[FR] Z Self Calibration

[nickweedon]

Is your feature request related to a problem? Please describe.

The problem that this PR would address is that every time a nozzle is changed or a Z homing probe is innacurate then the nozzle will not be at the correct Z position and may even scrape against the bed or cause the extruded to click or possibly even jam (i.e. if using TPU filament).

Are you looking for hardware support?

Ideally something like the Voron Sex Bolt (yes that is what this thing is called sadly) or an equivalent knockoff/clone.

Describe the feature you want

The ability to measure the Z offset automatically using a Z end stop switch attached to the bed.
This idea is best described by this already existing implementation for Klipper here: https://github.com/protoloft/klipper_z_calibration/wiki/What-It-Does

Additional context

I saw already that something similar to this was already proposed here #25657 but I would like to call out that support for multiple tool offsets does not address the problem described here. Moreover, if it did then it would be unlikely that both hardware and software solutions would have been produced already by Voron and Klipper.

I may actually be interested in working on this PR myself if there is no push back here (20+ year dev with experience in embedded systems including autonomous vehicles)

[studiodyne]

#define CALIBRATION_GCODE
I use it, but you need to be electronician or very geek.
The luzlbolt machine with two switching extruders , calibrate the machine with the marlin embeded calibration
I have made my own switching machine and i use it
The difficulty is to ensure electric conduction of the nozzle if you use a metal part
But you can use a switch if it is just for z. But take care if nozzle is cold or dirty , the z can be dramatically wrong

Add a z switch somewhere , and use calibration when changing nozzle(I don't know if calibration z only is possible , but you can modify the code, I use calibration for z and for x and y too)
I have it , it works fine
I have tweaked it to avoir tool swapping , because calibration swap 4 times between extruders and make the toolchange procedure eachtime...

You can! M425

[DerAndere1]

To elaborate on the comment by studiodyne: There are three Z offsets at play:

1. tool length offset (hotend offset),
2. workspace-offset: from the origin of the machine coordinate system to workpiece or bed surface: https://www.machinistguides.com/cnc-offsets/ .
3. In Marlin, we also have the nozzle-to-probe offset.

The implementation for RRF (https://github.com/pRINTERnOODLE/Auto-Z-calibration-for-RRF-3.3-or-later-and-Klicky-Probe) seemingly just auto-adjusts the tool-length offset (hotend offset), using a z limit switch ("z pin") as a tool-setter. They disregard workspace-offset and mash all offsets into the tool length offset. This is not desirable, because it makes the Gcode and offset-values difficult to interpret.

I suggest the following (untested) procedure for Marlin:

• configure your actual extruder as Extruder 1. Tool 0 (E0) should be a dummy, not used for printing. It only serves as a reference to have a constant nozzle-to-probe offset.
• Enable HOTEND_OFFSETS, CALIBRATION_GCODE and CNC_COORDINATE_SYSTEMS. HOTEND_OFFSET for tool 0 must be 0mm.
• Connect and configure your z-probe. Set your nozzle-to-probe offset. Ideally, tool 0 does not exist physically, so that the reference point for the nozzle-to-probe offset is at the gage line (for CNC mills) or at the underside of the toolhead without nozzle.
• Install the tool-setter: attach it to the frame, not to the printbed. Configure the tool-setter as the calibration object: I think CALIBRATION_OBJECT_CENTER is meant to be the position of the calibration object's center in the machine's native coordinate system. Note: The trigger-point of the tool-setter must be accounted for. If the tool-setter can only measure in z direction, disable all of CALIBRATION_MEASURE_RIGHT, CALIBRATION_MEASURE_LEFT, CALIBRATION_MEASURE_FRONT, CALIBRATION_MEASURE_BACK. The pin that is connected to the tool-setter's signal needs to be defined as CALIBRATION_PIN. Then, run this G-code:

T0; switch to reference tool
G28;  Home all axes.
G55;  Select workspace coordinate system 2
G1 X... Y... Z...;  move above the fixture / print-bed
M401; Deploy z-probe
G38.2;  probe towards the fixture / print-bed, stop when probe triggers 
G92 Z<-(nozzle-to-probe-offset)>;  G10 is not supported. use G92 to set the Z-position in the current workspace to the negative of the nozzle-to-probe offset. nozzle-to-probe-offset is the offset between probe and reference tool 0!!!!
T1;  Switch to Extruder 1.
M218 T1 Z<tool_length>;  G10 is not supported. Use M218 to roughly set the hotend offset. It must be within 5 mm of the true nozzle length. this is a limitation of the current implementation of G425.
G425 T1; Calibrate tool 1. This adjusts the hotend offset (tool length offset) for hotend 1
M500;  Save settings to EEPROM

Make sure you have workspace coordinate system 2 (G55) and tool 1 (T1) active during printing

[DerAndere1]

In addition to the above, if you want to use the tool-setter also to measure the nozzle-to-probe Z offset (not recommended because it disregards the trigger-point of the z-probe), configure the probe as an additional dummy tool (extruder 2). Measure and set the nozzle-to-probe offset as good as possible using an alternative measurement method. Set the hotend offset for tool 2 to the same value as the nozzle-to-probe offset. Then run the following G-code:

T0; switch to reference tool
G28;  Home all axes.
M218 T1 Z<tool_length>;  G10 is not supported. Use M218 to roughly set the hotend offset. It must be within 5 mm of the true nozzle length. this is a limitation of the current implementation of G425.
M401; Deploy z-probe
G425;  Calibrate all tools. This adjusts backlash compensation and the hotend offset (tool length offset) for all tools
M218;  Report hotend offsets. Note down the new hotend Z offset for tool 2 (the probe)
M851 Z<hotend_offset_T2>;  Set the nozzle-to-probe Z offset equal to the new hotend Z offset for tool 2
G55;  Select workspace coordinate system 2
G1 X... Y... Z...;  move above the fixture / print-bed
M401; Deploy z-probe
G38.2;  probe towards the fixture / print-bed, stop when probe triggers 
G92 Z<-(nozzle-to-probe-offset)>;  G10 is not supported. use G92 to set the Z-position in the current workspace to the negative of the nozzle-to-probe offset. nozzle-to-probe-offset is the offset between probe and reference tool 0!!!!
T1;  Switch to Extruder 1.
M500;  Save settings to EEPROM