The Smell of Molten Projects in the Morning

Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.

Tag: Repairs

If it used to work, it can work again

  • Plastic Screw-top Flashlight Fix

    As part of my clear-off-the-workbench effort, this flashlight emerged from the dark depths. It’s a few decades old and wasn’t a good design: the “switch” is a simple contact between the end of the cell casing and the reflector rim, activated by screwing the reflector tighter on the case.

    Broken flashlight case
    Broken flashlight case

    The failure is simple: the case cracks through at the stress raiser formed where the “switch” contacts rest on a sharp inside corner. That stiff little spring maintains pressure on the cells, so the case is always under tension and eventually fractures.

    Flashlight clamped in mill
    Flashlight clamped in mill

    I grabbed the broken pieces in the lathe, turned off the fractured plastic, and wound up with a pair of nicely mating surfaces (and a somewhat shorter flashlight, but it’s still long enough). Apply enough Plastruct solvent glue to soften the new faces, then clamp them together. The big manual mill knows how to apply a strong, steady vertical force to a project like this.

    It’s once again hanging by the basement door, where it gets used roughly once every other blue moon (yeah, it’s color-coordinated). This isn’t the first time this flashlight has failed that way, but it’ll be the last: next time, it’s in the trash.

    Honest, I swear it!

  • Screwdriver Bit Ball Repair: Rubberdraulics!

    Went to use a small multi-bit screwdriver and the bit fell right out: evidently, the ball wasn’t swaged tightly enough; it and the spring went walkabout. Given that I don’t know when or where that might have happened, there’s no chance I’ll ever see those parts again.

    Screwdriver bit with missing ball
    Screwdriver bit with missing ball

    But I do have some 2 mm steel bearings that aren’t grossly oversized, so all hope is not lost. Alas, I have no idea what sort of spring to put in there, other than that I don’t have one of those.

    Drilled hole with ball
    Drilled hole with ball

    This looks like an application for rubberdraulics: use compliant silicone snot rubber as a spring. Lautard described a use with a lock ring and an external screw to apply pressure, but here it’ll work fine to allow a small motion for a tiny ball.

    Drill out the recess barely larger than the ball: the slight clearance allows the cured rubber to squish out around the ball. I clamped it in the Sherline vise and jogged into position by eyeball, then poked a hole with G83 down 1.5 mm. The original recess was a bit over 2 mm deep, so there’s plenty of room for the silicone in the bottom.

    Then mush some silicone into the hole, install the ball, push it down until it stands barely proud of the surface, scrape off the excess rubber, and let it cure overnight.

    New ball in place
    New ball in place

    There, now, that wasn’t so bad, was it?

  • AA Cell Holder: Fragile Contacts

    Broken cell holder contact
    Broken cell holder contact

    It seems I applied a bit too much pressure to one of the contacts on a metal AA cell holder: the outer rim of the rivet holding the solder tab in place departed for the distant reaches of the Basement Laboratory.

    No big deal, I thought: pop another rivet in place and get back in operation…

    You really want the rivet to go in with the flat head inside the cell holder where the original flat head was. Unfortunately, the rivet yanker’s head won’t fit into the holder; I’m pretty sure the manufacturer has a Special Machine to make that happen.

    So I put the reinforcing washer and lumpy end inside. That meant switching the insulating washers to keep the overall distance from the negative cell contact about the same.

    Cell holder rivet - inside
    Cell holder rivet – inside

    The outside looks much better…

    Cell holder rivet - outside
    Cell holder rivet – outside

    For what it’s worth, these pix came from the Sony DSC-H5 with the flash turned down 1 EV. Much better results than the Casio EX-Z850, even with its flash set to Soft (whatever that is). The H5 has much better macro capability… and with the new Eneloop cells, it lasts long enough to make it usable in the shop.

  • Door Stop Bumper Fix

    After slightly over half a century, the rubber bumpers on the doorstops around the house have stiffened up and, occasionally, one falls off.

    Although I suppose I should just buy a new doorstop, molding a dab of silicone snot around the end of the nice brass post takes only a few minutes (plus an overnight cure). If what they tell us about silicone adhesives is true, this one is good until the sun goes dark…

    Re-bumpered door stop
    Re-bumpered door stop

    Another no-CNC repair!

  • Broken Tap Removal: The CNC Way

    Having successfully drilled and tapped eight 4-40 holes for the MOSFETs and two 8-32 holes for the heatsink clamps, I needed four more holes for the 6-32 standoffs that will mount the heat spreader to the base. As is always the case, the tap broke in the next-to-last hole…

    Broken tap
    Broken tap

    This is a three-flute tap, the break is recessed below the surface, and it looks like it’s cracked along one of the flutes. Bleh! I don’t have any tap extractors, mostly because I don’t do that much tapping, and I doubt the extractors work all that well on tiny taps.

    I tried something I’d never done before: slit the top of the tap with an abrasive wheel and unscrew it. That didn’t work, of course, but it’s a useful trick to keep in mind. I think the tap was cracked lengthwise and, in any event, a three-flute tap doesn’t have the proper symmetry for a slot. Better luck with larger four-flute taps.

    Slotted tap
    Slotted tap

    So I must dig the mumble thing out…

    Starting the moat
    Starting the moat

    The overall plan:

    • Clamp the heat spreader to the Sherline tooling plate
    • Helix-mill a trench around the tap
    • Grab the stub with Vise-Grips
    • Unscrew it
    • Repair the damage

    The clearance hole for a 6-32 screw is 0.1405 inch and that’s a 3/16-inch end mill: 70 + 93 = 163 mil radius, call it 0.170 inch. You really don’t want to kiss the tap flutes with the end mill, so you could make that the ID a bit larger.

    Manual CNC, feeding commands into Axis and using the history list to chew downward 20 mils on each pass. With the origin in the middle of the broken tap and the cutter starting at (-0.170,0), the code looks like:

    G2 I+0.170 Z=-0.020
G2 I+0.170 Z=-0.040
... and so on ...

    About 3000 rpm and 2 inches per minute feed; the feed was too slow, because the aluminum chips were much too fine. I actually used cutting lube for this job: the heat spreader got nice and warm.

    Coolant
    Coolant

    I stopped at Z=-0.100 and made a final pass around the bottom of the hole to clean out the ramp. Then, try unscrewing the tap…

    Tap stub - first attempt
    Tap stub – first attempt

    Of course, the stub broke off more or less flush with the bottom of the hole, so I continued milling downward to Z=-0.260, a bit more than halfway through the plate. This time, the needle-nose Vise-Grips got a good grip on an uncracked section and the remains twisted out with very little effort.

    Grabbing the stub
    Grabbing the stub

    Although the central pillar is outside the tap’s OD, leaving a solid aluminum shell, there’s not much meat to it. The shell broke off with the first twist and came out with the tap.

    Those are not, by the way, gold-plated Vise-Grips. It’s a flash picture and the worklight is a warm-white compact fluorescent: the color correction that makes the aluminum look neutral gray turns the reflected CFL into gold.

    Aligning replacement nuts
    Aligning replacement nuts

    I milled off the remains of the shell around the tapped hole, leaving a more-or-less flat bottom. If I cared enough, I’d machine a snug-fitting replacement aluminum plug, epoxy it into place, then (attempt to) drill-and-tap the hole again.

    Instead, because the hole was deep enough for a pair of 6-32 nuts and a washer, I simply aligned those on a screw and filled the hole with JB Weld epoxy.

    It doesn’t show in the picture, but the screw is well-lubricated with silicone grease to prevent it from becoming one with the nuts.

    I eased epoxy into the recess, chasing out the inevitable air bubbles, and then scraped off most of the excess.

    Epoxy fill
    Epoxy fill

    Let it cure overnight, scrub it on some sandpaper atop the sacrificial side of the surface plate, and it’s all good again…

    Sanded flat
    Sanded flat

    The little finger of epoxy sticking out to the front fills the end of the slit I carved into the top of the tap, which is visible in the other pictures if you look closely. The area around the hole isn’t stained; that’s smooth epoxy.

    Of course, the thermal conductivity of epoxy is a lot less than that of solid aluminum. I’m not really pushing the limits of TO-220 packages, so this kludge will work fine in this application. It’s also nice that the repair is on the bottom of the heat spreader, where nobody will ever know I screwed up…

    Now, to return to the project at hand, with even more motivation to avoid tapping holes in the future!

  • Arduino Mega: Showstopper Workaround

    The discussion following that post gave me enough impetus to figure this out. What I have here is not a complete solution, but it seems to solve the immediate problem.

    Downside: this will not survive the next regular system update that touches the gcc-avr package (yes, it’s the avr-gcc compiler and the gcc-avr package). Hence, I must write down the details so I can do it all over again…

    To review:

    The problem is that the avr-gcc cross-compiler produces incorrect code for Atmega1280-class chips with more than 64 KB of Flash space: a register isn’t saved-and-restored around a runtime routine that alters it. Simple sketches (seem to) run without problems, but sketches that instantiate objects crash unpredictably. Because Arduino sketches depend heavily on various objects (like, oh, the Serial routines), nontrivial sketches don’t work.

    The workaround is to patch the library routine that invokes the constructors, as detailed in that gcc bug report, to push / pop r20 around the offending constructors. The patch tweaks two spots in the libgcc.S source file, which then gets built into an assortment of chip-specific libgcc.a files during the compile.

    I was highly reluctant to do that, simply I’ve already installed the various gcc packages using pacman (the Arch Linux package manager) and really didn’t want to screw anything up by recompiling & reinstalling gcc from source. It’s certainly possible to update just the avr portion, but I don’t know exactly how to do that and doubt that I could get it right the first time… and the consequences of that catastrophe I don’t have time to deal with.

    So I elected to build the avr cross-compiler from source, verify that the as-built libgcc.a file was identical to the failing one, apply the patch, recompile, then manually insert the modified file in the right spot(s) in my existing installation. This is less manly than doing everything automagically, but has a very, very limited downside: I can easily back out the changes.

    Here’s how that went down…

    The instructions there (see the GCC for the AVR target section) give the overview of what to do. The introduction says:

    The default behaviour for most of these tools is to install every thing under the /usr/local directory. In order to keep the AVR tools separate from the base system, it is usually better to install everything into /usr/local/avr.

    Arch Linux has the tools installed directly in /usr, not /usr/local or /usr/local/avr, so $PREFIX=/usr. Currently, they’re at version 4.5.1, which is typical for Arch: you always get the most recent upstream packages, warts and all.

    Download the gcc-g++ (not gcc-c++ as in the directions) and gcc-core tarballs (from there or, better, the gnu mirrors) into, say, /tmp and unpack them. They’ll both unpack into /tmp/gcc-4.5.1, wherein you create and cd into obj-avr per the directions.

    I opted to feed in the same parameters as the Arch Build System used while installing the original package, rather than what’s suggested in the directions. That’s found in this file:

    /var/abs/community/gcc-avr/PKGBUILD

    Which contains, among other useful things, this lump of command-line invocation:

    ../configure --disable-libssp \
               --disable-nls \
               --enable-languages=c,c++ \
               --infodir=/usr/share/info \
               --libdir=/usr/lib \
               --libexecdir=/usr/lib \
               --mandir=/usr/share/man \
               --prefix=/usr \
               --target=avr \
               --with-gnu-as \
               --with-gnu-ld \
               --with-as=/usr/bin/avr-as \
               --with-ld=/usr/bin/avr-ld

    Yes, indeed, $PREFIX will wind up as /usr…

    Feeding that into ./configure produces the usual torrent of output, ending in success after a minute or two. Firing off the make step is good for 15+ minutes of diversion, even on an 11-BogoMIPS dual-core box. I didn’t attempt to fire up threads for both cores, although I believe that’s a simple option.

    The existing compiler installation has several libgcc.a files, each apparently set for a specific avr chip:

    [ed@shiitake tmp]$ find /usr/lib/gcc/avr/4.5.1/ -name libgcc.a
/usr/lib/gcc/avr/4.5.1/libgcc.a
/usr/lib/gcc/avr/4.5.1/avr35/libgcc.a
/usr/lib/gcc/avr/4.5.1/avr3/libgcc.a
/usr/lib/gcc/avr/4.5.1/avr51/libgcc.a
/usr/lib/gcc/avr/4.5.1/avr4/libgcc.a
/usr/lib/gcc/avr/4.5.1/avr6/libgcc.a
/usr/lib/gcc/avr/4.5.1/avr5/libgcc.a
/usr/lib/gcc/avr/4.5.1/avr31/libgcc.a
/usr/lib/gcc/avr/4.5.1/avr25/libgcc.a

    The key to figuring out which of those files need tweaking lies there, which says (I think) that the Atmega1280 is an avr5 or avr51. Because I have an Arduino Mega that’s affected by this bug, I planned to tweak only these files:

    /usr/lib/gcc/avr/4.5.1/libgcc.a
/usr/lib/gcc/avr/4.5.1/avr51/libgcc.a
/usr/lib/gcc/avr/4.5.1/avr5/libgcc.a

    I have no idea what the top-level file is used for, but … it seemed like a good idea.

    Now, I innocently expected that the libgcc.a files for a 4.5.1 installation would match the freshly compiled files for a 4.5.1-from-source build, but that wasn’t the case. I don’t know what the difference might be; perhaps there’s an embedded path or timestamp or whatever that makes a difference?

    The Arch Linux standard installation of gcc 4.5.1 has these files:

    $ find /usr/lib/gcc/avr/4.5.1/ -iname libgcc.a -print0 | xargs -0 ls -l
-rw-r--r-- 1 root root 2251078 Sep  4 16:26 /usr/lib/gcc/avr/4.5.1/avr25/libgcc.a
-rw-r--r-- 1 root root 2256618 Sep  4 16:26 /usr/lib/gcc/avr/4.5.1/avr31/libgcc.a
-rw-r--r-- 1 root root 2252506 Sep  4 16:26 /usr/lib/gcc/avr/4.5.1/avr35/libgcc.a
-rw-r--r-- 1 root root 2256310 Sep  4 16:26 /usr/lib/gcc/avr/4.5.1/avr3/libgcc.a
-rw-r--r-- 1 root root 2250930 Sep  4 16:26 /usr/lib/gcc/avr/4.5.1/avr4/libgcc.a
-rw-r--r-- 1 root root 2251846 Sep 27 12:58 /usr/lib/gcc/avr/4.5.1/avr51/libgcc.a
-rw-r--r-- 1 root root 2251550 Sep 27 12:58 /usr/lib/gcc/avr/4.5.1/avr5/libgcc.a
-rw-r--r-- 1 root root 2252458 Sep  4 16:27 /usr/lib/gcc/avr/4.5.1/avr6/libgcc.a
-rw-r--r-- 1 root root 2251474 Sep 27 12:57 /usr/lib/gcc/avr/4.5.1/libgcc.a

    The compilation-from-source using the gcc 4.5.1 tarballs has these files:

    $ pwd
/tmp/gcc-4.5.1/obj-avr
$ find -iname libgcc.a -print0 | xargs -0 ls -l
-rw-r--r-- 1 ed ed 2250258 Sep 27 15:51 ./avr/avr25/libgcc/libgcc.a
-rw-r--r-- 1 ed ed 2255798 Sep 27 15:51 ./avr/avr31/libgcc/libgcc.a
-rw-r--r-- 1 ed ed 2251686 Sep 27 15:51 ./avr/avr35/libgcc/libgcc.a
-rw-r--r-- 1 ed ed 2255490 Sep 27 15:51 ./avr/avr3/libgcc/libgcc.a
-rw-r--r-- 1 ed ed 2250110 Sep 27 15:51 ./avr/avr4/libgcc/libgcc.a
-rw-r--r-- 1 ed ed 2251838 Sep 27 15:51 ./avr/avr51/libgcc/libgcc.a
-rw-r--r-- 1 ed ed 2251550 Sep 27 15:51 ./avr/avr5/libgcc/libgcc.a
-rw-r--r-- 1 ed ed 2251638 Sep 27 15:52 ./avr/avr6/libgcc/libgcc.a
-rw-r--r-- 1 ed ed 2251474 Sep 27 15:52 ./avr/libgcc/libgcc.a
-rw-r--r-- 1 ed ed 2250258 Sep 27 15:51 ./gcc/avr25/libgcc.a
-rw-r--r-- 1 ed ed 2255798 Sep 27 15:51 ./gcc/avr31/libgcc.a
-rw-r--r-- 1 ed ed 2251686 Sep 27 15:51 ./gcc/avr35/libgcc.a
-rw-r--r-- 1 ed ed 2255490 Sep 27 15:51 ./gcc/avr3/libgcc.a
-rw-r--r-- 1 ed ed 2250110 Sep 27 15:51 ./gcc/avr4/libgcc.a
-rw-r--r-- 1 ed ed 2251838 Sep 27 15:51 ./gcc/avr51/libgcc.a
-rw-r--r-- 1 ed ed 2251550 Sep 27 15:51 ./gcc/avr5/libgcc.a
-rw-r--r-- 1 ed ed 2251638 Sep 27 15:52 ./gcc/avr6/libgcc.a
-rw-r--r-- 1 ed ed 2251474 Sep 27 15:52 ./gcc/libgcc.a

    The top-level files have the same size, but are not identical:

    $ diff ./avr/libgcc/libgcc.a ./gcc/libgcc.a
Binary files ./avr/libgcc/libgcc.a and ./gcc/libgcc.a differ

    Haven’t a clue what’s going on with different files in different spots, but I saved the existing files in the installed tree as *.base and copied the new ones from ./gcc/avr* into place. While there are many ways to crash a program, the AnalogInOutSerial demo program ran correctly on a Duemilanova (presumably with the existing libgcc.a) and failed on the Mega (with the recompiled libgcc.a). Save those files as *.rebuild just in case they come in handy.

    Manually change the libgcc.S source file (it’s only four lines, I can do this), recompile, and the build process recompiles only the affected files; that’s comforting. Copy those into the installed tree and, lo and behold, the demo program now runs on both the Duemilanova and the Mega.

    While it’s too soon to declare victory, the hardware bringup program I’m writing also works, so the initial signs are good.

    Thanks to Mark Stanley for blasting me off dead center on this. I didn’t do a complete install, but he got me thinking how to make the least disruptive change…

    And a tip o’ the cycling helmet to the whole Free Software collective for making a mid-flight patch like this both feasible and possible: Use The Source!

  • Tour Easy: Underseat Pack Repair Finished

    So, after a bit more than a year, I replaced the cracked backing plate in the other ERRC underseat pack on my Tour Easy. The first plate held up much better than I expected: hasn’t cracked or poked through the pack fabric.

    This repair followed the same outline, including cutting off the ripped netting on the outside of the pack and marching the pack into the clothes washer for a spin with a few shop rags. Reassembled everything, put it back on the bike, and … the new aluminum extrusion across top  of the plate smacked firmly into the water bottle holder clamped to the rear of the seat frame for the amateur radio.

    Underseat pack vs radio holder
    Underseat pack vs radio holder

    The extrusion is the lump running horizontally, just under the seat cushion. The corner of the pack extended rearward (left) of the water bottle holder’s black plastic body.

    The original flexy plastic pack plate simply bent out of the way, but that’s not going to work now.

    So I loosened the clamp, moved it a bit more to the right, and tightened it up again. I’d originally located it at the far right end of the straight part of the seat frame, so it’s now edging into the curved part that eventually forms the right side of the frame, but it’s good enough.

    My shop assistant says she wants another water bottle holder for an actual water bottle on her bike. I say she should just go to the shop and make whatever she wants, then install it. Negotiations continue…