CNC 3018XL: Pilot V5RT Pen Holder

It turns out my all-time favorite Pilot Precise V5 Extra Fine stick pen also comes in a clicky-top retractable version:

The cartridge is a nice 6 mm cylinder, eminently transformable into a plotter pen:

A few minutes with a caliper provides key measurements for a snout surrounding the business end:

The green letters & numbers give the nearest drill sizes. The “T” values along the bottom are the tailstock turns (at 1.5 mm/turn) required to poke the drills to the indicated depths, eyeballed when the body just enters the hole.

Having recently decomissioned the Thing-O-Matic and harvested its organs parts, I have a vast collection of 3/8 inch = 9.52 mm shafts and matching bronze bushings:

Bronze bushings have low stiction, at least when they’re co-axial, and are much shorter than linear ball bearings.

I chopped off a 70 mm length of shaft and faced the raw end:

The other end had a maker’s logo, but I don’t recognize it:

I really wanted an 8 mm bore around the snout, but it just didn’t work out. The ring around the 7.5 mm counterbore shows where the larger drill just … stopped:

A trial fit with the pen cartridge:

The top of the shaft gets a somewhat longer knurled ring for the 3 mm SHCS holding the cartridge in place:

The screw bears on a split collar turned and drilled from a Delrin rod:

The “split” came from a simple saw cut across one side and I milled a flat spot in the knurling to seat the screw. As usual, the knurled ring got epoxied to the shaft.

The snout started as a 3/8 inch aluminum rod, drilled as shown in the sketch, with a (scant) 7.5 mm section to fit the shaft. The carbide insert left a nicely rounded shoulder that required trimming to fit snugly into the shaft:

The compound can handle the shallow angle required to shape the snout:

A trial fit showed the snout was a bit too long for comfort:

Making something shorter doesn’t pose much of a challenge:

Another trial fit shows it’s spot on:

The critical part is having the snout support the plastic around the pen tip to prevent wobbulation.

Epoxy the whole thing together, add a suitable spring, tighten the screws & nuts for the reaction plate, and it’s all good. I write with about 50 g of force for these pens, so a light preload seemed in order:

If I’d weighed the full-up shaft + snout + collar + cartridge, I’d know if the Y intercept matches that weight. It seems a little lighter, but I’m not taking the thing apart to find out.

The first version of the 3D printed holder (shown above) is a straightforward modification of the LM12UU diamond drag bit holder, but, after building enough of these things, I realized the circular reaction plate should be triangular to get more clearance in front of the Z-axis stepper motor when installing & removing the holder:

It also has a recess for the serrated top of the bearing, to prevent the knurled collar from clicking annoyingly as the Z-axis rises at the end of each stroke.

Now, to see how well it draws!

The OpenSCAD source code as a GitHub Gist: