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: Improvements

Making the world a better place, one piece at a time

  • 3D Printer Design Conversation: Part 2

    Wherein I continue dumping my responses to a large-format 3D printer project …

    (Continued from yesterday)

    What do you mean by 12 hour mean time to failure

    In round numbers, the cries of anguish on the M2 forum seem to increase as parts require more than a dozen hours from start to finish; while you can print things that require 48 hours, that’s not the way to bet. There are more ways for things to go wrong than for them to go right, given the rather rickety collection of software & firmware making everything happen, plus the gummy nature of squeezing hot plastic into precise heaps.

    Most of the time, it works fine.

    much cheaper hardened polished rod system that the taz 6 uses?

    Unless they’re doing something non-obvious to make a kinematic assembly, two rods on four hard mounts with four one-degree-of-freedom slides will be severely overconstrained and, I expect, a continuing hunk o’ trouble:

    https://softsolder.com/2011/02/04/thing-o-matic-x-and-z-axis-rod-alignment/

    FWIW, linear slides don’t eliminate the need for a rigid and well-aligned frame. Even the slab atop an M2 can deform by more than 0.1 mm under belt tension, which is enough to wreck the nozzle-to-platform alignment across the length of the X axis.

    “Arduino-class firmware (Marlin, et. al.) is a dead end” Why is that?

    Marlin is a dead end: they’re trying to jam hard real time motor control, soft real time command parsing, and non real time UI control into an 8 bit microcontroller teleported from the mid 90s. AVR microncontrollers worked really well up through the Cupcake and have held back printer design & performance ever since.

    Which inexpensive all in one board would you go with

    Machinekit on a Beaglebone seems to be the least awful of the current alternatives, but I haven’t examined the field recently enough to have a valid opinion. You’ll find plenty of proprietary “solutions” out there, none of which I’d be interested in.

    Am I wrong?

    I think so, but, then, I may be wrong, too. [grin]

    It’s incredibly easy to slap together a bunch of parts that look like they should become a 3D printer. It’s remarkably difficult to engineer a reliable, stable, accurate device that actually produces dependable results.

    Mooching design cues and parts from here & there doesn’t get you to the goal; if it did, Kickstarter wouldn’t be a graveyard of cheap 3D printer projects.

    design a very rigid system for cheap

    If it’s for your personal satisfaction, have at it, but a one-off large-format printer won’t be any cheaper than, say, a Taz 6. Some diligent searching will uncover any number of homebrew printer projects along the lines of what you’re considering; learning from their mistakes will certainly be edifying.

    Anything is possible, but if you want to end up with a state of the art machine, you must begin with numbers showing how & why it actually meets the requirements. 3D printing now operates at accuracies, speeds, and controls comparable to CNC machines, with corresponding structural demands. There’s a reason high-end CNC machines aren’t made of sheet metal and don’t use 8 bit microcontrollers.

    You might want to start at the beginning of my blog and read through my adventures with the Thing-O-Matic, which will explain why I’m such a curmudgeon …

    (Continues tomorrow)

  • 3D Printer Design Conversation: Part 1

    I recently engaged in a wide-ranging email exchange with a guy planning to scratch-build a large-format 3D printer. He figured it would be a straightforward exercise and asked for some advice; I may be more cynical that he expected.

    Over the next few days, I’ll dump my side of the conversation so I can refer to it in other contexts. I’ve left his side of the conversation as the short quotes that prompted my replies, but you can probably infer what he was thinking.

    He’s well-acquainted with CNC machining and recently added a Makergear M2 to his collection …

    I’m hooked.

    All of sudden, you realize what you’ve been missing!

    In round numbers, I’ve been designing & printing one “thing” every week for the last five years. Granted, my “things” look a lot like brackets, because they go into other shop projects, but 3D printing is how I make nearly all the shapes I formerly bashed from metal.

    I loves me my 3D printer!

    an open source design with AFFORDABLE, EASILY ACCESSIBLE parts with a build platform of at least 150% X/Y volume of the MakerGear

    Some years ago, I had the same general idea. Then I bought an M2 (replacing my Thing-O-Matic), considered LinuxCNC / Machinekit for motion control, and realized there wasn’t much point; I didn’t want to devote far too much time & effort to solving an already solved problem.

    A larger build volume doesn’t buy you as much as you think, while imposing far too many hard constraints. Basically, good-resolution extruders run at 2 to 10 mm³/s, so large objects require print times beyond the 12-hour MTTF of the “printing system”: something will go wrong often enough to drive you mad.

    Bonus: plastic’s thermal coefficient guarantees bed adhesion problems. Using high-traction materials (PEI / hairspray / whatever) introduces problems in the other direction. There’s a limit to how big you can make things before they either don’t stick or stick too hard.

    Some the fundamental design problems that nobody recognizes until far too late in their design:

    • nozzle-to-platform accuracy < ±0.05 mm
    • XY axis speeds 30 mm/s to 500 mm/s
    • Z axis stiction & backlash < 0.1 mm
    • filament drive with excellent retraction control / speed
    • bed adhesion vs. part removal vs. Z accuracy
    • Arduino-class firmware (Marlin, et. al.) is a dead end
    • Windows is crap in any part of a machine-control problem

    Those are hard requirements. At a minimum, your design must satisfy all of them: miss any one and you’re not in the game. It’s easy to build a cheap and crappy fused-filament 3D printer (see Kickstarter), but exceedingly difficult to build one at the state of the art (see patent litigation).

    The M2 descends from the original RepRap design, with the Y axis slinging far too much mass back & forth. That kills nozzle-to-platform accuracy, introduces temperature instability, and soaks up bench space. On the other paw, look at the problems Makerbot (not Makergear) had with their direct-drive extruder on an XY platform; getting that right requires nontrivial engineering

    Bowden filament drives have improved, but really can’t provide enough retraction control / speed. Delta printers always use Bowden drives, because they can’t sling a direct-drive extruder with enough XYZ speed & accuracy. Bowden on an XY platform has the worst of both worlds: bad retraction and difficult mechanical design.

    I think the M2 occupies a sweet spot in 3D printer design: excellent results without excessive complexity or expense. It’s not perfect, but good enough.

    But, then, I’m a known curmudgeon …

    (Continues tomorrow)

  • Aluminum Armature Wire

    Sculptors build figures with aluminum armature (*) wire, because it’s dead-soft, bends easily, and holds its shape:

    Armature Wire assortment
    Armature Wire assortment

    The sizes: 1/4 inch, 3/16 inch, 1/8 inch, 1/16 inch. The latter came from my Big Box o’ Specialty Wire, with the others from Richeson via Amazon. You can certainly get better prices for larger quantities from metal suppliers.

    I’m thinking it might hold RGB LEDs around glass doodads, eliminating the need for epoxy, as the utter unreliability of those WS2812 chips has burned out my enthusiasm for permanent assemblies:

    Failed WS2812 LED - drilling
    Failed WS2812 LED – drilling

    Observations:

    • 1/4 inch wire is way too rigid, although a stalk might hold a display
    • The 1/8 inch wire looks much different than the others
    • 1/16 inch wire may work better inside a braided sheath with the LED conductors

    The wire is probably a 1000-series alloy, if only because anything else would start out too stiff and work-harden too quickly, although the sharp bends in the coils already feel hard. It’s possible to anneal aluminum by hand with some soap and a torch, with meltdown an ever-present hazard. Other references suggesting soaking at temperatures in the 300-400 °C range in a furnace I don’t have.

    (*) Armature wire has nothing to do with motor armatures!

  • Raspberry Pi: White OLED Display

    The white OLED displays measure 1.3 inches diagonally:

    RPi OLED Display - white on black
    RPi OLED Display – white on black

    They’re plug-compatible with their 0.96 inch blue and yellow-blue siblings.

    All of them are absurdly cute and surprisingly readable at close range, at least if you’re as nearsighted as I am.

    Some preliminary fiddling suggests a Primary Red filter will make the white displays more dark-room friendly than the yellow-blue ones. Setting the “contrast” to 1 (rather than the default 255) doesn’t (seem to) make much difference, surely attributable to human vision’s logarithmic brightness sensitivity.

    I must conjure some sort of case atop a bendy wire mount for E-Z visibility.

  • Monthly Science: Sonicare Recharge Intervals

    After replacing the NiMH cells in my Sonicare toothbrush in July 2012, they delivered about 21 days = 21 brushings between charges. After a year, I laid a sheet of Geek Scratch Paper on the windowsill (*) and noted pretty nearly every recharge:

    Sonicare recharge - 2013-10 - 2017-01
    Sonicare recharge – 2013-10 – 2017-01

    Anyhow, the original cells crapped out after 2-½ years, when these still delivered 13 days. After 4-½ years, they’re lasting 12 days between charges.

    Color me surprised, because they’re 600 mA·h NiMH cells. The originals were 2000 mA·h cells, which you’d expect would last longer, but noooo.

    No reason to change them yet, which is good news.

    FWIW, I recently bought some cheap brush heads from the usual low-end eBay seller. The OEM brushes have colored bristles which fade to tell you when to change brushes, although I run ’em quite a bit longer than that. The cheap replacements have never-fading colored bristles and, I suspect, all the bristles are much too stiff. The dental hygienist says I’m doing great, so it’s all good.

    Sonicare brush heads - cheap vs OEM
    Sonicare brush heads – cheap vs OEM

    High truth: at best, you get what you pay for.

    (*) Being that type of guy has some advantages, if you’re that guy. Otherwise, it’s a nasty character flaw.

     

  • Quartz Resonator Test Fixture

    A recent QEX article (Jan/Feb 2017 2016; sorry ’bout that), Crystal Measurement Parameters Simplified, Chuck Adams K7QO) suggested a simplified version of the K8IQY crystal parameter test fixture would work just as well for low-frequency quartz resonators:

    Quartz crystal resonance test fixture - schematic
    Quartz crystal resonance test fixture – schematic

    The resistive pads eliminate the fussy toroids and their frequency dependence.

    Tossing a handful of parts on a small proto board:

    Quartz crystal resonance test fixture
    Quartz crystal resonance test fixture

    I found two absurdly long hunks of RG-174 coax with BNC connectors, so that’s how it connects to the outside world; sacrificing a short SMA jumper would reduce the clutter, but that’s in the nature of fine tuning. At the frequencies this fixture will see, coax properties don’t matter.

    I can’t think of a better way to mount those AT26 cans than by soldering the wire leads directly to a pin header; pushing them under spring clips seems fraught with peril, not to mention excessive stray capacitance.

    Measure the actual in-circuit capacitance for the 33 pF cap (shown as 39 pF in the schematic, it’s not critical), which worked out to 34.6 pF.  That’s the external series capacitance Cx.

    The overall procedure, slightly modified from the original:

    • Measure C0 with resonator in capacitance fixture
    • Solder resonator to pins
    • Remove jumper to put capacitor Cx in series
    • Find series-resonant peak = Fc
    • Install jumper to short Cx
    • Find series-resonant peak = Fs < Fc
    • Remember the peak amplitude
    • Unsolder crystal
    • Install suitable trimpot = Rm in socket
    • Adjust trimpot to produce same output amplitude

    Crunch the numbers to get the crystal’s motional parameters:

    Rm = trimpot resistance
    Lm = 1 / [4 π2 (Fs + Fc) (Fs - Fc) (C0 + Cx)]
    Cm = 1 / [(2 π Fs)2 Lm]
    Q = [2 π Fs Lm] / Rm

    Then you’re done!

  • AADE LC Meter: AT26 Crystal Capacitance Fixture

    Crystals (or resonators) in AT26 packages have vanishingly small capacitances, so I conjured a little fixture for my AADE L/C Meter IIB (*) that holds them securely under little fingers snipped from an EMI shield:

    AT26 crystal capacitance fixture - Cpar detail
    AT26 crystal capacitance fixture – Cpar detail

    The finger on the right sits atop a snippet of rectangular brass tube so it need not bend so far.

    The base is a snippet of double-sided PCB with copper tape soldered around the edges. I drilled the holes slightly oversize and soldered copper tape there, giving the top foil a direct connection to the terminals. The raggedy slot looks like it came from a hacksaw; no false advertising there.

    The meter reports 6.5 pF of stray capacitance and nulls it to zero as usual. Without the fixture, it shows 2.5 pF.

    With the crystal in that position, the meter measures Cpar, the parasitic capacitance from both terminals to the can, which should be (roughly) twice the capacitance from either terminal to the can.

    Two more clips measure C0, the plate-to-plate capacitance:

    AT26 crystal capacitance fixture - C0 detail
    AT26 crystal capacitance fixture – C0 detail

    The meter drive is about 200 mV at 700 kHz, far away from resonance. Assuming the resonator’s effective series resistance is 25 kΩ (tuning forks aren’t crystals!), it’s dissipating 1.5 µW (and less as the ESR goes up). That may be slightly hot for some resonators, but it’s surely survivable.

    Some preliminary data on five 32.768 kHz crystals shows Cpar = 0.4 pF and C0 = 0.9 pF. I don’t trust those numbers very much, but they’re reproducible within 0.1-ish pF.

    (*) Almost All Digital Electronics and its website vanished after the owner died; the meter continues to work fine. The cheap knockoffs flooding eBay and Amazon may get you close to the goal.