One of the issues I have with the Thing-o-Matic and PLA is that Makerbot Industries uses a bolt pushing a Delrin plunger onto the filament to press it against the rotating drive shaft.
There is no spring involved.
My guess is that this leads to my issues of the filament pressure requiring constant human observation and adjustment as the filament diameter never stays perfectly constant.
So here I am, trying to replace the extruder with a spring loaded design (for the second time).
I tried this once in 2011. Back then I used a geared down design and the gears as downloaded where too small by 1-2mm and would not mesh properly.
This time I'll try a more simple, direct drive design first.
This also has the benefit of not requiring me to reverse the stepper motor as there are no gears to change it's direction of rotation.