Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
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..
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 Automatic Build Platform rollers have a small gap in the middle where pegs on the platform support the shaft. The belt must be sufficiently taut that it’s flat across the entire length & width of the platform, which means it’s so tight that it collapses into the gap and forms wrinkles in the most critical area.
Prior to installing an aluminum plate build surface, I wondered if adding a support in the gap would reduce the wrinkling, so I cooked up a small OpenSCAD script to print these things out:
ABP Roller Support
They’re at about the finest resolution the printer can produce; getting the fill between the walls seems iffy at best. The top set has obvious gaps that come from having walls too close together.
I finally printed them at 0.4 mm thickness and a width of 0.5 mm (w/t = 1.2) and that produced the lower set with adequate fill:
ABP roller center supports
Unsurprisingly, the holes are too small, but that’s easily fixed with a drill just slightly over 1/8 inch. The length of the stem also required a bit of fine-tuning; you can always make it shorter with a file:
Supports on rollers
Some preliminary testing says that the motionless supports might produce too much friction on the belt, but that was with a paper backing. Running slick tape around the middle of the belt’s inside surface might help, plus it would add a bit of stiffening. Adding some ridges to reduce the surface area in contact with the belt would probably just score the belt.
I’ve been experimenting with Kapton-on-paper belts, which remain much flatter than the endless belt, but are much more sensitive to the gaps. More fiddling is in order, after I get some one-off parts built on the aluminum plates.
The OpenSCAD script:
// ABP Central Shaft Spacers
// Ed Nisley - KE4ZNU - Feb 2011
//--- Extrusion dimensions
// Must be tall and skinny to get fill around the hole
ExtThickness = 0.50; // extrusion thickness
WTRatio = 1.20; // width-over-thickness
//--- Spacer dimensions
ShaftDia = 3.175; // metal rod = hard 1/8 inch = 0.125 inch
ShaftRad = ShaftDia/2;
RollerDia = 8.0; // approximate OD of in-situ rubber rollers
RollerRad = RollerDia/2;
OverAllLen = 7.6; // length along shaft = hard 0.300 inch
TaperLen = 2 * ExtThickness; // make it a few layers thick
Faces = 10; // polygonal shape for outside cylinders
//--- APB Interface
PlatformZ = 5.0; // thickness of ABP support under heater
PlatformGap = 2.0; // distance from roller to APB
//--- nophead's polygonal hole correction
// http://hydraraptor.blogspot.com/2011/02/polyholes.html
// Adapted to center the resulting cylinder
module polyhole(h, d, c) {
n = max(round(2 * d),3);
cylinder(h = h, r = (d / 2) / cos (180 / n), $fn = n, center=c);
}
//--- Odds & ends
FinagleLen = 1.0; // enough to make holes obvious
//-----------------------
// The Spacer!
difference() {
union() {
translate([0, 0, OverAllLen/2-TaperLen/2])
cylinder(r1=RollerRad, r2=RollerRad-TaperLen, h=TaperLen,
center=true, $fn=Faces);
cylinder(r=RollerRad, h=OverAllLen-2*TaperLen,
center=true, $fn=Faces);
translate([0, 0, -(OverAllLen/2-TaperLen/2)])
cylinder(r1=RollerRad-TaperLen, r2=RollerRad, h=TaperLen,
center=true, $fn=Faces);
translate([(RollerRad+PlatformGap)/2, 0, 0])
cube(size=[RollerRad+PlatformGap, PlatformZ, OverAllLen],
center=true);
}
polyhole(OverAllLen+2*FinagleLen, ShaftDia, true);
}
The general idea: a thermal switch detects an overtemperature condition and shuts off the power supply for the heaters and the extruder motor.
A sketch that should accomplish that goal:
Thermal Cutout Doodle
The -Power On pin is 14 in the 20-pin Motherboard connector and 16 in the default ATX 20+4-pin connector.
The thermal switches in my heap have their case connected to one lead, so it makes more sense to put ’em on the low side of the circuit, with the case grounded. That way you can attach ’em directly to, say, the Thermal Core… or at least not worry about inadvertent shorts.
How it works
When you turn the ATX supply’s AC power switch on, the +5VSB supply goes active and the AC On LED lights up. Nothing else happens: the relay remains open and the Thing-O-Matic appears to be dead. This is how it should work!
The +5VSB supply provides initial power to this circuit; all other ATX power outputs are disabled and all Thing-O-Matic boards are inactive. The AC On LED indicates that the standby supply is active.
The Test/Fault LED is lit in this condition to indicate that the relay is inactive; there seems to be no way to unambiguously indicate a thermal fault without at least one more relay. On the upside, you know the LED works.
Pressing the Power On pushbutton closes the DPST relay, assuming the Normally Closed (NC) 65 °C Thermal Switch is closed and the Estop button is not pressed. I happen to have such a thermal switch in my Parts Heap, but a higher temperature may be desirable; they’re stock items at Digikey / Mouser. See below for a discussion of the sensor location.
You can use a DPDT relay, which may be easier to find.
One pole of the relay bridges the Power On pushbutton switch, holding the relay active when you release the button. The other pole pulls the ATX -Power On input low to enable the ATX supply voltages to the rest of the Thing-O-Matic, which lights up and begins working as usual.
The +Power Good signal eventually goes high and turns on the Power OK LED.
If the temperature exceeds 50 °C, the Normally Open (NO) 50 °C Thermal Switch closes and the Heat Alert LED goes on. That temperature may be too low.
If the temperature exceeds 65 °C, the Normally Closed (NC) 65 °C Thermal Switch opens and releases the relay. That releases the -Power On input and immediately turns off all the ATX supply voltages: the Thing-O-Matic hardstops. Because the other relay pole no longer bridges the Power On switch, the relay remains off.
As suggested there, putting a high-temperature thermal fuse directly on the Thermal Core insulation blanket would be a Good Thing. That fuse goes directly in series with the 65 °C Thermal Switch as a deadman cutout: when it blows, you cannot turn the Thing-O-Matic on until you replace it.
The Test/Fault LED goes on when the 65 °C Thermal Switch cools off enough to close. That’s suboptimal, although the 50 °C Heat Alert LED will be on while it’s cooling.
Pressing the Estop pushbutton switch also releases the relay. That switch is Normally Closed (NC) because most wiring faults involve a wire not making contact. A Normally Open (NO) switch wouldn’t give any indication of the wiring problem, while an NC circuit simply won’t allow the TOM to power up.
As nearly as I can tell, the Motherboard ignores its own Estop switch input.
Important: don’t get clever by replacing the relay with a semiconductor. You want something dead simple that won’t suffer from static discharge damage or software failures. Relays, buttons, and switches tend to be very simple, easy to test, easy to verify, and not prone to weird failures.
Thermal Switch Locations
The ideal location has a physical attachment to the Thermal Core, rather than an air gap. The top of the Thermal Riser Tube seems ideal, but I measured the heatsink temperature as exceeding 90 °C with the Core at 220 °C.
What’s worrisome about that: the glass transition temperature for acrylic plastic is in the 85-165 °C range. The top of that tube is much hotter than it really should be, given the stresses imposed on the Filament Drive frame.
I’m thinking of adding a heatsink across the top, extending out of the openings under the top of the support structure. That should cool the Tube, make the acrylic happier, and provide a location where the temperature remains below 50 °C in normal operation. When the Core exceeds, say, 300 °C, the switches would trip.
I think that’s messy, but putting higher temperature switches on the existing Tube doesn’t avoid that problem. Obviously, running the TOM in a hot room will produce different results; there’s a tradeoff between false trips and burning the house down.
More study is needed…
ATX Power Supply Signals
These must be hijacked from the ATX power connector at the Thing-O-Matic Motherboard, maybe using square pins & heatshrink to fake a connector. You could repurpose the 4-pin +12 V / Ground CPU power connector, but then you’d be forced to come up with a mating cable connector.
Or just solder a cable to the Motherboard, cut the -Power On, splice in the wires, and be done with it.
The +5VSB (9 Purple) supply is active with the AC line switch turned on. This supplies power to bootstrap the protection circuit, much as it does in an ordinary PC.
The -Power On (14/16 Green) signal has an internal (and unspecified) pullup resistor. The Motherboard pulls that line to ground, so the new connection must be spliced into the middle of the existing ATX wire. The pin number is 14 in the 20-pin block and 16 in the 24-pin block.
The +Power Good (8 Gray) signal remains low until all output voltages get within the tolerance limits, then it goes high. There’s no spec for output current, but we assume it can drive an LED. Presumably this goes low when any voltage goes out of spec, so a glitch catcher should be instructive.
A Common or Ground (7 Black) line provides the circuit ground. The Motherboard has very low current requirements, so stealing any of the Ground wires will be OK.
This is a quick-and-dirty test to see how hot the neighborhood around a cartridge heater equipped MK5 might get, with the intent of determining where to put an overtemperature cutout switch.
I stuck one thermocouple inside the Kapton tape wrap, outside the ceramic wool insulation above the left-hand heater block, to get an idea of the actual surface temperature. Another thermocouple rests against the small heatsink at the top of the Thermal Riser, where it’s probably measuring a bit of the heatsink and some air temperature; it should be inside the small brass tube epoxied to the heatsink, but I was not going to tear the head apart for that.
As before, a pair of 25 W cartridge heaters raised the internal Thermal Core temperature to 225 °C in a bit under 15 minutes, according to the TOM’s usual thermocouple. I think baking the water out of the insulation wrap had a lot to do with the decrease, but the difference wasn’t more than a minute or two.
After cooking at 225 °C for 15 minutes, the outside of the insulation stabilized at 133 °C. Opening the front window of the build area let enough of a draft inside to affect the temperature, even under the tape wrap, so that’s definitely not a solid temperature.
The thermocouple at the top of the Riser Tube reached 83 °C and was also affected by drafts.
So.
The thermal cutout must be solidly mounted to either the Core itself or the Riser Tube, in order to prevent irrelevant temperature readings. I’m beginning to favor the Riser: it’s out of the way, shouldn’t get too hot in normal operation (because it’ll melt the filament), and has a solid thermal connection to the Core. A pain to get access in there, but you only need that occasionally.
The only question now is how to determine the actual temperature seen by a thermal switch in there. I think a clamp around the tube with a tab sticking out beyond the support structure is in order.
The Smell of Molten Projects in the Morning 