Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
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.
It’s generally accepted that laser cutter performance varies across the platform due to differences in path length, with (in my OMTech 60 W machine) the rear left corner having more power because it’s closest to the laser tube and the front right corner having less power because it’s farthest away.
HLP-200B Laser Power Meter – 60 W across platform measurements
I was mildly surprised at the minimal path length difference between the two corners and the center, but it’s due to the meter case reducing the distance along the X axis without a similar change along Y. In real life, you’d snuggle the HLP-200B sensor against the boundaries of the platform and measure the corresponding distances.
Given the size of the standard deviation bars, you can surely draw different conclusions, but the linear fit suggests the beam loses 3.5 W per meter of path length: 3.9 W from left rear to right front. Using meters for the distance multiplies the coefficient by 1000 and brings the digits up out of the noise; don’t believe more than two digits.
Although the beam diverges, the HLP-200B sensor is much larger than the beam and captures all the energy even in the front right corner, so beam divergence doesn’t matter and any square-law effect doesn’t apply.
If I had measured the power at the tube exit, it would be around 34 W and the error bars would surely justify that expectation, too.
Assuming the path loss in watts is proportional to the tube exit beam power, calling it 10% would be about right. That would definitely reduce the cutting performance in the front right corner if the power setting was barely adequate elsewhere on the platform.
Just to see if it worked, I tried measuring the path length between the laser tube exit and various spots on the platform with a laser distance measuring tool / rangefinder:
Laser Path Length setup – distance meter
That is a reenactment based on actual events.
The trick is to put a retroreflective panel at the tube exit:
Laser Path Length setup – retroreflector
The key under the tube comes from the key switch on the front panel, which is locked in the OFF position. That way, I can’t fire the CO₂ laser without opening the rear hatch to retrieve the key, whereupon I’ll most likely notice the retroreflective target I forgot earlier.
Protip: Always set things up so you must make two mistakes before the bad thing happens. I’m certain to make one mistake, but I can generally catch myself before making the second mistake.
Then it’s just a matter of positioning the base of the rangefinder on the laser head and convincing the targeting dot to go backward through the mirrors to the retroreflector:
Laser Path Length setup – retroreflector target
Which is a reenactment with a laser pointer through Mirror 2 to Mirror 1 to the reflector. If I had a few more hands, this stuff would be way easier.
Then drive the laser head around the platform and make measurements:
Path length measurements
The distances down the left side are at the Mirror 2 entrance aperture, the rest are at the Mirror 3 entrance on the laser head. I think the measurements are within ±50 mm of the “true” path length at any given spot, because I did not jog the head to exact coordinates. The two values in the front right corner suggest ±10 mm repeatability with my slack process and cross-checking the various differences along the axes comes out reasonably close.
Don’t believe all the digits.
Doing this for real would involve figuring the offset from the Mirror 3 entrance to the HLP-200B Laser Power Meter target, then positioning the rangefinder at that point:
HLP-200B Laser Power Meter – platform center
My rangefinder (an ancient Bosch GLR_225) can use four different measurement origins; I used the default “end of the case” setting, put that end flush-ish against the mirror entrance aperture, and declared it Good Enough™.
Those are flipped from the as-printed orientation: the orange ring builds upward, starting with two concentric threads on the platform.
The normal aligned joint is on the right above, with a closer look here:
Double Gear fidget toy – normal joInt
The scarf joint has a offset between layers:
Double Gear fidget toy – scarf joint
The PrusaSlicer visualization shows the effect, looking up from below the platform:
Double Gear fidget toy – scarf joint visualization
The blue PETG-CF parts have no visible seams anywhere with either setting, probably because the stuff swells slightly and obliterates any subtle differences.
Scarf joints don’t make much difference for a fidget toy, but should improve the outcome for more critical circular / spherical models.
I want to put the HLP-200B Laser Power Meter at the tube’s exit, just upstream from Mirror 1, where it can measure the laser’s power output before the mirrors get into the act. Reaching the Pulse button on the machine console requires much longer arms than any normal human can deploy, plus a certain willingness to lean directly over a laser tube humming with 15 kV at one end.
Perusing the KT332N doc brings up a hint, blocked in red so you can make some sense of it:
Another few minutes produces the box from Trocraft Eco, which is not quite thin enough for the switch (from my Box o’ Clicky Buttons) to snap into place, but a few dabs of hot melt glue hold it down:
Laser remote pulse button – installed
Double-sided foam tape sticks the box to the laser frame and the red-n-black cable snakes all the way across the back of the machine and through the electronics bay to the IN2 and GND terminals of the KT332N INPUT block:
Laser remote pulse button – Ruida KT332N wiring
With the laser head parked at a safe spot and all interlocks happy, it works:
Laser remote pulse button – demo
That is a re-enactment, because I lack sufficient dexterity to handle a phone with my left hand, poke the button with my right finger, and not damage anything important.
The general idea is to make it very difficult to inadvertently press that button: you must want to fire the laser with the tube compartment hatch up (it has no interlocks) and the control panel out of sight on the top-front of the machine.
Setting the power to 30% and putting the meter in harm’s way:
HLP-200B – Laser tube exit
Again, a reenactment based on actual events.
Five pulses later:
40.8
W
42.4
42.3
41.2
40.7
41.5
W avg
0.82
W std dev
For the record, those five pulses dumped about 5 × 42 W × 10 x ≅ 2000 W·s = 2 kJ into the meter, raising it from “chilly basement ambient” to “be careful where you hold it”, thus making the meter’s aluminum case the least-efficient handwarmer in existence.
The large standard deviations prevent firm conclusions, but, yeah, the power at the tube exit seems about right, before two mirrors and ≅800 mm of path length take their toll.
PrusaSlicer can recognize “things that look like logos” and process them with two different materials, so I tried it out with some plant signs:
Plant Signs – 50pct scale
They came out surprisingly well, particularly for characters with two adjacent filament threads:
Plant Signs – 50pct scale – 2-stroke
Smaller characters with single threads show more stringing, a characteristic of PETG, but it brushes off easily enough:
Plant Signs – 50pct scale – 1-stroke
While the existing text isn’t nearly as informative as real plant tags, they’re surely more durable and a chunkier font would improve both printability and readability.
I suggested Mary hand them out to any of her gardening cronies in need of a chuckle …
The Smell of Molten Projects in the Morning 
