Mainly: several degrees of hysteresis and dead-band errors in the
servos, torsion and tendency to non-parallel movement in multiple parts
of the mechanism, and generally, the fact the the whole thing is made of
ice-cream sticks and a clothes-peg.
I built my own A2 pen-plotter in the late '80s. One stepper motor
driving a toothed belt, moving the "head" along guide rods (all from a
defunct printer). Another stepper driving a shaft with home-made knurled
wheels to drive the paper back and forth (clamped between the wheels and
rubber wheels). A solenoid to lift and lower the pen.
The steppers were driven by LM-298 H-bridges (IIRC), which were
signalled by a Motorola 6821 PIA, with address block decoding by a
74LS138 and all connected to a Sinclair QL's expansion port.
It worked pretty well, but was soon made redundant by access to an
HP-7475A 6-colour pen plotter.
I did ask my parents whether the home-made one was still in the loft
recently, but they'd had a clear out.
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