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Archive for December 28th, 2017

MPCNC: Plotter Pen Holder Spring Constant

Watching the MPCNC plot Spirograph patterns led me to wonder about how much force the printed drag knife holder applies to the pen:

Spirograph - liquid ink pen - detail

Spirograph – liquid ink pen – detail

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:

MPCNC - Plotter pen force test

MPCNC – Plotter pen force test

Stepping the pen downward in 0.1 mm increments produced a set of numbers and a tidy linear fit graph:

MPCNC Plotter Pen Holder - Spring Constant

MPCNC Plotter Pen Holder – Spring Constant

I hereby swear I’m not making things up: the spring constant really is a nice, round 100 g/mm!

I set 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.

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