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.
If it used to work, it can work again
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…
Another no-CNC repair!
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…
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.
So I must dig the mumble thing out…
The overall plan:
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.
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…
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.
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.
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.
Let it cure overnight, scrub it on some sandpaper atop the sacrificial side of the surface plate, and it’s all good again…
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!
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/localdirectory. 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!
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.
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…
While walking home with the bike, I noticed that the odometer wasn’t matching up with reality. This generally means the front-wheel magnet sensor got whacked out of line and, given that I’d just laid the bike down on that side, that’s what I expected to fix.
As it turned out, the failure meant it was time for the more-or-less annual contact cleaning. The three tiny contact balls on the bottom of the Cateye Astrale tend to collect enough dirt over the course of a few thousand miles to become intermittent. The balls lead to the wheel and pedal sensors, with a single common wire.
You can see that they’re not shiny little factory-fresh bumps. Here’s a detail of the upper-right one on the base to the right. Even through the horrors of a tight crop from a hand-held shot, you can see the problem.
No big deal, just wipe ’em off and apply a bit of DeoxIT to make ’em happy again for another year.
On my way back from a ride around the block the back tire went pfft thump thump thump. I’m 1.5 miles from home: fix or walk?
The first step: always examine the tire to find the puncture, before you move too far. Finding something sticking out of the tire means you’re well on your way to fixing the flat. Lose the entry point and you’re left to blow up the tire and listen for escaping wind. So I picked up the butt end of the bike, spun the wheel, and this little gem heaved into view…
That area of the road has seen several collisions in recent months that left the shoulder littered with broken automotive glass. The shard in my tire glistened like a diamond, because one side was flat and mirrored; perhaps it’s from a headlamp reflector or side mirror. The pointy end went into the tire, of course…
Well, a single-point failure like that is easy to fix, so:
No pix of any of that, but suffice it to say I was astonished to discover that the glass penetrated the Marathon tire’s Kevlar belt just barely far enough to poke the Slime tire liner, but not enough to leave more than a hint of a mark on the tube. Definitely not a puncture and certainly nothing that would account for a sudden flat.
That glass shard is not why the tire went flat! Tire liners FTW!
Examining the rest of the tube revealed this situation a few inches anti-spinward of the glass fragment.
There’s a row of holes across the tube, with no corresponding tire or liner damage at all. As nearly as I can tell, the tube rubber simply pulled apart across that line, all at once, and the air went pfft just like you’d expect.
That’s not survivable, but I don’t carry a spare tube (well, two spare tubes: 700x35C rear and 20×1.25 front) on rides around the block. Long bike tours? Yup, spare tires & tubes because I’m that type of guy.
Anyway, I’ve got the tube in hand, so what’s to lose? Scuff it up with the sandpaper and yipes…
What’s not obvious in the picture is that all those little spots around the big holes are pinholes. The whole area of the tube must have gotten just barely enough rubber to cover the mold.
I know as well as you do this isn’t going to have a happy outcome, but I slobber on the cement, let it dry, squash on a big patch, install the tube & tire, fire a 16-gram CO2 cartridge into it, and … it doesn’t seal.
The tube is several-many years old, probably from whoever was supplying Nashbar at the time, and it served well, so it gets a pass. I’d rather tubes fail in the garage than on the road and sometimes they do, but that’s not the usual outcome.
My ladies were out gardening at the time and a long wheelbase ‘bent isn’t the sort of thing you can stuff into a friend’s car. Not to mention that my ladies had the magic phone.
So I walked home.
Sometimes a man’s gotta do what a man’s gotta do.
Memo to Self: Schwalbe tube at 8910. Reversed(*) the Marathon’s direction.
(*)They’re directional, but when they get about halfway worn I don’t see that it makes much difference. The rear tire on my bikes wears asymmetrically: probably too many tools in the left underseat bag.
When I replaced the kitchen counter & installed a new sink, I added a soap dispenser, mostly because the stainless steel sink had three holes that needed filling. After nigh onto a decade, the dispenser pump is now getting sticky: difficult to push down and reluctant to pop up.
The problem seemed to be that the O-ring wasn’t sliding nicely along the internal bore.
The catch is that both ends have ball check valves, so you can’t just squirt lube into the bore. I tried prying the thing apart, but the snap-together cap has a really aggressive closure.
So I shoved the exit valve ball (on the left of the picture) out of the way with a pin punch, wedged it into the end of the spring, and squirted the least amount of silicone lube I could manage into the pump. A bit of fiddling un-wedged the ball and got it back in position.
The pump works fine now, but I have my doubts as to how long the lube will last with continuous exposure to soap and constant sliding.
The thing probably needs a new O-ring and I’m certain of two facts:
The Smell of Molten Projects in the Morning 