to be able to print again in a very low-maintenance manner.

I'm still amazed at how one issue can magically burn away an entire day in what seems like a moment.

[Roxy]

G28 does not compensate for the Z_PROBE_OFFSET. Doing just a G28 without a G29 is going to cause offset issues (if you don't have a couple of small firmware tweaks). If you need more of an explanation, or have a real need to resolve this... Please just say so.

[sniffle]

Basically when g29 is enabled they are meant to be used in concert together. Not doing so leads to things not being quite right.

[paulpangrazzi]

Roxy and sniffle, thank you. That's the key I was missing... G28 must be followed by G29; they work together for proper ABL.
- I mean, it's obvious now that you say it... and it's obvious in practice... but while calibrating, I got stuck on an incorrect assumption and couldn't understand the big picture.

It's sorted now!