What is z offset doing here?

[bcsteeve 1]

I'm a bit confused by the Z0.9/0.75 figure because as far as I know, that's not anywhere in my settings... but it is the Z0.3 that is the concern of this topic.  0.3 is the initial layer height.  Z-offset is -0.15.  So why is the line after SKIRT not Z0.15?  The LCD on the machine also reports 0.3 during the first layer.   So my question is:  is the z-offset being ignored after whatever that 0.75/0.9 thing is?  Or is the Z0.3 really Z0.15 and the nozzle really is 0.15 lower than it otherwise would be?  

 

G1 F300 Z0.75 ;adjusted by z offset
G92 Z0.9 ;consider this the original z before offset
G0 F4500 X35.683 Y63.404 Z0.9
;TYPE:SKIRT
G1 F300 Z0.3
G1 F1500 E0

[bcsteeve 1]

If I'm to assume that it is correct and not being ignored, then I my best guess is that G1 physically moves the nozzle to 0.75 and then G92 tells the machine that it is at 0.9.  Following that, the next G1 has the machine move to what it THINKS is 0.3, but is in fact 0.15.

If I have that right, is the .9 figure arbitrary?

[bcsteeve 1]

Hmm, I just tried again but with -0.25 instead of -0.15 and I swear it made no difference.

[ahoeben 2,016]

5 hours ago, bcsteeve said:

my best guess is that G1 physically moves the nozzle to 0.75 and then G92 tells the machine that it is at 0.9.  Following that, the next G1 has the machine move to what it THINKS is 0.3, but is in fact 0.15.

This is correct, as explained here:

https://github.com/fieldOfView/Cura-ZOffsetPlugin

 

If this approach (which is fast and efficient since it only needs to add a couple of lines to the beginning of the G-code file) does not work for your or your firmware, then there is the "Extensive processing" option that actually goes through the whole G-code file and changes all the Z values.