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.

Category: Machine Shop

Mechanical widgetry

  • Thing-O-Matic: Lazy Susan Filament Spool

    All of those thermal tests on the MK5 head gave me plenty of time to ponder the problem of what to do with the filament bundle. Thingiverse has many plans for spools that fit over, under, or beside the printer, but they all seemed complex and fiddly. Besides, I didn’t have the printer running yet, so I couldn’t print up the parts… much less laser-cut anything.

    The Parts Heap disgorged a 4-inch Lazy Susan bearing, some double-layer corrugated cardboard, and odd bits of wood. The end result fits neatly atop the Thing-O-Matic printer:

    Filament spool - front view
    Filament spool – front view

    The base is 2 x 3-inch (actually measuring 1.5 x 2.5 inch) lumber, cut to exactly fit between the front and back plates of the printer box. The boards also butt against the socket-head cap screws securing the printer’s side plates, so they’re not moving. A scrap of 1/4-inch plywood bridges the two; it’s held in place with hot-melt glue atop the lumber. The weight of all that wood holds the assembly in place; making it lighter might not be productive.

    Filament spool - left side
    Filament spool – left side

    The filament coil rests on a hexagon of double-thick corrugated cardboard, cut about 12 inches across the flats and 13 inches across the points; you could glue two single-thickness sheets together. I laid it out with compass-and-ruler techniques, but do what you like.

    The pegs are 7/16-inch wood about 2 inches tall; the outer ones are on the hexagon points and the inner pegs are on a circle 1.5 inches inside the outer pegs. The rectangular caps on the inner pegs prevent the filament from creeping upward while feeding and are angled to let it slide off into the conduit. They’re held in place with hot-melt glue, of course, and a bit more glue stiffens the hexagon points.

    The only store-bought part is the 90-degree PVC elbow originally intended for electrical work: it’s a “1/2-inch Schedule 40 Rigid Nonmetallic Conduit” elbow. I slipped a spring inside the bore to prevent collapse, applied a hot-air gun until it was flexy, bent the second right angle to align the end bell with the incoming filament, and introduced it to Mr Belt Sander to angle the entrance bell more-or-less at right angles to the incoming filament.

    The Lazy Susan bearing must be centered on the top of the printer, but the hole for the conduit must be forward of center to align with the MK5 Extruder head’s filament entry. As it turned out, butting the conduit against the forward rim of the bearing (the non-rotating base part) worked perfectly. More hot-melt glue holds it in place.

    Filament spool - front detail
    Filament spool – front detail

    This front view shows an out-of-focus peg and filament pile at the top, the Lazy Susan bearing between the plywood and cardboard, and the filament dropping straight into the MK5 head.

    The spool easily rotates backwards when the extruder motor reverses. You can lift the spool off, put it down next to the printer, fiddle with the extruder machinery, then replace the spool without cutting the filament. Trust me on this, I’ve done it a lot.

    If I hadn’t dropped the filament bundle, it would probably have slipped right into the spool without any fiddling; the coils are about a foot in diameter as shipped. I devoted a few minutes to feeding the greatly enlarged and somewhat tangled mess neatly into the spool, after securing the bitter end to the cardboard with (wait for it) a dab of hot-melt glue.

    If I ever build another spool, I’ll replace the cardboard with either 1/4-inch plywood or acrylic, then print up some better-looking peg-like objects. A shot coat of paint couldn’t possibly hurt its appearance in the least, either…

  • Seating Knob Heads and Suchlike

    The Thing-O-Matic instructions suggest crushing the knob heads onto socket-head cap screws using pliers. That’s a desperation move for when you have no alternative.

    Instead, if you have a drill press (and you should!), do it this way: lightly grab the cap screw threads in the chuck and squash it into the head.

    Seating knob head on SHCS
    Seating knob head on SHCS

    The same trick works for pressing pulleys and drive splines onto motor shafts.

    Seating extruder drive on motor shaft
    Seating extruder drive on motor shaft

    You shouldn’t use your drill press as a heavy-duty arbor press, but for pressing small circular things onto shafts, it’s hard to beat.

  • Thing-O-Matic / MK5 Extruder: Cartridge Heater Doodles

    During the conversation following my original post on the MakerBot support forum, CodeRage suggested using cartridge heaters. I asked Eks about that and he said something along the lines of “Damn straight! We used ’em all over the place! Just do it!”

    CodeRage plans to retrofit his MK5 head with a pair of 230 V 150 W heaters running at 120 V to get a total of 75 W. I have qualms about running line voltage around the extruder head, but it’s certainly a better solution than toasting power resistors.

    The trouble with 1/2-inch models is that they don’t fit conveniently on the Thermal Core. I’d make an adapter block with a hole for the heater and two holes for the existing cap screws, but the screws don’t quite pass around a half-inch cartridge heater.

    He suggested 1/8-inch heaters from Sun Electric Heater Company, which look like just the ticket except that they’re nigh onto 40 bucks a pop. Ouch.

    High Temp Industries [Edit: new link 2013-12-27] has 1/4-inch heaters for under $20 that will fit in the space available. If I understand the configuration options, you can even get 12 V 30 W heaters (the same power as the existing resistors) with a 1000 °F (call it 500 °C) temperature rating.

    So I think what’s needed is to get some of those heaters, machine blocks to hold them on each side of the Core, and see how that works. The heaters will fit between the resistor screw holes and the Core is just about exactly 1 inch long. What’s not to like?

    This might work… except for the fact that HTI has a $150 minimum order, which is somewhat off-putting even for me. Anybody up for a group buy of ten cartridge heaters?

    Note that if you swap in some cartridge heaters, you really should do the separate +12 V supply Extruder Controller hack described there.

    [Update: Zach @ MBI has ordered a stack of cartridge heaters for their internal testing (he promises to send me some), plans a retrofit kit, and may become a retail source for the heaters. He reports the lead time to get heaters in bulk is something over two weeks, which is a lot longer than I expected.

    In light of that, I will hold the “group order” until I have a better handle on what’s needed to retrofit cartridge heaters into the existing MK5 head, how they’ll actually work, and what PID loop retuning may be required. Once I know more about all that, we can proceed.

    Having MBI handle the ordering & shipping makes sense to me!]

  • Thing-O-Matic / MK5 Extruder: Resistor Wrapup

    Extruder resistor wiring
    Extruder resistor wiring

    As nearly as I can tell, using a pair of 10 W power resistors as 30 W heating elements in the Thing-O-Matic’s MK5 Extruder Thermal Core isn’t going to work, at least if you want even minimal reliability.

    The fundamental problem is that the resistor specification limits the dissipation to a few watts, tops, near 250 °C, where they must run in order to melt any of the plastic filaments.

    The Thermal Core requires 20-30 W to maintain 225 °C, so each resistor must dissipate an average of 10-15 W at that temperature. That’s half of the MK5 extruder’s original design point and still nearly a factor of 10 beyond the resistor rating.

    The original design runs at less than 50% duty cycle to maintain 225 °C, which agrees with my measurements:

    • 50% of 60 W = 30 W
    • 33% of 60 W = 20 W

    If you want to run at lower power, it’s a drop-in replacement. Change the original 5 Ω resistors to 2.5 Ω resistors (from Digikey / Mouser / wherever), change the wiring to put them in series (not parallel!), and see how long they last. They’ll certainly fare better than at 30 W, but I wouldn’t expect more than a few hours of lifetime. The specs give them 1000 hours at rated power, which this certainly is not.

    A series connection means that when one resistor fails, the heat goes off. The original parallel connection left one resistor carrying the load and, at 30 W, it can actually get the Core up to operating temperature and keep it there. Many folks have been baffled by that, but the diagnosis is simple. Measure the resistance of the parallel resistors at the Extruder Controller end of the wires:

    • 5 Ω → one resistor has failed
    • An open circuit (infinite resistance) → both are dead

    The problem with the lower power dissipation, whether from a failed resistor in the original design or my suggested change, is that the extruder head has a thermal time constant of 10-11 minutes. Lower power means a longer cold-start time; 30 W should get it up to 225 °C in about 20-30 minutes depending on the insulation. That’s not really a problem if you’re printing a series of objects, but might be objectionable for quick printing sessions.

    However, when a resistor fails, the heat goes off, the plastic stiffens up, the DC extruder motor stalls, and the essentially unlimited motor current kills the A3977 driver on the extruder board. My incandescent lamp workaround may alleviate that problem: when the light goes on, check for a failed resistor.

    I picked up a stock of 2-to-3 Ω power resistors and will do some further experimenting with power levels, insulation, and suchlike. This is a short-term fix to get my Thing-O-Matic running, but there’s a better long-term way to go: cartridge heaters on a modified Thermal Core, which I’ll discuss shortly.

    If you arrived by search engine, jump there for my earliest guesstimates, go there to the beginning of the Thing-O-Matic hardware hackage posts, then read until you get back here. The story will, perforce, continue…

  • Thing-O-Matic / MK5 Extruder: Resistor Connections

    Having collected useful thermal numbers at low power levels, it’s time to fire that mother up and see what happens at temperatures around 200 °C. That, however, requires powering both resistors, rather than attacking one with clip leads as I’ve been doing. Given that I expect to change the resistors several times in the course of this adventure, soldering to the lugs seemed like a lot of effort.

    I mooched some solderless lugs suited for 2-56 screw terminals from Eks, pulled off the plastic insulating sleeves, lightly crimped them on 14 AWG solid copper wire, and silver-soldered the joints. The crimp handles most of the current, while the solder keeps the interior from accumulating oxidation products at high temperatures: a gas-tight joint is a happy joint.

    Crimped and soldered lug
    Crimped and soldered lug

    The resistor leads have holes just slightly too small for 2-56 screws, but a pass with a #41 drill does the deed; I think it’s an accumulation of solder rather than an under-sized hole.

    The leads are stamped to shape and two of them didn’t have quite enough room for the lug. You don’t want the joint to look like this:

    Misaligned lug
    Misaligned lug

    The briefest touch of a riffler file made them right, so as to look like this:

    Properly aligned lug
    Properly aligned lug

    Then it was ready for insulation:

    Extruder Head with lugs
    Extruder Head with lugs

    Note that the resistors are in series, not parallel (as per the Makerbot instructions), because I want a resistor failure to produce an unambiguous symptom: no heat. In addition, I expect to operate the heaters at much lower power, making higher resistances easier to drive from the +12 V.

    In truth, those screw-and-nut connections aren’t the most durable or reliable joints, particularly without lockwashers under the nuts to soak up the differential thermal expansion. But they’re good enough for what’s coming next.

  • Improved Sherline Probe Length Switch Repeatability: Selah!

    Re-running that probe length switch test a few weeks later produced these results for three trials over the course of two days.

    Probe Repeatability - Dec 2010
    Probe Repeatability – Dec 2010

    The Z-axis differences are all relative to the first reading on the first day, so this includes whatever Z-axis changes take place without doing anything else on the mill in between the tests. I turned the power off after making the first set of measurements, so the steppers restarted with up to a plus-or-minus one full step offset; that works out to:

    (0.050 inch) * (1 rev / 200 steps) = 0.00025 in = 0.0064 mm

    Because EMC2 doesn’t actually know where the stepper is, any uncommanded motion will show up as an offset when the probe switch trips, which is exactly what we see here.

    Two things of interest:

    • The -0.05 mm offset between the two days could well be part of a single step offset
    • Successive probe positions during a single test don’t change by hardly anything at all

    Conclusion: a cheap mechanical switch works just fine and an even cheaper switch was still good enough.

    The dataset looks like this…

    Trial	16 Dec	16 Dec	17 Dec A	17 Dec A	17 Dec B	17 Dec B
0	25.859616	0.000000	25.806118	-0.053498	25.810032	-0.049584
1	25.860282	0.000666	25.808900	-0.050716	25.810696	-0.048920
2	25.863610	0.003994	25.808214	-0.051402	25.813354	-0.046262
3	25.863610	0.003994	25.809368	-0.050248	25.812028	-0.047588
4	25.864276	0.004660	25.810032	-0.049584	25.812025	-0.047591
5	25.862945	0.003329	25.812162	-0.047454	25.814018	-0.045598
6	25.864941	0.005325	25.812690	-0.046926	25.812719	-0.046897
7	25.864276	0.004660	25.810696	-0.048920	25.813540	-0.046076
8	25.864276	0.004660	25.812690	-0.046926	25.814012	-0.045604
9	25.864276	0.004660	25.813354	-0.046262	25.812690	-0.046926
10	25.864941	0.005325	25.813477	-0.046139	25.814018	-0.045598

    The raw data, just for completeness…

    Probe test data - Dec 2010
    Probe test data – Dec 2010

    Selah!

    (I’m giving a talk and show-n-telling my Sherline CNC milling machine at Cabin Fever Expo right about now, so having this data readily available seemed prudent. The talk & handouts are there.)

  • Thermocouple Calibration: Isothermal Block

    Verily it is written:

    • The man with one thermometer knoweth the temperature
    • The man with many thermometers knoweth not the temperature
    Drilling the isothermal block
    Drilling the isothermal block

    Given the five thermocouples and their meters shown there, plus the Thing-O-Matic’s thermocouple, I had six different temperatures. They’re close, but we can do better than that.

    The general idea is to put all the thermocouple beads in close proximity so they share the same temperature, record their opinions to various temperatures, then figure out an equation that adjusts their disparate opinions to reflect consensus reality.

    I cranked out an isothermal block on the Sherline mill, using EMC2’s exceedingly handy polar coordinate notation to get a nice hexagon. Touch off XYZ=0 at the middle of the block, then center-drill and drill:

    G0 Z3
G0 @5 ^0
G83 Z-5 R3 Q1 F100
G0 ^60
G83 Z-5 R3 Q1 F100
G0 ^120
... etc ..

    For lack of anything better, 3000 rpm with a drill matching the ID of the brass tubes, plus dripping cutting fluid as needed.

    Thermocouples in block
    Thermocouples in block

    I used a 6 Ω 50 W resistor (the adult version of the resistors on the Thing-O-Matic / MK5 head) as a heat source, clamping the block to the resistors with plastic clamps to provide mechanical force and thermal isolation. Good idea, bad implementation: as you’ll see, those little red tips melt at a rather low temperature.

    The TOM thermocouple bead will fit into the empty hole.

    Next step: numbers!