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It occurs to me that I'm never really thinking about layer height when I calibrate my z-probe offset. This is obviously an oversight, because I'll want my nozzle to start higher for 0.3mm layers than for 0.1mm layers.

After this realization comes the question: Do I need to recalibrate every time I switch to a different layer height? Or can slicers store a reference layer height + probe offset and just deduce the proper starting height for every print? Or is it smartest to keep the 1st layer height constant throughout my prints to spare myself this frustration?

If it matters, my printer is the Printrbot Simple Metal, and my goto slicer is Cura.

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All modern slicers adjust the nozzle position for the first layer in accordance with your chosen layer height. You can see this in your gcode if you slice files with different layer heights. Before you add special slicer settings and offsets, if you print 0.1mm layers, the nozzle will start at Z=0.1mm, and if you print 0.3mm layers, the nozzle will start at Z=0.3mm.

There are two reasons this is more complex and less reliable than it seems:

Different slicers assume different initial tramming gaps. And your actual tramming gap may not match that assumption. If the slicer thinks your nozzle is leveled at Z=0 with a real physical gap of 0.1mm to start with, that means gcode Z=0.1mm is actually a 0.2mm gap that must be filled with plastic. So the slicer must compensate by starting lower than the nominal layer height.

So what works perfectly for one slicer won't necessarily work correctly for another slicer. And if you tram with a thinner object than the slicer expects (say a post-it note instead of business card) then your first layer will be off. This is why I personally prefer physical build plate leveling mid-print using screws while watching the strands go down. It bypasses all the assumptions about tramming gaps and just gives you the correct result. (Or you can do the same thing with babystepping in firmware that supports that.)

The other issue is that people use lots of weird, ad hoc slicer tricks to get their first layer to stick. Things like printing the first layer much hotter, or at half speed, or squashed way down and over-extruded, or at 60% layer height, or at 200% extrusion width, will all affect the extrusion volume calibration and the space-filling behavior of the molten plastic flowing onto the bed. The slicer doesn't really have the ability to understand "your" first-layer adhesion recipe.

The combination of incorrect/unknown tramming gap and person-specific first-layer settings is why the slicer can't always get the first layer height and extrusion volume correct across all layer heights. Within some fairly reasonable assumptions, the slicer is smart enough to always correctly relate extruder flow and nozzle position so it fills the space between the nozzle and whatever surface you're printing onto. But if you break those assumptions, it may perform differently for different settings.

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Calibrate to perfection for a specific layer height. When printing in a layer height that is different than what you calibrated for, just set the first layer height in the slicer.

That way, you avoid re-calibration as much as possible.

My experience: I've overlooked this issue in my experience too. I would usually print in 200 microns. Then for a specific piece I would try to print in 100 microns, and my first layer wouldn't stick. Manually calibrating sucks, setting the first layer height fixes this issue with no discernible drawbacks.

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  • $\begingroup$ Right, that's what I'm doing now. But if, for some reason, I'd like a different first layer height, it seems like something slicers could support. --- If, for example, we want nuzzle height above the bed to be 60% of first layer height, we would only need to tell the slicer that the probe is calibrated for 200 microns, and it could deduce the proper height for 100 and 300 automatically. --- But I don't think slicers support this. Of course, perhaps the equation isn't that clear-cut. $\endgroup$ – mhelvens Jun 7 '16 at 23:15
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The first layer is usually squished more to help with the adhesion. On top of that with ABS there's a bit of deformation even if the print doesn't lift. So spare yourself the frustration, FDM 3d printers aren't that precise themselves for you to care about this.

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  • $\begingroup$ Really? With a print surface like BuildTak it seems to be a very delicate balance between a print sticking too much and a print being too loose. It usually takes some playing around with wrenches for me to get the perfect bed adhesion. $\endgroup$ – mhelvens Jun 7 '16 at 23:19
  • $\begingroup$ Bed leveling (all points on the bed surface being the same height) is different from what you asked. $\endgroup$ – Leo Ervin Jun 8 '16 at 7:30
  • $\begingroup$ Yes. Where did I mention bed leveling? $\endgroup$ – mhelvens Jun 8 '16 at 10:01
  • $\begingroup$ when you say you have to play around with wrenches all I can assume you mean is moving the nuts up and down of the 3 or 4 screws on the edges of the bed which determine bed leveling $\endgroup$ – Leo Ervin Jun 9 '16 at 11:46
  • $\begingroup$ Nope. I meant moving the z-probe up and down. $\endgroup$ – mhelvens Jun 9 '16 at 15:58

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