Having twice failed to make music-wire springs work, I rummaged around in the Big Box o’ Small Springs with more diligence and unearthed a pair of coil compression springs that exactly match the pin ferrule OD. Twiddling the solid model produced this longer & flatter version with in-line springs and cylindrical plugs holding them in place:
A closeup of the pin arrangement, which now looks very clean and easy to build:
The OpenSCAD code will print out a quartet of plugs (pick the best two), but having thought of that too late, I turned a pair from a random acrylic rod:
I did remember to solder the wires before assembling the pins this time…
Because the pins now index on their shoulder with the springs at partial extension, I set the drills into the pin vice vise [Update: One can probably be arrested for pin vice] to produce depths displayed by the OpenSCAD program before reaming out the printed holes:
ECHO: "Depth to taper end: 24.72" ECHO: " ferrule end: 15.62" ECHO: " plug end: 4.62"
Then glue the pin plugs into the holder and the flat lid atop the case to capture the battery, clamping everything to the corner of the Sherline’s countertop:
And it Just Worked: nice travel between the limits, smooth operation, it’s the way I should have done it from the beginning*. You knew that all along, right?
Here are the three NB-5L Battery Holder versions, all snuggled up together. The longer and flatter coil-spring version sits on the right:
Now I can take some data…
The OpenSCAD source code:
// Holder for Canon NB-5L Li-Ion battery
// Ed Nisley KE4ZNU August 2011
include </home/ed/Thing-O-Matic/lib/MCAD/units.scad>
include </home/ed/Thing-O-Matic/lib/MCAD/boxes.scad>
include </home/ed/Thing-O-Matic/Useful Sizes.scad>
// Layout options
Layout = "Build"; // Case Lid Plugs Show Build Fit
//- Extrusion parameters - must match reality!
// Print with +2 shells and 3 solid layers
ThreadThick = 0.33;
ThreadWidth = 2.0 * ThreadThick;
HoleWindage = 0.2;
function IntegerMultiple(Size,Unit) = Unit * ceil(Size / Unit);
Protrusion = 0.1; // make holes end cleanly
BuildOffset = 3.0; // clearance for build layout
//- Battery dimensions - rationalized from several samples
// Coordinate origin at battery corner by contact plates on bottom surface
BatteryLength = 45.25;
BatteryWidth = 32.17;
BatteryThick = 7.85;
ContactWidth = 2.10;
ContactLength = 4.10;
ContactRecess = 0.85;
ContactOC = 3.18; // center-to-center across contact face
ContactOffset = 4.45; // offset from battery edge
ContactHeight = 3.05; // offset from battery bottom plane
AlignThick = 2.2; // alignment recesses on contact face
AlignDepth = 2.0; // into face
AlignWidth1 = 0.7; // across face at contacts
AlignWidth2 = 2.8; // ... other edge
//- Pin dimensions
PinTipDia = 1.6;
PinTipLength = 10.0;
PinTaperLength = 2.3;
PinShaftDia = 2.4;
PinShaftLength = 6.8;
PinFerruleDia = 3.1;
PinFerruleLength = 2.0;
PinLength = PinTipLength + PinTaperLength + PinShaftLength + PinFerruleLength;
ExtendRelax = 1.5 + ContactRecess; // pin extension when no battery is present
ExtendOvertravel = 1.0; // ... beyond engaged position
//- Spring dimensions
SpringDia = 3.1; // coil OD
SpringMax = 9.3;
SpringLength = SpringMax - 0.3; // slightly compressed
SpringMin = 4.5;
SpringPlugDia = 5.0; // plug retaining the spring
SpringPlugLength = IntegerMultiple(4.0,ThreadWidth);
SpringPlugSides = 12;
SpringTravel = ExtendRelax + ExtendOvertravel;
//- Holder dimensions
GuideRadius = ThreadWidth; // friction fit ridges
GuideOffset = 10;
WallThick = 4*ThreadWidth; // holder sidewalls
BaseThick = 6*ThreadThick; // bottom of holder to bottom of battery
TopThick = 4*ThreadThick; // top of battery to top of holder
ThumbRadius = 10.0; // thumb opening at end of battery
CornerRadius = 3*ThreadThick; // nice corner rounding
CaseLength = SpringPlugLength + SpringLength + PinLength - ExtendRelax
+ BatteryLength + GuideRadius + WallThick;
CaseWidth = 2*WallThick + 2*GuideRadius + BatteryWidth;
CaseThick = BaseThick + BatteryThick + TopThick;
//- XY origin at front left battery corner, Z on platform below that
CaseLengthOffset = -(SpringPlugLength + SpringLength + PinLength - ExtendRelax);
CaseWidthOffset = -(WallThick + GuideRadius);
CaseThickOffset = BaseThick;
LidLength = ExtendRelax - CaseLengthOffset;
echo(str("Depth to taper end: ",
(SpringPlugLength + SpringLength + PinFerruleLength + PinShaftLength + PinTaperLength)));
echo(str(" ferrule end: ",
(SpringPlugLength + SpringLength + PinFerruleLength)));
echo(str(" plug end: ",SpringPlugLength));
//----------------------
// Useful routines
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,
$fn=Sides);
}
module ShowPegGrid(Space = 10.0,Size = 1.0) {
Range = floor(50 / Space);
for (x=[-Range:Range])
for (y=[-Range:Range])
translate([x*Space,y*Space,Size/2])
%cube(Size,center=true);
}
//-------------------
//-- Guides for tighter friction fit
module Guides() {
translate([GuideOffset,-GuideRadius,CaseThickOffset])
PolyCyl(2*GuideRadius,(BatteryThick - Protrusion),4);
translate([GuideOffset,(BatteryWidth + GuideRadius),CaseThickOffset])
PolyCyl(2*GuideRadius,(BatteryThick - Protrusion),4);
translate([(BatteryLength - GuideOffset),-GuideRadius,CaseThickOffset])
PolyCyl(2*GuideRadius,(BatteryThick - Protrusion),4);
translate([(BatteryLength - GuideOffset),(BatteryWidth + GuideRadius),CaseThickOffset])
PolyCyl(2*GuideRadius,(BatteryThick - Protrusion),4);
translate([(BatteryLength + GuideRadius),GuideOffset/2,CaseThickOffset])
PolyCyl(2*GuideRadius,(BatteryThick - Protrusion),4);
translate([(BatteryLength + GuideRadius),(BatteryWidth - GuideOffset/2),CaseThickOffset])
PolyCyl(2*GuideRadius,(BatteryThick - Protrusion),4);
}
//-- Contact pins (holes therefore)
module PinShape() {
union() {
cylinder(r=(PinTipDia + HoleWindage)/2,h=(PinTipLength + Protrusion),$fn=6);
translate([0,0,PinTipLength])
cylinder(r=(PinShaftDia + HoleWindage)/2,
h=(PinTaperLength + PinShaftLength + Protrusion),$fn=6);
translate([0,0,(PinLength - PinFerruleLength)])
cylinder(r=(PinFerruleDia + HoleWindage)/2,
h=(PinFerruleLength + Protrusion),$fn=6);
translate([0,0,(PinLength)])
cylinder(r=(SpringDia + HoleWindage)/2,
h=(SpringLength + Protrusion),$fn=6);
translate([0,0,(PinLength + SpringLength)])
cylinder(r=(SpringPlugDia + HoleWindage)/2,h=(SpringPlugLength + Protrusion),$fn=SpringPlugSides);
translate([0,0,(PinLength + SpringLength + SpringPlugLength)])
cylinder(r=(SpringPlugDia + HoleWindage)/2,h=2*SpringPlugLength,$fn=SpringPlugSides); // extend hole
}
}
module PinAssembly() {
translate([ExtendRelax,ContactOffset,CaseThickOffset + ContactHeight]) {
rotate([0,270,0]) {
PinShape(); // pins
translate([0,(2*ContactOC),0])
PinShape();
}
}
}
//-- Case with origin at battery corner
module Case() {
difference() {
union() {
difference() {
translate([(CaseLength/2 + CaseLengthOffset),
(CaseWidth/2 + CaseWidthOffset),
(CaseThick/2)])
roundedBox([CaseLength,CaseWidth,CaseThick],CornerRadius); // basic case shape
translate([-ExtendOvertravel,-GuideRadius,CaseThickOffset])
cube([(BatteryLength + GuideRadius + ExtendOvertravel),
(BatteryWidth + 2* GuideRadius),
(BatteryThick + Protrusion)]); // battery space
}
Guides();
translate([-ExtendOvertravel,-GuideRadius,BaseThick])
cube([(AlignDepth + ExtendOvertravel),
(AlignWidth1 + GuideRadius),
AlignThick]); // alignment blocks
translate([-ExtendOvertravel,
(BatteryWidth - AlignWidth2),
BaseThick])
cube([(AlignDepth + ExtendOvertravel),
(AlignWidth2 + GuideRadius),
AlignThick]);
}
translate([(-ExtendOvertravel),
(CaseWidthOffset - Protrusion),
(CaseThickOffset + BatteryThick)])
cube([CaseLength,
(CaseWidth + 2*Protrusion),
(TopThick + Protrusion)]); // battery access
translate([(CaseLengthOffset - Protrusion),
(CaseWidthOffset - Protrusion),
(CaseThickOffset + BatteryThick)])
cube([(CaseLength + 2*Protrusion),
(CaseWidth + 2*Protrusion),
(TopThick + Protrusion)]); // battery insertion allowance
translate([(BatteryLength - Protrusion),
(CaseWidth/2 + CaseWidthOffset),
(CaseThickOffset + ThumbRadius)])
rotate([90,0,0])
rotate([0,90,0])
cylinder(r=ThumbRadius,
h=(WallThick + GuideRadius + 2*Protrusion),
$fn=22); // remove thumb notch
PinAssembly();
}
}
module Lid() {
difference() {
translate([0,0,(CaseThick/2 - BaseThick - BatteryThick)])
roundedBox([LidLength,
CaseWidth,CaseThick],CornerRadius);
translate([0,0,-(CaseThick/2)])
cube([(LidLength + 2*Protrusion),
(CaseWidth + 2*Protrusion),
(CaseThick)],center=true);
}
}
module PlugShape() {
difference() {
cylinder(r=SpringPlugDia/2,h=SpringPlugLength,$fn=SpringPlugSides);
translate([0,0,-Protrusion])
PolyCyl(PinShaftDia,(SpringPlugLength + 2*Protrusion),SpringPlugSides/2);
}
}
module Plugs() {
translate([0,ContactOC,0])
PlugShape();
translate([0,-ContactOC,0])
PlugShape();
}
//-------------------
// Build it!
ShowPegGrid();
if (Layout == "Case")
Case();
if (Layout == "Lid")
Lid();
if (Layout == "Plugs")
Plugs();
if (Layout == "Show") { // reveal pin assembly
difference() {
Case();
translate([(CaseLengthOffset - Protrusion),
(CaseWidthOffset - Protrusion + WallThick + ContactOffset + ContactOC),
(BaseThick + ContactHeight)])
cube([(-CaseLengthOffset + Protrusion),
(CaseWidth + 2*Protrusion),
CaseThick + BaseThick - ContactHeight + Protrusion]);
translate([(CaseLengthOffset - Protrusion),
(CaseWidthOffset - Protrusion),
-Protrusion])
cube([(-CaseLengthOffset + Protrusion),
(WallThick + GuideRadius + ContactOffset + Protrusion),
CaseThick]);
}
translate([ExtendRelax,ContactOffset,(CaseThickOffset + ContactHeight)]) { // pins
rotate([0,270,0]) {
%PinShape();
// translate([0,(2*ContactOC),0])
// %PinShape();
}
}
translate([CaseLengthOffset,ContactOffset,(CaseThickOffset + ContactHeight)])
rotate([0,90,0])
PlugShape();
}
if (Layout == "Build") {
translate([-(CaseLength/2 + CaseLengthOffset),-(CaseWidthOffset - BuildOffset),0])
Case();
translate([0,(CaseLengthOffset/2 - BuildOffset),0])
rotate([0,0,90])
Lid();
translate([CaseLengthOffset - SpringPlugDia,-CaseWidth/2,0])
Plugs();
translate([(CaseLengthOffset + SpringPlugDia),-CaseWidth/2,0]) // extra set of plugs
Plugs();
}
if (Layout == "Fit") {
Case();
translate([(-LidLength/2 + ExtendRelax),
(CaseWidth/2 + CaseWidthOffset),
(BaseThick + BatteryThick)])
Lid();
translate([ExtendRelax,ContactOffset,CaseThickOffset + ContactHeight]) { // pins
rotate([0,270,0]) {
%PinShape();
translate([0,(2*ContactOC),0])
%PinShape();
}
}
translate([CaseLengthOffset,
(ContactOffset + ContactOC),
(CaseThickOffset + ContactHeight)])
rotate([0,90,0])
Plugs();
}
(*) Modulo, of course, simply buying a $5 charger from eBay and gutting it. What’s the fun in that?
The Smell of Molten Projects in the Morning 