The extruder is working, but the quick and dirty solution to use full steps for the extruder stepper motor was causing some unwanted vibration, see previous post. The stepper motor driver IC TCA3727 (datasheet) is actually capable of full, half, quarter and "mini" stepping. Each phase of the stepper motor can be fed with 3 different current levels ("hold", "set" and "accelerate") and using all those different current levels between full steps will allow for a total of 24 phases in one complete cycle (full stepping uses 4 phases per cycle). Since all the wires controlling the current where already connected to the arduino only software needed to be changed. The "mini steps" are implemented now and the vibration is much less and the motor runs much smoother.
There is still no control of the extrusion motor, I start the motor manually just before a printing cycle and stop it when it is done printing. For some objects this is no problem, they come out fine, like this one:
Other objects which consist of layers in which there are separated "islands" will have bigger problems as thin threads of plastic will be formed when the extruder moves from "island" to "island". This is such an area:
In general PLA is working very well. The smell is much less, I have not found any warping of objects so far and the translucent nature gives for pretty objects. PLA is behaving differently than ABS. PLA is much harder when cold, but is very soft when an object is still warm. For small objects it is necessary to let them cool down before removing them from the base to avoid deforming them in the process. One type of object which is causing a problem here is a very thin tall object. Since the lower layers stay a bit soft, the friction of the extruder will cause the object to move with the extruder and gets ruined; this was supposed to be a straight, vertical cylinder about 5 mm in diameter.
I use the blue painter's tape on the base which is recommended widely and I find that very easy to use. The objects seem to adhere well to the tape, but the tape can be peeled of reasonable well.
I have not figured out yet what the best height is for the first layer. If the height is too high, the first layer will be messed up. When it is too low the first layer will be squeezed down, become super flat because of that, but it becomes a bit wider than the object should be. Check out the lowest layer of this object, it is on top in this picture as I turned it upside down. This is the same object pictured above from the top side.
As always, comments and questions welcomed.