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.
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.