Posts Tagged Baksheesh
Although the M2′s heated build platform works well enough, somebody who knows what he’s doing (you know who you are: thanks!) sent me an improved version. It’s a PCB heater, laid out to compensate for the usual edge cooling, firmly attached to a tempered glass plate with genuine 3M thermally conductive tape:
They designed the heater around the 30 VDC power supply used in their other equipment. Although I had
high moderate hopes that a boost power supply would convert the 24 V supply I already had for the stepper driver bricks into the 30 V for the heater, it was not to be. So there’s a 36 V 9.7 A 350 W supply arcing around the planet that (I think) should work better: adjust the voltage down as far as it’ll go, soak up another few volts in the solid-state relay, and Things Should Be Close Enough to 30 V. One can buy a genuine 30 V supply, but it costs surprisingly more than either 24 V or 36 V supplies on the surplus / eBay market and won’t really provide the proper voltage without upward tweaking anyway.
I replaced their standard 0.156 inch square terminals with Anderson Powerpoles, soldered a length of shielded cable to the 100 kΩ thermistor pads, and gimmicked up a connection to the 24 V supply; it delivered 23.7 V at the PCB terminals. The thermistor is 100 kΩ at 25 °C and 11.4 kΩ at 77 °C. The PCB heater is 5.9 Ω at 25 °C and 7.3 Ω at 77 °C; it dissipates 77 W at 77 °C (no, that’s not a typo).
The ultimate temperature looks to be about 90 °C with a 24 V supply, which isn’t quite enough for ABS (which I’m not using in the M2 right now, but probably will eventually). The time constant, assuming the 1-e-1 point is 66 °C, works out to about 9 minutes; it’ll be up to final temperature in half an hour. Those numbers aren’t quite as accurate as one might wish, because the heater power drops as the temperature rises and the copper resistance increases.
A 30 V supply would dissipate 120 W at 77 °C and rumor has it that the ultimate temperature is around 125 °C, which would be fine for ABS. Goosing the power a bit would produce more heat, but I’v been running the Thing-O-Matic at 110 °C and that’s good enough. More power, of course, gets it to the temperature setpoint faster, which is probably a Very Good Thing.
Obviously, you need PWM to control the temperature; given a 9 minute time constant, a bang-bang controller will work perfectly well.
The original data, including the thermistor resistance after I got my act together, plus a cute little temperature-vs-time graph:
The colored flyspecks are part of the paper; I salvaged a stack of fancy menu cards from a trash can and padded them up as geek scratch paper.
In the admittedly unlikely event you happen to be near the left-center part of Long Island this evening, drop in on my DIY 3D Printing & the Makerbot Thing-O-Matic presentation for the Long Island Linux Users Group meeting and pick up a tchotchke!
Many thanks to LILUG for ruthlessly eliminating all my objections to leaving the Basement Laboratory…
A long time ago, in a universe far away, I wrote a book that (barely) catapulted me into the ranks of the thousandaires. Time passes, companies get sold / fail / merge / get bought, and eventually the final owners decided to remainder the book; the last royalty check I recall was for $2.88.
Anyhow, now that it’s discontinued and just as dead as the ISA bus, I own the copyright again and can do this:
They’re both ZIP files, disguised as ODT files so WordPress will handle them. Just rename them to get rid of the ODT extension, unzip, and you’re good to go. Note, however, that I do retain the copyright, so if you (intend to) make money off them, be sure to tell me how that works for you.
The big ZIP has the original pages laid out for printing, crop marks and all, so this is not as wonderful a deal as it might first appear. The little ZIP has the files from the diskette, which was unreadable right from the start.
Words cannot begin to describe how ugly that front cover really is, but Steve’s encomium still makes me smile.
The text and layout is firmly locked inside Adobe Framemaker files, where it may sleep soundly forever. The only way I can imagine to get it back into editable form would be to install Windows 98 in a VM, install Framemaker, load up the original files, and export them into some non-proprietary format. Yeah, like that would work, even if I had the motivation.
If you prefer a dead-tree version, they’re dirt cheap from the usual used-book sources. Search for ISBN 1-57398-017-X (yes, X) and you’ll get pretty close.
Or, seeing as how I just touched the carton of books I’ve been toting all these years, send me $25 (I’m easy to find; if all else fails, look up my amateur callsign in the FCC database) and get an autographed copy direct from the source. Who knows? It might be worth something some day…
The back cover has some useful info:
MBI sent me a selection of 1/4-inch cartridge heaters to evaluate, seeing as how I’ve been such a pest on the subject of those poor aluminum-case power resistor heaters. Thanks, Zach!
I initially thought I could punch the cores out of the resistors and slip the cartridge heaters into the holes, but it turns out the resistor bodies aren’t quite the right size: slightly too short with slightly too large holes. So it goes. Some earlier thoughts live there.
This is a first pass at building mounting blocks to attach cartridge heaters to a stock MK5 Thermal Core. Ideally, you want a solid Thermal Core with a hole or two for the heaters next to the filament extrusion nozzle, but that requires fancier machining that I’m ready for right now. The fabled nophead shows how that looks for a ceramic power resistor.
The obvious question is whether you want a single high-wattage cartridge heater or a pair of low(er)-wattage units. I think a core-with-hole can get away with a single heater, which is also the lower-cost option. My thermal measurements suggest the Core is pretty much isothermal, so there’s no problem with distributing the heat evenly from one side to the other.
However, adding two lower-wattage heaters to a stock MK5 Thermal Core makes more sense, because the interface between the blocks and the Core seems to run a bit under 1 °C/W. A single 40 W heater would thus run 30-40 °C higher than the Core: call it 260 °C. IMO, that’s much too high for something an inch away from a plywood frame and an acrylic support structure.
A pair of 25 W heaters would run at 245 °C-ish. That’s still pretty hot, but every little bit helps. I’ll start with that arrangement and see how it works.
The blocks are ordinary steel from the Scrap Box: a convenient length of 1×1-inch bar stock that somebody else had made into something else a long time ago. I bandsawed off four 1×1-inch slabs, each about 5/8″ thick. A second bandsaw cut turned the square slabs into rectangles. I finished two blocks; the other two slabs await more experience with how these work.
I squared up the blocks with a flycutter in the Sherline, then sanded down the bottom surface a bit. The thermal tests suggest the contact is Good Enough with a reasonably flat surface, so I settled for a used-car finish: high shine and deep scratches. They’re actually smoother than the pictures would have you believe.
The Thermal Core has hard inch dimensions (minus cleanup cuts): 1 inch front-to-back and 13/16 inch tall. I generally work in metric, so the sketch at the bottom has everything in millimeters.
The mounting blocks have holes matching the resistor footprint. I drilled clearance holes for the heads of the original M2 socket head cap screws, ran an end mill down the hole to flatten the bottom, then drilled clearance holes for the threads. Those holes are perilously close to the edge, but the blocks really don’t want to be any taller. Perhaps use a less-generous clearance?
The alternative would be to mill a flange along the edge to match the resistor mounts and put the SHCS heads in free air, but that seemed like more work and it would cramp the thermal path from cartridge to block.
I also thought about chamfering the edges to make the blocks look less, well, blocky, but that’s in the nature of fine tuning.
The cartridge heaters slip-fit into a nominal 0.250 hole; the samples are 0.247 to 0.248 and (from what I read) the diameter tolerance stays on the minus side of 0.250. I don’t have a 0.250 reamer, which is how you get a precise hole ID, so I’ll go with drilled holes. Fortunately, I have a set of letter-size drills in nearly new condition:
- A drill = 0.234 to poke a hole in the block
- E drill = 0.250 to get the final diameter
The final holes worked out to be exactly 0.250 inch, to the limits of my measurement ability, which I will declare to be Good Enough. The cartridges have a loose slip fit with no side-to-side play.
The cartridges expand when heated and squeeze against the hole to make good thermal contact. While cool, however, they can slide out without much urging, so I added a 4-40 setscrew. It’s on the butt end of the cartridge heater shell, away from the leads, so if a cartridge becomes one with the block I can drive it out with a pin punch. Putting the setscrew at the end with the wire leads makes more sense (it’s cooler there), but then you’d be beating the entire length of the cartridge out past the setscrew hole.
The setscrew and the M2 SHCSs get a liberal dose of anti-seize grease before assembly.
Here’s what the holders looked like, just before bolting them in place:
Doodles with the more-or-less as-built dimensions: