Marlin: Z going below 0

[MoonshineSG]

In my Configuration.h i have #define Z_MIN_POS 0.
Home Z:

Send: G28 Z
Recv: X:105.00 Y:10.00 Z:3.60 E:0.00

I use a capacitive proximity sensor and M851 is

Send: M851
Recv: echo:Z Offset : -3.60

I can do a G1 Z0

Send: G1 Z0 F3000
Recv: ok
Send: M114
Recv: X:105.00 Y:10.00 Z:0.00 E:0.00 

The nozzle touching (or almost touching) the bed

All good so far.

But....

G1 Z-5
Recv: ok
Send: M114
Recv: X:105.00 Y:10.00 Z:-3.60 E:0.00 

Regardless what value I send for Z which is < -3.6 the head stops at -3.6, or whatever value I have set for M851.
Since I have #define Z_MIN_POS 0, why is the head moving below 0 ?

Is my understanding of Z_MIN_POS wrong ? Isn't that the min value for Z ?

(MarlinFirmware:RCBugFix, last update 15/05/2016)

[Blue-Marlin]

That's a problem we hope to have fixed in RCBugFix.
If not please check if you have defined

#define min_software_endstops true

[MoonshineSG]

I have #define min_software_endstops true in Configuration.h

I am in RCBugFix

[Blue-Marlin]

Do you home with the probe?
Does it make a difference if you make the test before or after G29? If so - mesh, grid or 3-point?

[MoonshineSG]

Do you home with the probe?

i only have the probe as Z stop.

No difference if it's before or after G29... Still goes down to the same value as M851

[Blue-Marlin]

Hmm ja. I see. The patch i meant only corrected to software endstops for G92.
Adding the probe-z-offset is no big thing - but finding the right conditions for that to not break something else.

[MoonshineSG]

I can imagine... Let me know if you need some help testing (when you have smth to test :D ) ...
For now I guess I have to make sure nothing breaks...

[jbrazio]

@Blue-Marlin will you care to submit a patch for this bug ?

[Blue-Marlin]

Likely the error is about here (https://github.com/MarlinFirmware/Marlin/pull/3829/files#diff-1cb08de130a6ece2d1b5b9c37bcfef48R1295). @thinkyhead is already in that area.

[jbrazio]

@thinkyhead I will assign this one to you then.

[thinkyhead]

Is my understanding of Z_MIN_POS wrong? Isn't that the min value for Z?

@MoonshineSG The software endstop for Z currently gets extended based on any negative probe offset and based on any negative home_offset (as set with M206). I'm pretty sure these adjustments to the software endstops are no longer required, so I'm removing them in #3829 (which is mainly to fix an issue where switching extruders could cause movement beyond the software endstops).

You could experiment with the same idea to see if it fixes the issue for you. Simply make this change to the clamp_to_software_endstops function in Marlin_main.cpp:

-    float negative_z_offset = 0;
-    #if ENABLED(AUTO_BED_LEVELING_FEATURE)
-      if (zprobe_zoffset < 0) negative_z_offset += zprobe_zoffset;
-      if (home_offset[Z_AXIS] < 0) {
-        #if ENABLED(DEBUG_LEVELING_FEATURE)
-          if (DEBUGGING(LEVELING)) {
-            SERIAL_ECHOPAIR("> clamp_to_software_endstops > Add home_offset[Z_AXIS]:", home_offset[Z_AXIS]);
-            SERIAL_EOL;
-          }
-        #endif
-        negative_z_offset += home_offset[Z_AXIS];
-      }
-    #endif
-    NOLESS(target[Z_AXIS], sw_endstop_min[Z_AXIS] + negative_z_offset);
+    NOLESS(target[Z_AXIS], sw_endstop_min[Z_AXIS]);

[MoonshineSG]

yes, that works. Now my Z doesn't go below 0 even if i have a negative z offset set by M851