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.

Category: Machine Shop

Mechanical widgetry

OMTech 60 W Laser: Path Length Measurements

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™.

2025-01-03
Christmas Non-Wrapped Box
Mostly to find out if I could do it:

Christmas Wrap custom box

The process:
- Fetch a printable wrapping paper pattern, ignore the watermark
- Resize / crop to fit a Letter page, print two copies on glossy photo paper
- Generate a suitable multi-sheet paper box in SVG format
- Import into LightBurn
- Rearrange / splice the parts to put the main box on one Letter size sheet and the end caps on another
- Fire The Laser to score (blue lines) and cut (red lines) the paper
- Assemble using a glue stick
- Fill with Christmasy stuff (your choice may vary)
- Tuck the flap, there’s no need to wrap!
The rearranged main box just barely fits across a Letter page:

Non-Wrapped Christmas Box – main piece – LightBurn layout

You can make many more end caps than you need:

Non-Wrapped Christmas Box – endcaps – LightBurn layout

Obviously, this makes no sense whatsoever in terms of box making, but the recipient smiled and that’s what I wanted.
2025-01-02
PrusaSlicer Scarf Joints

The release notes for PrusaSlicer 2.9 mention the addition of scarf joints on outer perimeters. Smooth joints seem like a Good Idea™, so I turned it on for comparison with a recent object:

Double Gear fidget toy – scarf vs normal

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.

2024-12-31
OMTech 60 W Laser: Manual Pulse Button

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:

KT332N Input bits

A few minutes with boxes.py produces a simple two-compartment box and a few minutes with LightBurn adds two holes:

Remote Switch Box – LightBurn layout

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 30% PWM measurements at the center of the platform came out slightly lower: 38.5 W average with a sample standard deviation of 2.2 W.

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.

The LightBurn SVG layout as a GitHub Gist:

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2024-12-30
Plastic Plant Signs

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 …

2024-12-26

HLP-200B Laser Power Meter: First Measurements

The HLP-200B Laser Power Meter arrives without much in the way of specifications:

The HLP-200B Laser Power Meter Handheld comes fully calibrated at 10.6 μm (CO2). Each laser power meter we calibrate is directly traceable to NIST absolute standards because we use GOLD standards as a reference for each calibration. You will obtain the most accurate result possible

A line in the description says “+/- 3% within the central section”, but that’s not much help. Back in the day, any error percentage referred to the meter’s full-scale value, which would be ±6 W for a 200 W meter.

So I plunked the meter in the middle of the laser platform:

HLP-200B Laser Power Meter - platform center — HLP-200B Laser Power Meter – platform center

Then took five measurements at each of ten power levels:

PWM %	10	20	30	40	50	60	70	80	90	99
°C	17.2	17.9	18.4	19.0	19.4	20.3	20.0	20.0	20.5	19.4
Tube Current	3	4	7	10	14	16	18	20	22	24
W	7.1	21.0	42.0	51.8	59.1	63.0	67.8	69.6	74.7	64.0
	6.0	19.8	37.2	48.9	52.7	56.0	65.1	69.6	72.4	71.8
	6.4	21.1	39.3	45.6	56.5	53.2	61.1	60.7	74.6	75.2
	5.6	17.8	37.1	40.4	55.3	53.2	55.1	64.2	74.9	73.5
	6.0	17.7	36.9	45.1	54.5	53.1	62.2	69.9	72.2	70.9
Avg Power	6.2	19.5	38.5	46.4	55.6	55.7	62.3	66.8	73.8	71.1
std dev	0.57	1.66	2.19	4.29	2.39	4.26	4.78	4.16	1.34	4.29

That’s easier to digest from a graph:

HLP-200B Laser Power Meter - 60 W platform center measurements — HLP-200B Laser Power Meter – 60 W platform center measurements

The absurdity of computing the sample standard deviation from five measurements taken at each power level does not escape me, but this just surveys the situation.

Earlier measurements of the tube current vs. PWM setting, using an RMS value computed by the oscilloscope’s firmware, produced a plot resembling the brown points (read the mA scale on the right) at the high end and differing greatly on the low end. These values come from the power supply’s digital meter, but the straight-line fit doesn’t look absurdly forced and the zero intercept seems plausible. I *assume* it’s actually measuring the tube current, rather than displaying a value computed from the PWM input, but I don’t know for sure.

The rather sketchy paperwork accompanying the laser had one handwritten “21 mA” seemingly corresponding to 60 W output, which looks approximately correct. The instruction manual has a table of power vs. current suggesting that 65-ish W corresponds to 18 mA, with 100 W at 23 mA; it’s unclear whether that is for the 60 W tube in the machine or applies to the entire range of available tubes. The manual recommends not using more than 95% PWM, with which I heartily agree.

Because my meter stand holds the target in the same position relative to the beam during successive measurements much better than I could by hand, I think the pulse-to-pulse variation comes from meter and tube repeatability.

Earlier measurements with a grossly abused Gentec ED-200 joulemeter suggested the laser has some pulse-to-pulse timing variation, down in the millisecond range, but produced roughly the right power for middle-of-the-range PWM settings. This meter integrates the beam power over about ten seconds, so I think variations will be due to (possible) tube power changes and meter repeatability, rather than timing errors.

Obviously, you must not depend on any single-shot measurement to fall within maybe 10% or several watts of the right answer.

With all that in mind and assuming the meter is delivering approximately the right numbers on average, the power supply overcooks the tube at any PWM setting above 50%. I’ve noticed some beam instability / defocusing over 80% while cutting recalcitrant materials, which is surely due to the tube not lasing properly. I generally avoid doing that.

The log fit to the measured power looks better than I expected, although I’m unprepared to compute natural logs in my head.

Hey, it’s my idea of a good Christmas present …

2024-12-25

HLP-200B Laser Power Meter: Holder / Stand

The overall measurement process for the HLP-200B laser power meter requires more coordination than I can muster on a dependable basis, so a third hand seemed in order:

HLP-200B Power Meter – target setup

In actual use, a pair of finger-crushingly strong magnets laid on the base hold it firmly to the honeycomb.

Because a CO₂ laser beam is invisible, the only way to know where it hits is to char a bit of paper:

HLP-200B Power Meter – target detail

With that evidence, I can jog the platform up-and-down and the gantry front-and-back to center the beam on the paper target and, thus, on the sensor behind it. That process happens at each test position across the platform:

HLP-200B Power Meter – targets

The meter shuts down a mere six seconds after completing each measurement, which means I must keep the lid open, listen carefully, and react quickly. Firing the laser thus requires defeating the lid interlock specifically wired to prevent that from happening:

Laser lid interlock sensor

Rather than install a switch to bypass the interlock, I taped a steel cover harvested from defunct electronics over the sensor:

Laser lid interlock sensor – bypassed

Which has the useful side effect of preventing me from closing the lid with the interlock defeated.

The holder is just slightly larger than the meter’s handle and some clamps produced a snug fit while the glue cured:

HLP-200B Power Meter – holder gluing

The holder keeps the meter sensor at the same position vertically and within about a millimeter horizontally. The laser beam seems to be around 5 mm in diameter (the scorches above come from the hottest central part), so the beam should hit the same position on the sensor during successive measurements, making them far more repeatable than my waving it around by hand.

The LightBurn SVG layout as a GitHub Gist:

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2024-12-23