Why does my extruder suck the filament back before printing?

Hi, I'm having problems with printing on my Prusa. When I start a print, the head moves to the starting position - usually in the middle of the table - and then, for no reason I can see, the extruder reverses about 10 - 15 mm of filament back out so effectively emptying the heater chamber. Consequently, when it starts trying to print, nothing comes out of the extruder until about half way through the second layer by which time the filament won't stick to the bed. This is the first few lines of a typical g-code file produced by Skeinforge (SFACT) up to where it starts printing.....


G28 E0
G90
G21
M103
M105
M113 S1.0
M108 S20.0
M104 S200.0
M104 S198.873
M108 S8.732
M108 S21.83
G1 X76.723 Y113.796 Z2.519 F60.0
G1 F798.0
G1 E5.0
G1 F60.0
M101
G1 X76.723 Y113.996 Z2.519 F60.0 E5.1176
G1 X76.723 Y126.541 Z0.576 F327.4646 E6.4856
G1 X78.797 Y128.615 Z0.576 F327.4646 E6.8016
G1 X78.797 Y110.886 Z0.576 F327.4646 E8.712
G1 X80.87 Y109.849 Z0.576 F327.4646 E8.9619

I can't see what causes the extruder to reverse and I'm also not sure where 'Z' gets the figure of 2.519 to keep going up to.

The machine has Marlin firmware and I use the latest version of Pronterface in Ubuntu 11.10 linux. I'm using 3mm PLA onto blue tape on an acrylic bed.

Once the filament gets a hold on the bed, the rest of the print is OK although often the second print of a run is not as good as the first - I think the machine only likes to do one print per day!!!

Any clues as to what might be causing these gremlins would be very welcome as its driving me nuts...!!

thanks,

Ian

---

Thanks, Ian

Replace G28 E0 with G92 E0. You might also need to move it after G21.

Can you explain what that does? I'm having similar issues right now.

Mine draws the first outline without extruding much (left over on tip?) then backs the filament all the way out.

This is on my first attempt at a print ever so I don't have any idea what those changes would do.

G92 E0 completely solved the issue - thanks brndr. I put it in the start.gcode file and its now automatically inserted at the start of every gcode file that Skeinforge creates. What it does is to make whatever position the extruder is at after its last print its new zero. i.e. it resets the software to zero for E. I had thought that G28 would home the extruder and so set it to zero but this isn't the case - G92 is what you need.

I now have other issues including trying to find where Skeinforge sets the initial positioning speed. When I start a print, the machine homes fine but then, as it moves the head to its start position, it just crawls across the build platform as, for some reason, Skeinforge has set this particular speed to 60mm/m instead of the normal speed of several hundred mm/m. FWIW, I have also found that the height of the head above the bed and the bed level are extremely critical in getting the filament to stick initially - I'm now wondering whether I should make a heated bed as my next priority..

---

Thanks, Ian

> I now have other issues including trying to find where Skeinforge sets the initial positioning speed. When I start a print, the machine homes fine but then, as it moves the head to its start position, it just crawls across the build platform as, for some reason, Skeinforge has set this particular speed to 60mm/m instead of the normal speed of several hundred mm/m.

I hit that issue too Actually solution is very easy - turn off limit, and initial speed will get back to normal. I do not know why it is enabled by default but it is there...