Filament slipping?

I am using this extruder: [www.reprap.org]. Direct drive, hobbed insert. 3mm ABS filament.
I am printing at 220C and PTFE filmed heated bed at 90C, sticks well.

The problem is, sometimes the filament aren't going through well - I thing some filament were stripped. It is much better if manually apply some force to push it downward. Attached is the photo of the insert and the calibration object.
My feedrate is rather slow, just about 25mm/s.

What are the advices that I could fix this problem?

I also wonder if anyone has tried using timing belt pulley in place of the hobbed insert?

I am not extruding at 225C, instead of 225C, which seems resolve the problem. But it probably need longer time to tell for sure.

Trying to use 3mm filament with a direct drive NEMA 17 extruder will not work well. There is a reason that current extruders for 3mm filament are geared down. A NEMA 17 just doesn't have enough torque to directly push 3mm filament, in fact it barely does for 1.75mm filament.

It may work consistently for very slow print speeds, but you might consider switching either to 1.75mm filament with your current extruder or switch to a geared extruder with 3mm filament.

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If it is slipping and not stalling the motor the spring is perhaps not strong enough.

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[www.hydraraptor.blogspot.com]

Thanks for the tips.
I don't think it's the torque yet because it does spin. I agree it won't work well at high speed.
As it can print "something" now I hope I could print a new extruder with this, a Greg's Wade prehaps.

Though I need to gather the vitamins for it...
I hate the hobbed bolt because something as unspecified as that is very hard to communicated with the workshop. For instance, I don't know what "hobbed" is in Chinese nor what the name of the tool that make it or how deep the grooves has to be, or chance for fixing it if something get wrong.

I think I have came across a M4 insert design somewhere? I might also experiment with some small timing belt pulleys I have on my hand.

Gotta get the printing dimensions right though.

Edited 1 time(s). Last edit at 07/22/2012 07:25PM by sam0737.

Here's a good hobbed bolt, including a drawing, that can be used as a reference. Basically you just need a round groove with sharp teeth cut into it.

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nophead made a good point that the spring is not hard enough.
I put a M5 screw in the spring effective reducing 4mm or 25% space of the spring, it seems made it pushing force double.

I thought the motor won't be able to spin, but to my surprise it still spins =) And the output flow is much more consistent, and it doesn't slip at all now.