Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
I set the layer speed at 250 mm/s = 4 ms / mm, then set PWM (a.k.a. “power”) for each test, and measured the results, which look like this for three power levels on corrugated cardboard:
Pulse Timing Pattern – cardboard – 10 20 30 pct
The scan interval of 0.2 mm produces distinct lines at 10% PWM, the lower limit of the laser’s range. The lines remain separate at 30%, although their width is definitely increasing.
Yesterday’s post explains the test wiring setup and the signals in the scope screenshots.
The 10% PWM current waveform looks like nothing you’d expect:
Tube Current – 10pct – 250mm-s – 5ma-div
The scope triggers at the start of a left-to-right scan line, with 50 ms devoted to ramping up the speed to 250 mm/s before the start of the vertical bar along the left edge and slowing down before reversing.
The green trace shows huge spikes in the laser current, not a well-defined DC current pulse, and they’re offscale beyond 30 mA at 5 mA/div. The baseline sits well above the 0 V line due to the AM502 amplifier’s breathtaking thermal drift; I occasionally touch it up, but the current really is zero between the pulses.
Similarly for 20% PWM:
Tube Current – 20pct – 250mm-s – 5ma-div
Even through there’s little visible difference between the 10% and 20% current waveforms, there’s a distinct difference in the actual beam power delivered to the cardboard.
At 30% PWM the beam current looks a bit more reasonable:
Tube Current – 30pct – 250mm-s – 5ma-div
The 2 mm = 8 ms bar on the right gives the current time to stabilize at 6 mA, but all of the pulses have at least 3 ms of spikes. The first pulse definitely looks worse, so it seems the power supply gets better as the scan line progresses.
At 40% PWM the beam current pulses look more like pulses:
Tube Current – 40pct – 250mm-s – 5ma-div
They still have 3 ms or so of those startup spikes, as seen in this closer look at the first pulse in a line, scaled at 10 mA/div (along with the PWM drive signal):
Tube Current – 40pct PWM first detail – 250mm-s – 10ma-div
The top of those spikes exceed 70 mA!
At 80% PWM, the current waveform looks like a damped tank circuit:
Tube Current – 80pct first – 250mm-s – 5ma-div
The 20 mA at the end of that pulse suggests the maximum tube current would be 25 mA, which is undoubtedly why OMTech recommends running at no more than 70% PWM = 17-ish mA.
The pulses start immediately after the L-ON signal goes active and stop promptly when it goes inactive, so there’s no question about the responsiveness. What baffles me is why the current looks the way it does.
I must figure out how to have the scope compute the RMS value of those spikes, with a sufficiently large mA/div setting to keep the entire range of the pulses on the screen.
Having seen some rather bizarre laser tube current waveforms from the replacement power supply (and an equivalent Cloudray supply I bought as a backup) in the OMTech 60 W laser, I finally got A Round Tuit for a closer look.
I tapped three signals from the Ruida KT332N controller by the simple expedient of crunching wires into the output terminal clamps along with their original ferrules:
KT332N controller – Tube Current test connections
From top to bottom:
X axis DIR: low = left-to-right motion = toward X+
Laser L-ON: low-active laser beam enable
PWM: pulse-width modulation laser power control
Those three cables pass through a small hole in the cabinet to the left of the hatch on their way to channels 1, 2, and 3 of the scope.
The PWM signal (cyan, channel 3) isn’t particularly useful, but a quick look confirmed it is an active-high signal ticking along at 20 kHz, with a duty cycle corresponding to the selected laser “power”:
Tube Current – 40pct PWM first detail – 250mm-s – 10ma-div
The bottom trace (green, channel 4) is the laser tube current, as monitored by a Tek A6302 Hall-effect current probe around the tube’s cathode (low voltage return) lead:
HV laser power supply – current probe setup
This time around, I poked a bight of that overly long wire through the hole in the cabinet (just above the power-line earth ground terminal) so I could keep the probe outside the cabinet and close the hatch.
Minus the PWM signal, the scope looks like this:
Tube Current – 40pct – 250mm-s – 5ma-div
The top trace (yellow, channel 1) is the DIR signal, with a high-to-low transition triggering the scope when the X axis begins moving from left to right.
The second trace (magenta, channel 2) is the L-ON laser enable; the high-voltage power supply drives current through the laser tube only when L-ON is low.
The third trace (green, channel 4) is, as above, the laser tube current. The Tek AM502 amplifier sets the gain, with the scope channel always set to 10 mV/div with a 50 Ω input impedance, so I must put the current scale in the screenshot file name (which becomes the caption here).
With all that in mind, the next few posts will make more sense … and I can remember what I did.
The red-dot pointer on the OMTech laser cutter has the same problem as my laser aligner for the Sherline mill: too much brightness creating too large a visual spot. In addition, there’s no way to make fine positioning adjustments, because the whole mechanical assembly is just a pivot.
The first pass involved sticking a polarizing filter on the existing mount while I considered the problem:
OMTech red dot pointer – polarizing filter installed
The red dot pointer module is 8 mm OD and the ring is 10 mm ID, but you will be unsurprised to know the laser arrived with the module jammed in the mount with a simple screw. Shortly thereafter, I turned the white Delrin bushing on the lathe to stabilize the pointer and installed a proper setscrew, but it’s obviously impossible to make delicate adjustments with that setup.
Making the polarizing filter involves cutting three circles:
OMTech red dot pointer – polarizing filter
Rotating the laser module in the bushing verified that I could reduce the red dot to a mere shadow of its former self, but it was no easier to align.
Replacing the Delrin bushing with a 3D printed adjuster gets closer to the goal:
Pointer fine adjuster – solid model
Shoving a polarizing filter disk to the bottom of the recess, rotating the laser module for least brightness, then jamming the module in place produces a low-brightness laser spot.
The 8 mm recess for the laser module is tilted 2.5° with respect to the Y axis, so (in principle) rotating the adjuster + module (using the wide grip ring) will move the red dot in a circle:
Improved red-dot pointer – overview
The dot sits about 100 mm away at the main laser focal point, so the circle will be about 10 mm in diameter. In practice, the whole affair is so sloppy you get what you get, but at least it’s more easily adjusted.
The M4 bolt clamping the holder to the main laser tube now goes through a Delrin bushing. I drilled out the original 4 mm screw hole to 6 mm to provide room for the bushing:
Improved red-dot pointer – drilling bolt hole
The bushing has a wide flange to soak up the excess space in the clamp ring:
Improved red-dot pointer – turning clamp bushing
With all that in place, the dimmer dot is visually about 0.3 mm in diameter:
Improved red-dot pointer – offset
The crappy image quality comes from excessive digital zoom. The visible dot on the MDF surface is slightly larger than the blown-out white area in the image.
The CO₂ laser hole is offset from the red laser spot by about 0.3 mm in both X and Y. Eyeballometrically, the hole falls within the (dimmed) spot diameter, so this is as good as it gets. I have no idea how durable the alignment will be, but it feels sturdier than it started.
Because the red dot beam is 25° off vertical, every millimeter of vertical misalignment (due to non-flat surfaces, warping, whatever) shifts the red dot position half a millimeter in the XY plane. You can get a beam combiner to collimate the red dot with the main beam axis, but putting more optical elements in the beam path seems like a Bad Idea™ in general.
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Applying a laser cutter to paper-like materials requires balancing two contradictory imperatives:
Hold the sheet flat to avoid distortions
Have nothing below to avoid schmutz on the bottom
This seemed like a good compromise:
Sheet Holder – Tek CC bottom deck
The orange 3D printed blocks hold aluminum miniblind blades:
Sheet Holder – steel sheet magnet pads
The curved slots hold the blades flush with the upper surface and align their top sides parallel to the laser beam, giving the beam very little blade to chew on near the focus point and allowing plenty of room below the sheet to dissipate cutting fumes.
The gold-ish squares are thin steel sheets covered with Kapton tape, painstakingly filed en masse from small snippets:
Sheet Holder – filed steel pads
The first iteration used precisely laser-cut refrigerator magnet pieces, in the expectation a crappy rubber magnet would provide just enough attraction to let a neodymium magnets hold the paper flat, without risk of blood blisters between fingers and steel:
Sheet Holder – ferrite magnet pads
As expected, contact with the neo magnet completely wiped away the alternating pole magnetism in the rubber sheet, leaving a weakly attractive non-metallic surface. Alas, the rubber had too little attraction through a laminated sheet of paper, so I switched to real steel and risked the blisters.
Most of the blocks are narrow:
Sheet Holder Bracket – solid model
The four corners are wider:
Sheet Holder Bracket – wide – solid model
They’re symmetric for simplicity, with recesses for the magnets / steel sheets on the top. The through-holes have recesses for M3 SHCS holding them to T-nuts in Makerbeam rails, with a slightly overhanging alignment ledge keeping them perpendicular to the rail.
The magnets come from an array of worn-out Philips Sonicare toothbrush heads:
Sheet Holder – magnet holders curing
They’re epoxied inside a two-piece mount, with the lower part laser-machined from 3 mm acrylic to put the two magnets in each assembly flush with the lower surface; the green area gets engraved 1 mm below the surface for the steel backing plate. The 1.5 mm upper frame fits around the plate and protrudes over the ends just enough for a fingernail grip:
Magnet Holder Cuts
The epoxy got a few drops of fuschia dye, because why not:
Sheet Holder – trimmed magnet holders
The garish trimmings came from slicing the meniscus around the lower part of the holder off while the epoxy was still flexy.
The holders must be flat for clearance under the focus pen:
Sheet Holder – focus probe clearance
Some experimentation suggests I can raise the pen by maybe 2 mm (with a corresponding increase in the Home Offset distance) , but the switch travel requires nearly all of the protruding brass-colored tip and there’s not much clearance under the nozzle at the trip point.
With all that in hand, it works fairly well:
Sheet Holder – Tek CC cutout
The lower deck has very little margin for gripping, which is why the four corner blocks must be a bit wider than the others.
The lamInator tends to curl the sheets around their width, so most of the clamping force should be along the upper and lower edges to remove the curl at the ends. This requires turning the whole affair sideways and deploying more magnets, which is possible for the smaller middle and upper decks:
Sheet Holder – Tek CC middle deck
Protruding SHCS heads on the four corners snug up against the edge of the knife-edge bed opening for Good Enough™ angular alignment.
Plain paper (anything non-laminated) seems generally flat enough to require no more than the corner magnets.
It’s definitely better than the honeycomb surface for fume control!
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The axis scale error, however, took me by surprise.The X axis travels on the order of 0.2 mm more along 250 mm, about 0.08%, than the Y axis, even after my tedious calibration. I must do that calibration again, because, as Miss Clavel observed in a different context, Something Is Not Right.
And, yes, that tiny difference is enough to misalign the last few fingers with their holes, to the extent of requiring somewhat more than Gentle Persuasion with a plastic mallet.
While running some finger-joint test pieces, this happened:
Detached laser lens holder
The knurled ring just below the Tight→ label worked its way loose and released the lens holder tube collet, whereupon the whole affair fell out and dangled on the air hose & wires as the gantry continued to zigzag along the finger pattern.
As is my custom, I was watching the proceedings and managed to poke the controller’s STOP button, which was a mistake. What I should have done was slap the EMERGENCY STOP mushroom switch, because the STOP button just tells the controller to cancel the current action and return to the home position, which resulted in dragging the lens holder across the plywood and platform.
The more typical laser cutter failure seems to be having the controller execute the Halt and Catch Fire instruction, resulting in at least a ruined workpiece, sometimes a ruined laser, and occasionally a serious conflagration.
Lesson learned: practice slapping the Big Red Switch every now and then.
Laser-cutting alignment pin holes in the most recent smashed-glass coaster raised the question of whether it’s feasible to engrave a deep recess around a hole with Good Enough accuracy for things like recessed screw heads.
The test image:
Scan vs cut offset
The top two rows create engraved recesses and cut holes from 1.0 to 1.5 mm and the next two rows run from 1.5 to 2.0 mm. The bottom row has 1.0 mm holes centered in engraved pits from 0.5 mm to 3.0 mm; obviously, the first hole will subsume its pit.
The first pass looked promising, although the edges of the engraved pits seemed ragged:
Scan vs cut alignment – first test
Perhaps the replacement power supply has different timing than the original one?
I’m still surprised that the core of a laser-cut hole falls right out of the sheet, right down to a sliver from a 1 mm hole:
Cut hole cores
Recalibrating the scan offset got the errors down to 0.1 mm in either direction:
Scan offset – 300 200 mm-s 0.15mm offset
The lines in the middle column are spaced 0.15 mm apart at scan speeds of 300 mm/s (top) and 200 mm/s (bottom).
Another test pattern puts an engraved rectangle inside a dot-mode cut line with 1 mm spacing on all sides:
Scan vs cut alignment – 300 mm-s 0.15mm
That’s wonderfully accurate!
A few more test pieces later:
Scan vs cut alignment – test pieces
Returning to the pits-and-holes test, with one engraving pass:
Scan vs cut alignment – holes x1 engrave
That’s lined up to be looking directly down the 3 mm pit in the lower right, which looks fine.
Two engraving passes makes the pits deeper (nearly through the 2.5 mm arylic) and somewhat messier, but still nicely aligned with the holes:
Scan vs cut alignment – holes x2 engrave
Engraving the recess before cutting the hole seems to produce a better result, perhaps because both the engraving and the cutting encounter uniform surfaces.
All in all, this worked out better than I expected.
The Smell of Molten Projects in the Morning 