A trio of Cutter Cutting Plotter Blade Holders arrived:

Collet pen holder

Despite the name, they’re not well-suited for drag knife blades, because they’re collets gripping a 2 mm shaft. The blade doesn’t rotate unless the plotter / cutter rotates the entire holder, which is actually a thing.

I got ’em because the snout of a common ball-point pen refill measures about 2 mm:

Collet pen holder – detail

The glob around the tip comes from plotting too fast for conditions; about 1500 mm/min works better for continuous lines and 250 mm/min improves text.

The stock MPCNC adapter has a single recess suited for Genuine Plotter Pens, but the knurled lock ring on these cheapies sticks out far enough to make them wobbly. This being an inconvenience up with which I need not put, a few lines of OpenSCAD tweak the stock STL:

MPCNC knife adapter mods – OpenSCAD model

The original STL is ivory, new cuts are cyan, and additions are reddish.

The two support beams are now 1.6 mm = four thread widths, for improved slicing with a 0.35 mm nozzle and a higher spring constant.

It’s by-and-large indistinguishable from the old adapter:

MPCNC – Pen Holder Detail

Which I was using upside-down, because the flange fit better.

The MPCNC works reasonably well as a pen plotter with a genuine ballpoint pen:

MPCNC Ballpoint pen plots

The OpenSCAD source code as a GitHub Gist: