Posts Tagged Mini-lathe

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!

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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 …

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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 …

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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.

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Mini-Lathe, Maxi-OD

This came about while tinkering up a shade for a repurposed LED downlight:

PVC fitting - boring setup
PVC fitting – boring setup

It’s a 4 inch DWV pipe coupling I bored out to fit the LED housing, which was ever so slightly larger than the pipe OD.

Cutting it off required as much workspace as the poor little lathe had:

PVC fitting - cutoff setup
PVC fitting – cutoff setup

Ignore the toolpost handle across the top. What’s important: the cutoff blade poking out of the QCTP, above the orange carriage stop lock lever, extending just far enough to cut through the coupling’s wall before the compound hits the coupling. The compound slide is all the way out against the cross-slide DRO, rotated at the only angle putting the tool where it needs to be and clearing the end of the coupling.

It ended reasonably well:

PVC fitting - LED floor lamp
PVC fitting – LED floor lamp

But, in retrospect, was hideously bad practice. Next time, I’ll make a fixture to hold the fitting on a faceplate.

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MPCNC Drag Knife Holder: Showing More Blade

Attempting to cut laminated cardstock decks for the Homage Tektronix Circuit Computer required a bit more blade extension than my LM12UU holder made available:

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

Shortening the 12 mm shaft wasn’t going to happen, so I knocked a little bit off the blade holder to give the knurled lock ring slightly more travel:

Drag Knife Holder - shortening stop
Drag Knife Holder – shortening stop

The lathe cutoff blade is a bit to the right of the new cut, but you get the general idea: not a whole lot of clearance in there.

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Mini-Lathe DRO Battery Life

The Mini-Lathe DROs eat a 390 alkaline coin cell a year, more or less:

Mini-Lathe DRO - battery life
Mini-Lathe DRO – battery life

The other DRO’s cell was 10 mV higher, so it might have survived another few weeks. I’ll call it a year, as the OEM cells failed half a year after I got the thing and these are the second set.

The last time I did this, I wedged a thin foam sheet below the display PCB to put a bit more pressure on the (+) contact tab sticking down from the middle of the plate:

Mini-Lathe DRO - battery compartment
Mini-Lathe DRO – battery compartment

The (-) contact is a pad on the PCB below the battery compartment. The glaring metal reflector is part of the curved cell retainer.

I still wish the DROs didn’t collide with the compound slide, but you can get used to anything if you do it long enough.

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