The Smell of Molten Projects in the Morning

Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.

The New Hotness

  • 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.