Mini-Lathe Metric Threading: 21 Tooth Gear

Mini-lathes sold everywhere except the USA have hard-metric leadscrews with a 1.5 mm pitch, so they can cut metric threads without any trouble at all. USA-ian mini-lathes have hard inch leadscrews with a 1/16 inch pitch and require gymnastics to cut decent metric threads.

For inch threads:

Mini-lathe - inch thread equation
Mini-lathe – inch thread equation

For metric threads, it’s upside-down and converted:

Mini-lathe - metric thread equation
Mini-lathe – metric thread equation

Including a 21 tooth change gear at A or C in the train will get closer to metric threads, but the stud holding the B-C gears on my lathe imposes a minimum B gear size. Here’s a 1 mm thread with a 21-30-45-50 train:

Mini-Lathe change gears - 1 mm - 21 vs 30 tooth - AB
Mini-Lathe change gears – 1 mm – 21 vs 30 tooth – AB

Oops.

You can interchange the AB and CD gear pairs, but the CD pair also has a minimum spacing:

Mini-Lathe change gears - 1 mm - 21 vs 30 tooth - CD
Mini-Lathe change gears – 1 mm – 21 vs 30 tooth – CD

If it worked, the 1 mm thread would be off by -125 ppm, which is surely close enough.

Without the 21 tooth gear, the LittleMachineShop gear calculator produces a 45-55-50-65 train that fits much better, albeit with +875 ppm thread error:

Mini-Lathe change gears - 1 mm - 45 55 50 65
Mini-Lathe change gears – 1 mm – 45 55 50 65

Dropping the 21 tooth gear into the LMS calculator produces a (barely) workable 21-50-60-40 train with -125 ppm error:

Mini-Lathe change gears - 1 mm - 21 50 60 40
Mini-Lathe change gears – 1 mm – 21 50 60 40

The very very snug fit of the screw (omitted here) on the 21 tooth gear nearly hits the 60 tooth gear. Your mileage may vary, of course.

Various sources suggest the gears are module 1, 20° pressure angle, 12 mm bore, and 8 mm thick (mine measure a scant 7.75 mm), with a 3 × 1.4 mm keyway (the key itself is 3 mm square, but it’s half-buried in the shaft).

Floor Lamp Height vs. Reach: Plumbing Fitting

The floor lamp with the invisible / non-tactile controls moved to a different chair, where it didn’t have quite enough reach and too much height. Knowing what was about to happen, I spliced a JST-SM connector into the wire inside the tube:

Floor Lamp - base wiring JST-SM connector
Floor Lamp – base wiring JST-SM connector

After trimming off all the extraneous bits, the larger half of the connector (male pins) fits through the tubing and the smaller half (female sockets) barely fits through the bottom bushings.

It turns out half-inch copper pipe fittings (ID = 15.9 mm) almost exactly fit the tubing (OD = 15.7 mm):

Floor Lamp - copper 45° elbow
Floor Lamp – copper 45° elbow

A quick test showed the 45° (actually, it’s 135°, but we’re deep into plumbing nomenclature) positioned the lamp head too high and with too much reach:

Floor Lamp - gooseneck exercise
Floor Lamp – gooseneck exercise

So shorten the tube attached to the head and deburr the cut:

Floor Lamp - tube deburring
Floor Lamp – tube deburring

The 45° fitting is too high and a 90° fitting is obviously too low, so cut a 20° slice out of a 90° fitting:

Floor Lamp - copper 90° elbow - 20° cutout
Floor Lamp – copper 90° elbow – 20° cutout

Cut a snippet of brass tubing to fit, bash to fit, file to hide, buff everything to a high shine, silver-solder it in place, and buff everything again:

Floor Lamp - copper 90° elbow - 20° fill strip
Floor Lamp – copper 90° elbow – 20° fill strip

The 5/8 inch aluminum rods serve to stiffen the fitting, smooth out the torch heating, and generally keep things under control.

Wrap the obligatory Kapton tape around the butt ends of the tubes to fill the fitting’s oversize hole, put everything together, and it’s just about perfect:

Floor Lamp - copper 70° elbow - installed
Floor Lamp – copper 70° elbow – installed

I immobilized the fitting with black Gorilla tape, but it really needs something a bit more permanent. One of these days, maybe, a pair of setscrews will happen.

The additional reach required a little more counterweight on the far side for security, so I added the broken stub of a truck leaf spring. It should be secured firmly to the base plate, but no tool I own can put a dent in those three pounds of spring steel. Maybe it’ll merit a fancy enclosure wrapped around the base?

Nissan Fog Lamp: RGB LED “Bulb”

After cleaning the fog lamp lens enough to be encouraging, I made an LED “bulb” from four WS2812 RGB pixels:

Nissan Fog Lamp - LED bulb standup
Nissan Fog Lamp – LED bulb standup

The small threaded hole has an M3 setscrew to let the brass post slide up & down to adjust the LED position inside the fog lamp’s reflector.

Despite my poor experience with the PCB-based WS2812 LEDs, the strip-mounted ones have been ticking along in the hard drive platter lamp basically forever, at least after I tamped down the heat problem.

The brass hex rod has plenty of thermal conductivity, particularly clamped into an aluminum disk connected more-or-less well to the fog lamp’s base.

Nissan Fog Lamp - RGB LED lamp
Nissan Fog Lamp – RGB LED lamp

The two short wires linking the two LED strips (the purple wire is data into the first LED) hold them in place around the hex, despite their desire to straighten out, pull free of their adhesive, and fall off.

The general idea was to put the LEDs at about the same level as the halogen bulb filament, thereby spreading enough light to fill the reflector housing:

Nissan Fog Lamp - LED vs halogen
Nissan Fog Lamp – LED vs halogen

I drilled a hole through the hex as a cable “conduit”, turned the end into a nice rod, then machined a stub of aluminum to fit:

Nissan Fog Lamp - parting off LED base
Nissan Fog Lamp – parting off LED base

A pair of slots milled along the sides of the aluminum disk fit the housing’s locating features:

Nissan Fog Lamp - LED bulb trial fit
Nissan Fog Lamp – LED bulb trial fit

Nissan used an elaborate spring latch to clamp the halogen bulb’s sheet-metal base in place, but its 50 mil wire didn’t have nearly enough give for my chunky aluminum disk. My version of a spring latch came from a length of 24 mil music wire, which definitely beats the epoxy I was planning to use.

Heat transfer seems to be a non-issue, as the LEDs get barely warm to the touch. Until they drop dead, I’ll assume it’s all good in there.

Two screws hold the lens in place, but the collision seems to have stripped their grip on the plastic and they didn’t un-screw:

Nissan Fog Lamp - lens retaining screw
Nissan Fog Lamp – lens retaining screw

Jamming a utility knife blade under the screw head and prying upward while turning the screwdriver persuaded them out of their sockets, after which the lens popped out of its form-fitted silicone gasket with surprisingly little effort:

Nissan Fog Lamp - reflector stains
Nissan Fog Lamp – reflector stains

The lamp spent a week or so beside the road, out in the weather, and shipped a few drops of rainwater through the rectangular hole under the spring latch anchor. Some delicate cotton-swab action removed most of the grime without too much damage, but the reflective film on those corrugations won’t ever be the same again.

Now it’s just a simple matter of software …

Mini-Lathe vs. Case-Hardened Shaft

While doodling a drag knife holder for the Sherline, I figured a 3/8 inch shaft would hold all the parts and fit neatly into a standard Sherline tool holder, which it did:

Sherline Diamond Drag Holder - installed
Sherline Diamond Drag Holder – installed

Having recently upcycled a 3/8 inch shaft from the Thing-O-Matic into a pen holder for the CNC 3018-XL, I cut off another section with an abrasive wheel, then tried to face it off:

Hardened shaft facing - abrasive step
Hardened shaft facing – abrasive step

Although the mini-lathe’s carbide insert gnawed at the shaft’s case-hardened shell, it obviously wasn’t making much progress against that step.

Back to the abrasive cutoff saw:

Hardened shaft facing - abrasive flattening
Hardened shaft facing – abrasive flattening

Which looked better, although it still wasn’t quite perpendicular to the shaft axis.

Back to the lathe:

Hardened shaft facing - lumpy face
Hardened shaft facing – lumpy face

Well, it’s better, but it sure ain’t pretty.

Put gently, the mini-lathe’s lack of rigidity doesn’t help in the least. The compound was a-reelin’ and a-rockin’ on every revolution and eventually turned a slight tilt into a distinct radial step.

Memo to Self: Dammit, use a brass rod!

CNC Kitchen Sink Strainer

Our Young Engineer recently rented a house, now knows why our sinks have CNC-machined strainers, and asked for something better than the disgusting stainless mesh strainer in the kitchen sink.

Being a doting father, I turned out a pair to get a pretty one:

CNC Sink Strainer - overview
CNC Sink Strainer – overview

They’re made from the same scrap smoked acrylic as the ones in our sinks:

CNC Sink Strainer
CNC Sink Strainer

They’re definitely upscale from the (not watertight!) 3D printed version I built for a Digital Machinist column to explain OpenSCAD modeling:

Strainer plate fill
Strainer plate fill

This time around, though, I rewrote the subtractive design in GCMC, with helical milling for all the holes to eliminate the need to change tools:

Sink Strainer - tool path simulation - CAMotics
Sink Strainer – tool path simulation – CAMotics

They’re done on the Sherline, because it has real clamps:

CNC Sink Strainer - on Sherline
CNC Sink Strainer – on Sherline

Four tabs eliminated the need to reclamp the stock before cutting the perimeter, but I should have ramped, not plunged, through the final cut between the tabs:

CNC Sink Strainer - tab surface fracture
CNC Sink Strainer – tab surface fracture

The handles come from the same chunk of hex acrylic as before, eyeballed to length, tapped 8-32, and secured with acrylic adhesive.

The GCMC source code as a GitHub Gist:

All in all, a pleasant diversion from contemporary events …

Metal-case 5T4 Vacuum Tube Opened

I’ve always wondered what’s inside a metal-case vacuum tube:

Dual rectifier tube 5T4 - metal case opened
Dual rectifier tube 5T4 – metal case opened

The cutter last saw action on the EMT used in the MPCNC, so it’s intended for use on steel tubes. I thought about parting the case off in the lathe, but a tubing cutter sufficed for a first attempt, even if it couldn’t cut quite as close to the flange as I wanted.

A 5T4 tube is a full-wave rectifier with two sections:

Dual rectifier tube 5T4 - upright
Dual rectifier tube 5T4 – upright

Unsurprisingly, the guts resemble those of glass-envelope rectifier tubes in my collection, like this 5U4GB:

5U4GB Full-wave vacuum rectifier - cyan red phase
5U4GB Full-wave vacuum rectifier – cyan red phase

The metal case would be far more rugged than a glass bottle and, perhaps, the flange locked the tube into its socket against vibration.

The filaments surely weren’t thoriated, so it’s all good …

MPCNC Drag Knife Holder: Lock Screw

While calibrating the MPCNC’s probe camera offset for the drag knife holder, this happened:

Drag Knife - vertical escape
Drag Knife – vertical escape

Well, at least it’s centered on the target:

Drag Knife - vertical escape - detail
Drag Knife – vertical escape – detail

This happened a few times before, because my fingers don’t fit neatly inside the drag knife holder to tighten the lock ring:

Drag Knife - LM12UU ground shaft - assembled
Drag Knife – LM12UU ground shaft – assembled

[Update: The lock ring keeps the holder at a fixed position inside the 12 mm shaft and doesn’t affect the blade directly. When the ring works loose, the threaded holder can rotate to expose more blade and, in this case, stab deeper into the target. ]

So I turned & knurled an aluminum ring, then tapped a 3×0.5 mm hole for a lock screw plucked from the Drawer o’ Random M3 Screws:

Drag Knife - lock screw - side
Drag Knife – lock screw – side

A view looking along the screw shows a bit more detail around the spring:

Drag Knife - lock screw - front
Drag Knife – lock screw – front

The general idea is to set the blade extension, then tighten the lock screw to hold it in place, without relying on the original brass lock ring, shown here while cutting a boss for the spring:

Drag Knife - turning spring recess
Drag Knife – turning spring recess

The lock screw’s knurled handle just barely kisses the NPCNC’s black tool holder ring, so my guesstimated measurements were a bit off. Clamping the knife holder one itsy higher in the tool holder solved the problem.

I cranked on 300 g of spring preload and, squashed like that, the spring’s rate is now 75 g/mm. Cutting at Z=-1 mm should suffice for laminated paper slide rule decks.

The original sizing doodle:

Drag Knife Holder - lock screw ring doodle
Drag Knife Holder – lock screw ring doodle

The short 18 mm section clears the inside of the LM12UU bearing, although it could be a millimeter shorter. The 19 mm section comes from the 3/4 inch aluminum rod I used, skim-cut to clean it up.

If I ever remake this thing, it needs a major re-think to get all the dimensions flying in formation again.