Category: Science

If you measure something often enough, it becomes science

PolyDryer Humidity vs. Spool Fill
The Basement Shop has 50±5% relative humidity, with the top held down by a hulking dehumidifier (plus a box fan stirring the air) and the bottom supported by being a basement. As a result, the 3D printer filament stabilized at about 50% RH, which seemed to work well enough for PETG.

Adding TPU to the stable called for better humidity control, so I set up a bunch of PolyMaker PolyDryer boxes with Auto-rewind spindles.

After a few weeks, though, I didn’t expect this:

PolyDryer humidity vs spool fill

That’s activated alumina desiccant, mostly because it’s reputed to have more capacity and a lower ultimate humidity than silica gel, but it likely doesn’t make much difference.

In addition to 25 g of desiccant in the PolyDryer meter case, I dropped five teabags holding 10 g each in the bottom of the box for more capacity. I measure the desiccant by putting 75.0 g into a cup, putting 25.0 g in the PolyDryer meter box (aided by a Polydryer Desiccant Funnel), 10.0 g into four teabags, and whatever’s left into the fifth teabag, thus eliminating rounding errors in the smaller quantities.

The stabilized humidity inside the boxes seems to depend on the amount of filament on the spool:
- Nearly full → 25% to 30% RH
- Half full → 20%-ish RH
- Nearly empty → 10% to 15% RH
I think the humidity level comes from the filament outgassing water vapor through its (limited) surface area on the outer layer around the spool. The difference between that rate and the desiccant’s ability to remove water vapor from the (unmoving) air in the box sets the stable humidity: more surface area → more water vapor → higher humidity.

After the filament eventually dries out, the humidity should decrease, but diffusion is a slow process. More likely, the humidity will remain stable as the printer pulls filament from the outer layer and exposes the somewhat wetter plastic within.

The heater and fan inside the PolyDryer base unit circulates hot air through the box around the spool, but depends on the desiccant to remove water vapor. Running the base unit for 6 or 12 hours makes little difference in the stabilized humidity, so I think the desiccant is doing the best it can as the filament outgasses more water vapor.

Using Air Exchanger vents seems to make no difference, likely because the desiccant must then pull more water vapor out of the incoming 50% RH basement air. A psychrometric chart says 50% RH air at 60 °F becomes 10% RH air at 120 °F, but moisture in the filament wrapped around the spool can’t escape any faster.

So, for example, a full spool of TPU starting at 25% RH:

PolyDryer humidity – TPU start

Six hours of drying pulls it down to 22%:

PolyDryer humidity – TPU finish

After sitting overnight it’s back at 25%:

PolyDryer humidity – TPU after 14 hr

Admittedly, that was with the vents in place, but the closed box started at 25% RH after sitting around for a week or so following a similar drying cycle.

The desiccant had absorbed 4 g of water since I put it in, so it hasn’t been entirely idle.

Which suggests 75 g of activated alumina desiccant is workin’ hard and doin’ swell in there, with the filament acting as an essentially infinite reservoir of water vapor.

I haven’t noticed any particular difference in PETG print quality and the TPU hasn’t gotten enough mileage to notice much trouble, but reducing the MMU3 buffer clutter was totally worth the effort.
2025-06-02
Sandisk 64 GB High Endurance MicroSD Card: End of Life

After about 7.5 years (!) the 64 GB card in my Sony HDR-AS30V helmet camera breathed its last:

SanDisk 64 GB MicroSD card – end of life

Over the course of several rides I noticed many video files ended prematurely or would not play. I gave up attempting to reformat the card in overwrite mode using the Official SD Card formatter after four hours, which says the wear leveler in the card has no spare capacity.

In round numbers, I ride 1700 miles a year at 12 mph, so the card recorded 1000 hours of 1920×1080 video at 60 frame/s, storing one 4.3 GB file every 22.75 minutes for a grand total of 12 TB of data.

Although that’s 188 times the capacity of the card, it rarely held more than an hour or two of data at any one time, because I copy the camera video files to a 3 TB USB hard drive after each ride. I don’t know how the exFAT file system interacts with the card’s wear leveling, but overall it’s much better than the non-high-endurance cards I’d been using way back when.

A new Sandisk 128 GB High Endurance card cost a third of what the 64 GB card did and, after setting the partition label to AS30V, it’s off to a good start:

Street Lamp Pole – Rombout House Ln – 2025-05-07

That’s the street lamp pole installed on the replaced base at the corner of Rt 376 and Rombout House Lane, with the barrels gradually being pushed closer and closer to the pole by turning traffic on the newly paved lane.

That pole is not going to see the end of this year.

Update: The barrels vanished this morning:

Street Lamp Pole – Rombout House Ln – 2025-05-08

Definitely the triumph of hope over experience.

2025-05-08
OMTech 60 W Laser: Engraving Wobbulation
Continuing the experiments on Y axis wobbling produced this shaky engraving:

Engraving – 100mm-s 0.25mm interval 9pct

The rectangle is 30×10 mm, with lines spaced 0.25 mm apart to simplify estimating distances (although I also have a measuring magnifier) and run at 100 mm/s to simplify converting distance to time. The lines alternate in direction, beginning with a left-to-right line at the bottom (which is bar-straight from the initial positioning move). The wobbles occur at the start of each line.

A closer look with blown contrast:

Engraving – 100mm-s 0.25mm interval 9pct – detail

The maximum error in the Y axis direction looks like 0.12 mm and damps out after 3 cycles. Each cycle covers 2.8 mm = 28 ms = 35 Hz.

The LightBurn Preview shows a 1.5 mm overscan distance and extrapolating the wobbulations leftward suggests the gantry starts the scan line with an overshoot due to the Y axis motion. The cycle-to-cycle damping is about 50%, so the initial overshoot (invisible in the overscan region) might be 0.25 mm, agreeing reasonably well with the 0.2 mm seen while cutting small squares.

The results above come from these settings:
- Layer speed: 100 mm/s
- Line interval: 0.25 mm
- Y acceleration: 2000 mm/s²
- Y start speed: 20 mm/s
I then made single-variable changes to the Engraving Parameters settings:

Line shift speed
- 500 mm/s
- 10 mm/s
Y Acceleration
- 200 mm/s²
Y start speed
- 30 mm/s
Today I Learned: The Y Start Speed (in mm/s) for engraving is capped by the Y Axis Jumpoff Speed (in mm/s², so perhaps the maximum change in speed), which is, in turn, capped at 80 mm/s.

Each of the variations produced a result visually indistinguishable from the image you see above: the error magnitude and oscillation frequency were identical.
One possible reason: None of those settings have any effect, because LightBurn doesn’t do whatever the Ruida controller defines as Engraving. However, changing both the Y start speed and the Jumpoff speed should have made at least a little change to the results and did not.

Another possible reason: Each 0.25 mm Y axis change requires 20.8 motor steps (either 20 or 21 at 12 µm/step), so the fancy tweaks lack space to take effect, the motor thumps 20-ish steps, and the gantry shakes the same way every time.

The closer you look, the worse it gets …
2025-02-25
OMTech 60 W Laser: Speed vs. Corner Radius Wobbulation

Experimenting with little squares showed the Y axis has a definite wobble:

Subpixel Zoo – Quattron RGBY Shifted – detail

Which suggested a simple test:

Cornering – overview

I adjusted the laser power to compensate for the speed, with the result being a line burned into white cardboard. The lines are a bit under 0.2 mm wide, roughly the width of the focused spot.

The controller settings for the X and Y axes:

KT332N – X Y Axis Parameters – 2025-02-18

The acceleration values may be affected by the limits in this section:

KT332N – Cut Engraving Parameters – 2025-02-18

Assuming the Y axis acceleration is 3000 mm/s², the RepRap calculator shows the Y axis speeds within the 30 mm distance along the vertical sides:

RepRap Accel Calculator – 3000mm-s2 30mm

Extracting the useful bits and lining them up for comparison:

Cornering – detail

The first column in the test results shows perfectly square corners have no problem at any speed, because the controller decelerates to nearly a stop before changing direction.

Rounding the corner to 0.5 mm introduces a distinct wobble in the Y axis that doesn’t change much, probably because the controller still decelerates as it approaches the corner.

The 1 mm radius corners show a distinct overshoot at all speeds. The peak overshoot doesn’t change much between 250 and 500 mm/s, because the RepRap calculator shows the machine barely reaches 250 mm/s by the middle of the side, so 500 mm/s isn’t any faster.

The first overshoot is about 0.2 mm, the first undershoot is a little over 0.1 mm, and the rest are barely visible.

The 2 and 4 mm radius corners have barely visible wobbles. Whether that is due to the head not flexing as much due to the lower acceleration around the larger radius I cannot say.

The machine may not follow the simple RepRap acceleration profile when approaching a corner, let alone a rounded corner.

I think attempting to reduce the overshoot by fiddling with the belt tension / hardware fasteners / whatever will be unavailing. The laser head runs on a linear rail along the gantry with plenty of unbalanced mass hanging off the bottom:

OMTech 60W beam alignment – head X plane

Moving the beam 0.2 mm on the platform by pivoting around the rail 6 inch = 150 mm above amounts to only 0.08°, far less than anything I can measure while adjusting the mechanics.

Slowing down doesn’t help nearly as much as I expected and rounding the corners makes it worse.

Word has it that much spendier machines behave better, which is both comforting and unhelpful.

2025-02-21
Laser-Engraved CD Stress Cracking

Given the cracking caused by vector patterns on CDs and DVDs, seeing stress cracks open up on large-area engravings came as no surprise:

Laser engraved CD cracking – D

They start smaller in the more closely engraved areas:

Laser engraved CD cracking – A

But eventually spread over the entire surface:

Laser engraved CD cracking – C

They’re not always straight:

Laser engraved CD cracking – B

And aren’t aligned with the engraving path:

Laser engraved CD cracking – B detail

My threat model says those discs are definitely unreadable …

2025-02-07
HLP-200B Laser Power Meter: Mirror Losses

With the manual laser pulse button in place, I measured the beam power at the entry and exit planes of Mirror 1 and Mirror 2, with the differences indicating something about the reflectivity (or lack thereof) of the molybdenum mirrors. Given that the losses are on the order of a few percent, tops, I expected this to be below the repeatability of the measurements.

The Mirror 1 entry point is basically the same as the laser tube exit:

HLP-200B – Laser tube exit

The Mirror 1 exit plane is perpendicular to that, just behind the mirror, but there is no way I can get a picture of the arrangement. Suffice it to say I do not want to ever put any body parts that close to an operating laser tube again.

The HLP-200B meter turned out to be exactly the right length to stand on its own in front of Mirror 2, although I needed a few test shots to figure out the lateral positioning:

HLP-200B Mirror 2 entry check

The Mirror 2 exit measurements were hand-held, with the meter braced against the mirror mount brackets on the gantry:

HLP-200B Mirror 2 exit

Without further ado, the results:

M1 Entry M1 Exit M2 Entry M2 Exit
35.5 31.2 30.3 32.9
28.3 30.6 29.1 32.6
31.8 22.8 27.8 28.9
30.3 29.0 29.4 28.5
26.9 28.4 28.7 27.0
31.1 31.7 28.6 26.9
30.7 29.0 29.0 29.5
2.99 3.27 0.84 2.67

The bold line gives the average of the six measurements at each position, with the sample standard deviation below that.

As expected, the pulse-to-pulse variations swamp any actual differences between the entry and exit power levels; Mirror 2 does not have a net power gain. A 2% loss in the mirror is 0.6 W at 30 W, obviously far too small for the HLP-200B meter to resolve.

I must once again set up the photocell to measure the stray IR scattered around the beam, measure the actual tube current, then see if the two vary as much as the HLP-200B says the beam power does.

2025-01-21
Laser-Engraved PETG / PETG-CF

Prompted by scruss’s report of successfully “engraving” PLA, I had to try this:

Laser engraved PETG-CF

It’s blue PETG-CF from the scrap box, done at 500 mm/s and 20% of a 60 W laser and came out looking really nice.

I did a pass at 10%, low enough that the laser barely fired, and the mark was, correspondingly, barely visible: no color change and only a slight depth. Obviously, you’d want to tune for best picture depending on whatever you were trying to achieve.

The results on black PETG, also from the scrap box, were somewhat less attractive:

Laser engraved PETG – bottom surface

That’s at 500 mm/s with power at 10% and 20, so the outcome definitely depends on the material. That surface was against the platform when it was printed on the Makergear M2, explaining the glossy smooth threads.

The other side was rougher and needed more power to punch a visible result into the plastic:

Laser engraved PETG – top surface

All in all, the PETG-CF result looks usable, particularly for small-ish annotations on a flat surface where full-on multimaterial printing would take forever without adding much value.

2025-01-07

M1 Entry	M1 Exit	M2 Entry	M2 Exit
35.5	31.2	30.3	32.9
28.3	30.6	29.1	32.6
31.8	22.8	27.8	28.9
30.3	29.0	29.4	28.5
26.9	28.4	28.7	27.0
31.1	31.7	28.6	26.9

30.7	29.0	29.0	29.5
2.99	3.27	0.84	2.67

Category: Science

PolyDryer Humidity vs. Spool Fill

Sandisk 64 GB High Endurance MicroSD Card: End of Life

OMTech 60 W Laser: Engraving Wobbulation

OMTech 60 W Laser: Speed vs. Corner Radius Wobbulation

Laser-Engraved CD Stress Cracking

HLP-200B Laser Power Meter: Mirror Losses

Laser-Engraved PETG / PETG-CF