Posts Tagged Mini-lathe
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:
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:
Which looked better, although it still wasn’t quite perpendicular to the shaft axis.
Back to the lathe:
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!
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:
They’re made from the same scrap smoked acrylic as the ones in our sinks:
They’re definitely upscale from the (not watertight!) 3D printed version I built for a Digital Machinist column to explain OpenSCAD modeling:
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:
They’re done on the Sherline, because it has real clamps:
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:
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 …
I’ve always wondered what’s inside a metal-case vacuum tube:
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:
Unsurprisingly, the guts resemble those of glass-envelope rectifier tubes in my collection, like this 5U4GB:
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 …
Well, at least it’s centered on the target:
This happened a few times before, because my fingers don’t fit neatly inside the drag knife holder to tighten the lock ring:
[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:
A view looking along the screw shows a bit more detail around the spring:
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:
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:
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.
This came about while tinkering up a shade for a repurposed LED downlight:
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:
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:
But, in retrospect, was hideously bad practice. Next time, I’ll make a fixture to hold the fitting on a faceplate.
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:
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.
The Mini-Lathe DROs eat a 390 alkaline coin cell a year, more or less:
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:
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.