Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
OK, somebody decided that the classic metal blade used on all plastic wrap boxes since the dawn of time cost too much, so they decreed that it be replaced with a plastic blade that costs essentially nothing:
Walmart plastic wrap – plastic cutter
Unfortunately, a thin plastic blade also bends easily and, after a few uses, cracks along the midline. After that, it simply doesn’t work; there’s no way to actually tear the plastic off the roll.
It turns out that a common hacksaw blade is exactly the right length and, oriented with the teeth pointing to the left, will rip through plastic wrap like, uh, a hacksaw through plastic:
Walmart plastic wrap – real cutter
That this hack should not be necessary goes without saying…
There’s a layer of double-stick foam tape between the box and blade. It’s probably removable, but I was in a hurry.
I carry the Canon SX230HS in my pocket, so as to have a decent camera ready when it’s needed; yes, it’s in a cloth case. Unfortunately, in recent weeks a tiny hair made its way into the lens stack, where it shows up as a slight blurring just left of center in high f/stop images:
Cheap cartridge heater insulation
With the camera attached to the stereo zoom microscope, the hair becomes painfully obvious:
Hair on SX230HX Sensor
Of course it’s in the middle of the image. [sigh]
A bit of searching turns up a bootleg technique to remove the front lens from the turret (basically, just twist and pull), but neither of the internal lens surfaces thus revealed lie near a focal plane and, in any event, were surprisingly clean. The hair is probably lodged just in front of the image sensor, most likely stuck to the back of the final lens where it casts a shadow on the sensor. If it wandered around you’d call it a floater.
Dismantling the entire camera and opening the lens stack seems fraught with peril, particularly as the camera pretty much still works fine for normal picture-taking. More pondering is in order…
Part of the Curvelicious Cookie Cutter effort involved making the thinnest possible cutter blade wall consisting of two adjacent threads, because that’s about what the Afinia printer was producing (from a different model). My OpenSCAD code, based on an Inkscape model derived from the as-printed Afinia cutter, enlarges the cookie shape by a specific distance with a Minkowski sum; the model ultimately becomes G-Code directing the extruder nozzle around the outline.
Obviously, that required a bit of fiddling:
Robot Cutter Variations
The pink cutter on the top came from the Afinia, complete with raft. The red cutters, all with short blades to speed up the printing, came from my M2.
The printer mechanics determine the step/mm values for all four axes: X, Y, Z, and the extruder. The effective diameter of the “gear” driving the filament into the extruder seems subject to some quibbling, but setting it so the thinwall box comes out with the proper filament width seems reasonable. Given those four values, the slicing software can control the extruder speed to produce the proper volume of plastic as the XY speed varies.
The slicing software must also know the raw filament diameter, which seems to be consistent within a few percent for the filaments in my collection. Because a 1% change in filament diameter produces a 3% change in extruded volume, a few percent is about all you can tolerate; broad-tolerance filament may require sensors and adjustments that printers don’t currently offer.
There is one remaining variable, essentially a Fudge Factor, which Slic3r calls the extrusion multiplier. This seems to be a linear factor applied to the extrusion volume, so that increasing the factor proportionally increase the flow rate. Given correct step/mm settings and the measured filament diameter, you (well, I) adjust the extrusion multiplier to get the proper extrusion flow. As it turned out, the multiplier I’ve been using with the M2 worked out to 1.00, although I’ve also used 0.97 on occasion. Although I haven’t read the Slic3r source code to verify this, varying the multiplier by +3% should fudge the diameter by about +0.017 mm = 1% of the measured 1.72 mm.
Note that the Makergear-modified Marlin firmware in the M2 will produce different results, as they use a different value for the extruder gear’s effective diameter. More discussion on that is there.
Soooo, I set up the extrusion multiplier to produce parts with accurate dimensions, because that’s what I care about, and didn’t worry too much about perfect surface finish, because I don’t really care about that. Cookie cutters, however, need a completely filled surface that prevents dough from collecting inside, but have essentially no dimensional accuracy requirements.
The quartet of stumpy cutters bundled together on the left of the top photo explored the effect of changing the extrusion multiplier. I used the same STL model for all the cutters and varied only the extrusion factor, so the results depend only on the plastic flow rate and the M2’s impeccable mechanical stability.
A sharp cusp at 0.96 has a slight opening:
Robot Cutter – 0.96 extrusion multiplier
The cusp fills in at 1.10:
Robot Cutter – 1.10 extrusion multiplier
The handle surface is slightly open at 0.96:
Robot Cutter – 0.96 extrusion multiplier
And filled in at 1.10:
Robot Cutter – 1.10 extrusion multiplier
In all those cases, the measured blade thickness varied slightly, but not enough to matter in this application. I didn’t record those numbers and no longer have the models, but … you just tune for best picture.
I picked up five 12 V 40 W cartridge heaters from the usual eBay source for some extruder experiments and did a quick check to make sure they actually worked:
Cartridge heater test
The bench supply is good for 3 A, which isn’t quite enough to light them up all the way, but at 8 V they drew anywhere from 2.67 to 2.20 A, declining by about 0.1 A as they heated over the course of maybe 5 s, which is about as long as you want to run them outside of whatever they’re supposed to be heating.
Those dissipations are a bit lower than I expected; at 8 V you’d expect to see about 27 W = 2/3 * 40 W, not the 18 to 21 W I actually measured. Current & power don’t scale linearly, so I must gimmick up a larger block and make some better measurements when I get the LinuxCNC hardware set up.
The insulating tubes on the wires emerging from the cartridge, inside the main sheath, show the usual attention to detail I’ve come to know and love from eBay suppliers:
Cheap cartridge heater insulation
Ah, well, it keeps my toy budget under control…
There’s a story behind the dark vertical smudge just to the right of the cartridge. More on that in a bit.
Late August is, as always, the season for giant orb-weaving spiders, one of which spun a web between two tall cactus plants on the patio and greeted us with this sight one morning:
Spider vs. Dragonfly – overview
We’re big fans of both spiders and dragonflies, but it was obvious who came out on top in this contest:
Spider vs. Dragonfly – front
These things are unimaginably weird:
Spider vs. Dragonfly – bottom detail
Even with the spider busy at lunch, she has four eyes to spare. They reflect the flash and appear as white-centered dark dots near the middle of the image:
Spider vs. Dragonfly – many eyes
I’m sure the red spinnerets are diagnostic:
Spider vs. Dragonfly – side detail
The spider tossed the empty husk over the side, then spent the next two nights and days parked in her lair, presumably digesting that big meal, and didn’t bother repairing the web:
Spider vs. Dragonfly – spider at home
She spun a fresh web on the third night and caught a more manageable insect:
Spider with smaller prey
All hand-held with the Sony DSC-H5, some with a 2x close-up lens. All the pix are tight crops, crushed to fit my arbitrary 750 pixel maximum and 200 kB size limit. If you need high-res original images for anything, drop me a note; I took far too many pictures of this encounter…
The Smell of Molten Projects in the Morning 