12 hours ago, Slashee_the_Cow said:The printer automatically takes the Z offset into account when reading the Z values from gcode. For a 0.3mm layer, it's not supposed to be exactly 0.3mm from the build plate, it's supposed to be 0.3mm + Z offset.
I think there might be some flexibility in terminology here, leading to misunderstanding or at least miscommunication (and re-reading my first post I can see I might have confused things).
I understand "Z offset" as a calibration, telling the printer where to position the Z motion for Z=0. So when told to position at Z=0.3, it goes to what the printer has been told is Z=0, plus 0.3mm. So far as the controller and the slicer are concerned, the nozzle is at 0.3mm above the datum plane.
Physically however, the gap is 0.3mm + 0.1mm*, +/- undulations in the build plate.
(* or whatever your "paper thickness" is, preferably equal to the maximum deviation in the build plate)
12 hours ago, Slashee_the_Cow said:Would be pretty bad in some cases if you had to take the Z offset into account when setting the height Z height of the gcode - I installed a linear rail upgrade on my printer which moves the print head up from its stock position, so right now I'm running at a Z offset of -2.2mm. Wouldn't want to try and print a 0.3mm layer at Z-1.9 because I'd prefer not to dig a trench in my bed.
(n.b. I have in fact dug a trench in my bed by forgetting to change the Z offset after I installed a new nozzle. It's not a fun experience, unless you consider buying a new build plate fun.)
Exactly. We are agreed that setting Z=0 as the nozzle in contact with the build plate is a very bad idea.
12 hours ago, Slashee_the_Cow said:So when the gcode says Z0.3, if you have a Z offset of 0.1mm, the printer will move to Z0.4 but it's still only going to extrude enough for a layer 0.3mm high.
But that is my point, and I'm surprised nobody else thinks this is an issue. It would be good to tell Cura to increase the extrusion on the first layer, so that the additional "safety margin" layer height is not under-extruded. I guess that's why the first layer is set to double the standard layer thickness, so as to reduce the percentage under-extrusion to an "acceptable" amount.
Wouldn't it be better to tune out the under-extrusion? Then the inherent errors in the build plate would result in a plus or minus extrusion error, rather than always biased towards under-extrusion.
Initially I was thinking to tune out the under-extrusion by telling Cura what initial gap to compensate for, but now I think specifying a percentage increase in extrusion might be the easier way to go, although the percentage will vary according to first layer height.
12 hours ago, Slashee_the_Cow said:If it didn't have that Z offset as a gap, there wouldn't be enough room for the filament to come out and form a layer of the correct height because it would squish up against the bottom of the nozzle (and potentially clog it as more filament comes out).
Sorry, I don't agree with that at all. All the other layers are printed at the calibrated gap, so I see no reason the first layer would not. Risk of gouging notwithstanding, if the nozzle were in contact with the build plate at Z=0, there would be a 0.3mm gap for a 0.3mm first layer.
If there's not sufficient gap, the extruder just slips (probably, at least on my printer) and little harm is done. There's no way to make up for too much gap though, and it seems to me the safety margin is leading to (overall) under-extrusion, therefore reduced adhesion and lower quality printing on the underside.
Pending some solution being built into Cura to provide an extrusion percentage increase for the first layer (presuming it's not there already and I just haven't found it), I think this might be another candidate for my first layer override solution when it came to reversing the inside-to-outside line ordering:
https://community.ultimaker.com/topic/46627-how-to-increase-first-layer-print-success-with-poor-bed-adhesion
Edited by CrazyIvan2
Grammar, clarification
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Slashee_the_Cow 541
No need to do that 🙂 The printer automatically takes the Z offset into account when reading the Z values from gcode. For a 0.3mm layer, it's not supposed to be exactly 0.3mm from the build plate, it's supposed to be 0.3mm + Z offset. So when the gcode says Z0.3, if you have a Z offset of 0.1mm, the printer will move to Z0.4 but it's still only going to extrude enough for a layer 0.3mm high.
If it didn't have that Z offset as a gap, there wouldn't be enough room for the filament to come out and form a layer of the correct height because it would squish up against the bottom of the nozzle (and potentially clog it as more filament comes out).
Would be pretty bad in some cases if you had to take the Z offset into account when setting the height Z height of the gcode - I installed a linear rail upgrade on my printer which moves the print head up from its stock position, so right now I'm running at a Z offset of -2.2mm. Wouldn't want to try and print a 0.3mm layer at Z-1.9 because I'd prefer not to dig a trench in my bed.
(n.b. I have in fact dug a trench in my bed by forgetting to change the Z offset after I installed a new nozzle. It's not a fun experience, unless you consider buying a new build plate fun.)
If you want to be precise about it, buy a feeler gauge. If you're a bit 😕 about the height of regular paper, try receipt paper. But first, try it after setting it with regular paper and see what results you get. If it's having trouble adhering to your build surface, consider reducing your Z offset.
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