Sunday, 19 June 2016

Getting printing sorted out, finally

After last post's success, it figures I would get a clog. I printed two more of the dog bag endstops, then tried doing the hole size test piece, during which the hotend drooled quite a bit while parked before printing, and the drool cooled down enough to harden and make it bump when starting the print. The print then started suffering from underextrusion on the very first layer and extrusion slowed down quickly and stopped. There was quite a bit of (pretty) drooling when pulling the extruder up:

Doing the Atomic was a lot easier with this filament. The end had a small air bubble in it, making me wonder if overextrusion is an issue. There was also a rim a bit further up, but not enough that it could be what's blocking extrusion.

The thin part of the filament, obviously the part that got stretched during the cold pull, also seems to have a hollow inside. Either retraction somehow brings in air pockets, or this filament has air in it, which would be disappointing for Innofill filament.

I should at the very least make sure the first layer isn't too low, that could cause this kind of early clogging. Let's try adjusting Z level upwards over a few prints. Also adjusted bed size 10mm to have it not start over the edge, and added an extra skirt.

I decided to re-calculate the E_STEPS_PER_MM settings. Before I had calculated (and then fudged, not shown).

#define E_STEPS_PER_MM ((3200 * 39.0)/(11.0 * 6.75 * 3.142))

This gives 535 steps/mm. But there are only 9 teeth on the small gear (as shown in below "selfie with gears"), and the hob diameter is actually only 3.7mm at the middle. So instead I should use 

#define E_STEPS_PER_MM ((3200 * 39.0)/(9.0 * 3.70 * 3.142))

Which gives 1192 steps/mm! Holy mackarel, Batman! That's a big enough difference that I'm going to question some givens, such as the basic steps/rotation and microstepping implicit in the 3200. By removing the gear parts of the equation and turning off PREVENT_DANGEROUS_EXTRUSION, I can just run the motor. Then I add a bit of tape to mark a point on the gear and tell it to extrude 1mm. It nicely made two complete revolutions - but only because I can't figure out to divide 32 by 2 and had asked for 32 * 200 steps/mm. At the appropriate 16*200, one revolution. So the 1192 steps/mm is apparently correct.

It's a nice touch that when done uploading new firmware, it runs the fan for a moment. Also when Pronterface connects. Nice subtle signal.

Almost forgot to re-enable PREVENT_DANGEROUS_EXTRUSION.

Now if the E_STEPS_PER_MM varies by print speed, as suggested by previous measurements, it depends also on how extrusion goes. But since extrusion differs by all manner of factors, that's not a useful setting.

The 100 mm test extrusion went totally smoothly, with the filament piling up in an amazingly regular circle, like I've never seen it before. But it extruded way too much. eating the mark I made at 120mm. With a 50 mm extrusion, it used 120-42=78mm, a factor 1.56. Odd. At least I can't blame that on the nozzle pushing back. And it's consistent. Adding in that factor gives a nice, consistent (up to my measuring ability) output.

I also managed to have it make a bistable system while it was extruding, because the first piece of filament rounded, then tumbled to form a little rocker. When enough filament landed on one side, it would rock to that side, then filament would fall on the other side, etc. Video (late in the process):

Test print with 0.3mm bed offset failed, the nozzle wasn't even close to the bed. How very odd. The Z max is now about 5mm off the bed. WTF? Recalibrating, but how can this happen? I checked that the Z rod holders weren't lifted. The hotend is securely fastened. The bed has nowhere to go. Is my Z end-stop flaky? The gap after homing is currently 2.82mm. Alternatively, the Z motors are wonky, I did pull some dirt out of the right-hand one. They sound good to me, though. 

When calibrating, some of my Z move commands got ignored. The Z height was still calculated correctly, so it's a command problem, not a mechanical mismatch.

I also started using hair spray on the surface.

Tried a few Z offset settings, though Slic3r does seem to take the thickness of the extruded parts into account for the first layer. I had a couple aborted attempts because restarting a print doesn't pick up Slic3r setting changes. At 0.2 mm, it only barely gripped on parts of it. At 0mm, it gripped somewhat loosely, and looking at the bottom afterwards, it clearly wasn't squished together enough to form a closed surface. At -0.2mm (!), it placed itself nicely on the bed, though didn't quite fill in, and there was a bit of corner upturn. At -0.3, it squeezed a little more together, I didn't notice any corner upturn, but I peeled it off while soft before checking. At -0.4, the bottom was quite nice. At -0.5, the print stopped when it was almost done, not quite sure why. After pause and resume it finished surprisingly nicely. The bottom was quite nice. There's perhaps a little bit of overextrusion going on.

Test cubes at Z offsets 0, -0.2, -0.3, -0.4, -0.5 (first three aborted early)

Test cube #43, -0.4mm Z offset.

Test cube #44, -0.5mm Z offset.
Tried the hole test by +Giles Bathgate and it came out really nice, even though it stopped partway through and stood drooling for a while - the nozzle pretty much just plowed through the lump. The holes from 2mm and up were consistently 0.05mm too small - not much, but enough that it would be a problem. 

That was with 2mm layer height, I'll try with 2.5mm and see if it squeezes outwards a little less. And it does! Actually enough less that make the holes roughly .2mm too large, and more variable in their error.

I made a somewhat more complex test cube, with round and square holes, a peg, and overhangs. It prints nicely at 220C [CC2#1] and 210C [CC2#2].

Complex test cube 2, #1 @220C

Complex test cube 2, #2, @210C
The first print at 200C failed to stick properly, possibly since I had done a few prints since last hairspraying. With some more spray, it stuck. Except one corner that curled up, otherwise perfect print, features look even better.  Solution: Print first layer at 220, rest at 200. Trying that with 0.2mm layer height, works nicely.

It's amazing that the print can stop for quite a while while the machine sleeps and yet (sometimes) not be much worse off. It's annoying that Pronterface can't keep the machine from going to sleep, though. I keep getting Python errors when it's trying.

My theory about the clogs and height changes is that the loose print earlier bumped the Z axis up a bit. After adjusting for that, the screw eventually fell down again. The print after that was close enough to the bed to cause backpressure and an early clog, as hinted at by the bottom layer already showing less extrusion. 

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