Extruder Motor Skipping Each Half Revolution

Cheddy

I am trying to calibrate the extruder on my delta printer. I'm testing with PLA and extrusion and retraction distances and feed rate are correct when the bowden tube is disconnected from the extruder hotend, but when I try to extrude through the hotend the filament appears to back up after a few millimeters and the extruder motor starts skipping. Interestingly, it only and always skips on one half of each revolution and runs cleanly on the other half. I have tried reducing the feed rate to 5mm/sec and I have tried increasing the extruder temperature, up to 250C, which helps slightly. Incidentally my two infrared thermometer guns only register about 180C on the nozzle and heater block when Rep-Host is reporting 260C.
I have dismantled and cleaned the extruder and nozzle, and I have played with varying the tension on the extruder motor bearing wheel, but neither of those helped. If I back off the bearing wheel to the point where the motor doesn't skip, then it fails to push the filament through.
I am wondering if it is a defective motor that loses torque during one of its phases, or is it a problem with the extruder.

Repetier

If motor moves full rounds without filament i would say motor is ok.

For temperature check by pressing filament through by hand at 3-4mm/s. If that is no big problem temperature is ok and nozzle good.

Now the question is what exactly happens, does the filament slip back or the motor rotate back.
If motor rotates back it has not enough torque and you might need to increase current i motor driver. Especially if motor stays cool it is not working at full power. What you see is the spring effect of bowden. At first you add force to the spring and the more you push the more force get back to motor until it starts extruding with same speed as motor runs. If the force exceeds and you do not slip the motor rotates back to free the force.

Cheddy

Thanks. I'll check the current on the stepstick driver and see if adjusting that helps. Stepstick drivers should read 1.6v, right?

BTW, does the length of the Bowden tube have any effect on the above? I have a suspicion I may have cut the tube too short. It's long enough to reach all points in the build space, but only just.

Repetier

You want the tube short, but long enough that it does not break your filament and no forces are put on xy motors pulling it back. Longer bowden have more spring effect and this effect makes the print results bad, requiring more retract etc.