Oh hey, Makerbot!

They have a new thing!

This means I can just buy it, wall it off, and point a hair dryer into it, right!?

As for The Next 3D Printer, I’ve been slow in finishing up the model because of a self-imposed lack of funding. I never like putting myself in debt working on personal projects, and have decided to take the month of IAP off to remove some of the “DC average” on my credit card built up from fall projects. There’s still about $200 in linear motion hardware, $100 in metal stock, and $400 to 200 in either MK4 or RAMPS control system (depending on which route I choose… oh yeah, not finishing the CAD until I know where the electronics are going is another factor).

In lieu of pursuing new things given that constraint, I’m probably going to keep trying to get tinycopter flying and when it finally snows some day (seriously?) I can finish working on the board controller for landbearshark. Otherwise, 2.007 work will also keep me entertained.

I Swear I’m Still Working On This Thing

Seriously! I promise I will poop out a new 3D printer by the end of IAP. Now that the holidays break is in full swing and everyone else is gone, I can focus on actually finishing up the design, rather than, say, be constant distracted by things happening at MITERS. I pushed out several orders for materials (including something like 30 square feet of acrylic sheets) and more mechanical parts before the Christmas Everything Shutdown, so hopefully I’ll have all materials in hand come January.

In the mean time, here’s another week or so of on and off work, including testing of the space heater platform and design of the XY gantry head. First, some more waterjetting which was not covered in the last episode.

I think I have made my first impossible-to-cut part. The heater grille has so many little things that can possible stick up into the machine path that, even with careful manual routing, I was unable to avoid interference accidents. And no, it does not have a powered Z axis so I can do head-lift traverses. I’m not that spoiled yet.

After writing off this piece, I tried again, but this time routing the machine to cut out everything but the little grille bits, then doing those last. That way, I can still have consistent mounting dimensions – who cares if the grilly bits are misaligned slightly? To make sure the piece didn’t float away after the separating cut, I inserted small tabs periodically to hold it in place:

I used a chisel and hammer to shear off the small tabs to separate the part. As can be seen, there was one bangup regardless, and there are parts of the resistor mounting holes which come close to the material edge as a result.

Oh well.

The proper method would have been to individually tab each one of those vents, and to manually route the path to start and stop on those tab cuts to minimize nozzle cycling. I wasn’t that patient.

0.22 ohm resistors mounted with a bit of thermal paste. I found out that no bolt head in existence fits in the little mounting tab given, so I had to drill out all of those mounting holes and actually tap the resistor’s aluminum body with M3 threads. They seem to work fine.

I actually bought one of those huge case fans seen in the Z platform models from xoxide. Actually no – I got 2, one to mess with and one to actually use, as per standard operating procedure. They move reasonable amounts of air for their size and low speed, and are indeed very quiet. The actual fan diameter is about 220mm – I found out which dimension is actually 250mm on these things, and the answer is the corner to corner measurement on the outside of the mounting tabs. Failsauce noodles.

Check out the new platform on top of MaB. This thing is going to be enormous – the standoffs mark the bounds of the build plate, which will be 250 x 250mm.

The big question after putting together the platform with wired resistors is whether or not it would dissipate the watts needed to bring the internals of the machine up to a high temperature. Fanning the resistors at room temperature showed that the 1.76 ohm arrangement (8 0.22 ohm resistors in series) isn’t very powerful at all. It draws about 80 watts, but it’s difficult to feel warm air even an inch away. Probably because the resistors are very, very well heatsunk and I just installed the sexually dimorphic father of all computer fans (this is the larger female of the species)

But future-MaB wasn’t going to be heating things up at room temperature – it’s going inside a box. For true thermal testing, I needed a box.

In this picture: a box

Conveniently enough, a box of essentially the right size was standing by.

I commandeered this early prototype of an arduino vending machine project by a certain Nancy (who is currently stomping it out in Asia – make sure to keep up with her blorg). It’s made of cardboard and very much glued and taped together. But hey, that’s all I need.

I actually love this form factor, by the way, so don’t be surprised if I just end up gluing everything to the inside of this

I stuck the platform on some big aluminum standoffs, hooked it up to 12 volts (uncompensated – I wanted a realistic simulation of losses in the wires), and closed the box up.

On the as-designed 80-watt setting, it was horrible. I think the temperature barely crested 40 celsius after 10 minutes, which is my benchmark for acceptable bootup time due to current MaB’s giant resistor board. Probably because the cardboard box leaks air everywhere and isn’t a good thermal insulator alone.  Sadly, I could not locate an IR thermometer (or even one of those stupid things you mount on your window… sadness), so the estimate is very much biased by my hand waving around in air, which given that I regularly act as my own soldering fixture, might be miscalibrated.

I went on Digikey and Mouser to see what the next value smaller resistor was available. I only have 8 mounting points, so I’d have to come up with a reasonable solution which was not just paralleling four resistors on each side of the board – that drew like 30 amps and was kind of insane. By going to 0.1 ohm resistors, I could get a power dissipation of roughly 180 watts.

The next step was to just pump up the power supply to 180 watts. When you’re just dumping into resistors, watts is watts, so the final setting turned out to be approximately 17.5 volts at the heater (the alligator clips eating like 2 volts on top) and 10 amps. This time, the results were much more promising. It certainly reminded me of the inside of a commercial printer, but again, without an IR or traditional thermometer, it’s hard to judge. I would estimate that it was over 60 celsius, but not with certainty.  I thought about hijacking MaB’s platform heater thermistor for a moment in order to get a good air temperature reading, but it was a little more work than I wanted to deal with at the moment.

The good news is that the shady case fan worked fine in the higher temperature environment.

My resistors are delayed due to the holiday break, but once MIT opens again briefly next week I should be able to get more testing done (as well as pick up a reasonable thermometer from somewhere)

a bit more design

I’ve been really lagging on the actual machine design this time, something which I took the past 3 days or so to address. The reason it took so long was because I’m a grad student that I… you know what, fine. I admit it. I’m a grad student. I spent more time thinking about different ways to design it rather than just merrily beasting along. Starting the first part file is always the hardest part of designing something entirely new, because you have to select yourself a ground point for the whole thing. At this point in the design, I was just aiming to have some kind of parallel gantry to stick Tinystruder to – dimensional adjustments and remodeling can come after I get a good look at everything.

Here’s some of the “mechanical penthouse” solid geometry. I picked an arbitrary square size – in this case, 400mm, selected for me by the fact that McMaster sells 400mm long shafting. The pulleys were selected from B&B Manufacturing and SDP-SI (who seem to gun for eachother and have “competitor” part cross references that are exactly eachothers’ part numbers) based on both price and diameter – the pulley diameter determines how far offset the X and Y rods have to be. This part was relatively simple.

What wasn’t as simple was designing the axis anchor blocks. Above is the most elaborately t-nutted thing I’ve ever designed…but it sucks. It requires 4 unique parts to be cut, and a cube only has 6 sides. When you have a waterjet, everything looks t-nuttable.

But what was important was that designing this thing gave me the critical dimensioned I needed for the part. After that, I could quickly whip up a better designed replacement:

Some times, thinking outside the box isn’t the right approach – the box must be thought about from a different axis. This axis anchor design is much better executed, requires fewer unique parts, and is fully constrained by 4 corner bolts. It even has provisions for a real belt-clippy-thing on the bottom (one axis runs the blocks inverted for clearance, so the thing on the left is still “Bottom”). It’s also smaller.

I legitimately thought about just machining a block of aluminum for this – the whole assembly can be replaced with a small cube that has two holes in it. One to hold the linear bearing, the other to hold the cross rod. But that wouldn’t be in the spirit of things, now would it?

The general idea explored in the anchor blocks was applied to the head mount. Because my cross rods don’t start in the same plane as my axis guide rods (unlike, say the Ultimaker), the end effector is a little tall. The rod separation is 40mm. There was no particular reason why I made the rods offset instead of in plane – it was just the first thing which came to mind.

And possibly because skew-offset rods are easier to install, and that further-spaced supports for the massive steel and aluminum testicle that is Tinystruder means more torsional stiffness along the axes.

Because Tinystruder sticks out more one way than the other, the gantry can’t be perfectly centered upon the build plate. This is a composite image of the maximum positions that Tinystruder can travel to at the moment, centered upon the build plate i.e. able to hit all corners. However, this puts the Z-axis in a slightly inconvenient position (halfway hanging out of the square base), so I might have to extend the platform arms even more to compensate for it. Or adjust some other dimension, or make the build volume rectangular, etc.

Now that the hard part of the machine design (in my opinion) is done, I hope there’s nothing to distract me in the near future so I can finish it off.


A Bunch of Waterjetted Things

Quick update on a few things as I manage to momentarily surface from intensive planning and building for next year’s 2.007 contest – which, given the whole point of the thing is to issue a challenge to students, I’m actually going to not say anything about until the spring semester begins in February. I promise it will be either a riot, a circus, or a shitshow… possibly some combination of the three.

I got a chance to finally cut out Tinystruder’s structure:

I found enough useful scrap plate to make 2 Tinystruders’ worth of structure. However, the second faceplate didn’t cut correctly since it turned out to be routed over a missing chunk of the scrap… oops.

All of the hardware that I need to finish Tinystruder has arrived from McMaster and others, including the tiny 693 bearings and the correct tube fittings that have a normal 10-32 thread on the end instead of some weird tapered NPT bullshit.

Also, steppers! I got the Polo(lololololololololo)lu 35 x 28mm steppers – they’re a bit shorter than I expected, but I most likely had the 35 x 36 size in mind. If they can crank the filament, then all is good. For convenience, I also got one of the Allegro A4988 driver chip breakout boards, which is incidently the same type that the Reprap RAMPS boards use as axis drivers.

I received my order from Makerbot that consisted of the custom 0.4mm nozzle (boy is it tiny) and the drive rollers. I also purchased one roll of 1.75mm ABS on a reel from buy3dink.com, which seems to be about the sketchiest place you can possibly get plastic filament from since there’s no business information, contact information, shipping information past a flat charge, etc… which would help you distinguish a legitimate vendor from some shady Eastern European place that steals your credit card information to fund organized crime. Well, I’m so far glad to say that the place is legitimate, and way cheaper than most 3d printer supply sites.

The only ingredient I’m now missing in this whole affair is the pair of cartridge heaters which Makerbot is still out of stock on – and nobody else seems to make 12 volt cartridge heaters.

No problem, I simply ordered some heaters of the same size (1/4″ diameter x 1″ long) off eBay, where they’re like 8 bucks each. Downside? They operate at 120 or 240 volts. I got a 120 volt variety and plan to just plug it into the wall through a variac for testing purposes. Not very scientific, but who cares? It either melts and extrudes or it doesn’t – I’m fairly confident those smaller steppers can crank the filament at 230 to 240 deg C.

scooter utensils

While I maintain that 3d printed scooter wheel forks are totally fine for average use – both RazErs use a 3d printed fork and fly over railroad crossings, cobblestone, and those blind people curb things with no problem – they are definitely less durable than equivalent metal ones. Case in point, I managed to totally shatter RRev’s fork by trying to mount a rough step curb cut. I totally forgot that I wasn’t on melon-scooter (which can handle a 1″ curb just fine, with its giant regularly-underinflated pneumatic tires), and the next thing I knew, I was riding on top of the front wheel.


With Make-a-Bot out of 3mm ABS filament, it was time to retire the printed fork and just make something metal. Of course I elect to take the utterly lazy bum way out instead of making all of, or even part of, the scooter fork from machined aluminum stock.


Maybe I should have added more t-nuts. But, the majority of the loads that this will see are upwards from the ends of the fork, supported by the Razor A2/3’s rubber block thing. Rearward bending moments are transferred into the steering fork through the crossing piece in the back.

These were last-minute shoehorned into the same batch that Tinystruder parts were being cut on.

I have yet to make the little spacers – this assembly was mostly forgotten in the rush of MEETERS. I’ll finish it off tomorrow and try it out on RazEr rEVolution – if it’s workable, I’ll probably make a pile in varying widths for future scooters.



In the spirit of eventually working towards running in-house developed equipment on all of my vehicles , I decided to man up and finally pitch Tinytroller at the final boss fight of scooter controllers: controlling the Turnigy C80/100 “melon” motor that runs melon-scooter.

Melon-scooter has been out of service for about a week and a half – the chopped Jasontroller worked extremely well until I let it out of my sight one Friday night at MITERS when a bunch of freshmen and new members were in attendance. When I attempted to leave later, I found that the motor was shorted through the controller and there was no response from it when powered on. And of course the froshlings had all quietly left by then, with nobody telling me that my scooter was behaving a little strangely and not like… going and stuff.


Anyways, I’ve found derpybike to be quite useful in the mean time. Since the failure was totally not under my control, I can’t quite tell what went wrong. When I opened the case of that controller, nothing appeared to be burnt or detonated, but some FETs are most definitely shorted through and the drive circuitry is dead. I’ll probably just order up another Jasontroller (or use one of those 500W bricks?).

The C80-100 is a pretty formidable control challenge for a homebrew motor driver because it has both very low resistance and very low inductance. I measured the line to line resistance to be around 20 milliohms – meaning any little twitch or fuckup by Tinytroller can send pulses of hundreds of amps through the system. The low inductance means the current through the stator cannot be modeled as approximately constant, especially at my relatively low (8khz) PWM frequency, wreaking havoc with non-robust current controllers. It’s built similarly to many other huge electric flight motors, so if I can control the Melon, I can probably take on other scarier airplane motors too.

Melon-scooter normally runs sensorless since I originally built it with a Hobbyking 100A airplane ESC, then subsequently a sensorless Jasontroller. To use it with Tinytroller (which is not yet sensorless), I had to append sensors in a similar fashion to Straight RazEr. It’s that red thing by the motor:

I bodged together Make-a-Bot’s heater one more time (last time, I swear!) and used the last 2 feet of my 3mm ABS filament to make this sensor mount. Unlike Straight RazEr’s mount, this is a two-piece since I needed to fit it into the very close gap between the motor and my frame.

Like so. Unfortunately I made this one a little too close – the ABS plastic actually rubs alot on the motor. It doesn’t seem to be affecting sensor operation, but it just makes an ugly scratchy sound. Oh well – it will have to do for now.

The first test was performed on 24 volts so I could (more) safely full throttle the motor in order to time the sensors properly. For a while, I was trying to find the absolute minimum point of phase current draw at no load, full speed, which corresponded to the point of optimal sensor timing. Wandering even a little outside this region caused the current to increase very quickly, some times up to 40+ amps no load… that’s 1000 watts dumping into the motor just spinning while sitting there. However, Tinytroller handled the mis-timed excessive current draw just fine – no fiery death like I expected.

I was able to get the motor current down to 7.5-8 amps no load, where it has generally been.

I did still have plenty of PLA plastic left, and I was going to print out a Nice Case for Tinytroller that enveloped the whole thing and had custom wire entrance and exit holes and whatnot, but decided PET film tape was enough for now. I made a little greenhouse (literally?) for Tinytroller which should keep most of the gunk out of it.

Plus, I figured it was going to explode anyway, so why waste time on a nice case?

All bundled up and connected.

I tried something a little different with this attempt at running melon-scooter. While Straight RazEr’s control scheme relied on a single throttle, with the bottom (released position) being a slight brake (negative current), neutral coast somewhere in the middle, and the top being full driving current, I put a handlebar throttle next to the thumb throttle on melon scooter and had Tinytroller read both.

The handlebar throttle controlled the amount of driving current and the thumb throttle controlled the variable regenerative braking. When neither was actuated, neutral coast (zero current) was commanded. Actuating one blocks out the reading of the other such that the readings don’t conflict and sum to a net zero, though that itself is a valid control scheme too.

After making sure it did indeed survive a no-load spin on the 12S battery pack, I threw the deck back on and went for a test ride. If it was going to explode, it might as well do it while the motor is running full bore on 40 volts. The no-load speed was measured to be 4330 RPM, a fair amount slower than even the Jasontroller’s 4700 RPMs. It could be attributed to sensored control with the sensors at the point of zero timing advance (sensorless will always tend to be faster) or it could be my battery being low after not being charged for a week.

The low speed terrible sound was still present – and boy was it ever noticeable on the melon. The “terrible sound” is a bug feature that has been with Tinytroller ever since I added the timer interrupt routine. It can be clearly heard as a clacking sound at very low speeds in the etek test video. I’m completely unsure as to where it comes from, and I can shift the Band of Terrible Sound up and down in the PWM output range if I add various length delays to the interrupt service routine & state changer. This just tells me my timers (1 and 2 on the ATmega) must be running into eachtoher somehow.

Regardless, Terrible Sound mode results in very high current draw for the duration of that “band”, and with the massive windings and rotor of the melon, it was felt as a very strong rumble or high frequency ripple torque. I can’t imagine it being too good for Tinytroller. As soon as the band of terrible sound is passed, the ride instant becomes smoother and more controllable, but transitioning back into the band results in the motor suddenly slowing and becoming rough. Considering that the Band of Terrible Sound occurs at a useful low cruising speed of about 4-5 mph, this is indeed quite a problem. I might have to dig deeper into the ATmega manual to find out what timer registers are being refreshed or reset when I dont’ expect them to be.

poor tinytroller

Melon-scooter managed to make it 90% of the way around the block before it suddenly flaked and shut off. I was able to cycle power and have it function again, but only for a very short while. After which it seemed that at least two low-side FETs were shorted, since the motor was reluctant to turn even with the power off.

Before that, during bench testing, I had noticed my big red key switch becoming flaky and occasionally shutting off or dithering on and off, power-cycling the controller many times. It very well could have been a flaky switch that shut off from vibration, and the sudden power kill would result in huge negative voltage spikes which could have destroyed components.

I’d hate to think that the only thing that took down Tinytroller this time was a flaky power switch, but the performance was fairly smooth and flawless once the Band of Terrible Sound was passed. Slowing down was difficult – re-entering the Band of Terrible Sound meant I  had to hold on to prevent the handlebar from punching me in the stomach as the motor suddenly acted like I had jammed a rock into it. Getting something reliably working is, in my opinion, 90% of the challenge of actually making a useful product or project, so I’ll just continue the Tragedy of the Tinytroller some time.

more 3d printers

I’ve also been sketching out some more designs for the Next 3D Printer. Adding a filament guide to the interior of the machine that had to be flexible enough to reach the far corners of the axes while folding up neatly and predictably has been a fun engineering exercise, and I now understand why commercial 3d plastic extrusion printers are so damn huge. I need alot of buffer space to run the set of wire and cable guides which hold the filament and the electric umbilical.

However, one thing I did decide on and finish designing was my new Z axis. In the first post about the new machine, I sketched out an idea for a combined chamber heater and build surface heater. Well, that idea has since been turned into reality Solidworks.

Hey, it’s like the exact same thing. The resistors are 10 watt types, currently spec’d to be 0.22 ohms each and to be run in series for a roughly 1.8 ohm string, which ought to net me about a hot 80 watts of heating power. The surface itself was made transparent for imaging purposes, and actually is supposed to be aluminum and not clear plastic or something.

I’m not a thermal systems engineer, so I just whipped up a radiator pattern for the resistors that kind of made sense in my head.

Some more design progress on the Z table. The parts here are “edge stitched” together in my usual style with tabs, slots, and interspersed t-nuts. Four LME12UU type linear bearings comprise the guide system, two on each side, held apart by spring washers. I’m reusing the central leadscrew nut from MaB because for some reason it cost $30 and I still have 5 more feet of the same leadscrew, and I’m not buying a whole new one. The structure is mostly 1/4″ aluminum beams and 1/8″ bracing plates – I’m trying to minimize the use of giant 1/4″ stock on this machine, but because the Z table is so big (250mm square build plate, with a total length of about 290mm front to back!) it was warranted here.

Did you know that they make PC case fans that are 250mm across? I didn’t know either until I accidentally found one on eBay while looking for real industrial 200-300mm class fans. In fact, they make case fans up to 360mm. Why the hell do you need a fan that big on your computer?

It turns out they don’t actually move much air at all – I ordered a sample one from xoxide for kicks, and it seems to be for case modders and PC builders who want to take the “large but slow moving air mass” school of case ventilation to the absurd limit.

But that’s actually exactly what I need – I’m not trying to build a hair dryer  or a heat gun, but something which will gently fan the sweat of my 8 power resistors onto the back of the build plate. Time will tell if they survive 60-70 celsius (I’m guessing not), but for now I have one designed in. More industrial grade fans are spec’d out if I need them, but if these fans do work out then more gaudy internal lighting for me. Maybe it’s time to start case modding your 3d printers.

More Tiny Things

This week has been filled with tiny things.


First, tinytroller testing on a tinykart.

But unfortunately it didn’t last too long. As in, all of like 20 feet or something.

I’m not sure why this trace ended up blowing out. The controller was behaving normally during wheels-up testing and under low speed driving with or without heavy acceleration command. However, as the speed picked up under sustained heavy throttle, something must have hiccuped, causing a very high current pulse to flow and detonate the trace.

Nothing was damaged – the gate drive and both FETs on that channel still function, the Arduino still works (and lights up!), and I patched that trace with a copper braid afterwards. Tinytroller’s current limit was set at 60 amps using the same software from before with no Serial writes, I swear, so I can’t imagine it was running substantially overcurrent for a long period of time. But then again, I haven’t ever seen trace failures that weren’t associated with total malfunction before.

Time to lob it on another vehicle and see if it does this again.


Oh yeah, I forgot about this:

Those are the phase voltage readings from Tinytroller while slowly commutating a Kitmotter. This was sampled once every slow loop just to test the phase sensors, so the values really do not reflect what’s going on in the motor (those would be updated 7800 times a second otherwise). If you squint at it, it looks vaguely trapezoidal.

That’s good. That means once I muster up the software balls to try it, Tinytroller can be SENSORLESS!!!… and maybe one day I’ll even have ArduSensorlessFOC or something…. SensorlessFOC-duino?

…and now for the more interesting thing.


I mentioned before I was playing with a native, integrated twin-head extruder design for the Next Sensation 3D Printer. Over the past few days, I’ve refined the idea a little and created a design which I will build and test to see if it’s valid. It mounts two NEMA 14 (not 17) motors side by side with a symmetric filament path and adjustable roller-based feed tension, which is something I wanted over the solid plunger of MaB’s current extruder.

I’m fairly positive the NEMA 14 steppers will have enough torque to push the filament, but to be even more certain, that’s why I’m going to build one.

Oh right, I’m doing this in SOLIDWORKS! I’ve always been a Child of Autodesk Inventor since they were the first to indoctrinate me in 2005 by sponsoring FIRST teams (and because they give out free 3 year student licenses with nary more than a .edu email address), but the chief solid modeling program around here is Solidworks. When approached with Solidworks questions, I’ve always tried to figure them out and give directions in terms of Inventor equivalent functions. Not all of these map 1 to 1, and SW still has several user interface tics I’m not used to, so in my capacity as future mother duck of engineering students I’ve decided to force myself to use SW and become acclimated to it. SW still seems a little rigid and less liable to letting me fudge everything, but that’s most likely because I haven’t discovered all the shortcuts and tricks yet.

Anyways, it took the longest time to get to this stage in the solid geometry modeling since I thought of several ways to keep the filament properly compressed between idler and drive rollers, as well as different ways of arranging the motors (face to face versus side by side, etc.).  I tend to mentally visualize a design completely before actually CADing it – since mistakes in creating geometry in CAD still implies starting over and wasting time, so this stage was accompanied by several cumulative hours of staring at the ceiling intently while rolling through maybe 3 or 4 unique designs.

I’m going to use teflon tubing (and quick fittings) as filament guides, so the two fittings are shown. They might not be the final ones, since McMaster doesn’t have many CAD files for its fittings. I ordered a bunch of candidates for measuring, so that part might change.  The “stalks” that the hot ends will be mounted to (3d printer speak: “thermal barriers”) are modified 1/4″-20 vented cap screws, their center vents drilled out to 2mm. While 5/16-18 screws have a native 0.08″ center vent (which is like 2mm anyway), I determined they were excessively large. The 1/4″-20 cap screw will be machined down such that its head is 6mm in diameter to clear the future filament tensioner.

While I would rather do the modern worldly hipster engineer thing and use 100% metric parts, McMaster doesn’t sell metric vented cap screws – the nearest source I found was Small Parts, and they cost $26 for 10. This, along with the fact that it is much easier for me to buy 1/8″ and 1/4″ aluminum instead of 3mm and 6mm, and there are no such things as miniature U.S. unit bearings, means the whole thing is an ugly mashup of metric and U.S. units. I might start totally over with U.S. unit components as much as possible to save having to do the ugly dual unit thing for the entire design – but we’ll see.

I already bought a ton of metric screws <:(

I yoinked the MK7 drive roller and cartridge heater models from the Stepstruder MK7 CAD files to fill in for placement. Since I think the Makerbot drive roller is the most nifty thing ever, I’ll definitely end up keeping that.

The filament tensioner is a cage-like assembly that slides over the drive roller and is pulled towards and away from it by an external thumbscrew. I had previously thought of using a cam-like assembly in the center, such that twisting a knob releases both filaments, but this method is more straightforward if not tightly integrated and cute. A 693 type miniature metric bearing (because they do not make miniature US bearings) is the compression element.

The cage is guided by the 6mm machined head of the cap screw and axially fixed by the threaded portion of the knob. The cage edges are also just wide enough to fit between the four motor mounting screws, constraining it (kind of) further. These three not-really-contraints would make MechE professors cry, but I think they’re enough.

Now with both cages in place!

I rarely insert screw models into parts unless I really need to test clearances, like in this case. This is where I’m glad McMaster-Carr has digitized something like all of their common screws. The modeling is almost excessively detailed. I mean, the screws have actual helical threads of the proper geometry. If I made an equivalently real-helically-threaded hole and turned on Collision Detection, Solidworks would let me Real-Screwdriver it in.

This is why I want a multitouch Minority Report interface for CADing. Right now.

The box has been closed off and some of the slotting work has been started. Tinystruder is too small to use a 40mm fan per motor – instead, I’m using just one. I might consider a high speed 20mm tall type rather than the conventional low speed, 10mm profile fans in popular use.

Now with more t-nuts. There aren’t many places to t-nut extensively on this, unlike my larger creations, so I partially depend on extending the fan mounting bracket towards the back using 50mm long M3 bolts to supplement the attachment at the edges. The crossing M3 bolts restrain the box at the points of highest bending stress – where the filament is being pushed downwards – and also keep the top and bottom together in the center.

I borrowed the MK7 hot end model for visual representation. You can’t get the heater blocks separately (grr), but they seem easy enough to duplicate with my own block of aluminum. I wouldn’t have been able to use the MK7 stock hot end anyway – my block needs to be half-threaded 1/4″-20 and half-threaded M6 x 1. Otherwise, the hot end is about as small as it can get – I briefly played with making the thing even narrower so the two heaters can sit in the middle between the stalks, but I began running into sub millimeter wall thicknesses on the parts.

So what is the fan blowing into? It’s just pressurizing a plenum in the above image and not really contributing to keeping the motors cool (as cool as 60-75C can be) or preventing 250+C extruder heat from seeping into the extruder body.

At least to address the second problem, the fan exhaust will blow through these vents on the bottom. The vent geometry is mostly for representation at the moment, and they might be widened or shrunk later. The idea is to have the center set act primarily as heatsink fins for the cap screw thermal barrier, which I will also turn down in diameter in the middle to minimize heat leakage. The exhaust also blows directly downwards and so can “hot chill” the part being printed to chamber temperature quickly. Certainly another argument in favor of a fan with some extra push to it.

If I were a better graduate student, I would actually heat-flow simulate this whole thing to validate the vent design.

I haven’t really thought about how to keep the motor temperature acceptable yet. In the final mounting scheme, there might be another set of fans at the back just to direct chamber air over the motors. They are very close together, however, so I lose two entire sides of convection area. Thus, Tinystruder might grow a little.

Up until now, I didn’t even think about how the thing would be mounted. Ideally, tinystruder will be hung under the head gantry, so I focused my attention on the top plate for mounting purposes. I didn’t want to directly thread into 1/8″ thick aluminum – it just seemed like a bad idea, nor could I reasonably mount a loose nut on the other side for a nut and bolt connection.

The answer came in the form of press-in nut inserts for sheet metal. These things have a splined section you hammer/press into an appropriately sized hole, and it acts as a hardpoint in your sheet metal assembly.

Final assembly, with virtual transparent faceplate so the action inside is visible. For some reason, my fan model suddenly became white. Hmm, weird Solidworks behavior…

The two thumbscrews will be the mounting interface to the not-CADed-yet gantry interface, which will have two little ears to accept the thumb screws. Otherwise, the top surface is flush on either side so the mounting ears have a seating location. For ultimate MechE trollage, I might try making this interface a kinematic coupling (as if the rest of the machine is precise enough to warrant it).

Tinystruder is 3.3″ (84mm) wide, 2.34″ (60mm) deep, and 2.84″ (72mm) tall from top plate surface to the tip of the nozzle, and the nozzles are space 24mm apart. According to SW, it should weigh about 17 ounces (480 grams).

I’ve already ordered all of the hardware and screws as well as nozzles and the drive rollers from Makerbot. I’ll need to locate 1.75mm filament somewhere else, since they seemed to be out of stock at the time. The frame parts will be waterjet-cut from stock I already have, so I’m hoping this can be built by the end of the week. However, more problematic is that the cartridge heaters were out of stock too – should have gotten them a week ago when I figured I’d needed them. I suppose I have no qualms about sticking a ceramic resistor in place of it or zip tying a soldering iron to the thing.

The big test for tinystruder is actually running inside a 60-75C ambient environment, which I will probably supply with a space heater in close proximity. If it can reliably push plastic for extended periods of time in such a test, then I’ll be satisfied.

By the way, mad props ducted fans to whoever pitched in 20 bucks the other day in the tips jar.  It’s well appreciated (and in fact it was absorbed into an order for stepper motors)!