Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
Category: Software
General-purpose computers doing something specific
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.
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.
Although the OMTech laser controls the laser power supply with a key-lock switch, there’s little visible difference between the OFF and ON positions. Having occasionally mistaken it in both directions, this seemed like a useful addition:
Laser Power Lock Indicator – installed
The strip of black duct tape below the lock muffles the rattle of the triangle hatch key against the metal cabinet.
Two snippets of foam tape hold the knob to the lock cylinder, making an admittedly tenuous connection, but the knob fits around the outside of the switch housing with minimal clearance and doesn’t shouldn’t suffer any torque or pulling, so it might work.
The solid model looks about like you’d expect:
Laser Power Lock Indicator – solid model
Unfortunately, it has no good orientation for printing, so I let PrusaSlicer generate support material inside the knob:
Laser Power Lock Indicator – Support structures
Suffice it to say: removing all that plastic did not go well.
I eventually grabbed the knob in the lathe and bored the interior out to its more-or-less proper dimensions, figuring nobody would ever notice the carnage, and it worked reasonably well. In the unlikely event I need another pointer, I’ll add a support spider to hold up the interior with minimal contact and less plastic.
Yeah, the laser really needs a stack light showing its condition and safety status …
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However, all that upward-directed light goes directly into my glare-sensitive eyeballs, so I added shades above the strips:
COB LED Shade – installed
They’re cut from corrugated cardboard because I have an essentially infinite supply and I’m still working out speeds and intensities. Eventually they’ll become something like black acrylic.
The brackets emerged from the vasty digital deep through the miracle of 3D printing:
COB LED Shade Brackets – slice preview
They’re stuck to the laser cabinet and the cardboard with double-sided duct tape. If you’re careful, they will line up along one edge of the tape, roll over neatly to stick their other face, then a single razor knife cut can separate each pair of neighbors.
The underside sports an aluminized mylar strip to redirect the wasted light in a more useful direction:
COB LED Shade – aluminized Mylar reflector
The tapeless sticky shipped with the laser holds the reflector in place, while its 20 mm width sets the 21 mm shade dimension. Although you want a reasonably smooth layer, it need not be mirror-flat.
Now it’s really bright in there:
COB LED Shade – overview
While I had my head under the hood, I stuck a fourth strip of COB LEDs on the lip along the rear edge of the opening; it’s bright enough to cast the shadow just forward of the laser head despite the OEM under-gantry LED strip. Because the rear strip is aimed downward, it didn’t need a shade.
The perforated cardboard sheet on the left is a spike plate: more about that later.
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What used to be a “light box” had become a “light pad” powered through a USB Micro-B connector on the side. Unfortunately, the pad’s 5 mm thickness allows for very little mechanical reinforcement around the USB jack, while providing infinite opportunity to apply bending force. Over the course of the last half-dozen years (during which the price has dropped dramatically, despite recent events), the slightest motion flickered the LEDs.
So I squished the jack’s metal shell back into shape, found a short right-angle USB cable, and conjured a reinforcing fixture from the vasty digital deep:
LitUp LED Light Pad
The plate fits under the light pad, where a strip of super-sticky duct tape holds it in place:
LitUp Light Pad USB jack reinforcement – bottom
The USB plug fits between the two blocks with hot-melt glue holding it in place and filling the gap between the plug and the pad.
I’d like to say it’s more elegant than the cable redirection for my tablet, but anything involving black electrical tape and hot-melt glue just isn’t in the running for elegant:
LitUp Light Pad USB jack reinforcement – top
On the other paw, that socket ought to last pretty nearly forever, which counts for a whole lot more around here.
The retina-burn orange tape patches on the connector eliminate all the fumbling inherent to an asymmetric connector with invisible surface features. The USB wall wart on the other end of the cable sports similar markings.
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Radial slits around the middle let it bend upward over the folded aluminum joint around the pillar:
Angel Food Cake Pan liner – detail
Ours claims to be a 10×4-½ inch pan, roughly the diameter at the top and the overall height. Your pan will surely be different: this one is, as the saying goes, old enough to know better.
Plotting the backlash / calibration target on both the CNC-3018XL and the MPCNC quickly showed, contrary to what I expected, the MPCNC was dead-on accurate, albeit with some wobbulation and a trace of backlash:
MPCNC – Backlash test – detail
Although it looks ug-u-lee, the (lower speed) drag knife cuts come out nice and, because the entry and exit moves match the main cut, the minimal backlash wasn’t a problem.
Turns out only the X axis on the 3018XL had a problem:
Cal Target – 400 step-mm – merged
Apparently the longer leadscrew I installed as part of the “XL” conversion has a small thread pitch error: about 1 mm short in every 250 mm of travel. I don’t have any (definite, non-handwavy) method to measure the pitch directly, other than by running the follower nut and measuring the results, but it’s consistently short.
Quite some time ago (after blowing up the OEM controller board), I set up the Protoneer CNC board in 1:8 microstep mode, making the GRBL $100 setting a nice, round 400 step/mm for a two-start leadscrew with 2 mm pitch and 4 mm lead:
After a few more measurements suggesting the leadscrew actually traveled 249.2 mm, the correct value will be:
401.28 step/mm = 400 step/mm × 250 mm / 249.2 mm
To verify I understood the problem and solution, I set $100 to a few integer values around the goal:
Cal Target – stacked – 399-402 step-mm
The top image shows the leftmost line at the 10 mm mark on the scale, because it’s easier for me to match the ink line with an engraved line, rather than the non-line at the end of the ruler.
The other images show the results for $100 set to 399, 400, 401, and 402 step/mm, respectively. The results last two results bracket the desired 250 mm outcome, with 401 step/mm being Close Enough™. GRBL accepts a floating point step/mm value, so I set $100 to 401.28, but I was unable to convince myself the result came out consistently different than 401.00.
Plotting both the tick marks (green) and the knife path (red) on the 3018XL, then cutting the bare paper on the MPCNC, showed the two machines now agree on where the knife should fall. The outer end of the tick marks extends 1 mm beyond the cut line to ensure small misalignments do not produce an obvious white gap around the edge of the deck.
The Y axis continues to match:
Tek CC – 2022-02-14 – Y detail
And now the X axis looks just as good:
Tek CC – 2022-02-14 – X detail
The drag knife corners are rounded, as you’d expect. The cut seems slightly offset from a small origin touch-off error, but the scales now match.
The Smell of Molten Projects in the Morning 
