The original makerbot MK4 extruder worked well until it broke, but like most extruders the starting and stopping of the plastic flow was not very accurate. When printing simple objects this is not a problem, a few loose strings here or there are easily removed with a sharp knife. The extruder motor did reverse a bit after extruding to try to reduce oozing out of plastic, but the gear motor was not really suited for that purpose. I read that now in the the latest version of ReplicatorG the reversing is not switched on by default to reduce the strain on the motor. One of the reasons for building the new extruder based on Wades's design a while back was exactly the extra control over the flow of plastic which a stepper motor would provide. The new extruder has been working fine so far, but one thing I had not done yet: the new extruder was running all the time using a separate Arduino processor and was not controlled at all by the printing software. This works ok, but of course it produces a large amount of strings between the different parts of a model, like in this part of the new Z axis support:
The part is fine after some cleanup with a knife as can be seen here.
The current standard software for the mendel includes support for a stepper motor in the extruder, but the control circuit which is used by default is a double motor steering on the extruder mother board which is not very good for this purpose. I think there are several pieces of software being developped which look promising for the near future (e.g. here), but I don't have a standard solution right now. So in order to get control over my extruder now, I hooked up the output of the extruder mother board which used to go to the MK4 extruder gear motor to my arduino board which is controlling my stepper motor. Since the extruder board is sending 12V signals I reduced the signal voltage with some diodes and resistors so it is safe to connect to the input of the arduino.
The arduino will now detect the extruder motor state from the 2 signals and determine if the motor should go forward, backward or stop. There are some limitations, but it works!
I printed this nice puzzle, a Philippe Dubois burr from thingiverse, which consists of 6 different parts. All parts where printed at the same time and the extruder stopped and started during moves between the different parts for each layer (first layer 1 for all parts is printed, then layer 2 for all parts etc.). As you can see the amount of strings is very little; strings are not completely gone, but no comparison to the z-axis picture above!
The puzzle looks like this when assembled:
The puzzle is much more difficult than I imaged. There is a good website describing the different forms of the puzzle and also includes animations of the solutions. This particular puzzle is what is described on the website as a level 6.4 burr puzzle. This means it takes 6 (!) moves before you can remove the first piece, after that it takes 4 moves before you can remove the second piece. After this the other pieces fall apart.