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MPCNC: Sakura Micron Pens
After shimming the corner posts, a plot with Sakura Micron pens came out nicely:

Spirograph pattern – Sakura Micron – Z-0.25 They’re all 01 size pens, with a nominal 0.25 mm line.
Just for fun, a plot done with four sizes of black Sakura pens at Z=-1.0 before the Great Leveling:

MPCNC – Sakura Micron black pen widths The 005 pen made a nearly rectangular single-pass tour around the perimeter of the plot, so you’ll see it passing through every legend.
The chunky-by-comparison 08 pen = 0.50 mm:

MPCNC – Sakura Micron 08 Black – detail The 05 pen = 0.45 mm looks much crisper:

MPCNC – Sakura Micron 05 Black – detail The 01 pen = 0.25 mm:

MPCNC – Sakura Micron 01 Black – detail The almost-can’t-see-it 005 pen = 0.20 mm:

MPCNC – Sakura Micron 005 Black – detail If you were doing this for a living, you’d probably use 05 pens, because plotter pens are hard to find.
Original HP plotter pens produced a 0.3 mm trace (with a hard to find un-worn tip) roughly equal to Sakura 03 pens, but I haven’t seen anything other than black at Amazon. There’s apparently a 003 pen with a 0.15 mm line; that’s just crazy talk.
Jamming Sakura pens into a plotter pen adapter for the MPCNC makes little sense, so I should gimmick up a specialized holder with some thumbscrew action to keep them from crawling upward out of the holder.