OMTech 60 W Laser: Head Alignment Puzzle

Having established that the laser beam comes out of the lens perpendicular to the honeycomb platform surface, this seemed peculiar:

OMTech 60W beam alignment - head X plane
OMTech 60W beam alignment – head X plane

The mirror 3 + lens tube has a distinct tilt when seen from the +X direction (the right side of the machine, so it’s in the YZ plane, I suppose).

This is not matched when seen from the -Y direction (front):

OMTech 60W beam alignment - head Y plane
OMTech 60W beam alignment – head Y plane

The tilt does not seen to correct for a misalignment in the X axis rail or the brackets attaching the head to the linear bearing.

The beam seems properly centered on the lens and nozzle, so I’m loath to twiddle the alignment just to see what happens. One of these days, for sure, that must happen …

OMTech 60 W Laser: Improved MDF Spikes and Stops

A bit of tinkering suggested I needed a way to repeatably position stock sheets on the honeycomb, so I conjured stops that would be slightly taller than the magnetic spikes:

Improved MDF Honeycomb Spikes - first pass
Improved MDF Honeycomb Spikes – first pass

Three of those form a corner into which you can tuck victims of the same general size:

Improved MDF Honeycomb Spikes - stock alignment
Improved MDF Honeycomb Spikes – stock alignment

Those pointy MDF spikes should start with slightly rounded tops, because that’s what they’ll look like after a few uses:

Improved MDF Honeycomb Spikes - alignment stops
Improved MDF Honeycomb Spikes – alignment stops

I also made a low-profile stop for victims lying directly on the honeycomb for engraving:

Please Close The Gate - engraved
Please Close The Gate – engraved

The SVG images include a nested version to tile across random MDF leftovers.

The SVG images as a GitHub Gist:

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OMTech 60 W Laser: Magnetic Honeycomb Spikes

When a cardboard base plate under metal spikes would pose a fire hazard, you can position magnetic spikes where they’re needed:

Magnetic Honeycomb Spikes - acrylic
Magnetic Honeycomb Spikes – acrylic

The 12 mm neodymium magnet is slightly larger than a single honeycomb cell, so it wants to center itself atop a cell. The stainless steel button head screw sits in the magnet’s countersunk hole and protrudes just enough to make sure the spike doesn’t slide sideways unless you want it to:

Magnetic Honeycomb Spikes - parts detail
Magnetic Honeycomb Spikes – parts detail

A cloud of combustible gas doesn’t pose a threat under there:

Magnetic Honeycomb Spikes - MDF
Magnetic Honeycomb Spikes – MDF

The thin red beam comes from the targeting laser on the back of the nozzle.

Storage is easy: just smush a handful of the things against the side of the laser cabinet:

Magnetic Honeycomb Spikes - storage
Magnetic Honeycomb Spikes – storage

Buy all the parts in lots of 100 to have supplies for other adventures!

Gentec ED-200 Optical Joulemeter: Oscilloscope Comparison

The little DSO-150 oscilloscope has a 1 MΩ || 20 pF input with a 200 kHz bandwidth that should be entirely adequate for the OMTech laser’s millisecond-scale modulation signals from the Gentec ED-200 Optical Joulemeter. There is, however, only one way to be sure:

Gentec ED-200 - scope test setup
Gentec ED-200 – scope test setup

The two scope inputs are in parallel, so the joulemeter over on the far right sees a 500 kΩ load, half of the specified 1 MΩ load, with at least twice the capacitance. If the two scopes display pretty much the same result, then it’s good enough.

A 50 ms pulse at half power looks the same on both scopes:

  • Gentec ED-200 - 50 ms - DSO-150
  • Gentec ED-200 - 50 ms - Siglent

A 50 ms pulse at full power doesn’t quite top out:

  • Gentec ED-200 - 11V 50ms - DSO-150
  • Gentec ED-200 - 11V 50ms - Siglent

Given that the pulse duration should be less than the detector’s 1.5 ms risetime, using a 50 ms pulse is absurd. Right now I’m just looking at the overall waveform and detector range, not trying to get useful numbers out of the poor thing.

All in all, the DSO-150 will do just fine.

OMTech 60 W Laser vs. Gentec ED-200 Optical Joulemeter: Long Pulse Duration Power

The Gentec ED-200 Joulemeter is severely underqualified to measure the OMTech 60 W laser’s beam power, because the laser’s 1 ms minimum manual pulse width isn’t much shorter than the sensor’s 1.5 ms risetime and the maximum beam power is far too high for the sensor’s health. With that in mind, I set the PWM power to 50% = 30 W (grossly too high) and looked at the peak output voltage for a series of (far too long) pulse widths:

Rounding the detector sensitivity to 11 V/J says the 1.3 V peak at 5 ms corresponds to 120 mJ and 24 W:

Gentec ED-200 - 60W 50pct 5ms
Gentec ED-200 – 60W 50pct 5ms

The 3.3 V peak at 10 ms is 300 mJ and 30 W:

Gentec ED-200 - 60W 50pct 10ms
Gentec ED-200 – 60W 50pct 10ms

The 3.4 V peak at 15 ms is 310 mJ and 21 W suggests the PWM power output is not nearly as constant as one might expect, although the pulse width looks fine:

Gentec ED-200 - 60W 50pct 15ms
Gentec ED-200 – 60W 50pct 15ms

The 6 V peak at 20 ms is 550 mJ and 27 W, although the on-screen display obscures the top:

Gentec ED-200 - 60W 50pct 20ms OSD
Gentec ED-200 – 60W 50pct 20ms OSD

Another 20 ms pulse without the OSD produces a peak eyballometrically close to 6.4 V for 580 mJ and 29 W:

Gentec ED-200 - 60W 50pct 20ms
Gentec ED-200 – 60W 50pct 20ms

The KT332N controller in the OMTech 60 W laser has a pulse duration setting showing tenths of a millisecond, but (based on some additional measurements) the beam power can vary by 25% for successive pulses in the low millisecond range, so the pulse width resolution doesn’t seem to provide useful control.

Despite the over-long pulses, the calculated power corresponds surprisingly well with the nominal laser output power.

The 1 ms pulses used in LightBurn’s Dot Mode are consistent enough to punch essentially identical 0.2(-ish) mm holes in manila paper to mark the graticule:

OMTech 60W laser - beam alignment - focus detail - 2022-03-22
OMTech 60W laser – beam alignment – focus detail – 2022-03-22

They’re on 0.25 mm centers, with slight variations showing the difference between stepper resolution and positioning accuracy. The shorter graticule lines have three holes on one side of the center lines and four on the other, despite the design’s 1 mm length on both sides; I think there’s a missing dot on the side where the head starts the line, perhaps due to a picket-fence error.

The large beam hole came from two 10 ms pulses, one at the focal point and another 10 mm lower.

MTD Snowthrower Friction Drive Rebuild

During the last snowstorm of the season, the venerable MTD snowthrower carved a trench out of the garage and across the driveway, then abruptly stopped moving. The motor roared and the auger turned, but the drive clutch handle had no effect, so I dragged its carcass into the garage and we completed the mission by hand.

Popping the belly plate on the next sunny day revealed the problem: the jam nut (part 34) anchoring the Friction Disk Wheel (part 28) to the Friction Wheel Bracket Assembly (part 32) had gone missing:

MTD Snowblower - page 26 - friction drive parts
MTD Snowblower – page 26 – friction drive parts

Worse, the Wheel’s threaded shaft spent some time rattling around in the Bracket while chewing up its thread:

MTD Snowthrower - friction disk wheel - damaged thread
MTD Snowthrower – friction disk wheel – damaged thread

This would ordinarily be No Big Deal, but what you see of the shaft is all you get: it rotates freely in the bearing embedded in the Wheel with no way to hold it while cleaning up its threads.

Having already promised to replace the Wheel, I installed the new Wheel using a castle nut secured with a generous dollop of red Loctite, then tapped two of its castellations into the shaft’s slot as a mechanical anchor:

MTD Snowthrower - friction disk wheel - castle nut
MTD Snowthrower – friction disk wheel – castle nut

I really wanted to lay a nice hard roll pin along that slot through the nut, but there’s no convincing way to secure such a thing without a second nut. Maybe next time?

While I had the drive train apart, the sad state of the Wheel Shift Rod Assembly (part 29) became apparent:

MTD Snowthrower - wheel shift rod - worn
MTD Snowthrower – wheel shift rod – worn

I scuffed up the shiny wear mark, turned a suitable acetal bushing, filled the trench with epoxy, and squished the bushing in place:

MTD Snowthrower - wheel shift rod - acetal bushing
MTD Snowthrower – wheel shift rod – acetal bushing

The flange might hold it in place against the Frame Shift Bracket (part 18), which snugly contains the rest of the bushing against the epoxy, so the whole affair might outlast the next season’s first snowstorm. We shall see.

A nice new R-clip secures the Friction Wheel Bracket Assembly in place against the old washer:

MTD Snowthrower - friction bracket R-pin
MTD Snowthrower – friction bracket R-pin

You might want to insert it the other way, but the black plastic housing above it extends just far enough to thwart your (well, my) desire.

Gentec ED-200 Optical Joulemeter: Specs

The Gentec ED-200 optical joulemeter from the Box o’ Optical Stuff is so thoroughly obsolete that no datasheet exists for it anywhere online:

Gentec ED-200 - measurement setup
Gentec ED-200 – measurement setup

The best I could come up with, after many dead ends, is a 2001 capture from gentec-eo.com at archive.org with the barest hint of specifications:

Gentec ED-200 specs
Gentec ED-200 specs

The Max Energy Density spec suggests longer pulses are allowed to deposit more energy, probably because more time gives thermal diffusion an opportunity to spread the heat across the target; at CO₂ laser wavelengths that may not apply.

With the platform lowered as far as it goes, the ED-200 is 130 mm below the laser nozzle where the beam diameter is about 6 mm for an area of 0.3 cm². Ignoring the ideal Gaussian beam profile by smearing 60 W uniformly across the circle gives a power density of 200 W/cm², which means the laser pulse must be less than 0.5 W·s / 200 W = 2.5 ms to stay inside the power density limit.

I sincerely hope Gentec overbuilt and underspecified their detector.

Also, there’s a useful overview document from Genetc-eo.com, wherein it is written:

The Voltage Response
The result is a voltage pulse that rises quickly with the response time of the device to a level proportional to the laser energy (Figure 2). It then decays exponentially over a longer period of time that is a function of the pyroelectric device and load impedance. Figure 2 also shows that there is a longer recovery time to return to the initial state of the detector. This is a function of thermal phenomena and is not affected by the load impedance as are the rise and decay times. The integrated pulse energy over this period is proportional to the peak voltage.

Pulse Width Versus Rise Time

Usually the applied laser pulse must be shorter than the rise time of the detector for all of its energy to be represented by the peak voltage. Pulse energy received after the detector voltage has peaked will not be fully integrated into that value. For very long pulses, the peak voltage will actually represent peak power rather than pulse energy.

Gentec Energy Detectors, page 2

Figure 2 shows the overall waveform:

Gentec Energy Detectors - Figure 2
Gentec Energy Detectors – Figure 2

Which looks a lot like this 10 ms pulse at 50% duty cycle:

Gentec ED-200 - 60W 50pct 10ms
Gentec ED-200 – 60W 50pct 10ms

The pulse was 10 ms long, much longer than the 1.5 ms ED-200 risetime spec, but the overall shape looks right. Dividing the 3.3 V peak by the detector’s 10.78 J/V calibration value (11 J/V works for me) says the pulse delivered 300 mJ = 300 mW·s. Dividing 300 mJ by 10 ms gives 30 W, a beam power astonishingly close to the expected value.

The OMTech laser has a nominal 60 W output, although the tube life drops dramatically with regular use over 70% = 40 W. Power does not scale linearly with the laser tube current displayed on the power supply milliammeter, with the maximum value presumably preset to the tube’s 20 mA limit producing 60 W. The 20 kHz PWM duty-cycle chopping applied by the controller should linearly scale the average power downward from there.

It looks like the ED-200 might deliver reasonable results for millisecond-scale pulses at low PWM duty cycles, but it was obviously intended for much milder lasers.

On the other paw, it’s fully depreciated …