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.
Making the world a better place, one piece at a time
Adding (fake) rivets to the COB LED shade brackets definitely improves their appearance:
I cut new shades from vintage clear acrylic sheet, with more aluminized mylar attached to the lower surface: you can barely see the COB LED strip through the reflecting surface.
Depending on how you arrange all the hardware hanging on the nozzle, the shades can collide with something at the home position in the far right corner:
Definitely a step up from cardboard …
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:
Three of those form a corner into which you can tuck victims of the same general size:
Those pointy MDF spikes should start with slightly rounded tops, because that’s what they’ll look like after a few uses:
I also made a low-profile stop for victims lying directly on the honeycomb for engraving:
The SVG images include a nested version to tile across random MDF leftovers.
The SVG images as a GitHub Gist:
When a cardboard base plate under metal spikes would pose a fire hazard, you can position magnetic spikes where they’re needed:
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:
A cloud of combustible gas doesn’t pose a threat under there:
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:
Buy all the parts in lots of 100 to have supplies for other adventures!
The Gentec ED-200 optical joulemeter from the Box o’ Optical Stuff is so thoroughly obsolete that no datasheet exists for it anywhere online:
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:
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:
Which looks a lot like this 10 ms pulse at 50% duty cycle:
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 …
The Box o’ Optical Stuff disgorged an ancient Gentec ED-200 Joulemeter:
It’s an optical pyrometer producing, sayeth the dataplate, an output of 10.78 V per joule of energy applied to its matte black absorber. Whether it’s accurate or not, I have no way of knowing, but aiming the business end toward the sun and waving my fingers over it produced a varying voltage, so there was hope.
It has a 1/4-20 socket on one side and my spare magnetic mount expects a 3/8 inch rod, so I drilled a suitable hole in a suitable aluminum rod and cut the head off a suitable bolt:
A dab of Loctite intended to secure bushings completed the assembly:
I later replaced the nut with a finger-friendly nylon wingnut.
Which allows a measurement setup along these lines:
The white disk atop the sensor is a homebrewed target to indicate the active sensor area and its center point:
The 1 mm graticule lines give a jogging suggestion to hit the center, assuming you (well, I) manage to hit anywhere on the target at the first shot. The beam is supposed to fill most of the central region, which is obviously not going to happen here, and it must not be focused to a pinpoint. The previous owner (or his minions) put a few scars on the surface and I expect to make similar mistakes.
Early results look encouraging:
The SVG image as a GitHub Gist:
I added the two mounting ears in anticipation of putting the joulemeter in the beamline between the mirrors to measure their loss.
Although the OMTech laser controls the laser power supply with a key-lock switch, there’s little visible difference between the OFF and ON positions. Having occasionally mistaken it in both directions, this seemed like a useful addition:
The strip of black duct tape below the lock muffles the rattle of the triangle hatch key against the metal cabinet.
Two snippets of foam tape hold the knob to the lock cylinder, making an admittedly tenuous connection, but the knob fits around the outside of the switch housing with minimal clearance and doesn’t shouldn’t suffer any torque or pulling, so it might work.
The solid model looks about like you’d expect:
Unfortunately, it has no good orientation for printing, so I let PrusaSlicer generate support material inside the knob:
Suffice it to say: removing all that plastic did not go well.
I eventually grabbed the knob in the lathe and bored the interior out to its more-or-less proper dimensions, figuring nobody would ever notice the carnage, and it worked reasonably well. In the unlikely event I need another pointer, I’ll add a support spider to hold up the interior with minimal contact and less plastic.
Yeah, the laser really needs a stack light showing its condition and safety status …
The OpenSCAD source code as a GitHub Gist:
| // Indicator for OMTech laser power lock | |
| // Ed Nisley KE4ZNU 2022-04-09 | |
| KnobOD = 35.0; | |
| KnobHeight = 22.0; | |
| KnobTaper = 4.0; | |
| PointerLength = 45.0; | |
| PointerThick = 3.0; | |
| TipOD = 2.0; | |
| /* [Hidden] */ | |
| //—— | |
| Protrusion = 0.1; // make holes end cleanly | |
| HoleWindage = 0.2; | |
| module PolyCyl(Dia,Height,ForceSides=0) { // based on nophead's polyholes | |
| Sides = (ForceSides != 0) ? ForceSides : (ceil(Dia) + 2); | |
| FixDia = Dia / cos(180/Sides); | |
| cylinder(d=(FixDia + HoleWindage),h=Height,$fn=Sides); | |
| } | |
| //———- | |
| // Create part | |
| // Plenty of magic numbers from actual measurements | |
| module Pointer() { | |
| difference() { | |
| union() { | |
| linear_extrude(height=PointerThick) | |
| hull() { | |
| circle(d=KnobOD,$fn=24); | |
| translate([PointerLength – TipOD/2,0]) | |
| circle(d=TipOD,$fn=12); | |
| } | |
| cylinder(d=KnobOD,h=KnobHeight – KnobTaper,$fn=24); | |
| translate([0,0,KnobHeight – KnobTaper – Protrusion]) | |
| cylinder(d1=KnobOD,d2=KnobOD – 3.0,h=KnobTaper + Protrusion,$fn=24); | |
| } | |
| translate([0,0,-Protrusion]) { | |
| PolyCyl(29.0,14.0 + Protrusion,24); | |
| PolyCyl(24.0,14.0 + 5.0 + Protrusion,24); // leaves clearance under pointer | |
| } | |
| translate([0,0,KnobHeight]) | |
| cube([12.0,2.0,2*KnobHeight],center=true); | |
| } | |
| } | |
| //———- | |
| // Build it | |
| Pointer(); |
And doodles giving the dimensions of the key lock, not all of which can be true at the same time:
With the solid state relay switching the air assist pump, an air flowmeter seemed like it would come in handy:
It’s stuck to the lip inside the top hatch on the right side of the cabinet, which may not be the most convenient location, but keeps it out of the way and doesn’t require much additional tubing.
The 6 mm tube kit included some (1/8 NPT?) push fittings that came heartbreakingly close to matching the flowmeter’s internal threads:
Given that the air pump doesn’t produce much pressure, two snippets of 1/4 inch silicone tubing suffice to couple the blue 6 mm tubing to the flowmeter’s barbs:
The run from the air pump to the flowmeter is now new blue tubing, with the original black tubing running through the drag chain to the laser nozzle:
Replacing a number of overly tight cable ties along the way may remove enough restrictions to counterbalance the additional tubing.
Opening the flowmeter’s valve all the way puts 14 l/m = 0.5 CFM through the nozzle. I have no idea of the proper rate, other than more is better while cutting and less is better for engraving.
Four years ago, Russ Sadler laid out the plumbing required to automatically select high and low flow air assist, which seems like a worthwhile project.
The Smell of Molten Projects in the Morning 
