The New Hotness

When in doubt, nuke ’em from orbit. It’s the only way to be sure:

When confronted with a zombie horde, though, nothing exceeds like excess:

In real life, they’re 12 gram CO₂ capsules, of the type used in tire inflators and air pistols. I knew I’d find something to do with the box of empties I’d been accumulating: they became (somewhat threatening) tchotchkes. This was inspired by that thing, although that STL file doesn’t render into anything and, as with many interesting Thingiverse things, there’s no source code.

These fins were an exercise in thin-wall printing: the outer square is one thread thick, the diagonal struts are two threads, and the ring around the nozzle has just a touch of fill inside, with a one-thread-thick base below the cartridge nozzle:

The solid model looks about like you’d expect:

The teeny little quarter-cylinders in the corners encourage Skeinforge to do the right thing: build each quadrant in one pass, leaving the corners unfinished. The diagonals must be exactly two threads wide to make that possible: each strut thread connects to the corresponding single-thread outer edge.

Now that I’m trying to be a subtractive kind of guy, that’s actually a fin block:

Minus the CO₂ cartridge that should fit inside:

It turns out that my box has several different types of CO₂ cartridges and the nozzle ends are all different. To get it right, there’s a template for matching the curves:

That end of the cartridge consists of a cylinder for the body, a sphere mated to a tangential conic section, another conic fillet, and then the cylindrical nozzle. Basically, you twiddle with the parameters until the template comes pretty close to fitting, then fire off a few trial fins until it comes out right.

They were a big hit at the Long Island Linux Users Group meeting…

The OpenSCAD source code:

// CO2 capsule tail fins
// Ed Nisley KE4ZNU - Oct 2011

Layout = "Show";			// Show Build FinBlock Cartridge Fit

include

//-------
//- Extrusion parameters must match reality!
//  Print with +0 shells and 3 solid layers

ThreadThick = 0.33;
ThreadWidth = 2.0 * ThreadThick;

HoleWindage = 0.2;

Protrusion = 0.1;			// make holes end cleanly

function IntegerMultiple(Size,Unit) = Unit * ceil(Size / Unit);

//-------
// Capsule dimensions

BodyDia = 18.70;
BodyRad = BodyDia/2;

BodyLength = 53.0;						// between hemispherical endcap centers
BodyBaseLength = 21;					// tip to endcap center

TipDia = 7.40;
TipRad = TipDia/2;
TipLength = IntegerMultiple(4.0,ThreadThick);

FilletLength = 5.0;						// fillet between tip and cone
FilletTop = TipLength + FilletLength;

FilletBaseDia = 8.60;
FilletBaseRad= FilletBaseDia/2;
FilletTopDia = 9.5;
FilletTopRad = FilletTopDia/2;

ConeTop = 16.0;							// tip to tangent with endcap
ConeLength = ConeTop - FilletTop;

echo(str("Cone Length: ",ConeLength));

IntersectZ = ConeTop;					// coordinates of intersect tangent
IntersectX = sqrt(pow(BodyRad,2) - pow(BodyBaseLength - ConeTop,2));

echo(str("IntersectZ: ",IntersectZ));
echo(str("IntersectX: ",IntersectX," dia: ",2*IntersectX));

//-------
// Fin dimensions

FinThick = 1*ThreadWidth;				// outer square
StrutThick = 2*FinThick;				// diagonal struts

FinSquare = 24.0;
FinTaperLength = sqrt(2)*FinSquare/2 - sqrt(2)*FinThick - ThreadWidth;

FinBaseLength = 2*TipLength;

//-------

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() {

$fn = 48;

  union() {
	translate([0,0,BodyBaseLength]) {
	  cylinder(r=BodyDia/2,h=BodyLength);
	  translate([0,0,BodyLength])
		sphere(r=BodyRad);
	}

	intersection() {
	  translate([0,0,BodyBaseLength])
		sphere(r=BodyRad);
	  union() {
		translate([0,0,(TipLength + FilletLength+ConeLength)])
		  cylinder(r=BodyRad,h=(BodyBaseLength - ConeLength));
		translate([0,0,(TipLength + FilletLength)])
		  cylinder(r1=FilletTopRad,r2=IntersectX,h=(ConeLength + Protrusion));
		translate([0,0,TipLength])
		  cylinder(r1=FilletBaseRad,r2=FilletTopRad,h=(FilletLength + Protrusion));
		}
	  }

	translate([0,0,FilletTop])
	  cylinder(r1=FilletTopRad,r2=IntersectX,h=ConeLength);

	translate([0,0,TipLength])
	  cylinder(r1=FilletBaseRad,r2=FilletTopRad,h=(FilletLength + Protrusion));

	translate([0,0,-Protrusion])
	  PolyCyl(TipDia,(TipLength + 2*Protrusion));

  }
}

//-------
// Diagonal fin strut

module FinStrut() {
  rotate([90,0,45])
	translate([0,0,-StrutThick/2])
	  linear_extrude(height=StrutThick)
		polygon(points=[
		  [0,0],
		  [FinTaperLength,0],
		  [FinTaperLength,FinBaseLength],
		  [0,(FinBaseLength + FinTaperLength)]
		]);
}

//-------
// Fin outline

module FinBlock() {
  union() {
	translate([0,0,FinBaseLength/2])
	  difference() {
		cube([FinSquare,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=StrutThick,h=(FinBaseLength + 2*Protrusion),center=true,$fn=16);
		}
	  }
	for (Index = [0:3])
	  rotate(Index*90)
		FinStrut();
	cylinder(r=IntegerMultiple((FilletBaseRad + StrutThick),ThreadWidth),h=TipLength);
  }
}

//-------
// Fins

module FinAssembly() {

  difference() {
	FinBlock();
	translate([0,0,ThreadThick])				// add one layer 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 == "FinBlock")
  FinBlock();

if (Layout == "Cartridge")
  Cartridge();

if (Layout == "Show") {
  FinAssembly();
  color(LG) Cartridge();
}

if (Layout == "Fit")
  FinFit();

if (Layout == "Build")
  FinAssembly();

The original doodles: