Archive for October 9th, 2018

MPCNC: Modified Drag Knife Adapter Spring Constant

Posted by Ed in Machine Shop, Science on 2018-10-09

The bars on the original MPCNC drag knife / plotter pen adapter had a 100 g/mm spring constant:

MPCNC - Plotter pen force test

MPCNC – Plotter pen force test

Making the bars slightly thicker improved their print-ability:

MPCNC knife adapter mods - OpenSCAD model

MPCNC knife adapter mods – OpenSCAD model

The reddish tint marks the new bars, with their location carefully tweaked to be coincident with the stock STL.

Shoving the pen into the scale with 0.1 mm steps produces another unnervingly linear plot:

Modified MPCNC pen adapter - Spring Constant data

Modified MPCNC pen adapter – Spring Constant data

Real plotter pens want about 20 g of force, so this isn’t the holder you’re looking for.

A bunch of plots at Z=-1.0 mm turned out well with the ballpoint pen insert, though:

MPCNC Modifed pen adapter - first plots

MPCNC Modifed pen adapter – first plots

The globs apparently come from plotting too fast for conditions; reducing the speed to 1500 mm/min works better.

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