Z-axis zero position and first layer height

Please correct me if I am wrong, but in my opinion z-axis position for first layer is not correct if set according to the instructions.
Let me explain…

When z-axis is homed, position is Z=0,00 naturally.
Now if the bed is adjusted to have a 0,35mm clearance at home position, actual layer height is 0,35mm if z=0.
However (at least in Curaengine) it seems impossible to set “first layer height” smaller than “layer height”! For 0,2mm layer height you will get at least 0,2mm z-value for first layer in g-code. If “first layer height” is other than “layer height” it sums these for first layer z value.

Now the problem to my understanding is that when first layer is printed at z=0,2mm, the true first layer thickness is actually 0,35+0,2 = 0,55mm which is waaay too thick and not what is should be.

Simply put: “actual first layer thickness” = “clearance” + “first layer height”
This means to me that first layer height cannot be set correctly.

I Think I will correct You … I think it is like this…
First layer is printed at Z==0 with layer height == “first layer height”.
Measure the distance between nozzle and bed at Z==0 (home pos in Z-dir, in center of X/Y) and put that value in Cura.
If you put a slightly higher value in Cura, then you got some more plastic extruded on the first layer.
2nd layer is printed with Z==0.2 (But is better to use 0.21 instead as layer height in Cura and for 0.1 mm , use 0.105).

I might try to oppose! First layer is definitely not printed at Z=0, but Z=“first_layer_height”, except when first layer is thinner than 0,2!

For example in this first layer was set to 0.3mm:
;TYPE:WALL-INNER
G1 Z0.300
G1 F9000 E0.00000
G1 F900 X148.389 Y172.912 E5.81854

In this first layer is set to 0.2mm:
;TYPE:WALL-INNER
G1 Z0.200
G1 F9000 E0.00000
G1 F900 X148.389 Y172.912 E3.87902

And now this gets confusing, here the first layer is set to 0,1mm:
;TYPE:WALL-INNER
G1 Z0.300
G1 F9000 E0.00000
G1 F1560 X148.425 Y172.904 E4.09093

You see my point?
It is not possible for the printer to print at Z=0.0 value, where the 0.21 (or whatever) spacing is actually measured.
Now when you set the 0.21mm gap at Z=0.0 value, you will get first layer thickness of 0.21+first_layer_thichkness, not the desired first layer thickness.
And thus cura does not calculate extruded volume correctly, because it does not know what is the “adjustment” or “clearance” thickness as it is not same as the set first layer thickness.

It is very helpful! Thanks for your knowledge
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