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

Micropositioner Rehabilitation: Z-axis Slide

With the planetary reducer off, I could remove all the bits and pieces holding the Z-axis slide to the rest of the positioner.

Rack drive casing

Note carefully the three spacing washers near each mounting screw. They hold the slide off the casting body by a very precise amount: they’re each 4 mils thick and prone to vanish in a light breeze. I discovered that each screw had three washers when I flicked one off the workbench and discovered two on the floor.

The metal plate holding the pinion in place has two flat-head screws to the left and two ramps to the right. The conical points of the two long setscrews on the right bear on the ramps, providing a convenient, if obscure from the outside, way to adjust the slide friction by clamping the pinion shaft. One of the setscrews was partially removed, so a previous owner had evidently tried to reduce the overwhelming stickiness.

With the washers in a safe place, the pinion cover comes off with only slight encouragement.

Pinion parts

Plenty of congealed lubricating oil to be found.

Even without the pinion gear, it was exceedingly difficult to urge the two parts of the slide apart: more congealed oil. Much to my surprise, the slide does not have adjustable gibs: it’s one of those precision-ground gadgets that Just Works. This picture shows all the parts in their gunky glory. Note the random dirt in the rack teeth, along with the goo on the pinion shaft.

Rack and pinion – disassembled

With everything apart, removing the gunk was a simple matter of toxic solvents and mechanical poking with wood picks and splints

I filed off the burrs on the shafts, thought briefly about grinding some flats for the setscrews, and decided to leave well enough alone.

A few dabs of clock oil here & there, reassemble everything in reverse order, and the Z-axis moves gracefully with minimal knob torque. It’s very sensitive to the clamping force of those pointed setscrews, though.

It’s now easy to discover that the planetary reducer has a 5:1 ratio and the Z-axis moves about 6 mm per turn. Because the reducer uses balls, it slips when the slide jams against something, rather than strip its gears.

I should clean the other two slides, but a dot of clock oil on each cheered them up enough to let me punt that for a while…

I like it!

Micropositioner

2010-01-04
Micropositioner Rehabilitation: Planetary Reducer

Micropositioner

An old 3-axis micropositioner recently found itself on my electronics workbench, where it should come in handy for SMD soldering, microscopic examination, and similar projects requiring the ability to move something in tiny, precise increments. This picture gives you the general idea; it’s mounted on a magnetic base stuck to a random chunk of sheet steel.

The knob on the front drives the vertical (Z) axis, with the other two controlling the front-to-back (Y) and left-to-right (X) axes. A rotary joint between the X and Y axes, plus another at the tip of the arm, mean you’re not restricted to orthogonal axes; that may be either a blessing or a curse, depending on what you’re trying to accomplish.

Unfortunately, the Z axis was essentially immovable: that big knurled knob took a remarkable amount of force to drive the slide. Some Quality Shop Time was in order.

Planetary reducer – cover

The thing is a chunk of old-school German engineering: nary a gratuitous plastic part to be seen. The planetary reducer has a cast metal cover secured to the torque arm with an acorn nut, which had obviously been removed several times before, as the cover was somewhat chewed beneath the nut.

I loosened the two setscrews holding the knob in place, gave it a pull, and … nothing. After a protracted struggle and considerable sub-vocal muttering, the knob came off to reveal a thoroughly scarred shaft. Contrary to what I expected, the shaft did not have flats below the setscrews, so the inevitable screw burrs locked the shaft to the knob.

Planetary reducer – torque arm

The picture to the left shows the planetary drive and torque arm after I filed off the burrs. Two plastic washers (the top one sits on the spring; it’s not shown here) provide smooth bearing surfaces that hold the knob under firm spring pressure, which prevents the Z axis from descending unless you turn the knob manually.

Planetary drive output shaft screws

Two more setscrews secure the planetary drive’s output bushing to the Z axis pinion shaft. The picture to the right shows that they’re pretty much inaccessible; one was directly behind a tab holding the drive together, the other was aimed at the shoulder of the casting holding the Z axis slide.

And, of course, even with the knob in place, I can’t turn the mumble shaft, which is why I’m doing this in the first place. The planetary drive uses balls, rather than gears, and the lubricating oil had long since turned into gummy varnish. I slobbered enough light oil into the drive to loosen the gunk enough to make the drive turn-able, albeit with considerable effort. I urged the input shaft barely enough this-a-way and that-a-way to get access to both of the screws.

Pinion shaft

As you’d expect, removing the drive required even more muttering and the application of dangerous tools. The pinion shaft was badly scarred in several places, so this poor thing has been dismantled several times before.

That was entirely enough for one day. Tomorrow, disassembling the Z-axis slide and cleaning things up…

2010-01-03
Tchotchke Repair

Tchotchke epoxy

Sometimes I get to do an easy one. This dust collector came with the house and sits on the fireplace; one of the little guys fell off when Mary went on a cleaning frenzy. As nearly as I can tell, he had a bad butt weld (using the exact term) with marginal penetration.

A dot of JB Weld, an uncomfortable overnight stay on the workbench, and he’s as almost good as new. I briefly thought about resistance-soldering him together, but came to my senses: epoxy to the rescue!

The balance point is sufficiently delicate that the additional weight of the epoxy pulls his side down a bit. I’ll call it art and leave it at that, although I should build a little circuit with a proximity sensor and an electromagnet to keep the thing in motion.

See-saw tchotchke repaired

Yeah, that’s my Tau Beta Pi Bent in the background… along with the little glass bead I made in the Corning Museum of Glass a few summers ago.

2009-12-29
Bicycle Tire Liner Abrasion

The front tire (a Primo Comet blackwall) on Mary’s Tour Easy was flat when we rolled out of the garage a few days ago. While a flat isn’t pleasant at any time, it’s much nicer to find one at home, before the ride, rather than out on the road!

I figured the tire ate something sharp that managed to work its way through the tire liner and into the tube; that’s rare, but it sometimes happens. These two pix of the tread show why we use tire liners: sidewall-to-sidewall nicks, cuts, gouges, and gashes, despite the fact that the herringbone tread has plenty of life left in it. Click the pix to enlarge, if you dare…

Tire cuts 1

And another section; it’s like this all the way around the tire. I think this one is the better part of a year old, so it has maybe 2000 miles on it. It handled 200+ miles along the Pine Creek Gorge rail-trail this past summer, which was sharp crushed gravel, but most of the cuts came from roadside debris on our ordinary utility rides around home.

Tire cuts 2

As it turned out, the tire liner had prevented all those punctures from reaching the tube, while killing the tube all by itself. The sharp edge where the the two ends of the liner overlap had worried its way through the tube.

Abrasion from tire liner

The tire liner wasn’t a genuine fluorescent green Slime strip, but some translucent brown thing. The difference: Slime liners are thinner and don’t have nearly this much abrasive power.

Alas, I didn’t have a Slime liner in my stash (remedied with the most recent bike parts order), so I put the brown liner back in with a few layers of genuine Scotch electrical tape to build the end up a bit. There’s really no good way to feather the end without making it into a ragged knife edge.

New tire and tube, of course. I’m not that crazy!

With any luck, the liner and tape will behave for another few years, until the tire wears out, and then I’ll replace everything. Other than this event, flats aren’t a big part of our riding experience.

2009-12-28
Failed Switch

Switch Innards

When I flipped this switch on, it started fizzing and emitting ozone-scented smoke while the lights it controlled flickered. This is not a nominal outcome. I toggled the switch a few times, but it continued to misbehave, so I installed a replacement switch and laid the old one out on the desk for an autopsy.

It’s an old-school mechanism, as suits the 1930-vintage structure it came from. The lyre-shaped arch with the spring swings back and forth on its tabs, which rest in the small recesses near the middle of the switch body. The peg on the toggle handle engages the spring, thus providing the over-center snap action.

The switch action takes place at the bottom of the arch, where those two very small tabs stick out. They wipe on the grubby-looking bottom tabs of the oddly shaped flat-brass doodads, the U-shaped ends of which surround the screws that clamp the copper wire to the switch.

I expected to find a scorched contact or perhaps an insect in the mechanism, but nothing seemed out of the ordinary. Apart, that is, from the layer of congealed grease covering everything inside. I suspect the grease was applied in the factory to help prevent contact corrosion, but the volatiles are long gone.

Switch Contacts

A closeup of the switch contacts shows (what I think is) the problem.

All the contact points are covered in grease, but the lyre-shaped gizmo looks like it’s been painted: its contact points were black and resisted cleaning by fingernail scraping.

As nearly as I can tell, all the current passed through a very few high spots that were wiped somewhat clean as the contacts closed. As those spots heated up, the grease melted and flowed over them, increasing the resistance and the heat.

The switch had been working for many decades, as the BX armored cable in the box had fabric-covered rubber (stiff rubber) insulation. I managed to install the replacement switch without breaking the insulation, but it was ugly in there.

2009-12-27
Failed LED

Dead LED

Doesn’t look like much, does it? It’s an ordinary blue LED that I used for the upper colon dot in a clock. Worked fine for a few dozen power-on hours, then it turned off a bit after 6:00 pm one day. Back on an hour later, more or less, then off again by the next morning, back on again, off again.

Might be a software error, as each colon LED is a separate TLC5916 display driver output. Might be a soldering problem, as my board doesn’t have plated-through holes. Might be (shudder) a burned-out transistor inside the TLC5916.

When it’s off, VCC appears on both sides, within a few tens of millivolts.

Resoldered the joints, after which it worked for a while. When it’s on, voltage measurements look normal: about 3.5 V drop across the diode and 1.5 V across the driver transistor.

No obvious code problems, but, then, code problems are never obvious.

Finally the thing stopped working for a few hours. I unsoldered it and there’s no continuity: it failed open. Peering deeply inside with a microscope shows nothing unusual: the flying gold wires look OK, the bonds look flat, and the chip has no burn marks.

Just a bad LED, I suppose. It’s surplus, of course, but that doesn’t mean much these days; there’s a lot of surplus going around.

Soldered in a replacement from the same batch and it’s all good.

So far, anyway.

2009-12-24
Whirlpool Refrigerator Shelf: Drawer Slide Repair

Refrigerator shelf bracket – inside

The bottom glass shelf in our Whirlpool refrigerator (the “Crisper Cover”) rests on an elaborate plastic structure that includes slides for the two Crisper drawers. Perhaps we store far more veggies than they anticipated, we’re rough on our toys, or the drawer slides came out a whole lot weaker than the designers expected. I’m betting on the latter, but whatever the cause, the two outside slides broke some years ago.

I don’t know what function the rectangular hole above the flattened part of the slide might serve, but it acted as a stress raiser that fractured the column toward the front. With that end broken loose, another crack propagated toward the rear, so the entire front end of the slide drooped when the drawer slid forward.

The minimum FRU (Field Replacement Unit) is the entire plastic shelf assembly, a giant plastic thing that fills the entire bottom of the refrigerator. You could, of course, buy a whole new shelf assembly, perhaps from www.appliancepartspros.com, but it’s no longer available. Back when it was, I recall it being something on the far side of $100, which made what you see here look downright attractive.

My first attempt at a repair was an aluminum bracket epoxied to the outside of the slide, filling the rectangular opening with JB Industro-Weld epoxy to encourage things to stay put. The plastic cannot be solvent-bonded with anything in my armory, so I depended on epoxy’s griptivity to lock the aluminum into the shelf. That worked for maybe five years for the right side (shown above) and is still working fine on the left side.

Refrigerator shelf bracket – bottom

The right-side bracket eventually broke loose, so I did what I should have done in the first place: screw the bracket to the shelf. Alas, my original bracket remained firmly bonded to the bottom part of the shelf and secured to the block of epoxy in the rectangular hole. Remember, the broken piece didn’t completely separate from the shelf.

So I cut another angle bracket to fit around the first, drilled holes in the shelf, transfer-punched the bracket, and match-drilled the holes. Some short(ened) stainless-steel screws and nuts held the new bracket in place and a few dabs of epoxy putty filled the gaps to make everything rigid.

That’s been working for the last few years. The refrigerator is going on 16 years with only one major repair (a jammed-open defrost switch), so I’ll call it good enough.

2009-12-23