Machine Shop

MPCNC: Ground Shaft Pen Holder

Drilling a pair of holes into a length of ground steel shaft turned it into a holder for a Sakura Micron pen:

DW660 Pen Holder - printed plastic vs ground steel
DW660 Pen Holder – printed plastic vs ground steel

The aluminum ring epoxied to the top keeps it from falling completely through the linear bearing.

The hole sizes are the nearest inch drills matching the pen’s hard metric sizes:

Ground 12 mm rod - Sakura pen drill diameters
Ground 12 mm rod – Sakura pen drill diameters

While I was at the lathe, I turned another layer of epoxy on the printed holder down to a consistent 11.95+ OD. It fits the bearing nearly as well as the steel shaft, although it’s not quite as smooth.

The steel version weighs about 20 g with the pen, so it applies about the same downforce on the pen nib as the HP 7475A plotter. The force varies from about 19 g as the Z axis moves upward to 23 g as it move downward, so the stiction amounts to less than 10% of the weight:

DW660 Pen Holder - ground shaft
DW660 Pen Holder – ground shaft

However, the more I ponder this setup, the less I like it.

When the Z-axis moves downward and the nib hits the paper, it must decelerate the weight of the pen + holder + ballast within a fraction of a millimeter, without crushing the nib. If the pen moves downward at 3000 mm/min = 50 mm/s, stopping in 0.3 mm requires an acceleration of 4.2 m/s² and a 20 g = 2/3 oz mass will apply 0.08 N = 0.3 oz to the nib. Seems survivable, but smashing the tip a few hundred times while drawing the legends can’t possibly be good for it.

Also, the tool length probe switch trips at 60 (-ish) g, which means the pen can’t activate the switch. Adding a manual latch seems absurd, but you can get used to anything if you do it enough.

10 thoughts on “MPCNC: Ground Shaft Pen Holder”

  1. I think I’m not understanding something. Where are the two holes in the ground steel shaft and how do you drill two holes in a ground steel shaft?

    1. I’ll take a guess: one from each side, maybe two different diameters. End result is one hole, but it starts as two.

    2. Jason’s got it: a hole from each end to fit the pen diameters. I added the shop doodle to clarify what I meant and record the sizes for later.

        1. The outside is case-hardened well enough to rebuff a carbide toolbit, so I used up a big Dremel wheel slicing off that chunk. Yuch!

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