There are 282 steps per mm of filament.
If you have the motor power on 1500mA you are running the motor out of spec, generating more heat on the axis, and potentially damaging the feeder motor. Also wondering how you did this, as the default limit is set to 1300.
... because I'd seen mention of it hereabouts. The print went well with my most reliable PLA right until the last couple of minutes: the print ends with a thin (~1.2mm?) wall that extends up several mm. I didn't get to the top of the wall before I started seeing signs of underextrusion.
wow - sounds complicated - a picture will help as it is normally something much simpler.
James
I gave you a link to the object on thingiverse, you can judge the complexity from that I suppose. Thanks for offering to look, but note that I didn't ask for help with the object itself, my question is about the inner workings of the firmware. I think I've done enough prints now to recognize underextrusion when it happens, I'm trying to get at why... especially since the same printer with the same filament had just got a perfect result with the cylinder extrusion test available elsewhere on this site.
@Daid: the 1500mA setting is recommended in the feeder replacement discussion. It's done using a manually placed gcode. The proposition being that the present limit is too conservative and causes skipping and hence underextrusion. I assume the current limit was not relevant here since it's at the opposite end of the extrusion rate spectrum.
I note your implied statement that 282 steps/mm is fine enough - I'll do some volume calcs on that tomorrow, however I notice that the question is still open: if <1 step was needed, would this be rounded down to 0?
282 steps/mm means that (if I'm doing this right) a single step is 0.02262mm3 or (according to Wolfram Alpha) less than half the volume of a grain of sand. I'm struggling to come up with a scenario where that little filament would be asked for.
I'm "sneakypoo" btw 
Well, that explains why I thought I remembered you claiming the design on these forums, but the only thing similar I could find was by some guy called Sneakypoo! 
Thanks for the design, and btw one reason I tried it was because I'd never printed an articulated object before. Very interesting!
Anyway, regarding what you're saying about each feeder step being a ridiculously small volume. I agree that intuitively that seems correct. However there are details I don't have. For example, at what point is the volume requirement calculated and transferred to the motor function? I can imagine that for each "pen move" I can easily calculate a volume of filament required, but some of those will be ridiculously small volumes too. So I guess they must be aggregated somewhere. ISTM that if any short move is rounded down to 0 filament, and the same move cycle is repeated long enough, you will run short of material, and eventually won't have enough.
Also, does Cura do these calculations to the same resolution as the feeder motor?
Ok, if I have the calculations right then one mm of filament is about 6.3794mm3. Divide by 282 gives about 0.0226mm3 per step of the feeder motor. If the layer thickness is 0.1 and the nozzle diam is 0.4 then one step of the feeder motor provides enough filament to lay down a 0.56555mm line. You might see rounding errors if the line is any small multiple of that length.
I guess there isn't much further to take this. I just found it odd that the UM2 could print out a large object with no extrusion problems at all, and then right after that fail on the small fiddly bits of this tiny thing I asked it to make.
Can someone elaborate on where this 282 steps/mm filament comes from?
I've been trying to dig up the specs of the UM2 feeder motor, and basically have not found anything explicit. I did find specs for the z axis motor which says it's 1.8deg/step. Are all 4 motors the same? Elsewhere I find that the feeder knurled wheel is around 7.95mm diameter, so is the following calculation not correct?
One step = 1.8 deg, so a full rotation is 360/1.8 = 200 steps.
One full rotation pushes pi*7.95mm filament, ~=25mm.
So steps/mm = 200/25 = 8.
That's a bit different from 282. Where am I going wrong? Normally I would look for some kind of gear reduction, but if the motor includes a gearbox I see no mention of it.
The stepper is a 0.9deg one, so 400 full steps/rev. But it's not doing full steps, it's doing microstepping. I'm not sure but I think it's doing 1/16 microstepping which means that for each full step it's doing 16 tiny ones. Then you have to take into account the compression on the filament as well.
Microstepping? I didn't know that was possible, but I'm still reading that link from yesterday.
[Edit] Oh I see. As I understand it, instead of switching each pole independantly (0->1), you can balance two poles.
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Can someone with a knowledge of the UM2 firmware comment on this? Does the feeder advance filament in discrete steps, and if so, how does it cope with fractional steps? Particularly, a long sequence of fractional steps.
Or is the question misplaced? Should I view the printer as a simple robot and refer the question to whatever generated the gcode, i.e. Cura?
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