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

Another CatEye Cadence Sensor Magnet Mount
The stock Bafang pedal cranks measure 170 mm on centers between the bottom bracket shaft and the pedal spindle. Having grown accustomed to the 165 mm cranks from Mary’s bike, I got a set of cheap 160 mm cranks to feel if there was any difference:

Bafang vs ProWheel crank forging

The bottom crank has a quick-and-dirty adaptation of the magnet mount for the Lekkie Buzz Bar offset cranks, but, of course, the 160 mm cranks have an entirely different profile. They are also heavier and more crudely forged, which is about what you’d (well, I’d) expect.

Also unlike the Lekkie cranks, neither the Bafang nor the Prowheel cranks correct the Bafang motor’s offset, so I’m using the left-side Kneesaver from the old cranks, which turns out to be close enough.

Modeling the profile started with an infrequently used contour gauge:

CatEye Magnet holder – ProWheel crank profile tests

The black 3D printed mount in the upper right fit the Bafang crank and appears in the top photo.

Transferring the new contour to paper and applying the Chord Equation got the radius of the not-quite circle:

CatEye magnet crank adapter – chord radius

Knowing the size of the magnet and the radius of the circle, drawing the profile in LightBurn was straightforward:

CatEye magnet crank adapter – framed

Applying the laser cutter to MDF produced the two successive test-fit pieces in the picture while figuring out how much stickout the magnet needed beyond the inner crank face to reach the sensor. LightBurn’s Node Editor simplified adjusting the size: drag-select a group of nodes, then move them in precise increments with the arrow keys.

Export the profile from LightBurn as an SVG file, import it into OpenSCAD, and extrude it to the proper length:
```
module CatEyeMagnet() {

Magnet = [19.0,14.0,8.5];

translate([0,75,0])
    linear_extrude(height=Magnet.y)
        import("CatEye magnet crank adapter.svg");

}
```
The translate puts the profile approximately at the XY origin. The center = true option moves the profile elsewhere on the XY plane, but does not center it, which may have something to do with the viewport used by LightBurn, the OpenSCAD version I’m using, or something else entirely.

In any event, the 3D printed mount fits the crank and puts the magnet where it will do the most good:

CatEye Magnet holder – ProWheel crank – installed

What looks like an obvious curvature mismatch comes from having the tape edge not quite squashed against the crank.

I should poke a channel through it for a cable tie around the crank, but that 3M foam tape is really good stuff and hasn’t failed me yet.
2023-04-03
Under-cabinet Dell Sound Bar Mount

Another bedroom rearrangement ejected the Raspberry Pi streaming media player in favor of a phone and Bluetooth speaker, which meant I could convert the under-shelf mount into an under-cabinet mount:

Under-cabinet sound bar mount

It’s basically ten identical identical spacers cut from 3 mm plywood, with a side benefit of dramatically reducing my scrap plywood stash, then skewered by a pair of absurdly long 4 mm self-tapping metal screws into holes drilled half an inch into the ¾ inch solid wood cabinet floor.

It clears some clutter atop the microwave and, at least to my deflicted ears, sounds much better. At some point I must screw the Raspberry Pi under the cabinet, too, but that awaits further rearrangement.

2023-03-30
Bafang DPC-18 Button Mollyguard

I got the Bafang DPC-18 display for my Tour Easy specifically to put the control buttons on the handgrip, rather than the buttons on the left of the 500C display on Mary’s bike:

Tour Easy Bafang – display 26 mi

The first pass put them on the left handgrip, just behind the thumb throttle:

Bafang DPC-18 control buttons – initial mount

This turned out to be a catastrophically bad position, because the “buttons” extend all the way to the edge of the mount and trigger when pressed a fraction of a millimeter: the dark line visible under the light gray top is the entire range of motion.

My resting hand position on the grip put the edge of my gloved index finger along the buttons, where it would inexorably nudge the + button until I was riding in assist level 9 (Rocket Sled) mode.

One ride was enough to convince me those buttons needed a Mollyguard:

PXL_20230321_233854755 – Bafang DPC-18 button Mollyguard – rear view

It is, of course, a laser-cut piece of 1.5 mil black acrylic, held in place with hot-melt glue. Because the button housing isn’t mounted symmetrically on the handlebar, I cut a few paper templates before getting the position and size right.

A view from the front shows the lip sticking up over the buttons:

Bafang DPC-18 button Mollyguard – front view

FWIW, the asymmetric mount put the buttons on the rider’s side of the flat handlebars found on contemporary upright city-rider style bikes. It makes perfect sense in that context, but didn’t help me in the least.

With the Mollyguard in place, I rotated the whole button assembly around the handgrip to allow pushing the buttons with my thumb in its natural position.

Now the assist level changes only when I want it to!

2023-03-28
Eyeglass Case Padding

The “live hinge” on my overnight eyeglass case shattered when it hit the floor (these things happen), which prompted me to finish a longstanding project of replacing the inadequate / worn out padding in my most-used cases to reduce rattles while in my pocket.

I’d long ago cut craft foam sheets to fit some of the cases, so I started by scanning a sample:

Zenni case pad

Admittedly, black foam on a white background isn’t much to look at, but it did fit one of the cases pretty well.

Rotate the image to make things simple, convert it into a monochrome bitmask, import it into LightBurn, fair some Bezier curves around it, duplicate and tweak for the other not-quite half of the case:

Zenni eyeglass case pads – LB Layout

I ended up with several different versions for various cases, but you get the general idea:

Zenni eyeglass case pads – installed

They’re all cut from 2 mm EVA foam sheets which, despite the “vinyl” in their name, do not contain chlorine and are suitable for laser cuttery.

Some of the deeper case halves required strips of adhesive sheet to secure the foam, but most sheets dry-fit in place.

Definitely no rattles!

2023-03-22
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