skeinforge/marlin bug??

I'm not sure if this is an issue with skeinforge or marlin' but I think the latter. I am using pronterface/skeinforge/marlin - all the latest just installed in the past day or two on a Ubuntu host and driving a reprap Mendel prusa with RAMPS1.4. I have been trying to get the extruder working to my satisfaction. I have a very simple job, and when I start it, the carriage homes, and then moves diagonally to the starting point, and then the extruder goes crazy retracting the filament for a good 3 to 5 seconds. The melted PLA wraps itself around the hobbled bolt and it's just a mess to clean up.

I THINK I,be tracked it down to a single code statement - G1 E1.0 - which is very close to the start of the job. I guess this is trying to retract 1mm, but like i,I've said it goes crazy. I've entered this command directly through pronterface and observed the issue. If I try it a second time it behaves normally. I'm surmising that the firmware doesn't know the extruder's current condition, so it mishandles the first request??

Does anybody have any suggestions as to what I can try here?

Thanks

You need to have the g-code reset the E axis length to zero. For some reason it is remembering here the E axis was at at the end of the previous print, and sending a G1 E1 will cause it to reverse back to 1.

G92 E0 at the start of your g-code should fix it.

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Thanks - I'll try that tonight.

Now if I can just figure out how to determine flow rate and feed rate. I am using a .25mm nozzle and I can't believe that nothing in the setup would change if I switched to a different size - and yet I see nowhere to specify the size. The only thing I can think of that might convey this is the ratio between flow and feed rates, but everything I've seen says to make this ratio 1. Something seems to be missing to me.

No the nozzle size doesn't affect any of the maths. The flow rate is defined by how fast plastic is fed into the extruder. It all comes out the end, regardless of the aperture size and the resultant bead size is defined by flow rate / feed rate.

The nozzle aperture does affect the maximum flow rate you can achieve because it affects the pressure. It also defines the minimum and maximum filament widths that you can use (because it affects how much the plastic is stretched). You specify those to Skeinforge but it doesn't need to know the nozzle diameter.

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Then is there a formula - or at least a set of guidelines - that would help me to determine the rates? I've learned that with a .25 mm nozzle, I'm limited to a layer height somewhere in the 0.15 - 0.25 range. I can set the height/width ratio and I guess that defines the bead size, but how does that relate back to flow/feed rates.

My firmware has a calibrated value for infeed rate - and that factors into the outflow rate. I just don't know how to derive one from the other or how to determine all of the parameter values.

Is there a website or a calculator that could help with this? I don't know if I should set the rate to 15 or 50. I'd just be throwing numbers out there.

Thanks

You set flow rate and feed rate to the same number in Skeinforge and it calculates the E values to feed the filament at the correct rate.

Yes with a 0.25mm nozzle the max layer height will be about 0.25mm with W/T at 1.5, depending on the die swell you get, which depends on the plastic. Above that it wont' be stretched so will not pull tight. You can go as thin as you want as long as you increase W/T so that the width is not much less than 0.25. Otherwise the plastic is stretched too much and will snap when spanning the infill gaps, let alone bridges.

The maximum feed rate depends on the mechanics of your machine, the quality / speed tradeoff you want, the amount of grip your cold end has and the size of the melt zone in the hot end, etc. I don' think it can be calculated. Start with moderate rates, like 20mm/s outlines and 40mm/s infill. Once you are getting good results you can increase the speed until something starts to go wrong. Maybe the extruder will run out of grip and strip the filament. Maybe your bot will shake too much and the quality will not acceptable.

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Thank you - this has given me some things to try.