Archive for category Machine Shop
Once again, the single moving part on my first-generation Kindle Fire stopped working. As before, the switch contacts accumulated enough fuzz & contamination to prevent any current flow, but this time the (soft) solder joints attaching the switch body to the PCB failed:
My joint cleaning & fluxing wasn’t up to contemporary standards, as shown by the obviously un-fused footprints left in the upper pads:
The switch frame seems to be unplated steel, which shouldn’t be an excuse.
So I dismantled the switch, cleaned the contacts and tactile bump plate, put it all back together, and did a much better job of surface preparation:
The other joint:
And, for completeness, the switch leads:
I don’t like the way the joint on the right looks, either, but we’ll see how long the whole affair holds together.
This may be the last time I can repair the Kindle, as a bypass cap came loose while I was working on the PCB, the screen has been accumulating dust at an increasing pace, and several latches securing the back of the case have cracked.
Methinks it’s getting on time for a new pocketable memory device; if only Pixel XL phablets had a bigger screen and didn’t cost night onto a kilobuck.
The Zire would power on whenever the switches clicked or that little joystick moved, which happened regularly enough to be annoying.
Mary made a small case that matched the other pouches I carry around:
She made the case to fit an HP48 calculator, but it was close enough for the Zire.
Time passed, the Zire died, I started carrying a Kindle Fire in another pocket, but the ABS slab provided a convenient stiffener between some Geek Scratch Paper and the various pencils / pens / markers / screwdrivers / flashlight filling the available space.
Unfortunately, minus the backup of an electronic slab, the protector finally failed along an obvious stress riser:
I cut a similar rectangle from a sheet of unknown flexy plastic, rounded the corners, clipped the pencils & whatnot to it, and maybe it’ll survive for a while.
A bag of sub-one-dollar resistive joysticks arrived from halfway around the planet:
A quick-and-dirty test routine showed the sticks start out close to VCC/2:
Welcome to minicom 2.7 OPTIONS: I18n Compiled on Feb 7 2016, 13:37:27. Port /dev/ttyACM0, 10:23:45 Press CTRL-A Z for help on special keys Joystick exercise Ed Nisley - KE4ZNU - May 2017 00524 - 00513 - 1
minicom on the serial port, as the Arduino IDE’s built-in serial monitor ignores bare Carriage Return characters.
The joystick hat tilts ±25° from its spring-loaded center position, but the active region seems to cover only 15° of that arc, with a 5° dead zone around the center and 5° of overtravel at the limits. This is not a high-resolution instrument intended for fine motor control operations.
The analog input values range from 0x000 to 0x3FF across the active region. Aim the connector at your tummy to make the axes work the way you’d expect: left / down = minimum, right / up = maximum.
delay(100) statements may or may not be needed for good analog input values, depending on some imponderables that seem not to apply for this lashup, but they pace the
loop() to a reasonable update rate.
Pushing the hat toward the PCB activates the simple switch you can see in the picture. It requires an external pullup resistor (hence the
INPUT_PULLUP configuration) and reports low = 0 when pressed.
Those are 0.125 inch (exactly!) holes on a 19.5×26.25 mm grid in a 26.5×34.25 mm PCB. Makes no sense to me, either.
The trivial Arduino source code as a GitHub Gist:
Having figured the mixing ratios, found the mixing trays, and donned my shop apron, I buttered up several iterations of the badge reel case to see how XTC-3D epoxy works on the little things around here.
In all cases, I haven’t done any sanding, buffing, or primping, mostly because I’m not that interested in the final surface finish.
A single coat produces a glossy finish with ripples from the printed threads:
Seen straight on, without the glare, a little speck toward the lower right corner shows that cleanliness is next to impossible around here:
An additional coat atop a Hilbert-curve upper surface comes out somewhat smoother:
Another view, with less glare, shows the pattern a bit better:
With no glare, the 3D Honeycomb infill shows through the surface:
Coating the surface with epoxy definitely makes it more transparent / less translucent by filling in the air gaps.
The sides of that part have only one coat and still show typical 3D printed striations.
Three coats wipe out the striations, along with all other surface detail:
The bolt head recesses collected enough epoxy to require reaming / milling, which certainly isn’t what you want in that situation. The bolt holes also shrank, although my usual hand-twisted drill would probably suffice to clear the epoxy.
Another view shows a glint from the smooth surface filling the upper-right recess:
Three coats definitely hides the 3D printed threads, although you can see some ridges and edges:
The epoxy isn’t perfectly self-leveling, probably due to my (lack of) technique:
Blowing out the contrast shows the surface finish:
Those scratches come from fingernails, after the overnight curing time. The surface is hard, but not impervious to scratching, which is about what you’d expect for a clear epoxy.
Slightly over-thinning the XTC-3D with denatured alcohol in a 0.7 : 0.3 : 0.3 by weight ratio produced a watery liquid that penetrated directly into the surface:
The finish depends critically on what’s below the surface and how much epoxy you apply. I tried to spread it uniformly with a foam brush, but the center came out somewhat rougher than the outer edge:
The striations along the sides filled in a bit, but surely not enough to satisfy anybody who worries about such things.
A specular reflection shows the changing surface smoothness:
Perhaps two coats of thinned epoxy would produce a watertight / airtight part, without changing the overall dimensions by very much. The mechanical properties depend almost entirely on the plastic-to-plastic bond, so I doubt a thin epoxy layer would improve its pressure-handling capabilities.
Few of the parts I make will benefit from an epoxy coating and I definitely don’t want to get into post-processing the parts just to improve their looks!
Schatz Manufacturing, a major bearing producer in Poughkeepsie, made a sample case to show off their wares:
You can tell by the yellowed backing paper that these have been around for a looong time.
It turns out that Poughkeepsie had two bearing manufacturers. Federal Bearings went into the products of other locally important industries:
A detailed look shows what was important, back in the day:
Out in the garage I still have a few grease pilots (*) from the final Schatz Federal downsizing / going-out-of-business / moving / whatever sale. A friend bought several sets of heavy-duty steel chests-of-drawers which contained, very much to his surprise, a huge assortment of grease pilots, ranging in size from fit-on-your-thumb to cover-a-dinner-plate, which he obviously had no use for. He unloaded them on me with a phrase that has lived on forevermore:
They’re a buck apiece, unless you take all of them, in which case they’re free.
You’ll find the sample cases on the top floor of Adriance Library, should you ever be in town.
Taken handheld in ambient light to avoid harsh flash shadows, then perspective-distorted to make them look like I was standing directly in front of the reflective plastic covers.
(*) Different from a “pilot bearing”. A “grease pilot” is a two-part circular steel assembly used to inject grease into the bearing races before snapping the shields in place. They’re painstakingly machined to cup the balls and fill the gaps, with a pipe fitting on the back surface for the grease pump.
The striations inherent in the DIY-grade 3D printing process don’t bother me all that much, but I got some XTC-3D epoxy to see what I’ve been missing. The impressive scrap pile from the badge reel holder provided test pieces:
Weighing the components seems the least-awful way to get the small quantities I need. The instructions recommend 100:43 by weight of resin (A) : hardener (B):
For these tiny parts, 2 g + 0.9 g was way too much and 1 g + 0.4 g seemed entirely adequate. If you slowly drool resin into the pan, drool slightly less than half that much hardener, and it’ll be about as close as you can get. The hardener is much less viscous than the resin: drool carefully.
The stuff might be self-leveling on larger parts, but on these small surfaces it’s better (IMO) to dry-brush multiple layers: you can see thicker and thinner sections in the first picture. The recoat time runs about 1-½ h.
The instructions recommend acetone or denatured alcohol as a thinning agent, at 10 to 25% of the resin volume, with curing times up to 24 h. Alcohol seems less likely to produce Bad Results, because it won’t evaporate instantly. Neither will affect PETG, but if you’re using another plastic, keep its solvent list in mind.
I tried alcohol with a by-weight amounts around 0.7 : 0.3 : 0.3 g, obviously overshooting both the hardener and the alcohol by a few drops. The end result resembled thick water, brushed on easily, and penetrated the surface easily.
A first coat of thinned epoxy should fill voids and unify the surface without changing the dimensions very much, with subsequent coats leveling the striations.
More pix after more layers and more curing …
So: jouncing over the larg(er) potholes / pavement discontinuities / debris on the roads around here wobbulates the front fender enough to pull the stays out of those tidy 18 mm = 6 diameter deep sockets on the fender clip.
Perhaps a generous application of heatshrink tubing will help:
Waving a heat gun around a 3D printed part seems fraught with peril, even with PETG’s glass transition temperature around 80 °C = 175 °F, as ordinary polyolefin tubing shrinks at 140-ish °C. Aiming the hot air stream more-or-less away from the clip (and the tire!) carried the day. PLA would surely have gotten bendy.
The proper solution surely involves screw clamps and suchlike. I really dislike fiddly hardware: I hope this hack survives.