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: Laser Cutter

Craptastic Kitchen Scale Tinkering
The health plan I use pays $100 toward the year’s over-the-counter healthcare stuff, although with a caveat: you can only buy the stuff from a specific website. As you might expect, what’s available consists of no-name generic products with absurdly high sticker prices and, just to rub it in, the hundred bucks gets paid in quarterly use-it-or-lose it installments.

Seeing as how it was free, I got a kitchen scale:

SmartHeart 19-106 Kitchen Scale – top view

It has two catastrophically bad design features:
- Terrible battery life
- Overly sensitive controls
It runs from a pair of series-connected CR2032 non-rechargeable lithium coin cells. Which would be fine, except that the blue LED backlight stays on for 30 seconds after each button touch and draws about 10 mA.

The battery lifetime is best measured in days.

The four control “buttons” on either side of the backlit LCD are touchless sensors using copper foil stickers:

SmartHeart 19-106 Kitchen Scale – NP-BX1 retrofit

The alert reader will spot those the empty CR2032 coin cell contacts over on the left and a pair of NP-BX1 batteries in the middle.

I figured there was no need to keep feeding it coin cells while I played with it, so I conjured a holder from the vasty digital deep. Normally, that would be an OpenSCAD solid model suited for 3D printing, but in this case the lithium cells exactly filled the space between the PCB and the bottom of the case, so it became a 2D design neatly suited for laser cuttery.

Kitchen scale – NP-BX1 holder – LB layout

I planned to stick the orange cutout (in 1.5 mm acrylic) as a stabilizer around the pogo pins making contact with the cell terminals from the red cutout (in 3 mm acrylic), but just melting the pins into the acrylic seemed sufficient for the purpose. Strips of adhesive sheet saved from the margins of previous projects affix the holder (not the cells!) to the scale’s upper glass layer.

As far as I can tell, the scale is perfectly happy running on 7.4 V, rather than 6.0 V. The PCB has two terminals marked +3V and +6V, so it probably depends on which LEDs they use for backlights:

SmartHeart 19-106 Kitchen Scale – PCB detail

The alert reader will notice a peculiarity concerning the sensor pad connections along the top edge.

More on that second bad design decision later …
2023-03-20
Medium Ricotta Lid Box: QR Code vs. Chipboard
Another box from the festi.info box configurator to collect more lids in a kitchen drawer:

Medium Ricotta Lids box

This was really a thinly veiled excuse for a deeper look at the QR code generator encoding the myriad parameters required to create the box and see what happens when you try to burn such a complex thing into chipboard.

Spoiler: chipboard has very low contrast and really does not work well with high-density QR codes.

Although the festi.info box generator can produce QR codes, I used qrencode (available in your Linux distro) on the command line to generate QR code image files with specific settings:
```
qrencode --level=M --size=6 --dpi=254 --output='Ricotta Lids 2 lb - chipboard.png' 'http://festi.info/boxes.py/ABox?FingerJoint_angle=90.0&FingerJoint_style=rectangular&FingerJoint_surroundingspaces=2.0&FingerJoint_bottom_lip=0.0&FingerJoint_edge_width=2.0&FingerJoint_extra_length=0.0&FingerJoint_finger=2.0&FingerJoint_play=0.0&FingerJoint_space=3.0&FingerJoint_width=1.0&x=200.0&y=120.0&h=60.0&outside=0&bottom_edge=h&thickness=1.4&format=lbrn2&tabs=0.0&qr_code=0&debug=0&labels=0&reference=0&inner_corners=corner&burn=0.04&language=en&render=0'
```
The key parameters:
- --level → error correction level
- --size → size of the smallest square (“module”) in pixels
- --dpi → DPI of the output image file
The default file type is PNG. The unusual 254 DPI makes each pixel exactly 0.1 mm wide and a peculiar 169.33 DPI = 0.15 mm came in handy for the first pattern.

The final parameter is the character string to encode, which you should definitely quote to prevent the shell from wrecking things while trying to help you.

A pattern with 4×4 pixel modules didn’t scan at all:

Chipboard QR code – 15pct 0.15mm 4×4 – overview

A closer look shows the modules have ragged edges due to laser timing variations during the engraving scans and gaps between successive scans because the spot size is less than the 0.15 mm scan interval:

Chipboard QR code – 15pct 0.15mm 4×4 – detail

Increasing the module to 6×6 pixels at a 0.1 mm scan interval :

Chipboard QR code – 15pct 0.10mm 6×6 – overview

A closer look shows the larger module reduces the relative size of the timing errors, while the decreased line spacing tidies up the blocks:

Chipboard QR code – 15pct 0.10mm 6×6 – detail

Reducing the power from 15% to 10% reduced the contrast to the point of illegibility:

Chipboard QR code – 10pct 0.10mm 6×6 – overview

A closer look shows the engraving barely punches through the surface and has somewhat more ragged edges due to the tube’s pulsating startu p current at very low power:

Chipboard QR code – 10pct 0.10mm 6×6 – detail

I also tried 5×5 modules with similar results.

The laser spot size sets the engraving scan interval, which then determines the DPI value for the QR code image. With all that matched up, you can send the images directly to the laser in Passthrough mode, without having LightBurn resample the pixels and change the module’s shape.

Looked at from a different angle: given the laser spot size and the module size, the QR code image size is not under your control.

From another angle: given a QR code image size in, say, millimeters, and the engraving scan interval, the module size is not under your control.

All this is moot if you print QR codes on a high-resolution / high-contrast printer. It’s just the gritty nature of laser cuttery that limits what you can accomplish.

And, of course, using a material less awful than chipboard will definitely improve the results.

If you want a similar box of your own, here ya go:

Medium Ricotta Lids – chipboard QR code – default

The URL is too ugly to print, but you can copy it from this link.
2023-03-16
Mini-lathe Chuck Jaw Holder

While swapping chuck jaws I realized I didn’t have to pile them on a shop rag atop the lathe headstock, no matter how neatly cut those rags might be:

Lathe chuck jaw holder – installed

It’s three layers of MDF cut to hold all six jaws from the 4 inch 3 jaw chuck, stuck together with wood glue.

You really need only four sockets: one empty for the jaw you just removed, then work your way around the chuck. But, hey, MDF is cheap and I usually remove all three at once anyway.

When it starts walking away, it’ll sprout silicone feet.

The LightBurn SVG layout as a GitHub Gist:

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view raw Lathe Chuck Jaw Holder.svg hosted with ❤ by GitHub

That was easy!

2023-03-15
Laser-safe PSA “Vinyl”: Proof of Concept

Mary’s ResMed AirSense 11 saves the data from every overnight breath she takes on an SD card, which she uploads to OSCAR once a week. I figured she needed an SD card to USB adapter / card reader of her very own:

PU PSA Vinyl test – installed

The lurid green letters aren’t backlit, they’re cut from a PVC-free “vinyl” (really, polyurethane) sheet with adhesive on the back.

N.B.: Laser-cutting real vinyl, made from PVC, releases enough chlorine gas to corrode the guts of your machine and, with poor fume extraction, rot your lungs along with it.

The LightBurn layout is pretty much what you’d expect, with the letter inside the outline of the USB dingus on a tool layer to get the size right:

PU PSA Vinyl test – LB layout

The red layer is a “kiss cut” through the vinyl (remember: polyurethane) that leaves the backing paper mostly undamaged:

PU PSA Vinyl test – dot mode cut detail

The cut uses Dot Mode, with the laser firing at 10% power for 2 ms, spaced every 0.1 mm along the cut. I found 0.1 mm spacing produced a more-or-less continuous cut in the PETG sheet for the Tek Circuit Computer cursor hairline, but this picture shows it’s definitely running in pulsed mode. In any event, Dot Mode is the only way a 60 W CO₂ laser can make a kiss cut, as a normal vector cut can’t run fast enough to prevent cutting all the way through the backing paper, even at 10% power, around those letters.

The edges of the letters are slightly melted with a raised border, although they look pretty good if you’re not peering at them through a microscope.

I cut the rectangular outline with scissors, peeled the waste vinyl away, and weeded the ‘a’ with tweezers:

PU PSA Vinyl test – weeded

Stick a snippet of transfer tape on top:

PU PSA Vinyl test – transfer tape

In theory, the transfer tape sticks more firmly to the cut letters than the letters adhere to their backing paper, so peeling off the tape also peels the letters off the backing paper.

Which did not go well:

PU PSA Vinyl test – transfer tape – peeling

The two adhesions obviously require a delicate balance to work properly and I would be unsurprised to learn different transfer tapes behave differently on each type of vinyl sheet, with no way to know the results without trying every possible combination.

A few retries got the “r” back in position on the transfer tape, but a bit of kink remains in the “M”.

A third adhesion balance occurs between the transfer tape and the USB card reader, where the tape must stick to the letters slightly less than the letters stick to the reader. Burnishing the tape + letters to the reader encouraged the letters to stick and the tape pulled off without dislodging them.

We deemed the result good enough for the purpose and the process taught me a few lessons along the way. Next time, maybe it’ll work out better.

2023-03-14
Please Close The Gate Signs: MDF Weathering

With the new signs in place, the old ones paused briefly for a photo op on their way to the trash:

Please Close The Gate – weathered MDF

That’s eight months of weathering on MDF covered with indoor urethane sealant and it’s not as awful as I expected: the MDF didn’t actually disintegrate, it just collected some mold / mildew / crud.

A closer look:

Please Close The Gate – weathered MDF – detail

The black paint survived surprisingly well.

I hadn’t paid ~~much~~ any attention to the edges, so they got covered with random amounts of black paint and urethane. It seems that’s where the disintegration starts:

Please Close The Gate – weathered MDF – side view

MDF definitely isn’t the right material for an outdoor sign and I knew that going in, but it’s cheap and readily available, which makes up for a lot.

For comparison, they looked nice right after installation:

Please Close The Gate – fresh painted

Ya learn something new every year around here!

2023-03-09
Please Close The Gate Signs: Paint Masking FAIL
A warm day let me shoot the engraved signs for the Vassar Community Garden gates with rattlecan black:

Please Close The Gate – masking tape peeled

The full sheet of orange acrylic arrived with plastic protective film on both sides, which I planned to use for paint masking. Alas, one side also had a wrinkle running its length that ended up on two signs, so I replaced that film with blue masking tape.

As fate would have it, the first side of the first sign I peeled had masking tape and produced what you see above.

Things went bad in a hurry. The paint had no adhesion whatsoever to the plastic film and fell off in flakes as I peeled the film away:

Please Close The Gate – plastic peeled

I assumed the flakes would just fall off the signs, perhaps with a little persuasion, so I peeled and weeded all the signs before cleaning them up.

Although the paint was fully dry, when the molecularly smooth surface of each paint flake touched the molecularly smooth surface of the newly exposed acrylic, the two instantly and permanently fused together.

There were a lot of flakes:

Please Close The Gate – plastic peeled – detail

Removal techniques that did not work:
- Vacuuming with a brush
- Gentle rubbing with a soft cloth
- Firm rubbing after spraying with acrylic cleaner
- Scraping with a plastic razor blade
So I deployed a P220 grit sanding block and wrecked the glossy surface of both sides of all six signs. I briefly considered trying to recover the finish by sanding them all up through about 2000 grit, then came to my senses: my sanding arm is weak.

Careful examination of the last picture shows several places around edges of the circle where the plastic film melted into a blob that blocked the paint, rather than vaporizing. I used enough power to engrave only about 0.3 mm deep (because they’re engraved on both sides), but the transition wasn’t fast enough for a clean edge.

They don’t look as nice as I’d like, but they’re good enough for the purpose:

Please Close The Gate – installed

The acrylic sheet is more see-through than I expected, at least when backlit by bright sunlight.

Please Close The Gate – seethrough

Next: we discover what happens to UV-stabilized orange acrylic and black outdoor paint over the course of a year in garden sunshine.
2023-03-08
Refresh Tears Eye Lube Storage Boxes

A recent Squidwrench meeting provided the opportunity to make a couple of racks for an assortment of Refresh Tears / Liquigel bottles:

Refresh eye lube – storage cases

I used chipboard to find out if the cross plates would stiffen the floppy 1.1 mm sheets enough for this light duty. Indeed, the overall structure becomes a nice rigid box, even though the feet and corners can’t withstand much abuse.

The finger joints use the default settings, which produce a lot of fingers along the edges. This turns out to be a Good Thing, as it gave the yellow wood glue plenty of opportunities to bond the sheets together.

Combining the default 5° slope with nine bottles along each level wastes a tremendous amount of vertical space. The adjacent racks hold three much larger cans per level, so roughly the same space doesn’t look like much. In retrospect, a 3° slope should work for smaller bottles.

And, yes, the squash on the lower shelf store nicely and become yummy meals all winter long.

The overstuffed URL generating the patterns:

http://festi.info/boxes.py/CanStorage?FingerJoint_angle=90.0&FingerJoint_style=rectangular&FingerJoint_surroundingspaces=0.0&FingerJoint_bottom_lip=0.0&FingerJoint_edge_width=1.0&FingerJoint_extra_length=0.0&FingerJoint_finger=2.0&FingerJoint_play=0.0&FingerJoint_space=2.0&FingerJoint_width=1.0&Stackable_angle=60&Stackable_bottom_stabilizers=0.0&Stackable_height=2.0&Stackable_holedistance=1.0&Stackable_width=4.0&fillHoles_bar_length=50&fillHoles_fill_pattern=no+fill&fillHoles_hole_max_radius=3.0&fillHoles_hole_min_radius=0.5&fillHoles_hole_style=round&fillHoles_max_random=1000&fillHoles_space_between_holes=4.0&fillHoles_space_to_border=4.0&top_edge=%C5%A0&bottom_edge=%C5%A1&canDiameter=30&canHeight=75&canNum=18&chuteAngle=5.0&thickness=1.15&format=lbrn2&tabs=0.0&qr_code=0&debug=0&labels=0&reference=0&inner_corners=corner&burn=0.04&language=en

And the eyeburning QR code:

Refresh Eye Lube – chipboard QR code

2023-03-02