Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
That big yellowed sheet is 9 mm = 3/8 inch thick, with an inch of warp, entirely enough to keep it out of the laser cutter.
So I cleared some floor space and loaded the sheet with a collection of scrap steel sufficient to bend it the other way:
Acrylic sheet unwarping
The main weight comes from a perfectly sized snippet of railroad rail, topped off with steel disks, angle iron, and a rugged scissors jack
The sheet didn’t touch the floor, so the weight kept stress on the plastic and it gradually flowed the other way:
Mostly unwarped acrylic sheet
The center remains 5 mm higher than the edges and, given that cold-flowing is at best an exponential process, I recently declared victory and added it to the stockpile. I’ll gnaw off small pieces for any given project, so the remaining warp won’t matter.
The rule of thumb says a CO₂ laser cutters needs 10 W per millimeter of acrylic, so my 60 W laser will be somewhat underpowered. Two or three passes should suffice and, for sure, nobody will kvetch about edge quality.
With the laser cutter set up as before and the scope set up to calculate the time integral of the tube current, this happens:
Tube Current – 40pct pattern – integ – 10 ma-div
The trigger is the Boolean AND of the top two traces:
DIR signal = low = left-to-right X axis motion
L-ON signal = low = laser power supply output enabled
The bottom trace is the laser tube current at 10 mA/div, which is, conveniently, also the scope vertical axis calibration, so you can read “amp” wherever you see “volt”. The four pulses correspond to a single scan line through the usual test pattern:
Pulse Timing Pattern – 1 mm blocks
Scanning at 250 mm/s, each 1 mm block occupies 4 ms and the 2 mm block on the right is 8 ms long.
With all that in mind …
The white line in the scope screenshot is the time integral of the current, scaled at 50 µV·s/div and read as 50 µA·s/div due to the 10 mV/div = 10 mA/div equivalence. The integral is pretty much a straight line up and to the right during each pulse, showing that the power supply delivers a nearly constant average current despite the random-looking spikes in the shorter pulses, the oscillations at the start of the longer pulse, and the reasonably flat section after the current settles down.
Say it again: Totally did not expect that.
A closer look at the first pulse of a different line:
Today I Learned: The per-division scale of the white integral line is completely bogus in Zoom mode. Although the display still shows 50 µA·s/div (magenta text obscured near the middle), the integral line rises seven divisions. Some fiddling around showed there is norelation between the calibrations of the normal display and those in Zoom mode.
What is important: seeing the integral as pretty much a straight line with a reasonably constant slope, if you’re willing to ease over the bumps due to the current spikes. The slope of that line (yeah, the derivative of the integral, for a chunky definition of derivative) gives the average tube current.
Referring to the non-Zoomed trace, the integral rises by about 40 µA·s during each 4 ms pulse (and twice that for the 8 ms pulse), for an average current of 40 µA·s / 4 ms = 10 ma. Having previously established 100% PWM corresponds to 24 mA, 40% of 24 mA = 9.6 mA seems about as close as one might expect.
Each square is 1 mm on a side and the pattern runs at 250 mm/s, so the laser will be enabled for 4 ms. For example, the test setup shows the result of a pass at 50% PWM:
The two cursors mark the duration of one block, with the laser current in the bottom trace starting off with the usual off-screen spikes, then settling down to a constant-ish 13-ish mA for the rest of the block. The 13.74 mARMS value (the AM502’s 10 mA/div matches the scope’s 10 mV/div, so you can read mV as mA) includes some part of those spikes (the higher gain clips the tips), but most of it comes from the stable-ish portion.
The whole measurement set as a slide show for your amusement:
I expected the line to pass through the origin, which it most certainly does not. One could make up a story about how the 30% and 40% PWM points are Close Enough to the line to sorta pull the bottom end over to the left a little, but even that doesn’t explain the known-to-be-weird results below 30% PWM.
A better story might be that 30-ish% PWM produces the minimum current required to fire the laser tube. Operating below that current works, in the sense that the laser produces a beam, but it’s out of spec. Running above that current eventually lets the power supply reach an agreement with the tube as to the operating point.
As before, those measurements do not account for the reasonably consistent results of scorching some cardboard:
Laser power settings of 10, 20, and 30% obviously produce different results:
Pulse Timing Pattern – cardboard – 10 20 30 pct
However, the scope traces for PWM values under about 25% all look pretty much like this:
Tube Current – 10pct – 250mm-s – 5ma-div
Rather than a simple constant current source, the power supply produces very high amplitude current pulses for low PWM inputs, with no visible differences between any of the PWM values.
The scope can compute the RMS value of (a section of) the trace, so I aimed it at traces captured from the upper left block of this test pattern:
Pulse Timing Pattern – 1 mm blocks
Because the pulses have such a high amplitude, I set the Tek AM502 current amp at 100 mA/div to capture the entire pulse. Measuring a part of the trace without a signal gives the baseline noise level:
The scope display is 10 mV/div, so 1 mVRMS (close enough to the 894.4 µV reported just above the bottom label row) means 10 mARMS of noise. Given that 100% PWM corresponds to about 25 mA (DC-ish during the pulse), the RMS numbers may not have any significant figures.
A slide show of the results so you can page through them:
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.
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
The mudflap on my front fender rides low enough to snag on obstacles and the most recent incident (about which more later) was a doozy, breaking the left strut ferrule and pulling the bracket off its double-sticky foam tape attachment. Fortunately, the repair kit now has plenty of duct tape.
The Smell of Molten Projects in the Morning 