While cleaning dead bugs out of the ceiling lamps, we discovered the kitchen light was missing one of the three nuts holding its cover in place. While spare nuts might be available, this seemed like a quicker & easier solution:
The stepped interior fits a brass insert with 8-32 threads (not metric, to my utter astonishment) rammed in place with a heat-set tool:
Using the nominal diameters seems to work fine, although I’m sure some finesse will be needed with smaller inserts.
Printed four just to be sure, rammed three inserts, and they’re ready:
The curved cap matches the original nut through the use of the Chord Equation to get the cap radius as a function of its height (sagitta) & base diameter. Admittedly, it looks kinda grotty with only a dozen layers, but it’s the thought that counts.
The original nuts are heavy knurled steel and the new ones are cheap plastic, but nobody will ever know:
Bonus: now I have two spare steel nuts for the next time …
The OpenSCAD source code:
// Nuts for LED ceiling light fixture
// Ed Nisley KE4ZNU
// 2024-09-27
KnurlLength = 7.4;
KnurlOD = 9.0;
CapOD = 9.0;
CapHeight = 2.0;
CapRadius = (pow(CapHeight,2) + pow(CapOD,2)/4)/(2*CapHeight);
echo(CapRadius=CapRadius);
NumSides = 1*(2*3*4);
$fn = NumSides;
Protrusion = 0.1;
difference() {
union() {
intersection() {
translate([0,0,KnurlLength + CapHeight - CapRadius])
sphere(r=CapRadius);
translate([0,0,KnurlLength])
cylinder(d=2*KnurlOD,h=KnurlLength);
}
cylinder(d=KnurlOD,h=KnurlLength);
}
// Ad-hoc 8-32 brass insert sizes
cylinder(d=5.5,h=8.0);
cylinder(d=5.9,h=5.7);
cylinder(d=6.2,h=2.2);
translate([0,0,-Protrusion])
cylinder(d=6.2,h=2.2);
}
The Smell of Molten Projects in the Morning 
Comments
5 responses to “Ceiling Lamp Nuts”
8-32 threads seem to be universal for a lot of electrical equipment, though 6 and 10 (both -32) make appearances. I think that one standard to rule them all actually happened. (And in the darkness find them… [grin]) My stash of ANSI screws has come in handy over the past few decades.
I have a little drawer of electrical fittings and really wanted to find a suitable nut in there, but it was not to be. Now it has a pair!
Lamp parts are weird: https://grandbrass.zendesk.com/hc/en-us/articles/203607534-Thread-Size-Chart
Apparently the 3/8 OD tube with 27 TPI threads in that drawer is really 1/8 Iron Pipe Standard, because gas: https://grandbrass.zendesk.com/hc/en-us/articles/201711490-What-is-IPS-and-Why-Do-We-Use-These-Size-Threads-for-Lighting-Parts
I had some suspicions about the lighting in the 1903 vintage house I grew up in… The wiring was, er, peculiar.
The Morse Estate (yes, that Morse) has electric lighting fixtures with the wires fed through the gas pipes, which definitely doesn’t meet contemporary NEC code but made perfect sense at the time.
Shudders. Dad had to do some wiring in that house (mid 1960s), using BX (metal spiral shield, no ground, inner wires had lousy insulation). No idea what the walls had. Knob and tube would have been the best available, I think.
Did remodels to a mid-century house and a room in a mid-30s house. Amazing how long it took for grounded outlets to become a thing. OTOH, when I did new construction up here, I actually knew how to do it (have NEC code, will wire).