Windell should know better, but my initial guess was that it might be a consequence of trying to follow the irregularities in human drawn lines since the carriage otherwise moves super smoothly when translating with the pen up.
I am also going to see if tightening the string and increasing the motor current has any effect.
I had completely missed the sliders. Are they custom designed or some kind of standard order part? I will try to get a closer look at them when I visit your shop next. I also got a little curious… does the friction matter where they rub against the wood? Are they perhaps rolling on bearings?
And the Polargraph drawing machines made my day… :-) thanks for sharing that!
That probably must be the simplest way to setup a pen plotter. I suppose Sandy could also have just used the EiBotBoard.