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.

Author: Ed

Thing-O-Matic: Work Flow

The reason I didn’t see the yellow low-overheat LED blink on when the Thermal Core ran away was that I’m usually upstairs except when actually printing.

My modus operandi involves sitting at my upstairs desk, fiddling with an OpenSCAD solid model until I like it, then exporting the STL file. This PC has larger screens, better graphics hardware, a fast CPU, a Comfy Chair, and ready access to the kitchen.

The Thing-O-Matic lives in the Basement Laboratory, connected to a dual-core Atom D520 PC running Ubuntu 10.04 LTS with ReplicatorG to control the printer. That PC dual-boots into the RTAI-patched kernel that runs EMC2 for the Sherline mill and is firmly cabled to the Sherline driver box. That PC’s monitor is up on the wall, the chair is a modified lab stool, and the miniature keyboard is barely suited for hunt-and-poke controls. Not a good place to sit and type.

That PC also has a USB webcam showing an interior view of the printer. I run XawTV, a minimal video capture program, to put that view on the PC’s desktop. I could set it up as a webserver camera, but that seems like too much work.

I use the Ubuntu desktop-sharing program to view / control the downstairs programs in a window on my (larger) upstairs monitor, so I can fiddle with RepG from the Comfy Chair. There’s a moderate lag due to stuffing the GUI through the network, but it’s tolerable for small changes & tweaks. The webcam view occupies one corner of the screen.

This is a staged reenactment showing the remote “downstairs” desktop in the left, with the “upstairs” desktop visible to the right:

Remote Desktop Screenshot – lowres

All the files live on our simpleminded server, which sits in the Basement Laboratory’s Computer Wing, and the PCs mount NFS shares from the server. I do all the bulk text editing & file fiddling from the Comfy Chair.

So I save the STL file from the upstairs PC, flip to the window showing the downstairs machine’s desktop, copy the STL to a local drive to avoid lag during operation, run RepG to open the STL and slice it into G-Code, fire the Thing-O-Matic, and trot downstairs to watch the proceedings.

As a rule, I don’t run the printer unattended, but now it looks like it’s a Bad Idea to run the heaters without being nearby. You knew that already, right?

2011-06-08
Thing-O-Matic: Thermal Runaway!
Well, it finally happened: the Extruder Controller jammed the extruder heater full on and the Thermal Core temperature went on an uncontrolled rise.

This changes the thermal runaway scenario from “It can’t happen here” to “Once is happenstance“.

I’d been tweaking an OpenSCAD model, slicing it, not liking the results shown in Skeinlayer, re-tweaking, re-slicing, and iterating around that loop for quite some time. When I figured I was close to having a good model & G-Code, I turned on the heaters to get the printer ready; heating from a cold start requires about 12 minutes due to the double aluminum build plates and hulking cartridge heater adapters.

I made several (well, many) more iterations through OpenSCAD and slicing, flipped to the Control Panel, and discovered the Thermal Core temperature was passing through 285 °C on the way up. Now, the temperature doesn’t rise abruptly, but it was already far higher than the original 210 °C setpoint; you cannot set an Extruder temperature over 260 °C (!) without passing through a confirmation dialog.

Oddly, the setpoint temperatures for both the Extruder and HBP showed 9 °C. The build platform was cooling off, as you would expect for a setpoint far below the actual temperature, but the Thermal Core heater was jammed on.

An LED indicator on the Z stage shows when the EC switches the cartridge heater on. That LED was lit, so I knew the EC had gone nuts: the heater was on, even though the Thermal Core temperature was far above the setpoint.

I passed up an opportunity for Science by punching the Emergency Shutdown button on the Thermal Lockout: the Thing-O-Matic went dark. Turned it back on, reconnected RepG, and ran a few yards of smoking hot filament out of the nozzle. Yes, the Thermal Core really was that hot… I ran the filament drive at 25 rev/min for 10 seconds and it sprayed filament like crazy.

The Thermal Lockout had not tripped, very much as expected. Back when I was figuring out where to mount the thermal switches, those measurements suggested that the Thermal Core would probably exceed 325 °C before the 100 °C NC switch at the top of the Thermal Rise opened. The Core hadn’t gotten close to that temperature, but it was on the way!

The 40 °C NO switch glued to the base of the filament drive had long since closed and lit the yellow LED, but even I agree that’s an absurdly low temperature for such a warning. I have no idea what the actual temperatures were, but I’m thinking of putting a pair of thermocouples in the obvious spots.

I’ve devoted considerable time and energy to eliminating erratic operation and glitchy behavior, to the extent that the printer has behaved flawlessly up to this point. Obviously, something glitched the Extruder Controller, which is basically an Arduino-class microcontroller monitoring two temperature sensors and driving two MOSFETs, while chatting with the Arduino Mega that lives under the Motherboard.

A power glitch will hard-reset the Mega (because I connected +Power Good to -Reset), but the (2.7, anyway) MB firmware has the disturbing property of not resetting the EC when it restarts. You can confirm this by turning the heaters on using the Control Panel, closing it, then blipping the Motherboard Reset button: the heaters remain on. I’ll connect +Power Good to the EC’s -Reset the next time I open the box.

The Thing-O-Matic worked fine (again) after the shutdown, but, as far as I’m concerned, all the effort I put into the Thermal Lockout on the Extruder Heaters has been justified. If the EC ran away once, it will run away again.

I don’t have an MBI MK6+ Thermal Core or their Safety Cutout Switch circuit, but a look at the schematic suggests a heads-up if you use that hardware. The Safety Cutout Switch has (IMO, anyway) several design flaws:
- The relay is not energized during normal operation. If it fails to energize when the thermal switch activates, the heater remains active.
- By design, the 12 V relay sees only 9 V when the thermal switch operates. The usual wiring resistance and MOSFET resistances, in combination with a sagging power supply, can reduce the available voltage below the relay’s 8.4 V must-operate voltage: it may not cut off the heater.
- The relay cannot operate when the ground connection in the Alarm cable isn’t connected, as is the case in a Cupcake without a matching E-Stop jack. In any printer, a disconnected Alarm cable or a broken ground wire will silently prevent the relay from operating.
I’m willing to be proven wrong on any of those points, but as nearly as I can tell, the cartridge heater in a MK6+ can operate with a completely non-functional “Safety Cutout” and you’ll never know there’s a problem. When installed exactly as directed and with the entire printer working properly, it’ll probably work… but if everything worked properly, you wouldn’t need it.

[Update: MBI recently issued a slipstream update that put the Safety Circuit at Rev B. The relay now attaches to the Extruder Controller MOSFET, thus eliminating the requirement for the Alarm cable’s ground. However, the relay now sees the heater’s high-current voltage drop on both terminals. Measure the actual voltage at the Safety Circuit’s input terminals with both heaters running. If that voltage is less than 11.2 V, the relay must operate below its must-operate voltage in order to cut off the Extruder heater.]

The rant at the bottom of that post gave some features that I considered vital when I was designing the Thermal Lockout for my printer. I thought it went without saying that a safety circuit should fail safe: if the safety circuit does not operate, the protected equipment must not operate. As long as the Thermal Switch functions correctly, my Thermal Lockout will fail safe: an inoperative relay, a broken wire, a disconnected switch, or a weak power supply will prevent the printer from starting.

My circuit is not completely fail-safe, of course, but the most common problems cause a hard shutdown.

It really does matter…

I’m now thinking that a thermal switch on the heated build platform is a Good Thing. I suspect the solder will melt and the connector will fall off before melting the acrylic in an ABP or charring the plywood in an HBP, but it’d be interesting to test that assumption, wouldn’t it?
2011-06-07
Thing-O-Matic: Filament Diameter
What with printing this and that and those and some other things, we ran through a pound of red filament in short order. That’s fine with me, because I could never figure out a good combination of settings to photograph the objects; the cameras I have on hand seem blind to saturated red and overexpose the daylights out of it, even with the flash turned down a stop or two.

We picked yellow as our least favorite of the remaining colors, yanked out the last few feet of red filament as a show-n-tell for presentations, rammed in the yellow, and extruded a red-to-orange-to-yellow plastic tangle over the platform.

The two filaments had somewhat different diameters:
- Red: 2.91 mm dia → 6.65 mm² area
- Yellow: 3.00 mm dia → 7.07 mm² area
The area ratio is 1.06, so the yellow filament puts 6% more plastic through the nozzle at a given extruder speed. I’d been running the red filament at 1.40 rev/min for 20 mm/s feed, so the yellow should produce the same results at 1.40/1.06 = 1.32 rev/min.

My Shop Assistant designed some beads for a cloak she’s repairing and we tried one at 1.25 rev/min (better to err on the low side, I figured) that came out thoroughly overstuffed. She deemed it usable and whisked it off for painting before I could get a picture.

I ran another one at 1.15 rev/min that was still a bit overstuffed. A pair (using Multiply) looked better at 1.10 rev/min. This is a group photo:

Bead – top fill 1.15 1.10 rpm

So I ran off two polyhole test sheets at 1.15 and 1.10 rev/min, the latter of which looked much better. At 1.10 rev/min, the holes run a consistent 0.3 mm smaller than the programmed diameter: 3% on the 9 mm hole. Close enough!

Polyholes – yellow 1.15 1.10 rpm

Then it’s time for some things from coasterman’s calibration set.

A thin-wall box at 1.10 rev/min and 20 mm/s came out with exactly 0.66 mm wall width, so that extrusion speed produces a physical thread that matches the Skeinforge thread parameters used to make the extrusion: 0.33 mm thickness and 2.0 w/t.

Thin wall box – yellow 1.10 rpm 0.33 x 0.66 mm

And then a perimeter test box came out perfect. After scraping off some Reversal zits, the small block fits into the recess in all possible orientations. The base is slightly larger than the top, but that just makes for a somewhat more snug fit. The block is upside-down in the recess so you can see both sides of the printed objects:

Perimeter Calibration Block – yellow 1.10 rpm 0.33 0.66 mm

So.

The initial extrusion speed ratio based on the filament area gets you close, but it still takes a few calibration objects to achieve perfect results. What’s really nice: after that calibration, it’s spot on!
2011-06-06
Thing-O-Matic: Fairing Clamp Plates

Although those pink clamp plates worked well enough, they did not provide, shall we say, a completely satisfactory user experience. I reprinted new sets in red while varying the extruder speed by 0.1 rev/min, with small tweaks to the overlap between the infill and the loop threads.

First, the big pictures with details scrawled on the back of the lower plate…

At 3.2 rpm, which is only slightly too fast:

Fairing Plate – 3.2 rpm

At 3.3 rpm, a bit overstuffed:

Fairing Plate – 3.3 rpm

At 3.4 rpm, there’s obviously too much plastic:

Fairing Plate – 3.4 rpm

Some closeups, in the same order…

At 3.2 rpm with 0.20 overlap, it looks OK:

Fairing Plate – 3.2 rpm detail

At 3.3 rpm with 0.25 overlap, which pretty much devours the inner loop thread:

Fairing Plate – 3.3 rpm detail

At 3.4 rpm with 0.25 overlap there’s serious overfill:

Fairing Plate – 3.4 rpm detail

In all cases, the extruder left a track while exiting upward from near the middle of the images. Even at 3. 2 rpm there’s slightly too much plastic.

My ladies don’t care about the fine details. They prefer red to pink and the clamps hold the fairings firmly in place…

2011-06-05
Thing-O-Matic: Miniature Swig Cup

A miniature version of the Swig Cup came out reasonably well. It’s supposed to be a platform-filling monster; at only 25 mm tall, it’s cute:

Miniature Swig Cup

There’s a bit of trouble with the overhangs at the bottom and top of the handle and the spouts at the top look a bit lumpy. What you can’t tell from that crappy picture is that the panels don’t quite seal to the cylinders: it’s not watertight.

You’ll also have trouble seeing the fine hairs connecting all the spouts. The Reversal plugin in combination with 100 mm/s moves makes the ooze hairs easily removable, although I tend to leave them in place for show-n-tells.

2011-06-04
Thing-O-Matic: Chainmail

This worked out better than I expected: printable chainmail!

ChainMail – top

The back view may be easier on the eyes:

ChainMail – bottom

As the writing says, printed at 20 & 100 mm/s, 0.33 mm thickness, 0.66 mm width, and bridge speed at 1.0 to 1.3 times the usual.

I tried a few variations and got decent results with the bars set to 3 threads wide (the pix show 4 × bars). Making it fairly tall (11 × thread thickness, IIRC) helps get enough clearance below the sagging bridges between the vertical pegs. I’m amazed it works as well as it does.

Dropping to a width of 2 threads doesn’t work: the vertical pegs simply disappear from the G-Code! Turning the pegs into cylinders might help.

A pair of flush-cutting wire nippers applied to the top of the pegs along one edge allows you to lace a pair of sheets together. Apply a micro-drop of plastic cement to each cut, put a roll of duct tape on the joint overnight, and it’s all good.

My Shop Assistant has some interesting ideas for this, although I was mostly interested in its build-ability. It’s wonderful to see the printer lay down a sheet of tiny vertical pegs, five layers tall, and clear the top of every one, every time, on its way back and forth.

I love it when a plan comes together…

2011-06-03
Thing-O-Matic: Graduation Day
OK, so I printed a Stanford Bunny to haul along to the 3rd Ward Make-A-Thon:

Stanford Bunny on platform

It’s one of the few objects I’ve printed with enough interior to show the honeycomb fill pattern:

Stanford Bunny – in progress

Obviously, this was before I sawed three segments off the LED ring light.

It had a bit of trouble with overhang under the ears, but I figure rabbits are soft and fluffy there anyway, so I’ll define this as a feature rather than a bug:

Stanford Bunny – ear overhang

What’s nice: all I did was slice the STL and build the rabbit. No muss, no fuss: It. Just. Works.

Some parameters:
- 0.33 mm thickness, w/t=2.0 -> 0.66 mm width
- 50 mm/s print, 75 mm/s move
- Reversal 25 rpm, 75 ms, early action
An organic object like this eliminates any problems with waviness due to axis instability.

If you look very closely, you can see early Reversal suckouts just to the left of the zit marking the start of the thread. This was the object that prompted me to turn off early Reversal action, but I still haven’t figured out how to get rid of the zits:

Stanford Bunny – Reversal suckouts and zits

(Is it just me or does that not look like part of a rabbit?)

All in all, though, this bunny marks the end of the Intense Thing-O-Matic Hackage era. The printer now works dependably, prints parts accurately, and doesn’t require a lot of babysitting. I’ll present some test pieces over the next few days that explore some variations.

While I didn’t quite jump & clap my hands, life is good…
2011-06-02