The New Hotness

A few days after installing the replacement cord caps, I bumped the bottom rail of the miniblind while opening the window and had one endcap disintegrate; apparently window hardware isn’t hardened against prolonged UV exposure. Who knew?

Fortunately, I can fix that:

Making the walls three threads wide provides enough room for a single solid infill thread:

Miniblind Endcaps - Slic3r Preview — Miniblind Endcaps – Slic3r Preview

The exterior shape comes from a hull wrapped around six circles: four to define the corner radius and a pair that bump the center out by the calculated chord height. The interior shape comes from a pair of chord-radius polygonal circles (they only have three facets across the length of the inside wall) that fit the bottom rail almost perfectly.

As always, natural PETG has a crystalline, slightly transparent, appearance:

I should spring for some opaque white filament, but that way lies madness; I might start caring what these things look like.

You can buy entire miniblinds for a few bucks a pop, but the last time we did that, they were different than the ones we had before. That wouldn’t matter if the standard miniblind mounting brackets fit our 1955 Anderson windows, but noooo they don’t: the custom adapters I machined for the first miniblind brackets, of course, didn’t fit the new miniblinds.

Now I can just snap the replacement endcaps (and cord pulls) in place, declare victory, and move on.

The OpenSCAD source code as a GitHub Gist:

	// Cap for miniblind cord and bottom rail endcaps
	// Ed Nisley KE4ZNU – September 2016

	Layout = "BaseEndCap"; // CordCap BaseEndCap

	//- Extrusion parameters – must match reality!

	ThreadThick = 0.25;
	ThreadWidth = 0.40;

	Protrusion = 0.1;

	HoleWindage = 0.2;

	//——
	// Dimensions

	OD1 = 0;
	OD2 = 1;
	LENGTH = 2;

	//———————-
	//- Build it

	if (Layout == "CordCap") {

	Cap = [9.0,16.0,25.0];
	Cord = [2.5,7.0,Cap[LENGTH] – 5];
	NumSides = 8;

	difference() {

	hull() { // overall shape
	translate([0,0,Cap[LENGTH] – Cap[OD1]/2])
	sphere(d=Cap[OD1],$fn=NumSides);
	translate([0,0,0.5*Cap[OD2]/2])
	sphere(d=Cap[OD2],$fn=2*NumSides); // round the bottom just a bit
	}

	translate([0,0,-Cap[LENGTH]/2]) // trim bottom
	cube([2Cap[OD2],2Cap[OD2],Cap[LENGTH]],center=true);

	translate([0,0,Cap[LENGTH] + 0.8*Cap[OD1]]) // trim top (arbitrarily)
	cube([2Cap[OD1],2Cap[OD1],2*Cap[OD1]],center=true);

	translate([0,0,-Protrusion])
	cylinder(d=Cord[OD1],h=(Cap[LENGTH] + 2*Protrusion),$fn=NumSides);

	translate([0,0,-Protrusion])
	cylinder(d1=Cord[OD2],d2=Cord[OD1],h=(Cord[LENGTH] + Protrusion),$fn=NumSides);
	}

	}

	if (Layout == "BaseEndCap") {

	Base = [25.2,9.0,12.2]; // base outside dimensions
	Edge = 8.0; // size of sqare ends
	CornerRadius = 0.75;
	Wall = 3; // wall thickness in threads

	ChordHeight = (Base[1] – Edge) / 2;
	ChordRadius = (pow(ChordHeight,2) + pow(Base[0],2)/4) / (2*ChordHeight);

	NumSides = 4*4;

	echo(str("Chord height: ",ChordHeight," radius: ",ChordRadius));

	difference() {
	linear_extrude(height=Base[2],convexity=2) {
	hull() {
	for (i=[-1,1], j=[-1,1])
	translate([i(Base[0]/2 – CornerRadius + WallThreadWidth),j(Base[1]/2 – CornerRadius + WallThreadWidth)])
	circle(r=CornerRadius,$fn=4*4,center=true);
	for (j=[-1,1])
	translate([0,j(ChordHeight + Base[1]/2 – CornerRadius + WallThreadWidth)])
	rotate(180/(2*4))
	circle(r=CornerRadius,$fn=2*4,center=true);
	}
	}

	translate([0,0,3*ThreadThick])
	linear_extrude(height=Base[2],convexity=2)
	intersection() {
	intersection_for (j=[-1,1])
	translate([0,j*(ChordHeight + Base[1]/2 – ChordRadius)])
	circle(r=ChordRadius,$fn=32*4,center=true);
	square([Base[0],2*Base[1]],center=true);
	}
	}
	}

view raw Miniblind cord cap.scad hosted with ❤ by GitHub

The original doodle with some dimensions that didn’t withstand careful measurements:

The New Hotness

Miniblind Bottom Rail Caps