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.

Tag: Laser Cutter

OMTech 60 W Laser: Engraving Scan Offset Adjustment

The fuzzy edges engraved on the acrylic test sample showed the need for scan offset adjustment:

Please Close The Gate – acrylic test piece

The problem arises from the finite delay between the controller turning the laser beam on and the rise time of the death ray energy at the focal point.

LightBurn can produce a calibration coupon (on Trolase laminated acrylic) to help explore the multidimensional parameter space:

Offset cal – zero offset – overview

The “Interval” value is the vertical (Y-axis) scan line spacing. The laser spot diameter is, at absolute best, about 0.2 mm on the focal plane, with the actual engraved line being smaller due to the energy distribution across the beamwidth and the power required to visibly damage the material, so a 0.1 mm interval should result a little bit of overlap between adjacent scan lines.

A closer look shows the serrated edges on the left and right sides of the engraved squares:

Offset cal – zero offset – detail

Peering at it through a measuring magnifier suggests the offset is a bit over 0.2 mm at 400 mm/s, corresponding to a 500 µs delay between laser turn-on in the rightward direction and turn-off in the leftward direction.

The LightBurn Scanning Offset Adjustment is half the measured distance, with an Initial Offset parameter to adjust the starting point of the first scan line. You measure the distance at each speed and fill in the table accordingly.

Iterating through offsets, speeds, powers, and intervals produces a series of test coupons slicing through the parameter space:

Offset cal – iteration

All in all, a 0.1 mm offset at 400 mm/s with 14% power (about 8 W) and 0.075 mm interval looks pretty good:

Offset cal – final offset – detail

Engraving various fonts:

Offset cal – text – overview

A closer look (left coupon on the top):

Offset cal – text detail

LightBurn linearly interpolates between table entries of offset values at specific speeds, so you must fill in several lines to give it something to munch on. The top text came from an offset table with two entries at 400 and 500 mm/s, which obviously wasn’t quite sufficient. The bottom text comes from a three-entry table:

LightBurn scan offset table

Which produces a better result, even at 500 mm/s and 20% power (12 W) on scrap acrylic:

Close Gate – test acrylic – overview

A closer look:

Close Gate – test acrylic – detail

Much better!

2022-05-16
Laser-cut Cutworm Collars

Mary, having had considerable trouble with cutworms in her gardens, routinely deploys cardboard collars around new plants:

Cutworm Collars – assembled

It seems cutworms trundle around until they find an edible plant, chew through the stem and topple the plant, then trundle off without taking another bite. A small cardboard barrier prevents them from sensing the plant: apparently, motivation to climb a short wall hasn’t yet evolved.

Up to this point, Mary applied scissors to tissue boxes, but I proposed an alternative with an adjustable fit to any plant:

Laser Cutting Cutworm Collars

A splayed cardboard box rarely lays flat, a condition enforced by a few MDF stops used as clamps.

Come to find out no two tissue boxes have identical dimensions, even boxes from the same brand / retailer, so lay out duplicates of the collar template to match your stockpile.

That was fun!

The SVG image as a GitHub Gist:

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2022-05-12
OMTech 60 W Laser: Adjustable Honeycomb Stops

When you (well, I) get fussy about angular alignment on the laser cutter’s honeycomb platform, an adjustable stop or two may come in handy:

Laser Honeycomb – Adjustable Pins

That’s a serving suggestion based on a true story, because I really wasn’t all that fussy about precise engraving alignment on those signs.

A more typical situation on a smaller scale:

Laser Honeycomb – Adjustable Pins – engraving

The scrap of MDF with three holes provides angular alignment for the little two-color acrylic test coupon, so you can tuck successive squares into the corner, hammer them with slightly different patterns, then compare the results.

The stops are an off-center hole (the ±3 text gives the offset) in an MDF disk with an acetal post:

Laser Honeycomb – Adjustable Pins – detail

The 3 mm SHCS provides a convenient way to turn the post and disk, so the threading isn’t critical. Sufficiently snug threading will let you turn the screw counterclockwise without loosening it, but that surely depends on how tightly the 8 mm section fits into the honeycomb. The larger top section is 9mm, cleaned up from the rod’s nominal 3/8 inch OD, for a jam fit into the 8.8 mm + 0.1 mm kerf hole.

The SVG images as a GitHub Gist:

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The ±5 mm offset disk may be more useful with larger items and now you know where those three holes came from.

2022-05-11
3D Elephant Head

Sometimes you gotta make silly things:

Trotec laser-cut elephant

It’s cut from 3 mm MDF, rather than the fancy Trotec Eco 1 mm cellulose sheet they recommend, which required embiggening the mounting slots in all the pieces.

It served as good, albeit tedious, practice for my atrophied GUI alignment and editing skilz.

2022-05-09
Please Close The Gate Signage

Making signs for the gates surrounding the Vassar Community Gardens provided an opportunity to test laser engraving power on MDF:

Please Close The Gate – 60 to 20 pct engraving – raw

The alert reader will observe MDF is totally the wrong material for outdoor signage, which is correct. I’ll be producing different signs as these disintegrate, with an emphasis on engraving different materials and applying different finishes along the way; nobody pays attention to signs, anyway.

With that in mind, the engraving power ranged from 60% on the top sign to 20% at the bottom, perhaps 40 W to 10 W, with a scanning speed of 500 mm/s. The highest power punched the engraving about 0.5 mm below the surface:

Please Close The Gate – 60 pct depth

They’re engraved on both sides, so those MDF locating pins came in handy:

Please Close The Gate – engraved

Alignment was obviously not critical.

Slathering the signs with polyurethane finish rated for indoor use improved the contrast on the deeper engraving:

Please Close The Gate – 60 to 20 pct engraving – finished

The bare sign (upper right) went on a distant / locked / rarely-used vehicle gate, where it will serve as an exposure control while turning into mush.

The small acrylic sign, a prototype for amusement value, clearly shows the need for offset correction at such high scan speeds:

Please Close The Gate – acrylic test piece

The MDF signs fit inside one vertical space of the “four inch” wire mesh on the gates, where they rest on the lower wire, and span three wires horizontally, so I could attempt to control the inevitable warping:

VCCG perimeter gate wire mesh

The mesh wire spacing is not mmmm a closely controlled manufacturing parameter, so the next iteration must be a few millimeters shorter to fit the smallest openings.

2022-05-06
OMTech 60 W Laser: Dual-path Air Assist

A tweak to the air assist plumbing of my OMTech 60 W laser produces much the same result as Russ Sadler’s Super Ultimate Air Assist, with somewhat less plumbing and cheaper Amazon parts:

OMTech Laser – air assist – plumbing

The overall doodle shows the electrical wiring and pneumatic plumbing:

Dual-path air assist diagram

The electronics bay now has two solid state relays:

OMTech Laser – air assist SSRs

The front SSR turns on the air pump when the controller activates the STATUS or AUX AIR outputs; the diode between the (-) terminals acts as wired-OR.

The rear SSR turns on the solenoid valve whenever the AUX AIR output is active. The diode turns on the other SSR to start the pump.

When the laser cutter is idle, both the STATUS and AUX AIR outputs are inactive, so the pump doesn’t run and the solenoid is closed.

The controller has a front-panel AUX AIR button that turns on its eponymous output, which turns on both the solenoid and the pump. I have turned it on to verify the circuitry works, but don’t do any manual cutting. I never was very good with an Etch-a-Sketch and the laser’s UI is much worse.

LightBurn includes an Air Assist setting for each cut layer, which should be OFF for engraving layers and ON for cutting layers. Basically, you go through the Material Library and tweak all the values , then It Just Works™ when you assign that material setting to a particular layer.

The solenoid valve must be a “direct acting solenoid valve“, as the air pump produces about 3 psi and cannot activate a “self piloted” solenoid valve. When the valve is open, the pump can push about 12 l/min through the plumbing to the nozzle:

OMTech Laser – air assist – 12 lpm flow

That’s noticeably lower than the 1 4 l/min without all the valves and additional plumbing.

The flow control valve is a manually adjusted needle valve to restrict the engraving air flow to maybe 2 l/min, just enough to keep the smoke / fumes out of the nozzle and away from the lens, when the solenoid valve is closed.

I set the controller to delay for 1 s after activating the air pump to let it get up to speed. There’s an audible (even to my deflicted ears) rattle from the flowmeter when the air assist solenoid opens.

The paltry 12 l/min seems to promote clean cuts and 2 l/min doesn’t push much smoke into the surface around the engraved area.

So far, so good.

2022-05-05
Laser-cut Punched Cards

I just had to do this:

Laser-cut punched card samples

It took several iterations to convince me I can’t quite pull it off yet, but the idea shows promise; the GitHub repo includes useful links to other variations and techniques.

The top card starts with hole locations / column numbers preprinted by the inkjet, then (nearly but not exactly) aligned in the laser cutter for “engraving” the variable text and “punching” the corresponding holes. The other cards represent various steps along the way, all of which demonstrate why a 60 W laser is the wrong way to print text on cardstock:

Laser cutter – engraved punched card

Aligning a preprinted sheet in the laser cutter with sufficient accuracy to hit all the holes turns out to be a significant challenge: the red dot laser pointer hangs off the rear of the nozzle with the beam at a steep angle:

Magnetic Honeycomb Spikes – MDF

Which means the red dot coincides with the main laser beam only at the exact focal distance below the nozzle after painstaking (and easily disrupted) alignment. A red dot laser coaxial with the CO₂ tube / beam should produce much better results, but that’s not what I have.

It Would Be Nice If™ I could cut the card outlines with the laser, print the hole positions on the inkjet, then align the “blanks” for “punching”, but I have yet to find any combination of parameters amid the unsteady ziggurat of Linux / CUPS printing configurations to produce properly aligned results on a custom paper size:

Misaligned punched card printing

You might think telling the printer it’s handling a #10 envelope with the image of the text carefully positioned to land at the proper spot on the actual card should work. You (well, I) would be wrong.

I’m pretty sure this can be coerced into working, but it must marinate on the to-do list for a while.

2022-05-04