Marlin 2.0.5.3 Z height issue.

Hi, I set the Z-offset to my proximity sensor with M851 Z0, G28, M211 S0, move nozzle down, M851 Z-xxx M500, M211 S1, G28 and, on the display, Marlin reports the right figure. However, when I start a print, Marlin then often - not every time - shows the offset as the figure I have inputted + 10 and tries to push the nozzle through the bed. I don't understand. I do run G29 in the file header each time and the process seems to run OK giving reasonable looking figures for each point but then when the print starts, it tries to put the head below the bed. The printer is one of the cheap Anet A8 knockoffs which I have beefed up and I'm still using the Anet V1-5 board (although when I have built a cabinet, I will be changing to an SKR 1.4 Turbo). Any ideas? Thanks

Disclaimer: I am not an expert. I have a CoreXY printer with a nozzle mounted endstop/probe. So all of this is my experience and logic. YMMV

I think there is something else going on here because "Marlin then often - not every time" is not acceptable operation.
I don't think it has anything to do with hardware. One thing to note is that Marlin does not report position to your control app except for a G28 and M114.
What I am not sure of is when using an off nozzle Z endstop, where does a G28 position the head?
I have a nozzle mounted probe so it is easy.
I will assume G28 goes to Z0+offset (xxx) and Marlin reports Z0 (at least it does on my Repetier Host).
Run G28, G29, M500, M211 S0, move to your Z0, M851 Z-xxx, M500, M211 S1, G28
At this point Z should be reported as =0. But since the endstop is your probe, it will actually be at Z0+xxx.
If you do G0 Z5 your Z should move up 5-xxx and be reported as Z=5, depending on your app and its settings.
You may need to do an M114 or configure an offset in the app. Repetier Host will do a relative move to Z5 and Marlin will execute a move considering the offset.
Yeah, it is confusing. A G28 stops at the trigger of the endstop.
The offset tells Marlin it is at Z0-xxx. What you see as Z depends on what you are looking at and what is configured.
Then a G0 Z0 will move to your desired Z0.

At least that is the way it happens with Repetier Host and my configs

Once you figure it out, write it down, which I did not. After a while it will become a big blur, so your notes will be important.

Thanks, Since I wrote the post, I have changed the mainboard to an SKR 1.4 turbo and the drivers to TMC2209s and also done a fresh install of Marlin. However, I haven't got rid of the problem - just acquired a few more which I have listed in another post. What was happening, and I assume still is since I have several more holes burnt into the bed sticker, is that G28 did indeed report Z0+the probe offset but then, for no apparent reason, when the print started, suddenly added 10 to the Z value and pushed the head down into the bed. This is my g-code file header and I can't see anything wrong here and I've been through the configuration.h and config.adv.h with a fine tooth comb without finding anything suspicious. Another post did refer to Z_MIN_POS maybe being set to -10 but mine isn't - its set to 0 so I'm still stumped.
G21 ;metric values
G90 ;absolute positioning
M82 ;set extruder to absolute mode
M107 ;start with the fan off
G28 X0 Y0 ;move X/Y to min endstops
G28 Z0 ;move Z to min endstops
G29
G1 Z15.0 F{travel_speed} ;move the nozzle up 15mm
G92 E0 ;zero the extruded length
G1 F200 E3 ;extrude 3mm of feed stock
G92 E0 ;zero the extruded length again
G1 F{travel_speed}
;Put printing message on LCD screen
M117 Printing...

Let's start this over. Please note, I can't watch this so you need to describe what you are seeing. Read my disclaimer.

Since I have a nozzle mounted probe and you have a separate probe I am not sure about its operation or how your probe deploys and undeploys.
Does it auto deploy for a G28?
Does it raise Z b4 homing X & Y?

did you do this?
Run G28, G29, M500, M211 S0, move to your Z0, M851 Z-xxx, M500, M211 S1, G28

What Z indication do you have at this point, and where is that indication, display, Repetier Host, Pronterface,...?

And when you did this:
If you do G0 Z5 your Z should move up 5-xxx and be reported as Z=5, depending on your app and its settings.

What Z indication do you have at this point? Does the physical position match the display?

Then when you do this
Then a G0 Z0 will move to your desired Z0.

Is it at the paper clearance height?

Upload your config files. It will be a lot easier for me if you flag your changes with something like //**
Probably a lot easier for you when you update also.

There may be a possibility, the issue resides in your app or slicer, but let's get thru this first.
What are you using?


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Notes about your file header: or maybe I don't understand what this is. I am assuming you mean this is a start script.

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none of this should be necessary
G21 ;metric values >> it is in metric mode unless you have set it to inches in config
G90 ;absolute positioning >> it is in absolute mode by default
M82 ;set extruder to absolute mode >> it is in absolute mode by default
M107 ;start with the fan off >> fan is off by default. Slicer will turn it on if so configured.

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This works
G28 X0 Y0 ;move X/Y to min endstops
G28 Z0 ;move Z to min endstops

but why not just this?
G28 >> that automatically does a G28 X Y Z == the 0s are also not needed.

========
without an M109 you are attempting to cold extrude
and
This is extruding 3mm b4 it starts to print and will just drag it to the start of the print an make a mess
G1 Z15.0 F{travel_speed} ;move the nozzle up 15mm
G92 E0 ;zero the extruded length
G1 F200 E3 ;extrude 3mm of feed stock
G92 E0 ;zero the extruded length again

Why not just let the skirt prime the hotend? That way the prime is stuck to the bed.
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