Bike Helmet Mirror: Brasswork Clamp

A bit of Quality Shop Time produced a slight improvement to the clamp holding the mirror to the stalk:

Helmet Mirror Ball Mount - mirror joint brasswork
Helmet Mirror Ball Mount – mirror joint brasswork

The general idea is to hold the wave washer (it’s mashed under the flat washer, honest) above those bumps on the plate holding the mirror and stalk balls. It’s a few millimeters from the end of a ¼ inch brass rod, drilled for the M3 screw, and reduced to 4.5 mm with a parting tool to clear the bumps.

While I was at it, I made two spare mirrors, just to have ’em around:

Helmet Mirror Ball Mount - new vs old
Helmet Mirror Ball Mount – new vs old

The new ball mount looks downright svelte compared to the old Az-El mount, doesn’t it?

I should replace the steel clamp plates with a stainless-steel doodad of some sort to eliminate the unsightly rust, but that’s definitely in the nature of fine tuning.

More AAA-to-AA Alkaline Adapters

Having a handful of not-dead-yet AAA alkalines and a bunch of LED blinkies built for AA alkalines, a pair of adapters seemed in order:

AAA-to-AA Alkaline Adapters - installed
AAA-to-AA Alkaline Adapters – installed

The blinkies need a somewhat wider base than they’d get from a pair of AAA alkalines, so it’s not quite as dumb as it may seem.

In any event, the positive terminal comes from a brass rod:

AAA-to-AA Alkaline Adapters - brass terminal
AAA-to-AA Alkaline Adapters – brass terminal

Nobody will ever see the fancy Hilbert Curve infill around the brass:

AAA-to-AA Alkaline Adapters - end view
AAA-to-AA Alkaline Adapters – end view

In this application, they’ll go from not-dead-yet to oh-it’s-dead faster than AA cells, so I can watch how the blinkies work with lower voltages.

Bike Helmet Mirror: Ball Mount

Nine years ago, I didn’t know how enough to design a bike helmet mirror with a ball mount, but even an old dog can learn a new trick:

Helmet Mirror Ball Mount - on helmet
Helmet Mirror Ball Mount – on helmet

However, it’s worth noting my original, butt-ugly Az-El mounts lasted for all of those nine years, admittedly with adjustments along the way, which is far more than the commercial mounts making me unhappy enough to scratch my itch.

The mount adapts the split spherical clamp from the daytime running light:

Helmet Mirror Mount - Ball
Helmet Mirror Mount – Ball

Scaling it down for a 10 mm polypropylene ball around the base of the 30 mm inspection mirror’s shaft simplified everything:

Helmet Mirror Ball Mount - drilled ball test
Helmet Mirror Ball Mount – drilled ball test

I’m reasonably sure I couldn’t have bought 100 polypro balls for eight bucks a decade ago, but we’ll never know. Drilling the hole was a complete botch job, about which more later. The shaft came from a spare mirror mount I made up a while ago; a new shaft appears below.

The solid model, like Gaul, is in three parts divided:

Helmet Mirror Ball Mount - Slic3r
Helmet Mirror Ball Mount – Slic3r

The helmet plate (on the right) has a slight indent more-or-less matching the helmet curvature and gets a layer of good double-stick foam tape. The clamp base (on the left) has a pair of brass inserts epoxied into matching recesses below the M3 clearance holes:

Helmet Mirror Ball Mount - inserts
Helmet Mirror Ball Mount – inserts

A layer of epoxy then sticks the helmet plate in place, with the inserts providing positive alignment:

Helmet Mirror Ball Mount - plates
Helmet Mirror Ball Mount – plates

The clamp screws pull the inserts against the plastic in the clamp base, so they can’t pull out or through, and the plates give the epoxy enough bonding surface that (I’m pretty sure) they won’t ever come apart.

I turned down a 2 mm brass insert to fit inside the butt end of the mirror shaft and topped it off with a random screw harvested from a dead hard drive:

Helmet Mirror Ball Mount - assembled - rear view
Helmet Mirror Ball Mount – assembled – rear view

At the start, it wasn’t obvious the shaft would stay stuck in the ball, so I figured making it impossible to pull out would eliminate the need to find it by the side of the road. As things turned out, the clamp exerts enough force to ensure the shaft ain’t goin’ nowhere, so I’ll plug future shafts with epoxy.

The front side of the clamp looks downright sleek:

Helmet Mirror Ball Mount - assembled - front view
Helmet Mirror Ball Mount – assembled – front view

Well, how about “chunky”?

The weird gray-black highlights are optical effects from clear / natural PETG, rather than embedded grunge; it looks better in person. I should have used retina-burn orange or stylin’ black.

This mount is much smaller than the old one and should, in the event of a crash, not cause much injury. Based on how the running light clamp fractures, I expect the clamp will simply tear out of the base on impact. In the last decade, neither of us has crashed, so I don’t know what the old mount would do.

The clamp is 7 mm thick (front-to-back), set by the M3 washer diameter, with 1.5 mm of ball sticking out on each side. The model has a kerf one thread high (0.25 mm) between the pieces to add clamping force and, with the screws tightened down, moving the ball requires a disturbingly large effort. I added a touch of rosin and that ball straight-up won’t move, which probably means the shaft will bend upon droppage; I have several spare mirrors in stock.

On the other paw, the ball turns smoothly in the clamp and it’s easy to position the shaft as needed: much better than the old Az-El mount!

The inspection mirror hangs from a double ball joint which arrives with a crappy screw + nut. I epoxied the old mirror mount nut in place, but this time around I drilled the plates for a 3 mm stainless SHCS, used a wave washer for a bit of flexible force, and topped it off with a nyloc nut:

Helmet Mirror Ball Mount - mirror joint
Helmet Mirror Ball Mount – mirror joint

I’m unhappy with how it looks and don’t like how the washer hangs in free space between those bumps, so I may eventually turn little brass fittings to even things out. It’s either that or more epoxy.

So far, though, it’s working pretty well and both units meet customer requirements.

The OpenSCAD source code as a GitHub Gist:

// Bike helmet mirror mount - ball joint
// Ed Nisley KE4ZNU 2020-09
/* [Layout options] */
Layout = "Build"; // [Build, Show, Plate, Base, Clamp]
//-- Extrusion parameters
// Extrusion parameters
/* [Hidden] */
ThreadThick = 0.25;
ThreadWidth = 0.40;
HoleWindage = 0.2;
function IntegerMultiple(Size,Unit) = Unit * ceil(Size / Unit);
function IntegerLessMultiple(Size,Unit) = Unit * floor(Size / Unit);
Protrusion = 0.1; // make holes end cleanly
inch = 25.4;
ID = 0;
OD = 1;
//- Basic dimensions
MountDia = 30.0; // footprint on helmet
BallDia = 10.0;
BallRad = BallDia / 2;
WallThick = IntegerMultiple(2.0,ThreadWidth);
FloorThick = IntegerMultiple(2.0,ThreadThick);
CornerRound = 2.0;
Insert = [3.0,4.0,4.0]; // threaded brass insert
Screw = [3.0,5.5,25.0]; // clamp screw
Washer = [3.7,7.0,0.7]; // washer
ShowGap = 2.0;
BuildGap = 5.0;
//-- Helmet Interface Plate
ScrewOC = BallDia + 2*WallThick + Screw[ID];
echo(str("Screw OC: ",ScrewOC));
Clamp = [ceil(Washer[OD]), // barely holds washer under screw
ScrewOC + Washer[OD], // minimal clearance for washer
BallDia +2*FloorThick // screw fits through insert
Kerf = ThreadThick;
echo(str("Clamp: ",Clamp));
HelmetCX = 60.0; // empirical helmet side curve
HelmetMX = 3.0;
HelmetRX = (pow(HelmetMX,2) + pow(HelmetCX,2)/4)/(2*HelmetMX);
HelmetPlateC = MountDia;
HelmetPlateTheta = atan(HelmetPlateC/HelmetRX);
HelmetPlateM = 2*HelmetRX*pow(sin(HelmetPlateTheta/4),2);
echo(str("Plate indent: ",HelmetPlateM));
HelmetPlateThick = max(FloorThick,0.6*Insert[LENGTH]) + HelmetPlateM;
echo(str("Screw length: ",Clamp.z + Insert[LENGTH]));
MountSides = 2*3*4;
// 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,$fn=Sides);
// Clamp frame around ball
module ClampFrame() {
difference() {
union() {
for (i=[-1,1], j=[-1,1]) {
translate([i*(Clamp.x/2 - CornerRound),j*(Clamp.y/2 - CornerRound),Clamp.z/2 - CornerRound])
translate([i*(Clamp.x/2 - CornerRound),j*(Clamp.y/2 - CornerRound),-Clamp.z/2])
for (j=[-1,1])
sphere(d=BallDia + HoleWindage,$fn=48);
for (j=[-1,1])
module ClampSelect(Section) {
XlateZ = (Section == "Top") ? Clamp.z/2 :
(Section == "Bottom") ? -Clamp.z/2 :
intersection(convexity=5) {
cube([2*Clamp.x,2*Clamp.y,Clamp.z + 2*Protrusion],center=true);
// Concave plate fitting helmet shell
module HelmetPlate() {
difference() {
translate([0,0,HelmetPlateThick - HelmetPlateM + HelmetRX])
for (j=[-1,1])
translate([0,j*ScrewOC/2,-Protrusion]) {
PolyCyl(Insert[OD],0.6*Insert[LENGTH] + Protrusion,6);
// Base of clamp ring
module MountBase() {
difference() {
union() {
translate([0,0,FloorThick + Clamp.z/2])
for (j=[-1,1])
PolyCyl(Insert[OD],0.6*Insert[LENGTH] + Protrusion,6);
// Lash it together
if (Layout == "Plate") {
if (Layout == "Base") {
if (Layout == "Clamp") {
if (Layout == "Show") {
translate([0,0,ShowGap]) {
translate([0,0,Clamp.z/2 + FloorThick + ShowGap/2])
translate([0,0,Clamp.z/2 + FloorThick + ShowGap])
if (Layout == "Build") {
translate([MountDia/2 + BuildGap,0,0])
translate([-(MountDia/2 + BuildGap),0,0])
translate([0,MountDia/2 + BuildGap,Clamp.z/2])

The original doodles include a bit of dress-up fairing that didn’t make the cut:

Helmet Mirror Ball Mount - doodles
Helmet Mirror Ball Mount – doodles

Lathe-straightened Copper Wire

I formerly straightened the copper wire into “bus bars” for the astable multivibrators by whacking it with a slide hammer, but someone whose name is lost in the mists of time told me the right way to do it:

Lathe-straightening Wire - setup
Lathe-straightening Wire – setup

Yup, grab a piece of wire at both ends in Tiny Lathe and give it a few low-speed turns while pulling firmly on the tailstock.

No muss, no fuss, no drama, just bar-straight and slightly work-hardened copper wires:

Lathe-straightening Wire - results
Lathe-straightening Wire – results

I slide-hammered the top wire before remembering the clue. The bottom two wires have peppermint-stick swirls.

Thank you, whoever you were!

Mini-Lathe ER Collet Chuck Drawbar

The ER-16 and ER-32 collet chucks use an M12×1.75 bolt to snug their MT3 tapers in the Mini-Lathe spindle. As nearly as I could figure, I needed a 190 mm bolt to get enough thread engagement, but the nearest available sizes were either too short or too long.

Fortunately, making round things is what a lathe is all about:

MT3 drawbar - assembled
MT3 drawbar – assembled

The aluminum bellyband adds 30 mm to the length and aligns the bolt sections, with the threaded section from a long 5/16-18 bolt inside holding the metric bolt together:

MT3 drawbar - parts
MT3 drawbar – parts

Although I got it right on the first try (!), the bellyband lets me fine-tune the length as needed.

The original dimension doodle and some in-flight updates:

ER Collets - MT3 drawbar bolt - dimension doodles
ER Collets – MT3 drawbar bolt – dimension doodles

The fancy brass / bronze washer comes from a battered rod with mushroomed ends. A pair of V-blocks let me cut a chunk off one end with negligible drama:

Bronze Bar Stock - support fixture
Bronze Bar Stock – support fixture

It’s clamped firmly to the right block and a few licks with a file knocked off enough of the mushroom on the left end to put it flat(-ish) into the V; the near side of the right block is barely raised off the surface.

Face off the mushroom to get a flat spot for a center drill:

MT3 drawbar - battered bronze rod
MT3 drawbar – battered bronze rod

Some peaceful turning & boring produces a pretty washer:

MT3 drawbar - washer cutoff
MT3 drawbar – washer cutoff

The bore needed a bit of relief to seat the bolt head squarely on the outer surface:

MT3 drawbar - spindle washer
MT3 drawbar – spindle washer

And then It Just Fit™:

MT3 drawbar - installed
MT3 drawbar – installed

Loctite on the inner bolt threads should keep everything together.

Screw Thread Measurement

While I was cutting threads for the Floor Lamp poles, I tried measuring my progress over wires:

Floor Lamp - tube fitting - thread measurement
Floor Lamp – tube fitting – thread measurement

Those are three lengths of music wire, slightly bent from their storage roll, held in place with a precision clamp metric micrometer. Given the crudity of the setup, the uncalibrated wire diameter, and my lack of thread-fu, the results came out both close and unconvincing.

A set of real thread measuring wires being cheap & readily available, I’m prepared for the next time around this block:

Thread Measuring Wires - eBay set
Thread Measuring Wires – eBay set

The 185 mil “wires” (they’re all allegedly ground rod) will let me cut threads matching things like a Jesus nut; they’re suited for 3 TPI / 8 mm pitch screws. Mostly, wires from the front row will be all I ever need.

Which look like this in action:

Thread Measuring Wires - eBay setThread Measuring Wires - detail
Thread Measuring Wires – eBay setThread Measuring Wires – detail

The black doodad (the set includes half a dozen for all the wire sizes) fits over the micrometer anvil and holds two wires betwixt anvil and screw, leaving me to manipulate the screw, the third wire, and the micrometer with my remaining hands. Hence the vise holding the micrometer, which is known to be Very Bad Practice.

From the side:

Thread Measuring Wires - overview
Thread Measuring Wires – overview

All of the smaller wires measure 0.5 mil too thin, which is likely due to my lack of calibrated measurement equipment:

Thread Measuring Wires - scant 24 mil
Thread Measuring Wires – scant 24 mil

The few thread pitch diameters I measured also came out slightly too small, again likely due to calibration and screw tolerances.

The LittleMachineShop description of measuring threads over wires seems entirely adequate.

To forestall link rot, a slightly rearranged version of their tables of wire constants:

Thread Wire Measurement Constants
Thread Wire Measurement Constants

The lower table has metric thread pitches with the wire sizes in inches.

You measure the distance over the recommended wire (in inches or millimeters, as appropriate), subtract the constant, and get the pitch diameter in the same units. Conversely, add the constant to the desired pitch diameter to get the target over-wire distance, carefully cut the thread until it measures a bit less than that, back up sixty seconds, and cut it spot on.

Verily, it is written: there is no UnDo key (⎌) in machine shop work.

Floor Lamp: Threaded Fittings

The reshaped copper elbow on the floor lamp now has the right angle, but lacks threaded connections to the tubes. The OEM tube threads are close to M15×1, thus prompting the change gear exercise persuading Tiny Lathe™ to cut metric threads.

Chuck up a length of 5/8 inch aluminum tube, clean up the end, and poke a thread runout slot into it:

Floor Lamp - tube fitting - thread runout
Floor Lamp – tube fitting – thread runout

Turn the soon-to-be-thread OD to 14.7 mm, well under the minimum 14.794 mm major thread diameter. I figure it’s better to match the existing not-quite-standard tube threads than to get all fussy about tolerances:

Floor Lamp - tube fitting - thread OD
Floor Lamp – tube fitting – thread OD

Drill out the tube to 27/64 inch = 0.422 inch = 10.7 mm, a bit larger than the OEM fittings, to easily pass the JST-SM connector I added so I could take the lamp apart:

Floor Lamp - tube fitting - drilling bore
Floor Lamp – tube fitting – drilling bore

Yeah, you’re not supposed to let the swarf build up like that, but it’s hard to stop when you’re getting good chip.

Break the sharp edges:

Floor Lamp - tube fitting - ready for threading
Floor Lamp – tube fitting – ready for threading

Set up for threading:

Floor Lamp - tube fitting - external threading setup
Floor Lamp – tube fitting – external threading setup

That’s a really nice Warner laydown threader I won as a Cabin Fever door prize quite some years ago.

A comprehensive discussion of threading may be handy.

The compound is at 90° to the cross slide, because the DRO housing doesn’t let the compound swivel to the proper angle for thread cutting. I’m just ramming the threader straight into the tube, taking sissy cuts, and hoping for the best.

Kiss the OD with the cutter, set the cross slide DRO to zero, position the cutter just off the end of the tube, close the split nuts around the leadscrew, engage the threading dial at a conspicuous mark:

Mini-Lathe Threading Dial - aligned
Mini-Lathe Threading Dial – aligned

The first real pass looked good:

Floor Lamp - tube fitting - first thread pass
Floor Lamp – tube fitting – first thread pass

The runout slot is 1/16 inch = 1.6 mm wide and I’m running the lathe dead slow, so there’s plenty of time to punch the STOP button as the cutter enters the slot and let the spindle coast down. Flip the switch to REVERSE, crank the cross slide out a turn (1 mm with 0.3 mm of crank backlash), run the cutter back to the starting point, crank the cross slide in, and iterate until the fitting screws into one of the OEM lamp tubes:

Floor Lamp - tube fitting - final thread
Floor Lamp – tube fitting – final thread

The 5/8 inch tube is just a smidge too small for the copper fitting, so knurl the fitting to enlarge the OD slightly more than a smidge:

Floor Lamp - tube fitting - knurled
Floor Lamp – tube fitting – knurled

Break the knurl edges, part off the fitting, clean up the new end, and do it all over again:

Floor Lamp - tube fitting - threaded adapters
Floor Lamp – tube fitting – threaded adapters

The knurls got filed down to an exact slip fit in the copper elbow and will eventually be epoxied in place.

The cut-off tube on the lamp head also needs internal threads, so bore out the interior to flatten the weld seam:

Floor Lamp - tube fitting - cleaning tube bore
Floor Lamp – tube fitting – cleaning tube bore

No pix of the threading, but you have the general idea; the tube wall is a scant 0.6 mm thick, so this isn’t the place for full-spec threads. I stopped when the OEM tube screwed in place.

Apart from the hideous solder job, it came together pretty well:

Floor Lamp - tube fitting - unpainted
Floor Lamp – tube fitting – unpainted

It’s much more stable than Kapton-wrapped tubes jammed into a bare copper fitting, although that’s not saying much.

A rattle-can finish seems appropriate …