Top Surface Skin affects other layers?

[sg000 0]

So I just did an experiment. Ver 4.2.1. I sliced 2 ways, the ONLY setting change is "Top surface skin layers" set to 1. What I found was that the extrusion amounts are slightly higher for ALL commands above the first skin layer with the setting on for all the NON-top-skin layers. (E1187.42799 vs E1139.49951) And its exactly the same amount each time for a number of layers, 47.92848. Not much, but curious why this would be. Shouldn't the setting have no effect on any of the completely internal layers? It didn't affect the bottom most layers. 

[gr5 2,099]

The "E" value is an absolute position - not relative position.  "E1187.42799" means you have extruded 1.187428 meters of filament since you started the print (actually after so many meters it resets back to 0 as Marlin can't handle numbers over some value).

 

So just like X100 means X is at 100mm, E100 means the extruder is at 100mm.  It's a position.  Not an amount to extrude (which would be a relative value).

 

So just looking at one E value doesn't help you much.  You have to subtract 2 E values.

[gr5 2,099]

You probably already know this but "top" in cura has a specific meaning.  If you are printing a cube then it matches the normal definition: the top layer is all top skin.  But if you have say a pyramid then *every* layer has some top skin out near the edges of the pyramid.  A sphere has a ring of top skin on every layer of the top half of the sphere and a ring of bottom skin on every layer of the bottom half of the sphere.

[sg000 0]

2 hours ago, gr5 said:

The "E" value is an absolute position - not relative position.  "E1187.42799" means you have extruded 1.187428 meters of filament since you started the print (actually after so many meters it resets back to 0 as Marlin can't handle numbers over some value).

Thats a huge help! I hadn't taken that into consideration. Thank you so much!

[sg000 0]

2 hours ago, gr5 said:

You probably already know this but "top" in cura has a specific meaning.  If you are printing a cube then it matches the normal definition: the top layer is all top skin.  But if you have say a pyramid then *every* layer has some top skin out near the edges of the pyramid.  A sphere has a ring of top skin on every layer of the top half of the sphere and a ring of bottom skin on every layer of the bottom half of the sphere.

Yes, good point, but in this particular case, my "inner" layers are truely that except for 1, and its this layer that I'd like to be extruded normally and not with the skin setting. So I ended up merging 2 files worth of gcode. The code for no skin as the base of the file and then the last 2 layers from the file with the skin. Works almost flawlessly except for a tiny blob at the layer start which I think can be attributed to the relative vs absolute mentioned in the other comment. A little tweak and it'll be perfect.

 

--Scott

[gr5 2,099]

When I merge 2 gcodes what I do is a G92 command in between.  Like this

.
.
G0 X333 Y333 E1013.7

G92 E871.3  #ADDED between

G0 X333 Y334 E871.3
.
.

I add the G92 which resets the E position and I make it the same value as the first E value in the pasted code (or I back off a tiny bit - I could have done E871.2)

 

G92 resets the position of any axis listed to the value in the gcode.  It doesn't actually move any servos - just changes the current position indicator inside the printer.

[gr5 2,099]

The blob was probably because you didn't do the G92 and the newer code had a higher E value so it overextruded right at the switchover.