Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
It is in series with the lower switch on the side panel:
OMTech Laser – rocker switch lit
Although I would have labeled those switches differently, the “Control Switch” handles the 120 VAC line voltage to the HV power supply. As you’d expect, when its light is ON, the power supply is also ON and the laser is ready to fire.
Those two pictures show the situation after I turned the laser power on a few days ago: key lock switch OFF, HV laser power supply stubbornly ON.
Whoops.
The “Control Switch” still does what it should, so I can shut the HV supply off when it’s not needed, but the key lock switch has definitely failed ON.
As far as I can tell, the moving contact bar jammed at the bottom of its travel against the terminals. Pulling the switch out of the laser jostled it enough to release the bar and it’s now at the top of its travel:
OMTech Laser – key lock – side view
If it failed once, it’ll fail again.
OMTech’s Customer Support agrees it shouldn’t behave like that; a replacement should arrive in a few days.
Wiping down a tool or wiping up a mess with a small rag and then throwing it out simplifies cleanup:
Shop wipes
Long ago, I applied scissors to old towels / t-shirts / whatever to get randomly sized squares, but when Mary began using rotary cutters for her sewing projects I immediately saw the light. A few times a year, I lower the scrap box level and restock the shop wipes boxes.
A laser cutter is even better:
Shop Rags – LB camera layout
Flatten the rag on the honeycomb, drag a few rectangles into place, and fire the laser:
Shop wipes – laser cut
Something like 50 mm/s at 60% power works for all the fabrics I’ve tried, from worn-out towels and dead sweatpants to napkins and t-shirts. Thinner fabrics can be stacked, but wrinkles and seams get in the way of clean cuts.
Rounded-corner rectangles are easy enough to draw and the scrap cloths have different shapes, so I don’t see much point in saving a file with any specific layout. Your scrap box may be more orderly.
A clean cut lets the outer cloth just lift away:
Shop wipes – on honeycomb
The wipes give off a distinct smell of charred cloth, but running them through the clothes washer in a big mesh bag with everything else solves that problem.
Obviously, one couldn’t possibly justify a laser cutter to make shop wipes, but if you happen to have one just standing around, well …
The OMTech laser manual specifically warns against allowing clutter to accumulate atop the cabinet:
It is highly recommended to have an extra work table nearby in order to avoid placing objects on or directly adjacent to the machine, which could become a fire or laser hazard.
OMTech USB570c Cabinet Laser Engraver User Manual
The Basement Shop lacks the floor space for their recommended “extra work table”, so the laser cabinet now sports a pair of wings:
They’re a convenient 9 inches wide, just right for general clutter. That stubby screwdriver encroaching on the lid shows I haven’t been entirely successful.
Each white shelf bracket has three self-tapping machine screws driven into the wood and a single 4 mm SHCS through a hole drilled into the cabinet with a nyloc nut & washer on the inside. If I understand the somewhat abbreviated instruction sticker correctly, I installed them upside-down in order to put the longer end under the wood where it would do the most good; they’re entirely rigid enough for the purpose.
Putting two lines of the most-readable font inside an outline reverse-engineered from a few handwritten samples let me cut out a bunch of plant markers from white-on-black Trolase acrylic:
Plant Markers – cutting
Which look downright dignified in real life:
Plant markers – African Violet
Admittedly, sweet potato slips don’t require such extensive documentation:
Plant Markers – sweet potatoes
Cutting the sheet flat on the honeycomb platform worked well, modulo Sadler’s warning about cutting acrylic, and a few smudges on the back of the markers will go unnoticed.
The codes give the position and format for text fields in a CSV file containing one line for each tag:
Austrocylindropuntia subulata,Eve’s Pin Cactus
Euphorbia,abyssiniaca
possibly G. Carinata,var. Verucosa
African Violet,Maui
Sansevieria trifasciata,Mother in law’s tongue
Plectranthus,'Mona Lavender'
The rules governing quoted strings and suchlike remain to be explored, but single quotes in the CSV file pass through unchanged.
Putting a tab at the point of the marker will prevent it from falling free when cut out, should you want to try raising the sheet above the platform to reduce the amount of crud accumulating on the back side.
I thought a pine plank would cut faster than oak, but they’re equally stubborn.
Maple requires slightly more power, with the glued butt joints between the slabs putting up a stiff resistance.
A sheet of 3 mm MDF cuts well at 20 mm/s 60% and I expect 3 mm plywood might need similar numbers.
A pervasive odor of burned wood seems to be the only downside; if you think a wood stove is a good idea, you’ll love laser cutting the stuff. Sanding the blackened perimeter and sealing the surface surely helps, but it’s feasible only for the kind of simple convex shapes you don’t really need a laser to cut.
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:
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.
The Smell of Molten Projects in the Morning 
