printrbot is driven by a belt. Rather than having a fixed loop, the ends are attached to the carriage directly (I'm uncertain how it was intended, but it seemed to make sense to loop the ends this way and use zip ties to secure them... This ends up working surprisingly well (I will probably make special clamps for tensioning later).
I'll look at the quality of the signals later but it's a good idea to check the current limits for the output from the drivers before everything is wired up. My board was set to send a bit too much current (the stepped got way too hot) so I backed it off to the point where the steppers failed to turn reliably and then gave them a bit more... don't overdrive steppers.
After a bit of not having anything stick at all I found out that making certain the bed is leveled (much more accurately than I expected) and having the limit switch for the z-axis exactly right (piece of paper underneath has light tension no matter where the tip is on the hotbed) -> printing/ticking first layer is 100% reliable and repeatable.
I'm still printing at only 0.4mm layers here and will play with increasing the resolution after I have replacement parts made. I have not flatted my shafts and I suspect that might be a requirement for higher speeds with might be needed for playing with higher resolutions.
As a side note, the two times I had to rethread the filament were because I hit extrude before the temp was high enough... what happens is that the hobbed bolt shreds the filament and the grooves fill with dust so nothing grabs... Very easy to fix -> heat the extruder -> run backwards to clear the filament -> open the tensioner -> flick out the dust with a needle from the groves in the hobbed bolt -> blow the dust away -> rethread -> CALIBRATE AGAIN -> and you're all set to print.
This is really a fun little bot... once I have replacement parts printed I'll see just what kind of quality I can get out of it.