Every now and again, an upshift to the large chainring on my Tour Easy would go awry and drop the chain off the outside, where it would sometimes jam between the pedal crank and the spider. In the worst case the flailing chain would also jam in the TerraCycle idler, but I fixed that a while ago.
Contemporary chainrings (i.e., anything made since the trailing decades of the last millennium) generally have a chain drop pin positioned against the crank specifically to prevent such chain jamming.
Making a chain drop pin is no big deal if you’ve got a lathe and an M4 tap:
A closer look:
That’s a 10 mm length of 5/16 inch brass rod drilled with a recess to fit the head of a 10 mm M4 socket-head cap screw.
The pin should be a micro-smidgen shorter, as it just touches the crank, but, if anything, moving the chainring inward by one micro-smidgen improved the upshifts and I’m inclined to go with the flow.
Making it fit required enlarging an existing recess to fit the spindle plate, a straightforward lathe job with the plate grabbed in the 3 jaw chuck’s outer jaws:
Carbide inserts don’t handle interrupted cuts very well, but sissy cuts saved the day. The plate is kinda-sorta cast iron, so the “chips” are dust and a vacuum snout reduces the mess; you can see some chips inside the bore.
A faceplate for the mini-lathe lathe located three holes matching the spindle plate, after I noticed the amazing coincidence of both parts having 26 mm bores. Making an alignment tool from a scrap of 3/4 inch (!) Schedule 40 PVC pipe was an easy lathe job:
Transfer-punching those holes produced pips on the chuck side of the adapter plate:
I thought about freehanding the holes, but came to my senses:
Of course, the Sherline lacks enough throat for the plate, so each hole required clamping / locating / center-drilling / drilling / finish drilling. With all three drilled, hand-tapping the threads was no big deal:
Those are M8×1.25 studs from LMS (although the ones I got look like the 30 mm version), with the long end sunk in the adapter plate to put the other end flush with the nut on the far side of the spindle plate:
And then it fits just like it grew there, although the jaws don’t have much clearance inside the interlock cover:
Now I’m ready for the next set of coasters and, if the jaws stick out too far, I can gimmick the interlock switch for the occasion.
The description of the 4 inch chuck seems inconsistent with its listed dimensions, which may be why I ended up with the larger chuck in the first place. You can never have enough chucks: all’s well that ends well.
Lacking a 4-jaw chuck for the lathe, this should suffice:
Which is just the Sherline 4-jaw chuck chucked in the lathe’s 3-jaw chuck, with both chuck Jaw 1 positions lined up and marked on the acrylic disk fixture. The picture is a recreation set up after the fact, because I lack a good picture of the overall scene.
Now it’s easy enough to center the fixture, stick the coaster in place with reasonable accuracy, then tweak the Sherline chuck to center the coaster:
Because the bottom layer is a laser-cut disk, eyeballometrically aligning its edge to a simple pointer worked surprisingly well:
Turning the OD down to match the bottom disk meant I could finally get decent results with zero drama:
From the bottom, this one has a 3 mm mirror, the 3 mm fluorescent green frame + petals, and a 1.6 mm top sheet:
If I never tell anybody, they’ll think the slightly granular look if the tape was deliberate; it looks OK to me.
And, for completeness, the crash test dummy from the start of this adventure:
I don’t know how to avoid the bubbles, as the usual torch-the-top and pull-a-vacuum techniques pop bubbles at the epoxy-air interface. These bubbles are trapped under the top acrylic sheet, even though I was rather painstaking about easing the layer down from one side to the other while chasing bubbles along.
Maybe I can define bubbles as Part of the Art?
Definitely fancier than chipboard, although not nearly as absorbent.
A first pass with an optical wipe removed most of the crud:
Gentle touch-up with a little more isopropyl alcohol cleared the rest:
The focus lens required similar attention, but there is no way to get meaningful pictures of a transparent lens.
Realigning the mirrors went well (top before, middle during, lower after):
The diagonal results at Mirror 3 show the XY axes aren’t quite square, but AFAICT it’s close enough. The rightmost tape shows good beam centering in the nozzle and the Focus target shows excellent Z alignment over about 50 mm of travel.
AFAICT, that’s the default layout for all similar power supplies.
The H and L pins are the High- and Low-active enable inputs that, when it’s working right, control the laser output. The KT332 controller (and, most likely, all RuiDa controllers) produce a low-active output, so you just wire the controller’s output to the L input and you’re done.
That was the original failure that got me to this point: the power supply ignored its L input and turned the beam on at whatever power the PWM signal on the IN terminal called for. Having that happen was surprising, having it happen with the cabinet lid open was … disturbing.
The P input is intended for the Water Protect signal from the flow sensor on the laser cooling plumbing. When the water is flowing, the IN terminal will be low and the power supply will pay attention to the L input.
The power supply arrived with a jumper between the P input and the G ground / common terminal:
The jumper holds the P input low = active, meaning the power supply thinks the water is always flowing.
It turns out that the Water Protect signal goes only to the controller. When it’s inactive = no water flowing, the controller will refuse to fire the laser and also sound an alarm. Running the signal directly to the power supply would result in a puzzling failure-to-fire with no diagnostic from the controller.
I removed that jumper and added a (green) wire from the Lid Interlock signal at the controller:
To the power supply’s P input:
In principle, if this power supply fails the same way as the previous one (with its L input always active), then at least it won’t fire with the lid up.
Believing that may display a childish naivety, but at least the thing seems marginally safer than it was before.
So I clamped it to the Sherline’s tooling plate and milled off the rim:
Given the Sherline’s cramped work envelope, all the action took place along the rearmost edge, requiring eight reclampings indexed parallel to the table with a step clamp.
The cutter cleared off everything more than 0.3 mm above the surface of the glass chunks. I could probably have gone another 0.1 mm lower, but chopping the bit into the edge of a shattered glass fragment surely wouldn’t end well.
Polishing the dark gray milled surface might improve it slightly, at the risk of scuffing whatever poured epoxy stands slightly proud of the glass:
Perhaps if I define it to be a border, everybody will think it was intentional.
The petals stand slightly proud of the black top frame, as the colored sheets were marginally thicker than the black sheet, but it looks OK in person. They’re all epoxied to a transparent base plate, so the bottom view is pretty much the same:
Because the bottom is perfectly smooth, I think it looks better than the top, which shows irregularities around the petals where the epoxy didn’t quite fill the gaps. There is one small bubble you won’t notice if I don’t tell you about it.
I laid a small bead of epoxy around the perimeter of the base, laid the black frame in place, ran a bead along the midline of each petal shape plus a drop in the round part, laid the petals in place, and hoped I didn’t use too much epoxy. It turned out all right, with only a few dribbles down the edge that wiped off easily enough.
I peeled the protective plastic off the top while the epoxy was still tacky, which pulled far too many fine filaments across the surface:
After the final cure, I managed to scrape most of them off with a thumbnail; I hope to never make that mistake again.
As you might expect, acrylic plastic’s pure saturated colors wipe the floor with Sharpie-scribbled white chipboard:
The black frame makes the whole thing overly dark, so the next attempt should use white or perhaps a transparent layer atop a mirror base.