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.


  1. #1 by Benjamin Coburn on 2017-12-28 - 11:53

    I think that is the straightest fit line I have ever seen in experimental data, I didn’t think actual physical objects worked that way.

    • #2 by Ed on 2017-12-28 - 12:05

      I’ve run across a few other instances of wonderfully linear fits, all of which triggered the Happy Dance.

      Of course, nearly anything looks linear over a small-enough range …

      • #3 by scruss2 on 2018-02-10 - 22:44

        “ut tensio, sic vis”, after all. Most springs will be super linear over their working range because that’s what we design ’em for. Most non-balance weigh scales rely on linear response, even load cells.

  2. #4 by allted on 2018-01-08 - 00:24

    So for a softer touch on the pen tip I could make a longer mount (longer spring arms)?
    Freaking amazing blog I have seen it many times and it always impresses me. I put a link to you on the homepage hope you don’t mind.

    • #5 by Ed on 2018-01-08 - 11:54

      Either longer arms at the same thickness or thinner arms at the same length would soften it, but 100 g/mm (in PETG, anyhow) should handle any reasonable platform un-flatness. I wrecked a couple of Sakura pens through carelessness; the poor things haven’t yet had a chance to wear out from over-pressure.

      If a drag knife needs more pressure, then you’d want thicker arms to let you “plot” at about the same “depth” with more force. At some point I’ll set up the drag knife, although from what I’ve read on the forums, a simpleminded vacuum table for paper is a must-do … which means I gotta get the DW660 working!

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