The Smell of Molten Projects in the Morning
Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
Cruisin’ the streets
It has nothing to do with the load o’ tools in the left underseat bag on my Tour Easy. It has everything to do with the fact that we ride on the shoulder of crowned roads.
Here’s a cross-section of one defunct Schwalbe Marathon rear tire with the tread obviously thicker on the left side, which would be the right side of the mounted tire:
Maybe I should change the tires more often?
Printing went smoothly after two preliminary passes to work out the sizes and alignments; this is the second pass, which you can tell because the mirror shoulder has three supports instead of the two shown in the solid model:
One view of the parts, with the mirror shaft in place:
Another view, showing the bottom of the Elevation Plate with the recessed nut:
Assembling the two glue joints required an overnight clamping:
Then a layer of double-stick foam tape affixes it firmly to the helmet:
It’s a bit too big and way ugly, but works pretty much as expected.
Two lengths of heatshrink tubing now lock the mirror shaft sections in place; they tended to rotate slightly under normal vibration.
The OpenSCAD code and model have a few modifications from this object. The next one won’t have the third section of mirror shaft, which makes the shoulder and Az Mount smaller, and the Az Mount is 1 mm closer to the El Body. That shaves a few millimeters off the whole thing.
The mirror clamp out there on the end is much too large and has too many fiddly parts. I think a little printed doodad would work, but that’s in the nature of fine tuning.
After a bit of OpenSCAD twiddling, those doodles turned into a printable model. This view shows what it looks like all neatly assembled:
The tiny hole on the top of the Elevation Body accepts a 2-56 setscrew that grabs the arc protruding from the Elevation Plate and locks the up-and-down setting. The Azimuth Mount pivots on the 3-48 screw holding it to the Elevation Mount.
Both of those pivots must be loose enough to move when you bump the mirror and tight enough to stay put in normal use. It’s a delicate balance and I’m not convinced this will work for the long term, but it’s a brassboard.
The 2-56 stud on the end of the mirror shaft screws into a socket in the rear side of the Az Mount. Another 2-56 setscrew in the Az Mount (facing the El Body), grabs the side of the shaft and prevents it from rotating.
All the parts lay out on their backs for printing, with a grid to show how they fit on the build platform:
The mirror shaft shoulder on the Az Mount (front center) sticks out in mid air and requires a little bit of support.
The El Mount (left rear) builds surprisingly well with its curved top surface downward. If it’s rotated 90 degrees with the curve facing to the left, Skeinforge grumps about not being able to do something or another and generates totally bogus G-Code.
The Helmet Plate has a 3 mm deep depression that more-or-less corresponds to the helmet’s surface. It’s gouged out by a huge sphere sitting on the plate, with a radius calculated from the measured helmet curvature.
The OpenSCAD source code has two useful parameters near the top:
You’ll need the MCAD and Visibone libraries to make this work. It’s the original code, without the tweaks to the grid mentioned in the comments there:
// Helmet mirror mount
// Ed Nisley KE4ZNU June 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/lib/visibone_colors.scad>
//-- Layout Control
Layout = "Show"; // Build Fit Show None
Examine = "None"; // AzMount ElMount ElBody ElPlate HelmetPlate None
//-- Extrusion parameters
ThreadThick = 0.33;
ThreadWT = 2.0;
ThreadWidth = ThreadThick * ThreadWT;
HoleWindage = 0; // enlarge hole dia by this amount
//-- Useful sizes
Tap2_56 = 0.070 * inch;
Clear2_56 = 0.082 * inch;
Head2_56 = 0.156 * inch;
Head2_56Thick = 0.055 * inch;
Nut2_56Dia = 0.204 * inch;
Nut2_56Thick = 0.065 * inch;
Tap3_48 = 0.079 * inch;
Clear3_48 = 0.096 * inch;
Head3_48 = 0.184 * inch;
Head3_48Thick = 0.058 * inch;
Nut3_48Dia = 0.201 * inch;
Nut3_48Thick = 0.073 * inch;
Tap4_40 = 0.089 * inch;
Clear4_40 = 0.110 * inch;
Head4_40 = 0.211 * inch;
Head4_40Thick = 0.065 * inch;
Nut4_40Dia = 0.228 * inch;
Nut4_40Thick = 0.086 * inch;
//-- Azimuth Mount
AzMountDia = 12.0;
AzMountLength = 14.0;
AzFacets = 30;
echo(str("Azmuth mount dia: ",AzMountDia," length: ",AzMountLength));
//-- Mirror sizes
MirrorShaftDia = 3.60;
MirrorShaftOffset = -1.5; // vertical offset from center of AzMountBody
MirrorShoulderLen = 3*MirrorShaftDia;
MirrorShoulderDia = min(AzMountDia,MirrorShaftDia + 6*ThreadWidth);
MirrorStudDia = Tap3_48;
MirrorStudLen = 2.0;
//-- Elevation Mount / Body / Plate
ElMountDia = AzMountDia;
ElMountLength = 2.0 + ElMountDia;
ElMountBase = 2.0;
ElMountRounding = 2.0;
ElMountFacets = AzFacets;
ElBodyWidth = ElMountDia;
ElBodyBlockLength = ElMountLength + AzMountLength/2 - MirrorShaftOffset;
ElBodyThick = 8.0;
echo(str("Elevation body overall: ",(ElBodyBlockLength + ElBodyWidth/2)," width: ",ElBodyWidth));
ElPlateTall = ElBodyBlockLength + 0.70*ElBodyWidth;
ElPlateWidth = 1.25 * ElPlateTall;
ElPlateThick = ceil(4.0 / ThreadThick) * ThreadThick;
ElPlatePlusX = ElPlateThick + (ElMountDia/2 + ElMountBase) + ElBodyThick;
echo(str("Elevation plate tall: ",ElPlateTall," width: ",ElPlateWidth));
ElArcRadius = (3/4) * ElBodyBlockLength;
ElArcThick = 4*ThreadWidth;
ElArcHeight = (1/2) * ElBodyThick;
ElArcAngle = 35;
ElArcFacets = 32;
ElPlateFacets = 52;
//-- Helmet Interface Plate
HelmetCX = 60.0;
HelmetMX = 4.0;
HelmetRX = (pow(HelmetMX,2) + pow(HelmetCX,2)/4)/(2*HelmetMX);
HelmetPlateC = max(ElPlateTall,ElPlateWidth);
HelmetPlateTheta = atan(HelmetPlateC/HelmetRX);
HelmetPlateM = 2*HelmetRX*pow(sin(HelmetPlateTheta/4),2);
HelmetPlateThick = ThreadThick*(ceil(HelmetPlateM/ThreadThick) + 1);
//-- Bearing Interfaces
BearingWidth = 3*ThreadWidth;
BearingOverlap = 3*ThreadThick;
BearingClearance = 1*ThreadThick;
BearingStudDia = min(AzMountDia,ElBodyWidth) - 2*BearingWidth;
//-- Convenience values
Protrusion = 0.1; // make holes look good
PegSize = 1.0;
//----------------------
// Useful routines
module PolyCyl(Dia,Height) { // based on nophead's polyholes
Sides = ceil(Dia) + 2;
FixDia = Dia / cos(180/Sides);
cylinder(r=(FixDia + HoleWindage)/2,
h=Height,
$fn=Sides);
}
module ShowPegGrid(Size) {
for (x=[-5:5])
for (y=[-5:5])
translate([x*10,y*10,Size/2])
cube(Size,center=true);
}
//----------------------
// Azimuth Mount
module AzMount() {
difference() {
union() {
cylinder(r=AzMountDia/2,h=AzMountLength,$fn=AzFacets); // body
translate([0,0,AzMountLength/2 + MirrorShaftOffset])
rotate([-90,0,0])
cylinder(r=MirrorShoulderDia/2,
h=MirrorShoulderLen,$fn=AzFacets); // mirror shaft shoulder
if (Layout != "Fit")
for (y=[0:1]) // shoulder support
translate([-AzMountDia/2,(4*y + AzMountDia/2 + ThreadWidth),0])
difference() {
cube([AzMountDia,2*ThreadWidth,AzMountLength/6]);
translate([AzMountDia/2,-Protrusion,AzMountLength/2 + MirrorShaftOffset])
rotate([-90,0,0])
cylinder(r=MirrorShoulderDia/2,h=ThreadWidth + 2*Protrusion);
}
}
translate([0,-Head3_48/2,AzMountLength/2 + MirrorShaftOffset])
rotate([-90,0,0])
PolyCyl(MirrorShaftDia,(AzMountDia + MirrorShoulderLen)); // mirror shaft
translate([0,-(Head3_48/2 - Protrusion),AzMountLength/2 + MirrorShaftOffset])
rotate([90,0,0])
PolyCyl(MirrorStudDia,MirrorStudLen+Protrusion); // mirror stud
translate([0,0,
Head3_48Thick - (AzMountLength - MirrorShaftDia)/2 + MirrorShaftOffset - Protrusion])
PolyCyl(Head3_48,AzMountLength + Protrusion); // mounting screw head
translate([0,0,-Protrusion])
cylinder(r=(Clear3_48 + HoleWindage)/2,
h=(AzMountLength + 2*Protrusion),
$fn=ceil(Clear3_48)+2); // mounting screw clearance
translate([0,0,AzMountLength/2 + Head3_48Thick + MirrorShaftDia/2 + MirrorShaftOffset - Protrusion])
cylinder(r1=(Head3_48/cos(180/7) + HoleWindage)/2,
r2=Clear3_48/2,
h=(3*ThreadThick + Protrusion),
$fn=7); // overhang support
translate([0,0,AzMountLength/2 + MirrorShaftOffset])
rotate([0,90,0])
PolyCyl(Tap2_56,AzMountDia/2 + Protrusion); // setscrew hole
translate([0,0,AzMountLength - (BearingOverlap + BearingClearance)])
PolyCyl(BearingStudDia,
BearingOverlap + BearingClearance + Protrusion); // bearing surface
}
}
//----------------------
// Elevation Mount
module ElMount() {
difference() {
union() {
translate([(ElMountDia/4 + ElMountBase/2),0,(ElMountLength/2 + BearingOverlap)])
rotate([0,90,0])
cube([ElMountLength,ElMountDia,(ElMountDia/2 + ElMountBase)],
center=true); // mounting block
translate([0,0,BearingOverlap]) {
// color([0.4,0.3,0.3,0.7])
cylinder(r=ElMountDia/2,
h=ElMountLength - ElMountDia/2,
$fn=ElMountFacets); // cylinder to Az
// color([0.3,0.4,0.3,0.7])
translate([0,0,ElMountLength - ElMountDia/2]) { // curved interface
intersection() {
cylinder(r=ElMountDia/2,h=ElMountDia/2,$fn=ElMountFacets);
translate([0,ElMountDia/2,0])
rotate([90,0,0])
cylinder(r=ElMountDia/2,h=ElMountDia,$fn=ElMountFacets);
}
}
}
cylinder(r=(BearingStudDia - HoleWindage)/2,h=BearingOverlap); // bearing stud
}
translate([0,0,-Protrusion])
PolyCyl(Tap3_48,(3/4)*ElMountLength + BearingOverlap + Protrusion); // AzMount screw
}
}
//----------------------
// Elevation Body
module ElBody() {
difference() {
union() {
translate([-ElBodyBlockLength,-ElBodyWidth/2,0])
cube([ElBodyBlockLength,ElBodyWidth,ElBodyThick]);
translate([0,0,ElBodyThick])
cylinder(r=(ElBodyWidth - 2*BearingWidth)/2,h=BearingOverlap);
cylinder(r=ElBodyWidth/2,h=ElBodyThick,$fn=ElMountFacets);
}
PolyCyl(Clear3_48,ElBodyThick + BearingOverlap + Protrusion);
translate([0,0,-Protrusion])
PolyCyl(Head3_48,Head3_48Thick);
translate([-ElArcRadius,0,ElBodyThick - ElArcHeight/2])
rotate([0,-90,0])
PolyCyl(Tap2_56,ElBodyBlockLength - ElArcRadius + Protrusion);
translate([0,0,ElBodyThick - (ElArcHeight + BearingClearance)])
difference() {
cylinder(r=ElArcRadius + (ElArcThick/2 + BearingClearance),
h=ElArcHeight + BearingClearance + Protrusion,
$fn=ElArcFacets);
cylinder(r=ElArcRadius - (ElArcThick/2 + BearingClearance),
h=ElArcHeight + BearingClearance + Protrusion,
$fn=ElArcFacets);
}
}
}
//----------------------
// Elevation Plate
module ElPlate() {
union() {
difference() {
translate([ElBodyWidth/2 - ElPlateTall/2,0,0])
scale([ElPlateTall,ElPlateWidth,1.0])
cylinder(r=0.5,h=ElPlateThick,$fn=ElPlateFacets);
translate([0,0,-Protrusion])
PolyCyl(Tap3_48,ElPlateThick + 2*Protrusion);
translate([0,0,ElPlateThick - (BearingOverlap + BearingClearance)])
PolyCyl(BearingStudDia,(BearingOverlap + BearingClearance) + Protrusion);
translate([0,0,-Protrusion])
cylinder(r=Nut3_48Dia/2,h=(1.1*Nut3_48Thick + Protrusion),$fn=6);
}
translate([0,0,ElPlateThick])
difference() {
cylinder(r=ElArcRadius + ElArcThick/2,
h=ElArcHeight,
$fn=ElArcFacets);
cylinder(r=ElArcRadius - ElArcThick/2,
h=ElArcHeight + Protrusion,
$fn=ElArcFacets);
rotate([0,0,90 - ElArcAngle])
translate([ElArcRadius + ElArcThick,0,ElArcHeight/2])
cube([2*ElArcRadius + ElArcThick,
2*ElArcRadius + ElArcThick,
ElArcHeight + Protrusion],
center=true);
rotate([0,0,-(90 - ElArcAngle)])
translate([ElArcRadius + ElArcThick,0,ElArcHeight/2])
cube([2*ElArcRadius + ElArcThick,
2*ElArcRadius + ElArcThick,
ElArcHeight + Protrusion],
center=true);
}
}
}
//----------------------
// Helmet Interface Plate
module HelmetPlate() {
difference() {
scale([ElPlateTall,ElPlateWidth,1.0])
cylinder(r=0.5,h=HelmetPlateThick,$fn=ElPlateFacets);
translate([0,0,HelmetRX + HelmetPlateThick - HelmetPlateM])
sphere(r=HelmetRX,$fn=256,$fs=0.1);
}
}
//----------------------
// Lash it together
if (Examine == "AzMount")
AzMount();
if (Examine == "ElMount")
ElMount();
if (Examine == "ElBody")
ElBody();
if (Examine == "ElPlate")
ElPlate();
if (Examine == "HelmetPlate")
HelmetPlate();
if ((Layout == "Build" || Layout == "Show") && Examine == "None") {
translate([-10,-20,0])
rotate([0,0,90]) // mis-align top fill from ElMount
AzMount();
translate([-10,20,ElMountLength + BearingOverlap])
rotate([0,180,-90])
ElMount();
translate([0,0,0])
rotate([0,0,0])
ElBody();
translate([10,15,0])
rotate([0,0,215]) // mis-align top fill from ElBody
ElPlate();
translate([20,-20,0])
rotate([0,0,-45])
HelmetPlate();
if (Layout == "Show")
ShowPegGrid(PegSize);
}
if ((Layout == "Fit") && Examine == "None") {
translate([0,0,-(AzMountLength/2 + MirrorShaftOffset)])
color(MFG) AzMount();
translate([0,0,AzMountLength/2 - MirrorShaftOffset - BearingOverlap])
color(DHC) ElMount();
// color([ 0/255, 204/255, 204/255,0.5]) ElMount();
translate([ElMountDia/2 + ElMountBase,0,0])
rotate([0,90,0])
color(DFC) ElBody();
translate([ElPlatePlusX,0,0])
rotate([180,90,0])
color(LHC) ElPlate();
translate([ElPlatePlusX,0,ElPlateTall/2 - ElBodyWidth/2])
rotate([0,90,0])
color(LWM) HelmetPlate();
}
This 2-56 stud will hold the mirror shaft into whatever helmet mount I eventually decide on. It’s a pan-head screw that miraculously fits snugly inside the cut-down shaft section, held in with a delicate epoxy dribble around the edge.
The head abuts the end of the smaller shaft section, so the two no longer slide. I think a length of heat-shrink tubing will stabilize them in rotation, although perhaps I should have just slobbered more epoxy into that joint.
After the epoxy cured, I sliced off all but 2 mm of the screw thread with an abrasive wheel and cleaned up the wreckage with a file. I actually remembered to spin on a nut before cutting, which ensured I finished the threads properly.
The business end of the mirror has far too many moving parts: two indented plates for the balls on the mirror and shaft, a screw, and a nut. That’s one too many ball joints, at least, and Wouldn’t It Be Nice If the mirror had a watertight seal around its perimeter?
For now, I just epoxied the nut in place after scuffing up the plate and nut with some sandpaper to give the epoxy something to grip:
You can’t see the new washer and rubber grommet under the screw head that provides a bit of compliance to hold the balls more securely, plus a dot of low-strength Loctite in the nut to discourage things from falling apart on the road.
With those doodles in mind, I applied an abrasive cutoff wheel to the shaft of an inspection mirror (from the usual eBay supplier) about 15 mm behind the second joint. That puts a short section of the third tube inside the yet-to-be-built helmet mirror mount.
The two copper-colored springs center the smaller tube inside the larger one and provide enough friction to make the whole thing work. The tubes seem to be chrome-plated brass and the springs might be phosphor bronze. I suppose they’re Matryoshka-sized from one end to the other.
I’d never taken one of those shafts apart before; now we both know.
Having had many bike helmet mirrors disintegrate over the miles and years, I’ve had a background project bubbling along to build something more durable. Whether that’s feasible or not remains to be seen, but here’s another go at it.
A full-up ball joint seems to be more trouble than it’s worth and, in any event, requires far too much precision to be easily duplicated. That renders those doodles, mmm, inoperative.
These doodles aren’t workable, either, but they convert the ball joint into two orthogonal rotating joints that could be 3D printed with some attention to detail.
The general idea:
What’s not to like:
A few more days of doodling produced something that seems better. The az-el joint axes and the mirror shaft axis now meet at a common point, so the mirror shaft moves as the radius of a sphere. The elevation screw hides behind the azimuth mount, out of the way, which makes it awkward to adjust the tension.
The helmet mount plate must be concave to more-or-less match the helmet curvature. I’ve been securing mirrors using double-sided foam tape to good effect, but it requires a fairly large pad to provide enough adhesive force.
Two glue joints make everything buildable and should have basically the same strength as the parts themselves. The helmet plate builds concave face up. The az and el mounts build with the bearings upward, as do the mating surfaces on the other parts. Maybe the screws need actual nuts embedded in the mating parts, in which case there may be problems.
The setscrew holding the mirror shaft can crush the tube; I think they’re thin brass, at best. Putting a stud screw on the end will hold the shaft in place, leaving the setscrew to prevent rotation. Perhaps the stud can reinforce the tube.
What’s not to like:
But maybe something will come of it.
A gust of wind blew Mary’s bike helmet off the seat and, by the conservation of perversity, it landed on the mirror with predictable results:
I affixed the two ends with solvent glue, then epoxied a brass tube around them to stiffen it up. While I had the epoxy and brass out, I added a splint over a previous repair near the mirror ball:
After taking that picture, I heated and bent the remaining shaft just slightly to put the ball near the middle of its range. There’s no possible way this can survive this year’s cycling, so I must get cracking on building some durable mirrors. A 3-D printer should come in handy for something in that project!
The Smell of Molten Projects in the Morning 