Hello !
My printer is an experimental Delta, dual extrusion, single nozzle. The firmware is Smoothieware.
After months of loyal service, Simplify3D suddenly adopted a surprising behavior: after homing, it drops to print height (X0 Y0 Z0.24), and there, before any other movement, it begins by retracting from 10 to 40 cm of filament, by the active extruder. The following is obviously not good ...
Here is the beginning of the Gcode file, as generated by Simplify3D:
G90
M82
M106 S0
M140 S100
M104 S235 T0
M104 S235 T1
G28; home all axes
G92 E0
G1 E-2.8000 F2700
G1 Z0.640 F12000
; layer 1, Z = 0.24
T0
; tool H0.240 W0.480
; skirt
G1 X0.057 Y-5940 F18000 ...
I suspect a contradictory definition of absolute and relative movements between the two extruders, but I am not, for now, very familiar with the dual extrusion, as I just installed it. Nevertheless, I obtained a few very satisfactory 2 colors prints, before this sudden anomaly.
Many thanks for any help.
Re: Aberrant massive retraction at the beginning of the prin
Well, the gcode file doesn't lie 
There's nothing in that file that would cause an excessive retraction. It very clearly does a simple 2.8mm retraction at the beginning of the print. Nothing more. So if the extruder motor is doing all kinds of weird things, then it must be a firmware issue. Honestly, I wouldn't be surprised as I encountered dozens of bugs in the few short weeks that I attempted to use smoothie. I quickly moved on to something else more stable and never looked back.
There's nothing in that file that would cause an excessive retraction. It very clearly does a simple 2.8mm retraction at the beginning of the print. Nothing more. So if the extruder motor is doing all kinds of weird things, then it must be a firmware issue. Honestly, I wouldn't be surprised as I encountered dozens of bugs in the few short weeks that I attempted to use smoothie. I quickly moved on to something else more stable and never looked back.
