Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
The NYS DOT recently spiffed up Raymond Avenue in Arlington, installing three small diameter rotary intersections and rows of street lights with lenses cunningly contrived to glare directly in your eyes. Why installing black, non-reflectorized bollards two feet away from the travel lane at each crosswalk seemed like a good idea escapes me, too.
Anyhow, it seems one of those streetlights was a bit too close to the corner. The ruts across the top of the picture came from a truck that evidently clipped the pole and neatly fractured the entire aluminum (?) base casting without bending anything else.
Fractured street lamp post
Further south, this wall in Wappingers Falls shows why flat-roof building aren’t such a great idea in snow country:
Ice flow across wall
That isn’t a new problem, by the way. Last year it looked like this:
This didn’t work out, but it came close. Eventually I’ll figure out what material can replace the boot, at which point I’ll need to remember these steps…
That LED flashlight + laser pointer has a rubber boot over the push-on / push-off switch stem that makes it sorta-kinda waterproof. Although I wouldn’t trust it in more than a sprinkle, it’s my pocket flashlight and tends not to get soaked very often.
Anyhow, the rubber boot wore through:
Broken switch boot
Taking it apart, now that I know how, was easy enough:
Switch button parts
Note that the mushroom part goes on the outside, which means the stem will vanish if the boot rips apart.
I planned to mold a boot from acrylic caulk, so I wrapped narrow strips of electrical tape to match the stem to the mushroom head, then wrapped a bit around that to make the final boot fit loosely:
Wrapped switch stem
A thin layer of oil served as mold release, over which I smoothed a blob of caulk. This looks awful, but the majority of the blob at the bottom will get trimmed off:
Switch stem covered with acrylic caulk
Unfortunately, the cured caulk turned out to be remarkably fragile. Each individual blob felt tough, but it’s really not designed to form thin membranes; I got about what I expected.
Pourable silicone rubber seems like the right hammer for the job: make an outer mold to surround this thing (or a 3D printed replica) and pour it on. I must get some of that, one of these days.
So I put the flashlight back together with the mushroom on the inside to keep the stem in place… and I generally avoid getting more than knee-deep in liquids, so not having a good seal won’t matter too much.
Extruding at a rather low 200 °C with that pair of 25 W cartridge heaters in the 14 °C Basement Laboratory Machine Shop Wing, the heaters exhibit fairly consistent timing:
47-49 seconds OFF
59-66 seconds ON
So, in round numbers, a 50 W heater has a 56% duty cycle with a 111 second period, with the temperature varying ±2 °C around the setpoint. The head has a fairly substantial ceramic wool insulation blanket, not the stock ceramic tape wrap, so your results will be different. One side of the build chamber is open to the ambient air, so it’s not as warm in there as usual.
The average dissipation of 28 W is half of the original MK5 head’s 58.8 W dissipation, which agrees reasonably well with what other folks have reported for the duty cycle of stock MK5 heads. More insulation is better, but a substantial fraction now escapes up the Thermal Riser tube, so doubling the blanket thickness might not be worth the bulk.
A single 40 W heater would run at 70% duty cycle. The only downside of lower power is a longer delay from power-on to extruding; a lower stuck-on overheat temperature seems like a Better Thing.
I’m deliberately using a relatively low extrusion temperature to explore the lower bounds of what’s practical. I think another 10 °C would improve the thread’s stickiness; right now some spots seem not so well glued together..
In the Mysteriously Missing Label category, we find three similar products with only one shelf label. Which one is the better deal?
Caffeine unit pricing
You might think it’d be the Wal-Mart Equate house brand. In order to find out, you’d have to haul all three offerings down the aisle and around the corner to the price scanner, which would reveal it’s the one on the far left, by a considerable margin. Oddly, that one says “Double Strength” even though it has the same 200 mg dose as the others.
In the Bizarre Units category, we have two very similar products with completely different unit-price units of measure. Seeing as how a “fluid ounce” is a unit of volume and a “pound” is a unit of weight (or, for the pedantic, force), even the dimensions aren’t compatible. Clicky for bigger pictures.
Vanilla unit pricing – 2 oz
And, just to show that wasn’t a one-off mistake that could happen to anyone, the smaller size containers continue the theme.
Vanilla unit pricing – 1 oz
Given that nothing in a Wal-Mart store happens by accident, someone was directed to remove two of those three labels and another someone deliberately chose incompatible units.
Of course, anyone I’ve ever asked has no idea why that would happen…
These Fiskars scissors[Update: they’ve moved to the Gardening section. Try there or there. ] seem to be intended for sewing & quilting, but they work just fine for snipping plastic filament, cutting tape, and severing hangnails…
Fiskars Softouch Scissors
The titanium nitride coating probably doesn’t add much value to the mix, but that’s what they had at JoAnne Fabric when I bought ’em.
Fiskars scissors tip detail
This detail of the tip shows why they’re so great for detail work: each blade ends in a two-way taper to a genuine cutting point. Of course, that means they’ll survive exactly zero falls to the shop’s concrete floor, but they’re fine while they last.
The trick is to sign up for JoAnne sale flyers, which regularly deliver “40% off any one item” discount coupons, then make a targeted shopping expedition. Those coupons account for the green self-healing cutting mat that’s in the background of so many pictures around here, too…
The advantage of an aluminum build plate is that it’s flat, but it must also be parallel to the XY axis movements: the nozzle should have a constant altitude across the entire surface of the plate. There’s a tool for measuring that: a dial test indicator.
Measuring build plate alignment
I still don’t have solid way to mount the DTI to the Z axis stage, but the bar clamp works reasonably well. The DIT has a full-scale range of about 30 mils = 0.76 mm, with half on either size of the zero center point. Obviously the probe isn’t at right angles to the DTI body, but it’s close enough for differences of a few mils.
The G-Code routine (see below) positions the Z stage in the middle of the platform and prompts you to mount the DTI and set the reading to 0.0. That requires a bit of delicate fiddling and anything within a few mils should be fine. Don’t adjust the leadscrew by hand, because all this depends on repeatable positioning.
With that in place, the G-Code will raise the DTI, move the stage, lower the DTI, pause for five seconds while you note the reading, then repeat. For my DTI, the readings are in mils = 0.001 inch and, while I could record half-mil values, it’s not worth the effort.
You’ll get nine numbers showing the height across the plate, spaced 20 mm in X and 25 mm in Y:
0
3
6
2
2
2
1
-2
-4
Subtract the minimum number from all the rest to remove the height offset and get everything referenced to zero:
Minimum
-4
4
7
10
6
6
6
5
2
0
Looks like the plate isn’t quite a planar surface (it’s bent!) and it tilts upward to the right rear, but the total difference amounts to 10 mils = 0.010 inch = 0.25 mm. I think that’s smaller than the variation caused by jitter and vibration and general creakiness in the X and Y stages. The repeatability seems to be within two or three mils, which is probably the limit of the hardware.
Bottom line: good enough for now!
The flat aluminum plate reveals a definite front-to-back bow in the heater plate. Clamping the two tightly together would fix that and improve heat transfer, but then the aluminum plate wouldn’t be easily removable when it’s hot.
Put this G-Code routine (call it Flatness.gcode) in the ReplicatorG scripts/calibration directory and you’ll be able to run it from the menu:
(Measure surface flatness)
(MakerBot Thing-O-Matic with ABP and aluminum plate)
(Tweaked for TOM 286)
(Ed Nisley - KE4ZNU - Feb 2011)
(-- The usual setup --)
G21 (set units to mm)
G90 (set positioning to absolute)
(-- Home axes --)
G162 Z F1500 (home Z to get nozzle out of danger zone)
G161 Y F4000 (retract Y to get X out of front opening)
G161 X F4000 (now safe to home X)
(-- Set coordinate zeros --)
G92 X-53.0 Y-58.0
(G92 Z115.3) (set Z for ABP with belt)
G92 Z112.8 (set Z for ABP with aluminum sheet platform)
(-- Get height gauge set up --)
G0 X-10 Y10 Z25 (center gauge probe on platform)
M1 (Attach gauge, set to 0.0 mm)
G92 X0 Y0 Z0.0
G0 Z2.0 (traverse height)
(-- Begin probing --)
G1 Z0.0 (denter)
G4 P5000
G0 Z2.0
G0 X-40.0 (left center)
G1 Z0.0
G4 P5000
G0 Z2.0
G0 Y-50.0 (left front)
G1 Z0.0
G4 P5000
G0 Z2.0
G0 X0.0 (mid front)
G1 Z0.0
G4 P5000
G0 Z2.0
G0 X40.0 (right front)
G1 Z0.0
G4 P5000
G0 Z2.0
G0 Y0.0 (right center)
G1 Z0.0
G4 P5000
G0 Z2.0
G0 Y50.0 (right rear)
G1 Z0.0
G4 P5000
G0 Z2.0
G0 X0.0 (mid rear)
G1 Z0.0
G4 P5000
G0 Z2.0
G0 X-40.0 (left rear)
G1 Z0.0
G4 P5000
G0 Z2.0
G0 X0.0 Y0.0 (center again)
G1 Z0.0
G4 P5000
(G0 Z5)
This is a variation of Thing 6384: an aluminum plate sitting atop the Automated Build Platform’s bare heat spreader, minus the belt. HIs truly ingenious idea was to cover the plate with a thin layer of ABS to ensure adhesion: an ABS filament bonds very well to ABS!
Aluminum build plate in action
I started with a big sheet of 3/32 inch aluminum, a bit thinner than the 1/8 inch sheet he used, which is what I had in the Parts Heap. Bandsawed three chunks to rough shape, squared up the edges on the Sherline with manual CNC:
Squaring the sheets
That was complicated by the Sherline’s cramped work envelope. The 5/8 inch lathe bit on the right sits at exactly right angles to the X axis and serves as the reference plane. To make it happen:
Stack the three plates, clamp to table aligned against lathe bit
Whack off the far edge
Put clean edge against lathe bit
Whack off another edge
Measure / scribe 120 mm from each new edge (thus the blue stripes)
Align & cut
That actually worked quite well, although you’d think the angular error would build up as I rotated the plates. I checked and tweaked the angle after the first cut and it was all good.
Tight hole clearance
Then drill six clearance holes for the socket head cap screws holding the heater plate to the ABP; a #1 drill gave a few mils clearance, which is all it needs. The holes are 4 mm in from the edges of the 120 mm square, with the two middle ones at, yes, 60 mm.
However, there’s not much meat between the edge of the plate and the holes: call it 1.1 mm. If you do this, using 122 mm plates would produce less scary-close results. That’s why I like manual CNC for this stuff: no need to lay it out, tap in the numbers and it just Works.
My APB heater has a static drain connected to the heat spreader, so I milled a 2 mm recess around the right-hand screws to clear the lugs, wires, and Wire Glue blob. The silicone wiper gets its own cutout, which I made a snug fit so that the rubber would push the plate against the screw heads and hold it in place.
Milled recesses
I machined recesses on only one plate, so I could incorporate any changes in the other two. The initial setup was atop a scrap plastic sheet which, as it turned out, wasn’t particularly flat. The edges of that not-quite-complete hole on the left were nasty-sharp.
Thin-shaved plate edge
Then clean off the ink with xylene, scrub the plate with a 220-grit sanding sponge, and it looks really nice. Impossible to photograph a uniform gray surface, though: the autofocus goes nuts.
While all that was going on, I’d dumped some MEK into a polyethylene jar along with a handful of calibration cubes and similar debris. I used MEK, rather than acetone, because it’s somewhat less aggressively flammable while still being a good solvent for ABS. Right now, the gunk has the consistency of thin honey, which may be too thick to spread easily; I’m still figuring this out. I apply the gunk with a folded coffee filter: scrape the puddle around to cover the whole plate, then let it dry. This is best done outdoors, except that right now it’s well below freezing out there.
Here’s what the film looks like under the start of a quartet of dodecahedrons I ran off to see if they stuck properly:
ABS coating on aluminum build plate
The bottom surface looks like it was machined: dead flat,nice edges, good thread definition. The parts stick like they were glued to the surface, with no tendency to pull up at the corners.
The Outline thread shows some adhesion trouble for the first 10 mm or so. After that, it’s nailed right to the ABS film. That’s why I use Outline, at least until I figure out a better way to start the thread.
After I finish the next two plates, I’ll have a somewhat quick-change build platform: pull the hot plate off (holding it with pliers!) and slap a new one on. Not as convenient as the ABP, but much better for building precision parts like gears and extruder motor mounts.
The Smell of Molten Projects in the Morning 