Autolevelling z_probe_offset what is this value used for?

The value in autolevel settings Z_PROBE_OFFSET_FROM_EXTRUDER what is it actually used for?

The reason I ask as it does not seem to make any difference what value I enter here. After homing z with my proximity sensor Marlin sets the z coordinate to 0 at the probe's trigger point. When I print I still need to enter a z-offset either in slic3r or in firmware via the LCD or by using the m851 command. This offset is the actual difference in z height between the nozzle and the bed after homing z. So what is this value in the firmware actually doing? Is it there to help avoid a head crash during the auto levelling process itself? Or was the intention for it to lower the nozzle by this amount automatically when printing begins - because if this was meant to be the purpose it does not actually do this.

I am aware that having travel limits set might limit the ability of the firmware to lower the head below z=0 so my travel limits have z=-3mm to allow the head to be lowered below z=0.

I am in the process of writing a guide for the reprap wiki on proximity sensors and auto levelling so it is important that if I incorporate the answer I get here that is is factually accurate. I need to establish the procedure for implementing autolevelling and so far it seems as though mechanically setting the sensor height (trigger height) relative to the nozzle, then accurately measuring the offset from the nozzle to the bed after homing, then using that value to achieve a good first layer by setting z-offset in slic3r/LCD/m851 is important but that having an accurate value for z-probe-offset in marlin is unimportant.

Grateful for the help - as will be the many confused reprappers trying to get their sensors to do something useful.

In Marlin setting the #define min_software_endstops false lets you go negative after homing.

Z_PROBE_OFFSET_FROM_EXTRUDER is for the difference in the height between the probe and hotend. If you have min_software_endstops set to true, then that setting won't work as expected.

Okay so if min_software_endstops is disabled then what does the z probe offset do? Will it lower the head that distance before printing begins?

If this is the way it works then I will try it and then go with two options. 1) leave min_software_endstops off and measure the offset and enter it into z_ probe offset in marlin. Or 2) with min_software_endstops enabled. Leave z probe offset set to 0 and enter the offset in slicer, or using m851 or Lcd.

Sound right?

Quote
DjDemonD
Okay so if min_software_endstops is disabled then what does the z probe offset do? Will it lower the head that distance before printing begins?

It sets the Z height to the positive of the offset distance. If the offset is -1.3, then Z is set to 1.3. So if a G1 Z0 is sent, the nozzle goes down to the bed with whatever clearance you set it up with.

Don't forget to do a M502/M500 if you have the EEPROM enabled. The Z offset is one of the items saved.

I think I'm running into a similar misunderstanding with Z_PROBE_OFFSET_FROM_EXTRUDER. I perform a bed level G29 as part of my start script and need to manually set the Z-offset to -0.5mm, via LCD. And for the next print right after that, offset needs to be set to -1.0mm because another bed level was done with Z-offset at -0.5mm.

Apparently, the remedy is to add to the start script: zero the Z-offset, via M851, just before bed levelling, and set Z-offset to -0.5mm right after. I'm not seeing any other way in the header files to work around this. Was this the conclusion given the current state of things?

Sorry for going so far back in time with this.