The HP 7475A plotter spec calls for 19 g = 0.67 oz of downward force on the pen, so, in an ideal world, one might want to use one’s collection of aging plotter pens in a similar manner.
Plotter pen, meet digital scale:
Stepping the pen downward in 0.1 mm increments produced a set of numbers and a tidy linear fit graph:
I hereby swear I’m not making things up: the spring constant really is a nice, round 100 g/mm!
plot_z = -1.0 in the GCMC program, with Z=0.5 touched off atop a defunct ID card on the paper surface to compensate for any tabletop warp / bow / misalignment, plus any errors from the tool length probe. An eyeballometric scan against a straightedge shows pretty nearly no misalignment, which means the holder mashes the pen against the paper with about 100 g of force, five times the HP spec.
A distinct case of pen abuse rears its ugly head.
It’s time to conjure a height probe for the tool holder.