Posts Tagged Mini-lathe
The front rim on my Tour Easy developed a distinct bulge, of the sort usually caused by ramming something, but I’m not Danny McAskell and the bulge got worse over the course of a few weeks, suggesting the rim was deforming under tire pressure. Having ridden it upwards of 35 k miles with plenty of trailer towing and too much crushed-stone trail riding, the brake tracks were badly worn and it’s time for a new rim.
An Amazon seller had an identical (!) rim, except for the minor difference of having a hole sized for a Schraeder valve stem, rather than the Presta valves on the original rims. One can buy adapters / grommets, but what’s the fun in that?
The brake track walls are 1.5 mm thick on the new rim and a scant 1.0 mm on the old rim, so, yeah, it’s worn.
A few measurements to get started (and for future reference):
If you don’t have an A drill, a 15/64 inch drill is only half a mil larger and, sheesh, anything close will be fine.
Introduce a suitable brass rod to Mr Lathe:
Break all the edges and drop it in place:
One could argue for swaging the adapter to fit flush against the curved rim, but commercial adapters don’t bother with such refinements and neither shall I.
The 7.0 mm length got shortened to fit flush with the center of the rim:
It’s brass, because the rim is heaviest on the far side where the steel pins splicing the ends live, and, with the tube & tire installed, the rim came out almost perfectly balanced. Which makes essentially no difference whatsoever, of course.
The shiny new rim sports shiny new reflector tape (from the same stockpile, of course).
That was easy …
Drilling a pair of holes into a length of ground steel shaft turned it into a holder for a Sakura Micron pen:
The aluminum ring epoxied to the top keeps it from falling completely through the linear bearing.
The hole sizes are the nearest inch drills matching the pen’s hard metric sizes:
While I was at the lathe, I turned another layer of epoxy on the printed holder down to a consistent 11.95+ OD. It fits the bearing nearly as well as the steel shaft, although it’s not quite as smooth.
The steel version weighs about 20 g with the pen, so it applies about the same downforce on the pen nib as the HP 7475A plotter. The force varies from about 19 g as the Z axis moves upward to 23 g as it move downward, so the stiction amounts to less than 10% of the weight:
However, the more I ponder this setup, the less I like it.
When the Z-axis moves downward and the nib hits the paper, it must decelerate the weight of the pen + holder + ballast within a fraction of a millimeter, without crushing the nib. If the pen moves downward at 3000 mm/min = 50 mm/s, stopping in 0.3 mm requires an acceleration of 4.2 m/s² and a 20 g = 2/3 oz mass will apply 0.08 N = 0.3 oz to the nib. Seems survivable, but smashing the tip a few hundred times while drawing the legends can’t possibly be good for it.
Also, the tool length probe switch trips at 60 (-ish) g, which means the pen can’t activate the switch. Adding a manual latch seems absurd, but you can get used to anything if you do it enough.
Mary mentioned the pivot pin supplied with a quilting ruler tended to hang up on the layers of fabric and batting in the quilt squares she’s been making. A quick look showed the pin bore a remarkable resemblance to an ordinary thumb tack:
I reset the pin shaft perpendicular to the head, grabbed a small brass tube in the lathe tailstock, inserted pin in tube, grabbed the head in the chuck, ignored a slight radial offset, and attacked the pin with fine files and sandpaper:
The lathe chuck seemed the easiest way to firmly hold the head; I rotated the chuck by hand while filing.
Most of the remaining scratches go mostly parallel to the pin, but it really didn’t work much better than before. We decided polishing the pin wouldn’t improve the situation enough to make it worthwhile.
That’s the difference between sharp and keen, which cropped up with the cheap ceramic knife from a while ago. The point may penetrate the fabric, but the shaft can’t get through the tight weave.
She’s now using a scary thin and pointy embroidery pin, having successfully rebuffed my offer to mount it in a suitable base.
The MAXTEMP error killing the M2 while printing the bar clamp mounts (probably) came from a short in the thermistor pellet that lowered the thermistor resistance and raised the calculated temperature. I manually heated the extruder and, although the temperature stabilized at 250 °C, the history plot showed irregular downward jogs from increasing resistance. Whenever this constellation of symptoms appears on the M2 forums, I always recommend ordering another thermistor or two, so …
Start by turning a 1/8 inch OD brass tube down to 3.00 mm, parting off a suitable length, facing the ends:
Countersink the ends just for pretty.
The tube should be a slip fit in the hot end:
While I had the hot end on the bench, I scuffed the nozzle to remove (most of) the baked-on crud:
The plan is to seal the thermistor bead inside the tube with JB Weld epoxy, which I’ve verified (!) to work at extrusion temperatures, depending on the epoxy to insulate the wiring and immobilize all the pieces.
Harvest the original wire harness from the defunct thermistor, solder to the bead, lay out guide lines:
Slobber epoxy over everytyhing, fill the tube, insert bead into tube, stabilize with tape:
Verify connectivity through the thermistor and isolation from the brass tube, then return upstairs to
warm up thaw out while the epoxy cures.
At this point, the observant reader should be thinking “Uh, Ed, that bead looked a tad large. Are you absolutely sure … ?”
Halfway up the basement stairs I realized I’d meticulously entombed a 10 kΩ thermistor, not the 100 kΩ thermistor used in the M2’s hot end. You can easily verify the resistance, as I did, with a quick web search; I have hella-good SEO for some specific topics.
Back to the lab …
Fortunately, JB Weld has a pot life over an hour, so extract the wrong bead, unsolder, install the right thermistor using snippets of insulation harvested from the original wiring, realign components:
Re-verify resistances, return upstairs, fast-forward through the night, have another good idea …
For reasons not relevant here, I had to wake up a New Old Stock bottle of an eye drug dispensed as a suspension in a small bottle:
Two more umbrella struts snapped and required the same repair, but, having drained all the suitable snippets from the Box o’ Brass Cutoffs, some lathe work was in order:
I used the carbide insert in the mistaken belief it’d be less grabby, then applied the cutoff tool.
Break the edges, slide splints over the ribs, slobber epoxy on the struts, slide splints into place, apply masking tape for a bit of compression & alignment, and let it cure:
Three down, five to go …
Pure almond butter comes with the somewhat stilted admonition “Must stir product. Oil separation occurs naturally.” I’d just opened a new jar and was busily (and laboriously) stirring when I realized we have the technology:
I installed the chuck’s outside jaws to grab the jar lid.
About three hours at 50 rpm, the lathe’s slowest speed, did the trick. We now have the smoothest, creamiest, best-mixed almond butter ever.
In a month or so, I’ll chuck up an unopened jar to see how well it works without any manual intervention.