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:
| // Diamond Scribe in linear bearings for CNC3018 | |
| // Ed Nisley KE4ZNU – 2019-08-9 | |
| Layout = "Build"; // [Build, Show, Base, Mount, Plate] | |
| /* [Hidden] */ | |
| ThreadThick = 0.25; // [0.20, 0.25] | |
| ThreadWidth = 0.40; // [0.40, 0.40] | |
| /* [Hidden] */ | |
| Protrusion = 0.1; // [0.01, 0.1] | |
| HoleWindage = 0.2; | |
| inch = 25.4; | |
| function IntegerMultiple(Size,Unit) = Unit * ceil(Size / Unit); | |
| ID = 0; | |
| OD = 1; | |
| LENGTH = 2; | |
| //- Adjust hole diameter to make the size come out right | |
| module PolyCyl(Dia,Height,ForceSides=0) { // based on nophead's polyholes | |
| Sides = (ForceSides != 0) ? ForceSides : (ceil(Dia) + 2); | |
| FixDia = Dia / cos(180/Sides); | |
| cylinder(r=(FixDia + HoleWindage)/2,h=Height,$fn=Sides); | |
| } | |
| //- Dimensions | |
| PenOD = 6.1; // pen refill shaft, max OD | |
| Bearing = [(3.0/8.0)*inch,16.0,10.6]; // linear bearing body, ID = shaft diameter | |
| BearingFlange = [Bearing[OD],17.2,1.0]; // flange around end of bearing | |
| Spring = [8.5,9.5,15.5]; // compression spring around shaft, LENGTH = uncompressed | |
| SpringRecess = 4*ThreadThick; | |
| WallThick = 4.0; // minimum thickness / width | |
| Screw = [3.0,6.75,25.0]; // holding it all together, OD = washer | |
| Insert = [3.0,4.2,7.9]; // brass insert | |
| //Insert = [3.0,5.0,8.0]; | |
| //Insert = [4.0,6.0,10.0]; | |
| Clamp = [43.2,44.0,34.0]; // tool clamp ring, OD = clearance around top | |
| LipHeight = IntegerMultiple(2.0,ThreadThick); // above clamp for retaining | |
| BottomExtension = 15.0; // below clamp to reach workpiece | |
| MountOAL = LipHeight + Clamp[LENGTH] + BottomExtension; // total mount length | |
| echo(str("Mount OAL: ",MountOAL)); | |
| Plate = [PenOD + 4*ThreadWidth,Clamp[ID] – 0*2*WallThick,WallThick]; // spring reaction plate | |
| echo(str("Screw length: ",Spring[LENGTH] + Plate[LENGTH] + Insert[LENGTH])); | |
| NumScrews = 3; | |
| ScrewBCD = Bearing[OD] + Insert[OD] + 2*WallThick; | |
| echo(str("Retainer max OD: ",ScrewBCD – Screw[OD])); | |
| NumSides = 9*4; // cylinder facets (multiple of 3 for lathe trimming) | |
| // Basic mount shape | |
| module CNC3018Base() { | |
| translate([0,0,MountOAL – LipHeight]) | |
| cylinder(d=Clamp[OD],h=LipHeight,$fn=NumSides); | |
| translate([0,0,MountOAL – LipHeight – Clamp[LENGTH] – Protrusion]) | |
| cylinder(d=Clamp[ID],h=(Clamp[LENGTH] + 2*Protrusion),$fn=NumSides); | |
| cylinder(d1=Bearing[OD] + 2*WallThick,d2=Clamp[ID],h=BottomExtension + Protrusion,$fn=NumSides); | |
| } | |
| // Mount with holes & c | |
| module Mount() { | |
| difference() { | |
| CNC3018Base(); | |
| translate([0,0,-Protrusion]) // bearing | |
| PolyCyl(Bearing[OD],2*MountOAL,NumSides); | |
| translate([0,0,-Protrusion]) // bearing flanges | |
| PolyCyl(BearingFlange[OD],BearingFlange[LENGTH] + Protrusion,NumSides); | |
| translate([0,0,MountOAL – 1.5*BearingFlange[LENGTH]]) // sink into surface | |
| PolyCyl(BearingFlange[OD],2*BearingFlange[LENGTH],NumSides); | |
| for (i=[0:NumScrews – 1]) // clamp screws | |
| rotate(i*360/NumScrews) | |
| translate([ScrewBCD/2,0,MountOAL – Clamp[LENGTH]]) | |
| rotate(180/8) | |
| PolyCyl(Insert[OD],Clamp[LENGTH] + Protrusion,8); | |
| } | |
| } | |
| module SpringPlate() { | |
| difference() { | |
| hull() | |
| for (i=[0:NumScrews – 1]) | |
| rotate(i*360/NumScrews) | |
| translate([ScrewBCD/2,0,0]) | |
| cylinder(d=Screw[OD] + 4*ThreadWidth,h=Plate[LENGTH],$fn=24); | |
| translate([0,0,-Protrusion]) | |
| PolyCyl(Plate[ID],2*MountOAL,NumSides); | |
| translate([0,0,Plate[LENGTH] – SpringRecess]) // spring retainer | |
| PolyCyl(Spring[OD] + 4*ThreadWidth,SpringRecess + Protrusion,NumSides); | |
| for (i=[0:NumScrews – 1]) // clamp screws | |
| rotate(i*360/NumScrews) | |
| translate([ScrewBCD/2,0,-Protrusion]) | |
| rotate(180/8) | |
| PolyCyl(Screw[ID],2*MountOAL,8); | |
| } | |
| } | |
| //—– | |
| // Build it | |
| if (Layout == "Base") | |
| CNC3018Base(); | |
| if (Layout == "Mount") | |
| Mount(); | |
| if (Layout == "Plate") | |
| SpringPlate(); | |
| if (Layout == "Show") { | |
| Mount(); | |
| translate([0,0,MountOAL + Plate[LENGTH] + Spring[LENGTH]]) | |
| rotate([180,0,0]) | |
| SpringPlate(); | |
| } | |
| if (Layout == "Build") { | |
| translate([0,-0.75*Clamp[OD],MountOAL]) | |
| rotate([180,0,0]) | |
| Mount(); | |
| translate([0,0.75*Plate[OD],0]) | |
| SpringPlate(); | |
| } |
The Smell of Molten Projects in the Morning 