A trio of N2O cartridges / capsules made their way into the Basement Laboratory and cried out to be fitted with fins:
My original model tinkered up a cartridge from solid object primitives, but I’ve since discovered that cheating produces a much better and faster and easier result for cylindrical objects:
The trick is getting an image of the original object from the side, taken from far enough away to flatten the perspective:
Then overlay and scale a grid to match the actual length:
The grid has 1 mm per minor square, centered along the cartridge’s axis, and zeroed at the tip; I rotated the cartridge image by half a degree to line it up with the grid.
Print it out on actual paper so you can eyeball the measurements and write ’em where you need ’em:
Which becomes an OpenSCAD polygon definition:
RADIUS = 0; // subscript for radius values HEIGHT = 1; // ... height above Z=0 at seal flange //-- N2O 8 g capsule CartridgeOutline = [ // X values = measured radius, Y as distance from tip [0.0,0.0], // 0 cartridge seal tip [2.5,0.1], // 1 seal disk [3.5,0.5],[4.0,1.0], // 2 tip end [4.2,2.0],[4.3,3.0], // 4 tip [4.3,6.0], // 6 chamfer [4.5,8.0], // 7 taper [4.9,9.0], // 8 [5.5,10.0], // 9 [6.0,11.0], // 10 [6.7,12.0], // 11 [7.1,13.0], // 12 [7.5,14.0], // 13 [8.0,15.0], // 14 [8.4,16.0], // 15 [8.8,17.0], // 16 [9.0,18.0],[9.0,58.0], // 17 body [0.0,65.0] // 19 dummy end cone ]; TipLength = CartridgeOutline[6][HEIGHT]; TipOD = 2*CartridgeOutline[5][RADIUS]; BodyOD = 2*CartridgeOutline[17][RADIUS]; BodyOAL = CartridgeOutline[19][HEIGHT];
Because the rounded end of the cartridge doesn’t matter, I turned it into a cone.
Twirl that around the Z axis and It Just Works:
module Cartridge() {
rotate_extrude($fn=CartridgeSides)
polygon(points=CartridgeOutline);
}
Which then punches a matching dent in the fin structure:
The lead picture doesn’t quite match the Slic3r preview, as I found the single-width diagonal fins weren’t strong enough. Making them two (nominal) threads wide lets Slic3r lay down three thinner threads in the same space:
That’s letting Slic3r automagically determine the infill and perimeter thread width to make the answer come out right. As nearly as I can tell, the slicing algorithms have become smart enough to get the right answer nearly all of the time, so I can-and-should relinquish more control over the details.
The OpenSCAD source code:
// CO2 capsule tail fins
// Ed Nisley KE4ZNU - October 2015
Layout = "Build"; // Show Build FinBlock Cartridge Fit
//-------
//- Extrusion parameters must match reality!
// Print with +0 shells and 3 solid layers
ThreadThick = 0.25;
ThreadWidth = 0.40;
HoleWindage = 0.2;
Protrusion = 0.1; // make holes end cleanly
function IntegerMultiple(Size,Unit) = Unit * ceil(Size / Unit);
//-------
// Capsule dimensions
CartridgeSides = 12*4; // number of sides
RADIUS = 0; // subscript for radius values
HEIGHT = 1; // ... height above Z=0 at seal flange
//-- N2O 8 g capsule
RW = HoleWindage/2; // enlarge radius by just enough
CartridgeOutline = [ // X values = measured radius, Y as distance from tip
[0.0,0.0], // 0 cartridge seal tip
[2.5 + RW,0.1], // 1 seal disk
[3.5 + RW,0.5],[4.0 + RW,1.0], // 2 tip end
[4.2 + RW,2.0],[4.3 + RW,3.0], // 4 tip
[4.3 + RW,6.0], // 6 chamfer
[4.5 + RW,8.0], // 7 taper
[4.9 + RW,9.0], // 8
[5.5 + RW,10.0], // 9
[6.0 + RW,11.0], // 10
[6.7 + RW,12.0], // 11
[7.1 + RW,13.0], // 12
[7.5 + RW,14.0], // 13
[8.0 + RW,15.0], // 14
[8.4 + RW,16.0], // 15
[8.8 + RW,17.0], // 16
[9.0 + RW,18.0],[9.0 + RW,58.0], // 17 body
[0.0,65.0] // 19 dummy end cone
];
TipLength = CartridgeOutline[6][HEIGHT];
TipOD = 2*CartridgeOutline[5][RADIUS];
CylinderBase = CartridgeOutline[17][HEIGHT];
BodyOD = 2*CartridgeOutline[17][RADIUS];
BodyOAL = CartridgeOutline[19][HEIGHT];
//-------
// Fin dimensions
FinThick = 1.5*ThreadWidth; // outer square
StrutThick = 2.0*ThreadWidth; // diagonal struts
FinSquare = 1.25*BodyOD;
FinTaperLength = sqrt(2)*FinSquare/2 - sqrt(2)*FinThick - ThreadWidth;
FinBaseLength = 0.7 * CylinderBase;
FinTop = 0.9*CylinderBase;
//-------
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);
}
module ShowPegGrid(Space = 10.0,Size = 1.0) {
Range = floor(50 / Space);
for (x=[-Range:Range])
for (y=[-Range:Range])
translate([x*Space,y*Space,Size/2])
%cube(Size,center=true);
}
//-------
// CO2 cartridge outline
module Cartridge() {
rotate_extrude($fn=CartridgeSides)
polygon(points=CartridgeOutline);
}
//-------
// Diagonal fin strut
module FinStrut() {
intersection() {
rotate([90,0,45])
translate([0,0,-StrutThick/2])
linear_extrude(height=StrutThick)
polygon(points=[
[0,0],
[FinTaperLength,0],
[FinTaperLength,FinBaseLength],
[0,(FinBaseLength + FinTaperLength)]
]);
translate([0,0,FinTop/2])
cube([2*FinSquare,2*FinSquare,FinTop], center=true);
}
}
//-------
// Fin outline
module FinBlock() {
$fn=12;
render(convexity = 4)
union() {
translate([0,0,FinBaseLength/2])
difference() {
intersection() {
minkowski() {
cube([FinSquare - 2*ThreadWidth,
FinSquare - 2*ThreadWidth,
FinBaseLength],center=true);
cylinder(r=FinThick,h=Protrusion,$fn=8);
}
cube([2*FinSquare,2*FinSquare,FinBaseLength],center=true);
}
difference() {
cube([(FinSquare - 2*FinThick),
(FinSquare - 2*FinThick),
(FinBaseLength + 2*Protrusion)],center=true);
for (Index = [0:3])
rotate(Index*90)
translate([(FinSquare/2 - FinThick),(FinSquare/2 - FinThick),0])
cylinder(r=2*StrutThick,h=(FinBaseLength + 2*Protrusion),center=true,$fn=16);
}
}
for (Index = [0:3])
rotate(Index*90)
FinStrut();
rotate(180/12)
cylinder(d=IntegerMultiple(TipOD + 6*ThreadWidth,ThreadWidth),h=TipLength);
}
}
//-------
// Fins
module FinAssembly() {
difference() {
FinBlock();
translate([0,0,2*ThreadThick]) // add two layers to close base cylinder
Cartridge();
}
}
module FinFit() {
translate([0,0.75*BodyBaseLength,2*ThreadThick])
rotate([90,0,0])
difference() {
translate([-FinSquare/2,-2*ThreadThick,0])
cube([IntegerMultiple(FinSquare,ThreadWidth),
4*ThreadThick,
1.5*BodyBaseLength]);
translate([0,0,5*ThreadWidth])
Cartridge();
}
}
//-------
// Build it!
ShowPegGrid();
if (Layout == "FinStrut")
FinStrut();
if (Layout == "FinBlock")
FinBlock();
if (Layout == "Cartridge")
Cartridge();
if (Layout == "Show") {
FinAssembly();
color("LightYellow") Cartridge();
}
if (Layout == "Fit")
FinFit();
if (Layout == "Build")
FinAssembly();
The Smell of Molten Projects in the Morning 