Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
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
The same trick works for pressing pulleys and drive splines onto motor shafts.
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.
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!]
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…
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
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
The briefest touch of a riffler file made them right, so as to look like this:
Properly aligned lug
Then it was ready for insulation:
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.
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
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
(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.)
The man with one thermometer knoweth the temperature
The man with many thermometers knoweth not the temperature
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:
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
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.
Extruder head with thermocouple mounts – epoxy curing
As I described there, the resistors on the Thing-O-Matic MK5 Extruder Thermal Core operate in impossible conditions. To summarize, each resistor is rated to dissipate 10 W at 25 °C, but is actually dissipating nearly 30 W at well over 225 °C. Ouch!
I wanted to figure out just what was going on inside the Extruder Head, which means some instrumentation was in order, which meant I had to figure out how to attach a set of thermocouples to the Core. This picture shows one approach: epoxy a set of small brass tubes to various parts of the MK5 Extruder Head.
JB Industro-Weld epoxy is rated to 500 °F = 260 °C, which is barely adequate for the job at hand.
The general idea is that each tube provides an isothermal mount for a thermocouple bead, without the inconvenience of drilling holes in various metal bits and messing with high-temperature thermal compound. I am assuming that putting the beads inside the tubes, heating the Core, then waiting for the temperature to stabilize will produce meaningful results.
I have a motley assortment of meters that allegedly read temperature from Type K thermocouple beads. The business end of those thermocouples looks like this:
Thermocouple beads
The twisted one in the middle has a completely non-standard red-black insulation color code, but as long as the meter it came with is happy, I’m happy. The two on the right have industrial-strength wires, as befits the fact that they plug into a Fluke 52 dual-thermocouple meter; them, I trust.
I skinned down the insulation a bit so they’d all reach into the middle of the tubes and filed down the bead on the right just a smidge.
It shouldn’t come as any surprise that each of the five thermocouples reported a different number for the ambient temperature, which meant a calibration run was in order.
The Smell of Molten Projects in the Morning 