Z-height, first layer height, and part height

[benavery]

Hypothetical situation: Say I'm printing a part that is 1mm high with 0.1mm layers, and the z-stop is calibrated at 0.2mm above the glass.
It seems the optimal gcode here would print the first layer with the z-position set to 0, but extrude for 0.2mm layer, then print 8x 0.1mm layers to get the final piece working.

Is this what happens ?
Does Simplify3d optimize for some general z-stop setting, or does it assume that z=0 is actually *on* the glass, so prints with the head 0.3mm above the glass ? If I am concerned about z-accuracy of a print it is recommended to adjust my part prior ?
What exactly does the 'first layer height' in S3D or Slic3r do ? If I set it to 50% does it print a layer at half the height and half the plastic ? Or half the height but all the plastic ? Ie, is this the setting to tweak to change how smushed the first layer is ?
Does this setting relate to how far/close my z-stop is calibrated ?

[jsc]

Theoretically, you should aim to set your z stop to have the nozzle touching the glass at z=0, when everything is up to printing temperature. Setting up with a gap is to allow for thermal expansion. If you print with 100% layer height and 0.1mm layers and take a look at the gcode, you will see that it says "layer 1, Z = 0.1", and then shortly after a G0 Z move to that effect. In practice, you end up setting the Z stop so that your first layers "look right".

If you print with a lower layer height percentage, what Simplify3D used to do in the last version is keep the same first layer height, but extrude more. I know, confusing. If you specified 50% first layer height, it would extrude 200% more at 0.1mm height. Even more confusingly, if you printed with a first layer height percentage > 100%, it would raise the layer height but extrude at 100%. In the latest version, I don't think it does the second part (>100%) any more, but I haven't double checked the g code lately, so I'm not sure what it does. I think it may just print as if you had specified layer_height * percentage for the first layer. I think it still does the weird <100% behavior.

[benavery]

Awesome. Thanks.
I can't fathom how that <100% behavior makes any sense, but now that I know, at least I can tweak it appropriately.

[jsc]

I think the idea is that people set first layer to <100% because they want to "squish" the filament. Keeping the same layer height but extruding more accomplishes this squishing without bringing the nozzle closer to the bed, which at low layer heights depends on good setup.

[j-rod]

you also can't really get the printer to put the nozzle closer to the bed when it's already hit the z stop can you? or does the firmware allow that to be overridden?

[Josh]

There are multiple ways of achieving that. The quickest and dirtiest is to simply turn the Z knob a "click", which will raise or lower the bed slightly (clockwise will raise, counter-clockwise will lower). You could also set a Z home offset, either by sending, say, "M92 Z0.1" after homing to tell the printer that it's actually .1mm higher than 0 at the home position; or, if your firmware has it, M206.

[Capt. John]

Thank you Makergear, for having their lead tech support expert chime in.
This USA company is to be congratulated!

[j-rod]

yeah i've used the knob a few times to get that first line started properly. for some reason the extruded plastic doesn't normally seem to want to catch until its moved about half an inch from the start point. i've re-leveled the hell out of the build plate, but it could still be off a bit.

using the z-height offset, we should use a negative number to get it close to the nozzle, correct?