Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
The pedal on Mary’s most recent Kenmore 158 lost its low-speed control, which meant I must add a few more graphite / carbon disks to the stacks:
Kenmore 158 – carbon disks
The contacts needed a bit of attention, too:
Kenmore 158 – carbon contact plates – detail
Contrary to what I found in the previousrheostats, these stacks end with a double-thick graphite disk backed up by a disk of brass shimstock, all of which needed cleaning, too. No broken disks, none severely eroded, no debris, just a general shortening of the stacks; I think the disks gradually turn into carbon dioxide.
Each stack has 42 graphite disks that average 0.79 mm thick, the double-thick disks measure 1.5 mm, and the brass shims are 0.30 mm = 12 mil. The punched contacts on those brass plates stand 0.95 mm proud of the surface.
With the big graphite plugs in place, the ceramic housing had 37 mm deep holes for the disk stacks. Subtracting the 0.95 mm contact leaves about 36 mm and, seeing as how the stacks add up to just under 36 mm overall, there’s barely room for one additional disk. I added one to each stack, buttoned the pedal up, and it works perfectly again.
As before, I put the larger spool on the floor under the lathe and let the thread spill straight off the top toward the smaller spool. This time, I didn’t have a twist accumulating in the loose thread between the two spools:
Grab longer lengths of the loose thread
Absolutely no slippage between the fingers!
Put more tension on the thread at the takeup spool
As nearly as I can tell, the thread still has a slight twist coming off the larger spool, but grabbing longer lengths captures the twist and more tension lays it on the smaller spool. After cutting the thread, what was left had maybe three turns of twist, which was no big deal and obviously hadn’t accumulated.
Mary has been working on the Splendid Sampler project, with 56 completed blocks (*) stacked on her sewing table. We agreed that those blocks would make a nice background for our Christmas Letter, but the labor involved to photograph all the fabric squares and turn them into a page seemed daunting.
Turned out it wasn’t all that hard, at least after we eliminated all the photography and hand-editing.
The 6½x6½ inch blocks include a ¼ inch seam allowance on all sides and, Mary being fussy about such things, they’re all just about perfect. I taped a template around one block on the scanner glass:
Quilt block in scanner template
Then set XSane to scan at 150 dpi and save sequentially numbered files, position a square scan area over the middle of the template, and turn off all the image enhancements to preserve a flat color balance.
With “picture taking” reduced to laying each square face-down on the glass, closing the lid, and clicking Scan, the scanner’s throughput became the limiting factor. She scanned the blocks in the order of their release, while tinkering the auto-incremented file number across the (few) gaps in her collection, to produce 56 files with unimaginative auto-generated names along the lines of Block 19.jpg, thusly:
Block 19
The “square” images were 923×933 pixels, just slightly larger than the ideal finished size of 6 inch × 150 dpi = 900 pixel you’d expect, because we allowed a wee bit (call it 1/16 inch) on all sides to avoid cutting away the sharp points and, hey, I didn’t get the scan area exactly square.
With the files in hand, turning them into a single page background image requires a single Imagemagick incantation:
I figured the -geometry value to fill the 8 inch page width at 150 dpi, which is good enough for a subdued background image: 8 inch × 150 dpi / 7 images = 171 pixels. Imagemagick preserves the aspect ratio of the incoming images during the resize, so, because these images are slightly higher than they are wide, the height must be slightly larger to avoid thin white borders in the unused space. With all that figured, you get a 1197×1384 output image.
Bumping the contrast makes the colors pop, even if they’re not quite photo-realistic:
Quilt block montage – contrast
I’ll lighten that image to make the Christmas Letter text (in the foreground, atop the “quilt”) readable, which is all in the nature of fine tuning.
She has 40-odd blocks to go before she can piece them together and begin quilting, with a few other projects remaining to be finished:
Mary quilting
(*) She’s a bit behind the block schedule, having had a year of gardening, bicycling, and other quilting projects, plus whatever else happens around here. Not a problem, as we see it.
Although I’m not the type of guy who thinks twinkly LEDs will enhance his apparel, one of Mary’s quilting thread sources had a closeout deal on their “wearable electronics”, including a large cone of stainless steel thread / yarn:
Stainless steel thread
… CR2032 lithium cells & holders, plus assorted LEDs on small PCBs.
The usual advice for connecting the thread seems to involve knotting it through the PCB holes, then sewing it to the backing fabric. Alas, I’m bad with knots and the stainless steel yarn isn’t all that cohesive:
Emerald LED – Stainless steel thread – knotted
The holder has an even smaller hole, but Mary gave me a needle threader that helped:
CR2032 – Stainless steel thread – knotted
Some advice found on The InterTubes suggests using copper crimp beads (perhaps with solder) to prevent the thread from completely unraveling and keep the thread loop tight around the PCB hole:
Rose LED – Stainless steel thread – Crimp bead – Wire Glue
Beadworkers use crimping pliers that leave a tidy dent; I mashed the beads with a needlenose pliers and called it good.
The LEDs seem to be white LEDs with filters or, perhaps, blue / violet LEDs with different phosphors: their forward voltages look more blue than red or green. Everybody in this field depends on the minor miracle that lithium cell voltages match blue LED forward drops closely enough that you can get away without a ballast resistor.; the cell’s 20-ish Ω internal resistance doesn’t hurt in the least. An interesting white paper (SWRA349) from TI explores the effect of current on cell capacity and how to size a parallel capacitor that reduces the peak battery current.
The black gunk is Wire Glue, which costs about five bucks for a lifetime supply in a small jar (or nigh onto 15 bucks via Amazon Prime) and is basically carbon powder in a water-based binder. Apply a dab to the connection and the water evaporates to leave the carbon + binder behind.
That works better on joints that don’t move, which is precisely what you don’t have in a wearable electronic situation. You can see the crumbling Wire Glue after the trip back from a Squidwrench meeting:
I also picked up a Permatext Rear Window Defogger repair kit (09117, if you’re looking) that seems to be a staggeringly expensive way to get a tenacious high-current conductive adhesive. More on that later.
The yarn runs 3.5 Ω/ft, much lower than Adafruit’s three-ply yarn (10 Ω /ft), and suggests itself for flexible connections, EMI gaskets, and suchlike.
Those LEDs are taped to the kitchen window, where they cast a cool light over the table, with the battery holders sitting on the sash. I’d just replaced some data logger CR2032 cells, so they’re running from nearly dead lithium batteries.
For future reference: 2.77 V and falling, pushing less than 2 mA through the LEDs.
Mary has been doing Ruler Quilting and wanted a pencil guide (similar to the machine’s ruler foot) to let her sketch layouts before committing stitches to fabric. The general idea is to offset the pencil by 1/4 inch from the edge of the ruler:
Ruler Adapter – solid model
That was easy.
Print three to provide a bit of cooling time and let her pass ’em around at her next quilting bee:
Ruler Adapter – Slic3r preview
Her favorite doodling pencil shoves a 0.9 mm lead through a 2 mm ferrule, so ream the center hole with a #44 drill (86 mil = 2.1 mm) to suit:
Ruler quilting pencil guides
The outer perimeters have 64 facets, an unusually high number for my models, so they’re nice & smooth on the ruler. Even though I didn’t build them sequentially, they had zero perimeter zits and the OD came out 0.500 inch on the dot.
The chamfers guide the pencil point into the hole and provide a bit of relief for the pencil’s snout.
If I had a laser cutter, I could make special rulers for her, too …
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However, she wanted more light on the right side of the needle, so now she has it:
Needle LEDs – front
That’s without any LEDs along the front and back of the arm, hence the dark pool beyond the sewing machine’s base.
Those are the same 5050 warm white LEDs I used on the other side:
Needle LEDs – lower right
Seen without the glare:
Needle LEDs – bottom
They’re mounted on a 32 mil brass strip from the shimstock stash, carefully hand-bent and twisted to match the curvature of the arm, and held in place with JB Kwik steel-filled epoxy for good heat conduction to the aluminum arm. One can argue with the epoxy oozing out from under the brass, but it’s invisible from above.
No construction photos, alas, because I made this in a white-hot frenzy one afternoon and managed to not take any pix during the entire session. Call it working in the flow, OK?
All four SMD LEDs sit in epoxy blobs that isolate them from the brass strip, with 26 AWG solid wire “bus bars” soldered to the top of their terminals and a length of that lovely PTFE-insulated miniature coax leading off into the endcap. More epoxy encloses all the wiring & connections to provide a surprisingly smooth surface that shouldn’t snag the fabric.
The power supply uses an 18 W 120 VAC to 12 VDC brick intended for small LED installations:
Needle LEDs power supply – exterior
The AC comes from the same zip cord that formerly supplied the original 15 W incandescent bulb in the endcap, so the new lights behave the same way: push the power button to turn on the machine and the LEDs pop on just like they should. I put quick-disconnect terminals in the AC line to make it removable, although those need some sort of insulated plug to cover the exposed blades inside their housing.
Inside the black box, a small boost supply steps the voltage up to just under the nominal operating level of 21 VDC:
Needle LEDs power supply – interior
You can just see the adjusting screw hole in front of the AC brick in the overall view.
The DC output exits in the middle of the far side, through a coax jack epoxied to the base.
As before, all six LEDs run in parallel at (for now) 18.5 VDC and maybe 50 mA each, for a total of 300 mA, and seem fearsomely bright even at that. We can now tune for best light as needed.
This is a major major major improvement over the previous tangle of wires stuck on the outside of the machine, with all the wiring internal to the arm and the power supply out of sight under the sewing table.
After an hour, the arm above the four LEDs runs 13 °C above ambient and the endcap over the two LED heatsink is 6 °C over ambient. The AC supply runs at 104 °C and its plastic case offers no provision for heatsinking. All in all, things are warm and not hazardous.
I haven’t retrofit this machine with LED strips along the front & back of the arm, as those may not be needed with the intense needle lighting; the NisLite desk lamp may suffice for area illumination.
The power switch on Mary’s “embroidery” Kenmore Model 158 sewing machine became exceedingly stiff, to the extent she said it was painful to push. Buying a shiny new switch seemed iffy, because a cursory search through the usual reputable electronic suppliers suggested there’s no way to specify how stiff the button might be, nor how that might feel in actual practice.
The switch harvested from the pulse-drive machine felt somewhat less stiff, so I decided to (try to) loosen it up and, if that worked, swap it for the stubborn one.
A pair of rivets hold the two halves of the switch together, obviously intended as a permanent solution. A carbide burr in the Dremel tool dealt with them easily enough:
Model 158 Power Switch – grinding rivets
Inside, the actuator drives a rotating brass contact:
Model 158 Power Switch – rotor
Two stationary brass contacts are spot-welded to the wires:
Model 158 Power Switch – contacts
The actuator under the button consists of a helix-twisted steel rod, a rather stiff spring, and a four-vaned phenolic blade that engages those two little flaps on the rotor. The rivet holes exactly fit plain old 1-72 screws:
Model 158 Power Switch – actuator stem
Not seeing anything obviously fix-able inside, I wiped the excess oil off and reassembled it in reverse order:
Model 158 Power Switch – reassembled
Astonishingly, that bit of attention loosened it up: the button now presses easily!
I swapped it with the too-stiff switch and declared victory…
The Smell of Molten Projects in the Morning 