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
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 4x4 - overview
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 4x4 - detail
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 6x6 - overview
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 6x6 - detail
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 6x6 - overview
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 startup current at very low power:

Chipboard QR code - 10pct 0.10mm 6x6 - detail
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
Medium Ricotta Lids – chipboard QR code – default

The URL is too ugly to print, but you can copy it from this link.

Gas Price Signage: FAIL

The big price displays at the Mobil station on the corner have always behaved oddly, but these replacements began failing within a week of their installation:

Mobil price sign - north face
Mobil price sign – north face

That doesn’t look too bad, until you notice the number of dead LEDs in both red displays.

The south face is in worse shape:

Mobil price sign - south face
Mobil price sign – south face

The green LEDs seem to be failing less rapidly than the reds, but I don’t hold out much hope for them.

The previous display had seven-segment digits made of smooth bars, rather than discrete LEDs. This one appeared after the segments failed at what must have been more than full brightness; the red LEDs were distracting by day and blinding by night.

Maybe they got the LEDs from the same folks selling traffic signals to NYS DOT? The signals around here continue to fail the same way, so I suppose DOT doesn’t replace them until somebody enough people complain.

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
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
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
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
Please Close The Gate – fresh painted

Ya learn something new every year around here!

Tree vs. Guide Rail: Sheared Bolt

Spotted on a walk along the Mighty Wappingers Creek after a storm with plenty of gusty winds:

Tree-smashed guide rail
Tree-smashed guide rail

The tangle of branches and logs came from a tree that fell across the road from the far right side and put that crease into the guide rail. The vertical stump seems unrelated to that incident.

A bit of rummaging at the base of one post produced a victim:

Tree-smashed guide rail - sheared bolt - side
Tree-smashed guide rail – sheared bolt – side

The impact produced enough force to turn the rail brackets into guillotine metal shears against the posts:

Tree-smashed guide rail - sheared bolt - end
Tree-smashed guide rail – sheared bolt – end

It’s not a clean shear cut, which isn’t surprising under the circumstances.

An ordinary ½-13 Grade 8 bolt has a 17 k pound proof load: popping that bolt required a mighty oomf.

Memo to Self: stay indoors during windy storms!

Laser Engraving vs. Acrylic Mirror: Scattershot

The improved Holly Mirror Coaster looks pretty good:

Holly Coaster - overview
Holly Coaster – overview

Until you realize some of those specks aren’t surface dust and take a closer look:

Holly Coaster - mirror speckles 1
Holly Coaster – mirror speckles 1

The surface scratches are doubled by their reflection in the bottom mirror. The little dots that aren’t doubled reveal marks in the mirror surface itself.

An even closer look:

Holly Coaster - mirror speckles detail
Holly Coaster – mirror speckles detail

As nearly as I can tell, those are random speckles caused by the laser tube firing when it shouldn’t, due to the chaotic nature of the gas discharge going on inside.

In this case, they cause defects in the mirror coating allowing alcohol from the fat-tip permanent markers coloring the engraved areas to hit the acrylic. The starbursts come from stress cracks around the punctures.

Peering even closer shows similar cracks along the edges of the colored areas:

Holly Coaster - mirror speckles tight detail
Holly Coaster – mirror speckles tight detail

Not much to do about the random speckles, but it’s obvious I must up my coloring game.

Which would be significantly easier if rattlecan spray paint sprayed at winter temperatures …

Laser Cutter vs. Mirrorshades: Rear

Continuing with the same battered sunglass lens:

Laser vs sunglasses - beam front overview
Laser vs sunglasses – beam front overview

It’s now oriented with the back side of the lens toward the unfocused beam going into the laser head.

The front surface remains undamaged after two pulses at 500 ms 50% power:

Laser vs sunglasses - beam rear - front overview
Laser vs sunglasses – beam rear – front overview

The red disk in the middle of both wounds is new this time.

As seen from the rear, the first pulse shattered the rear glass layer:

Laser vs sunglasses - beam rear - A
Laser vs sunglasses – beam rear – A

The image is about 7 mm from side to side.

A chip of glass popped out of the upper part of the fracture, but the other pieces remained in place.

The distinct blue ring is 3 mm OD and marks the inner boundary of a purple disk surrounding the central burn. The disk appears only in reflected light (which is impossible to photograph with any setup I can manage), suggesting it comes from diffraction in a surprisingly uniform air layer blown between the glass and the plastic polarizing sheet.

Also seen from the rear, the second pulse produced a neater wound:

Laser vs sunglasses - beam rear - B
Laser vs sunglasses – beam rear – B

The blue ring is again 3 mm OD and the image is 7 mm across.

The central red spot probably comes from damage to the polarizing sheet.

The most surprising things, at least to me, didn’t happen:

  • the glass lens didn’t disintegrate
  • the laser beam didn’t punch completely through

Protip: Don’t depend on ordinary glasses, even fancy sunglasses, to protect your eyes from CO₂ laser beams.

Laser Cutter vs. Mirrorshades: Front

Well, a shattered lens found beside the road on a walk:

Laser vs sunglasses - focused overview
Laser vs sunglasses – focused overview

The battered frame has enough information to suggest they were once rather fancy. At this point, all that matters is they have two glass layers separated by a dark plastic polarizing film, with a gold-ish metallized front glass surface.

I fired the two pulses (on the left side of the obvious crack) at the front of the lens, both at 100 ms / 70% power:

Laser vs sunglasses - overview
Laser vs sunglasses – overview

Neither pulse penetrated the lens.

The smaller zit was fired in the position shown in the first picture, with the focal point more-or-less at the top surface of the lens. As seen from the front:

Laser vs sunglasses - focused front
Laser vs sunglasses – focused front

The outer part of the damaged area is about 0.5 mm in diameter. The heat around the damage seems to have cleared away all the schmutz on the lens; those things that look like scratches are oily smears and road dirt.

Seen from the rear:

Laser vs sunglasses - focused rear
Laser vs sunglasses – focused rear

The rear surface is blistered, but doesn’t have a hole, so I think the beam melted the glass and inflated a cavity along its path.

I then perched the lens in the unfocused beam path, with paper taped over the laser head opening to keep any fragments off the mirror and focus lens:

Laser vs sunglasses - beam front overview
Laser vs sunglasses – beam front overview

The beam produced the larger scar and also blasted off a ring of crud around the wound, as seen from the front surface:

Laser vs sunglasses - beam front
Laser vs sunglasses – beam front

The beam seems to have shattered a thin layer under the metallization, but didn’t do any deeper damage. The rear surface is undamaged and the paper didn’t have a scorch mark.

They’re not laser safety glasses, but at least they didn’t disintegrate.

Protip: do not lie on the laser platform and stare upward into the laser head, even while wearing fancy polarized mirrorshades.