The simplest way to push a pen (or similar thing) downward with constant force may be to hold it in a linear bearing with a weight on it, so I gimmicked up a proof-of-concept. The general idea is to mount the pen so its axis coincides with the DW660 spindle, so as to have the nib trace the same path:
The puck mimics the shape of the DW660 snout closely enough to satisfy the MPCNC’s tool holder:
The pen holder suffers from thin walls constrained by the 10 mm (-ish) pen OD and the 12 mm linear bearing ID, to the extent the slight infill variations produced by the tapered pen outline change the OD. A flock of 16 mm bearings, en route around the planet even as I type, should provide more meat.
In any event, 3D printing isn’t noted for its perfect surface finish, so I applied an epoxy layer and rotated the holder as it cured:
After letting it cure overnight, I ran a lathe tool along the length to knock down the high spots and set the OD to 11.9+ mm. Although the result turns out to be a surprisingly nice fit in the bearing, there’s no way epoxy can sustain the surface load required for the usual precision steel-on-steel fit.
A plastic pen in a plastic holder weighs 8.3 g, which isn’t quite enough to put any force on the paper. Copper weighs 9 g/cm³ = 9 mg/mm³ and 10 AWG wire is 2.54 mm OD = 5 mm², so it’s 45 mg/mm: to get 20 g, chop off 450 mm of wire.
I chopped off a bit more than that, straightened it, annealed it, and wound it around a random contestant from the Bucket o’ Sticks with an OD just over the pen OD:
The helix is 13.5 mm down the middle of the turns and 14 turns long (trimmed of the tail going into the chuck and fudging the tail sticking out as a partial turn), so it’s 593 mm long and should weigh 26.7 g. It actually weighs 27.6 g: close enough.
Which is enough to overcome stiction due to the holder’s surface roughness, but the mediocre epoxy-on-balls fit allows the pen point to wander a bit too much for good results.
The prospect of poking precise holes into 16 mm drill rod seems daunting, but, based on what I see here, it will produce much better results: rapid prototyping FTW!
The OpenSCAD source code as a GitHub Gist: