[BUG] M851 Z-Offset is ignored

[rotarypower]

Did you test the latest bugfix-2.1.x code?

Yes, and the problem still exists.

Bug Description

I'm upgrading a old printer with BLtouch and a BTT SKR1.4 Board.
No matter what I set my NOZZLE_TO_PROBE_OFFSET to the Z value is always ignored. When changing X and Y changes are applied but no matter what I set the Z value to the bed will always move to the same physical location.
Config.zip

Bug Timeline

No response

Expected behavior

No response

Actual behavior

No response

Steps to Reproduce

No response

Version of Marlin Firmware

2.1.2.4

Printer model

No response

Electronics

No response

LCD/Controller

No response

Other add-ons

No response

Bed Leveling

ABL Bilinear mesh

Your Slicer

None

Host Software

None

Don't forget to include

• A ZIP file containing your Configuration.h and Configuration_adv.h.

Additional information & file uploads

No response

[ellensp]

Please test the bugfix-2.1.x branch to see where it stands. If the problem has been resolved then we can close this issue. If the issue isn't resolved yet, then we should investigate further.

[rondlh]

no matter what I set the Z value to the bed will always move to the same physical location.

Where is this location?

[rotarypower]

It is 3.8mm below the nozzle. When sending M114 the Z value reads 10

[ellensp]

NOZZLE_TO_PROBE_OFFSET values are stored in eeprom,

When the firmware is first run the eeprom is populated with the default firmware settings at that time.

Since you dont have EEPROM_INIT_NOW set, any attempts to update NOZZLE_TO_PROBE_OFFSET in configuration.h are ignored. The first set eeprom values take precedence.

Either enable EEPROM_INIT_NOW so that the eeprom data is updated to Config file default every time you upload new firmware

or

Force a eeprom to new config values with M502 - Factory Reset then M500 - Save Settings after every new firmware upload.

but this is only a guess as your Steps to Reproduce is empty!

[rotarypower]

I'm sorry I forgot to mention that I did M502 -> M500 after the firmware update.
But changing the offset via M851 has no effect either, altough values are updated correctly when checking with M503

[rondlh]

Your Z-offset is "#define NOZZLE_TO_PROBE_OFFSET { -10, 0, -3.8 }"
So I would expect that your bed moves to 3.8mm below the nozzle (like mentioned), then the Z-probe is triggered, and Z homing is complete. Afterwards the bed will move up a bit (Z_AFTER_PROBING 10). So everything seems to work as expected.

The key now is, where does the bed go when you move to Z=0?

Typically the procedure I use is like this:

1. Set Z-offset to 0: M851 Z0
2. Heat the bed and nozzle to typical printing temperatures
3. Home the printer: G28
4. Move the probe to the centre of the bed (if not there already after G28)
5. Slowly bring the nozzle and bed closer with babystepping (M290Z-0.01) (Use a sheet of paper)
6. You should see you Z-offset changing (M851) because you have BABYSTEP_ZPROBE_OFFSET enabled.
7. Now save your Z-offset (M500)

[rotarypower]

I'm sorry for not getting back to you for a couple of days.
The issue has been resolved, I re-did the configurations from scratch, that solved it. Must have had a mistake in there