Longboat Prusa - Extruder Problem

I am having a problem with my extruder. Best shown by this video.

Extruder Problem Video

Apologies, this video has been converted to divx .mkv file, so I am not sure how compatible that is for all of you.

Basically when I hit 'Extrude' from within Pronterface it works for the first time, maybe a second time but then it 'sticks'. It still manages to do the full extrude, or appears to but you can see from video something is wrong.

Fast forward video to 25 seconds to avoid seeing me connect etc and to show it working fine for the first time.
At 1 minute 15 secs you can start to see it 'stick'. It does this reliably from this point onwards.

I have swapped the connectors over on my board so that the extruder motor is wired to the z-axis driver and the z-axis motor is wired to the extruder driver. When I click 'extrude' the z-axis turns as expected but it does the same, it sticks. When I click on the Z-axis the extruder motor whirs away quite happily. Problem with the extruder driver on the board?

Cheers

Mark

That sounds strange. I had issues with my extruder which are covered here and hopefully helps a bit:-
[forums.reprap.org]

However the fact that you switched them over and the problem moved makes it seem likely that you are right, though obviously the Z-axis and extrudes are set for different speeds and accelerations in the firmware so this might be a factor the Z axis runs pretty slow and direct drive while the extruder is fast and geared, so when you switched the extruder would probably find it easier running slower and the Z axis find it hard to run faster...

I was surprised to find that the temperature needed to be a lot higher than the Pronterface default (I use 205C now which seems to work well) and before this I had an issue like you describe (though I cant see your video)...

Hope this helps

George

My guess is the current on the extruder driver is set too high. After a while it overheats and shuts down for a while.

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[www.hydraraptor.blogspot.com]

Mark
You can adjust the current to the driver by turning the scew on the stepper driver board.
You can measure the voltage (which more easily measured than current) by measuring between ground and the small hole just next to the driver chip on the driver board.
My longboat prusa setup works fine when measuring 0.47 volts at this point which I understand means the current is at the top end for the extruder but not too high that it will overheat.
For reference I use 0.41 volts on the other driver boards as those motors don't need as much current.

Thanks guys problem sorted!