So I clamped it to the Sherline’s tooling plate and milled off the rim:
Smashed Glass Coaster – meniscus removal
Given the Sherline’s cramped work envelope, all the action took place along the rearmost edge, requiring eight reclampings indexed parallel to the table with a step clamp.
The cutter cleared off everything more than 0.3 mm above the surface of the glass chunks. I could probably have gone another 0.1 mm lower, but chopping the bit into the edge of a shattered glass fragment surely wouldn’t end well.
Polishing the dark gray milled surface might improve it slightly, at the risk of scuffing whatever poured epoxy stands slightly proud of the glass:
Smashed Glass Coaster – leveled edge
Perhaps if I define it to be a border, everybody will think it was intentional.
The petals stand slightly proud of the black top frame, as the colored sheets were marginally thicker than the black sheet, but it looks OK in person. They’re all epoxied to a transparent base plate, so the bottom view is pretty much the same:
Cut Acrylic Coaster – bottom
Because the bottom is perfectly smooth, I think it looks better than the top, which shows irregularities around the petals where the epoxy didn’t quite fill the gaps. There is one small bubble you won’t notice if I don’t tell you about it.
I laid a small bead of epoxy around the perimeter of the base, laid the black frame in place, ran a bead along the midline of each petal shape plus a drop in the round part, laid the petals in place, and hoped I didn’t use too much epoxy. It turned out all right, with only a few dribbles down the edge that wiped off easily enough.
I peeled the protective plastic off the top while the epoxy was still tacky, which pulled far too many fine filaments across the surface:
Cut Acrylic Coaster – frayed top
After the final cure, I managed to scrape most of them off with a thumbnail; I hope to never make that mistake again.
As you might expect, acrylic plastic’s pure saturated colors wipe the floor with Sharpie-scribbled white chipboard:
Chipboard coaster – rounded petals – front vs back cut
The black frame makes the whole thing overly dark, so the next attempt should use white or perhaps a transparent layer atop a mirror base.
Having run the Newmowa NP-BX1 batteries through my old Sony HDR-AS30V helmet camera a few times, a plot seemed in order:
Newmowa NP-BX1 video duration vs charge
The cluster of dots shows most of our rides last about an hour.
The line is an eyeballometrical fit, slightly coerced to pass through the origin because that’s where it should go.
The 9.1 mA·hr/min slope is in reasonable agreement with past results, given different batteries and charger. The Keweisi meter emerged first from the box.
Straining the hr/min dimensional nonsense out of the slope suggests the camera averages 550 mA and 1.9 W. Derating those by a few percent to account for the recharge efficiency might be in order, but they’re surely in the right ballpark.
While I had the hatch open, I thought it would be interesting to look at the HV supply’s current waveforms:
HV laser power supply – current probe setup
The Tek current probe over on the right measures return current through the cathode wire, the point in the circuit where you might be tempted to install an ordinary analog (moving-coil) panel milliammeter, oriented so (conventional) current returning from the tube will produce a positive voltage.
Unfortunately, an analog meter isn’t up to displaying anything meaningful for this nonsense:
HV laser power supply – 5 mA-div – 50 ms 10 pct pulse
Admittedly, that’s a 50 ms pulse, during which an analog meter would barely twitch. The vertical scale is 5 mA/div, so the highest peaks exceed 35 mA, more than twice the tube’s recommended “14-15 mA”.
A closer look at the pulse startup waveform:
HV laser power supply – 5 mA-div – 50 ms 10 pct pulse – detail
HV laser power supply – 5 mA-div – 50 ms 10 pct pulse – tight detail
That’s at 10% PWM, close to the threshold below which the laser just won’t fire at all. The power supply must ramp up to produce enough voltage to fire the tube while simultaneously limiting the current to prevent the discharge from sliding down the negative resistance part of its curve.
Apparently this supply isn’t quite up to the task.
A 10 ms pulse at 50% PWM gives the supply enough time to stabilize the current:
HV laser power supply – 5 mA-div – 10 ms 50 pct pulse
The 14-ish mA at the tail end of the pulse (note the baseline offset) matches my previous 13 to 14 mA measurements as closely as seems reasonable. That 2 ms of hash on the leading edge suggests the start of each cut or engraving line will be a bit darker than you might expect.
Another 10 ms pulse, this time at 99% PWM:
HV laser power supply – 5 mA-div – 10 ms 99 pct pulse
The peak 24-ish mA matches the previous measurements. Note that the peaks in all the previous pictures exceed the 99% PWM current level.
AFAICT, all PWM values below about 25% produce equivalent results: random current spikes with unpredictable timing and amplitude. Changing the PWM value does not affect the (average) tube current or laser output power in any predictable way.
Some samples to illustrate the point, starting with a different 50 ms pulse at 10% PWM than the first one up above:
HV laser power supply – 5 mA-div – 50 ms 10 pct
A 50 ms pulse at 15% PWM:
HV laser power supply – 5 mA-div – 50 ms 15 pct
A 50 ms pulse at 20% PWM:
HV laser power supply – 5 mA-div – 50 ms 20 pct
A 50 ms pulse at 25% PWM:
HV laser power supply – 5 mA-div – 50 ms 25 pct
Now, that last one is different. After the hash during the first 8 ms or so, the power supply actually produces a stable 5 mA beam current, which is roughly what I measured using the power supply’s meter.
However, the other three are pretty much identical: the 10% PWM pulse does not delivers half as energy as the 20% PWM pulse. The waveforms may be different, but not in a meaningful or consistent way: the two 50 ms 10% pulses are different, but you’d (well, I’d) have trouble separating them from the 20% pulse.
To summarize:
The first several millisconds of any pulse will consist of randomly distributed spikes with very large tube currents.
For PWM values greater than 25%, the tube current will settle down to the corresponding current after 5 to 10 ms. Before the current settles down, the tube will be firing those random spikes.
For PWM values less than 25%, the tube current never settles down: the entire pulse, no matter how long, will be short, high-intensity spikes, without a consistent DC-ish level.
No matter what an analog meter might show.
I have no way to know if this power supply is defective, but I’ll certainly ask …
I think the Total job laser on time line says the power supply failed after firing the laser for a little over eight hours. The OMTech manual says the laser tube should last 1000 to 2000 hours (low vs high power), which suggests I should stock up on power supplies.
Its replacement just arrived:
OMTech replacement HV supply
It (bottom) seems to be a knockoff of the original ZYE Laser supply (top), with a similar model number and a “serial number” resembling a date from last year. All the connectors matched up, which isn’t too surprising.
The three most interesting inputs:
L = controller’s active-low L-ON enable output
IN = controller’s PWM output
P = jumper to G (circuit ground) — not water flow sensor
Also note the two AC power-line terminals directly adjacent to the TEST button, then consider insulation and stand-off distances before poking the button with your index finger.
The power supply has a digital current meter, so I plotted output current against PWM input:
Laser Power Supply – mA vs PWM – overview
Taking more points at the low end, with vertical bars indicating single-digit flicker on the meter:
Laser Power Supply – mA vs PWM – 0 to 20 PWM
I have little reason to believe the meter reading indicates the true current with any accuracy and I know CO₂ laser output power does not scale linearly with the current.
But it’s cutting again, which is a step in the right direction.
The Smell of Molten Projects in the Morning 