Archive for July, 2012


A New Pushybot

Jul 24, 2012 in Null Hypothesis


My annual robot party, Robot Battles @ Dragon*Con, is right around the corner. Again.

This is a competition I’ve been attending since 2002 and competing in since 2003 – it’s been my “end of summer festivity” for every year since then, except 2007 when I was retained by the mits for freshman orientation.

For most of my RB history prior to the MITs, I’ve had a primarily drivetrain-centric entry, Test Bot, that worked all the time and was undefeatable so long as I didn’t mess up. After that, beginning with Test Bot 4 in 2006 and really kicking off with Uberclocker version 1 in 2008, I began focusing more on cool actuators and weaponry.  That probably explains why I haven’t won anything since, or really have a robot that can put on a performance without being unreliable. Cool weapons and whirly things are funny until you can’t get it to move straight across the arena to beat the other guy senseless with it.

I’m kind of sick of that. Lack of any real motivation to fix the drivetrain issue with Überclocker Remix thoroughly has led to its losses in 2009, 2010, and 2011. That’s 3 straight years of fail, and I can even name the reasons why. In 2009, the robot lost drive functionality early on because I dared to think that using $2.50 drive motors was a good idea (and brought no spares, also a good idea). I upgraded the gearbox and motor rig to a custom DeWalt solution for 2010, but it slipped a poorly-made output shaft and I didn’t care enough to try and ninja-fix it during the actual event. And finally, last year, with the drive motors finally reliable, the robot ditched chains left and right. Literally. Both of them. Poor tensioning results in the chains catching on… something – I couldn’t tell what was actually going on, but it cost me the tournament. Again, what just totally sucks about it is that it was no fault of the design itself, but just my inattentiveness or ambivalence.


So I’m starting a new 30lb build which will be 100% drivetrain. The goal of this build, really, is to just come up with something that’s fun to drive and run into things with again. Piloting the (comparatively well-handling) Cold Arbor test rig around a few weeks ago reminded me of how important it is to keep your robot short, dense, and 4WD with the center of gravity smack in the middle. Soft wheels helps too, as does an immense amount of horsepower in the motors. This 30lber will act as a baseline to compare all my future designs with. The drive will be modular, with one motor per wheel, quickly replaceable and with easy to find spares (something Clocker has never been very good at). The frame will be pretty damn near indestructable. If I can’t beat the all-drivetrain pushy-brick with a new design, then anything else is pretty much hopeless. It will function as the null hypothesis test of robots – to be a proven design, it has to reject (/defeat) the null hypothesis.

Incidentally, that’s exactly what I’m calling it. Introducing Null Hypothesis:

Well gee, it looks like I’m almost done designing it already. Usually my CAD posts start off with a rectangular extrusion or something equally simple, but I’ve been slowly digesting this design for a little while.

Short rundown of the design: Four 18v Harbor Freight 900rpm type drills (now on sale again as item 68239) direct-driving four 40A durometer “McMasterbots” wheels. 1″ thick UHMW bar frame, and polycarb top and bottom. True 0.75″ ground clearance each side. Classic indestructable pushbot! The outside of the bot measures 18″ x 18″, which is a reasoanble 30lber size.

The design above is the “first pass” appraisal of the concept. It has a solid UHMW front wedge that was intended to be carved from a 3 x 4″ block. That turned out to be $150 of UHMW, which seems to have gotten way pricier since my last UHMW brick in 2006, because UHMW is just condensed natural gas or something.

Note how it gets a little weird and irregular out back – I wanted this bot to have no rear traction gaps. A classic “box” frame leave you a finite, usually less than 45 degree, tip angle before the frame lifts the wheels off the ground (for Test Bot 4.5 this was only about 25 degrees because it’s so damned low). One solution was to cut up a pipe and round off the end of the frame, but this left me little space to mount the motors. Hence, I settled for 45 degree ‘facets’ that can get pushed further back to let me add mounting features for the drill gearboxes.

The ground clearance is called “true” 0.75″ because the top and bottom covers are inset into the frame. This leaves me with a 2.5″ frame that has 2.25″ of internal space given the choice of 1/8″ polycarbonate top and bottom, which actually turns out to be not that roomy.

Here’s a “second pass” of the design. When I do a “first pass”, it’s generally just to puke ideas and generate shapes. Some times I find that the shapes end up being nonsensical or impossible.

There wasn’t such an impasse with this design, but I decided to get rid of the solid UHMW brick front. As funny as it might be, I really can’t justify spending $150 on UHMW just to use maybe $70 of it. The wedge ‘sections’ are made of the same barstock now as the frame. A 1/8″ 4130 steel plate forms the business end – I got a plate of the alloy for reasonably cheap, but there might not be an oven big enough for this thing with immediate reach to fully heat treat and take advantage of its strength.

The battery and its retaining mechanism is now shown too. It’s actually really hard to make an all-UHMW 30lb bot – UHMW is just not dense enough, really, so the weight needs to be made up. In this case, it’s with theoretically enough  batteries to run 4 or 5 matches back-to-back. The intended voltage of the system is going to be 25.6 nominal, 8S A123 cells, with 4 effective cells in parallel, physically arranged as two separate 8S2P packs. This should yield a decent top speed of 15mph: not too quick, but also not sluggish. Middle of the road pushybot.

The battery packs will be made with shock mounting padding, then captured inside a 3/8″ or 1/4″ polycarbonate ‘cage’ inside (hard to see, with polycarb being modeled as transparent). The cage is part of the top and bottom plates.

Electronics-wise, I’m trying to decide between the Botbitz controllers that I got to test for Arbor, or my full-custom board (yes, again):

The hell is that thing? I christened it RAGEBRIDGE because, somehow, I have never managed to produce a reliable small H-bridge controller. Landbearshark’s custom controllers were a miserable failure for several now-obvious reasons. I’m also really in need of a synchronous-rectified current-control-capable H-bridge for an outside consulting project – which is really what this board was developed for.

So I took another stab at it. I took every precaution to not loop my grounds and cross my logic with power, so I hope this can become a reliable hardware base.

That is, if i ever receive it. It’s been nearly a month, MyroPCB. The excuse I got was that my order was ‘damaged in fabrication’ and had to be redone. Well hhmm, I hope it wasn’t all those vias….

The reason I’m still holding out hope for these things is because the BB controllers can’t do “true 24v” systems – the Hobbyking ESCs they’re based off of are rated to only 6S lithium polymer cells, or around 26v peak. There are 35v parts all over the place, including the main power capacitors, and the FETs are 30v parts. In other words, a freshly charged 8S A123 battery at 28.8v is most likely too much for them. And I’m not going to back down from that – NH will be too slow if I only run 6S.

Worse come to worst, I have all of those Victor 883s (classic ones, mind you… The FET layout above is clearly borrowed from the Victor883107-7.).

Here’s the front end of the bot. I’m most likely going to just stencil-paint (or laser-etch?) the H_0 onto the front, instead of making it a cut-through, because that weakens the steel too much. Else, I might weld a backup plate behind it.

The top and bottom (and wedge) hardware on this bot is all 1/4″-20 countersunk cap screws, with the side/frame screws being 3/8″-16. Big, meaty threads. I considered using giant lag bolts for maximum grip strength, but they were not available in anything other than “giant oversize decking and timber-clamping Hex head”.

Random little electrical details will be added soon, when I can decide on the choice of electronics.

If you’re wondering where the hell the drill motors are disappearing into…

Hey, first real picture of the build!

Here’s what’s going on. The McMasterbots wheel will use the nose portion of the black flanged piece as a secondary ‘bearing’. This is a scheme to avoid hanging the entire wheel solely off the 3/8″ shady drill steel axle (as is common in drillboxes). The D-shaped nose of the drill gearbox is additionally supported by the mounting flange, giving it more plastic meat. The motor is prevented from torquing and moving axially using fine-threaded set screws (the cross holes in the picture). The flange itself is screwed into the UHMW side rail.

The black plastic piece is made of ABS, rapid-prototyped on the Lab Replicator. The final version will also be made of ABS, and incidentally also to be rapid-prototyped on the Lab Replicator. On a side note, most of my complaints in that post seem to have been wrapped up in the latest firmware for the Replicator – they were probably common enough that an official fix was distributed.

I’ve ordered pretty much everything I’m going to need to make the mechanical bits of this bot in the coming two weeks or so. I have eight drills now, and the buffer pile on my desk is becoming insufferable.

I have to make a decision about how to build the frame. While I was initially going to buy and mill slots in UHMW barstock, I snagged a large plate (48 x 8″) of UHMW on eBay for very cheap. It has all of the square footage needed to build the frame, but separating it into chunks will be a bitch.

While my normal tactic when faced with a giant plate and needing bars and shapes would be to Just Waterjet It, UHMW is well known to be so soft and abrasive resistant that it cuts poorly.

I laid out some test parts (these are the side bumper things) using different qualities to see if any were remotely tolerable. Anything short of high quality (or in Omax terms, “Quality 5″) just doesn’t even make it through all the way, and I can’t get the part out of the material! Even the above piece, which was on Q5, needs all the holes drilled out. The outline is reasonably acceptable, though, and I may make the parts which don’t need to be critically square using the cheap plate. This include those side bumpers as well as the little wedge triangles.

I bought a 4 foot stick of UHMW for the parts that can’t be waterjet-cut, but I might have to get friendly with this plate and a slitting saw in the near future.

This is not it. I have several other robot updates on deck as August rolls in, including:

  1. My kinda-open-secret project of creating a new arena hazard for the new Atlanta Bot Arena
  2. The last round of updates to Überclocker before it will be retired after this D*C event
  3. Maybe a new beetleweight design that I’m tossing around.


Kitmotter 0002 Rework and Rewind

Jul 22, 2012 in Kitmotter!, Project Build Reports

Last time I left off with Kitmotter 0002 suffering from melty-hub syndrome; during test riding, the stator had gotten so hot as to melt its sintered nylon hub partially, causing it to lose grip on the shaft and shear my wire insulation off, causing a short. It was clear that I would have to replace the hub with something more durable and high-temperature.

As much as I would love to include all of the common rapid prototyping / digital fabrication processes on one device, and have a hub 3D printed out of stainless steel, there are cost and practicality concerns. Since half of Kitmotter is made of MDF already, I elected to continue using the material for now. If I could come up with a hub that works when made of MDF, then it should also work when made of aluminum.

This is the layered hub design. Luckily, having to stick to Z-axis thickness quanta of 1/8″ means that this hub is exactly 1″ long, so a few different materials can be used to finely tune costs. I added more ‘hub features’ which fit the HP Laserjet 8000 motor’s stator, as well as a deeper keyway. This hub will be assembled on the shaft and glued in place first, then the stator slipped over it and glued again. With a multi-piece design like this, I didn’t want to risk relying on a press fit (especially since the shaft is a stock chunk of keyed drive shaft and has no ‘placement features’), hence the added mechanical coupling.

I elected to laser this one in-house for quick results. I got a little off on the tolerances, so this is going to end up a little tight, but that will be fixed in the design files before I release them.

Pressing out the old stator hub, which was totally melted in place, required a little attention to not damage the windings. I used an arbor press with a giant wad of shop towels under the copper windings for the initial shove, then just grabbed it and wiggled the assembly apart. You can clearly see there how the normally powdery surface of the SLS nylon part has been melted.

The outside tolerances were correct, so the stator slipped on properly. I used ultra-thin CA glue to retain it – the stuff wicks into the thin and porous MDF gap very well. A higher temperature solution would be coating both stator bore and hub in epoxy, but this was quicker.

It turns out that the new hub’s projections to grip the stator’s “keyways” did not play well with how the phase windings crossed the stator, so I had to cut the crossing strands and solder in jumper wires.

I repaired the damaged heat shrink insulation (with more heat shrink) and tossed it back onto Johnscooter. I was able to get around campus with it without the motor displaying signs of overheating (though the shaft did get very hot). This thing is still really underpowered.  With Kitmotter 0002 coming in at a dismal 0.41 ohms line-to-line, it means I was (again) losing half of every watt put into the motor as heat.

making it betur

As I mentioned before, the winding on Kitmotter 0002 was kind of bullshit – like Kitmotter 0001, I made it quickly without any intention of it actually producing torque. There’s an immense amount of open space still in those slots. 30 turns of my Chibikart hex-28 gauge produced a reasonable torque per amp, but it came at the cost of high resistance – i.e. not that many amps to be had. For motors of the same size, a higher resistance motor tends to heat up more and run slower just due to the I*R penalty of no-load current draw coursing through it. It also won’t produce as much stall torque because the R limits the stall current.

Well that’s ass. If hex-28 worked, then nona-28 should also work:

I ordered 3 more reels of magnet wire for my “Hobbykinging Rig” that I made for winding the Chibikart motors. 9 28 gauge strands in parallel are equivalent to a roughly “18.5 gauge” winding in terms of “circular mils” of copper. It should reduce my resistance by a third, putting me more in the .28 ohms range. I hope.

That’s much more like it. See those filled slots? 30 turns on this core is at the edge of sanity (and windings falling out sideways – the bad version of “Hobbkinging” you don’t want to experience in your motor). Maybe I could have done 31 or 32 if I pulled harder, but 30 turns is on par with the previous winding. The same torque per amp, but less resistance, yields a motor with more maximum output power. Unfortunately the resistance turned out to be more like 0.31 ohms. I suspect the extra is due to the “end turns” effect, where successive layers of windings need to travel around the end of the coil for a longer distance to reach the active length of the stator again. For this reason, busbar windings and holy-shit-how-did-you-wind-that Crazy German Guy 12 gauge magnet wire windings win over Hobbykinging, still. But hey, reduction of resistance by about 25%.

The stator pictured above is another RH7-1260 stator I had (it’s not the one inside Kitmotter 0002). I performed a “stator swap” after finishing this winding. The result is that Johnscooter doesn’t necessarily have more torque to launch with, since the Jasontroller current limits to about 25 amps, but it can keep accelerating for longer because the lessened resistance means the motor’s back-EMF can keep building (accelerating) further before the sum of it and the I*R voltage drop equals the supply voltage from the controller.

Once Kitmotter 0002 gets a few more miles on it, I intend to upload the design files to my site (the Useful Stuff section really needs more love); both 3D models and ready-to-cut DXFs will be provided.

Site Updates Again

Jul 19, 2012 in Stuff

I’ve taken some time to bring all the project pages back up to date. First, Johnscooter and RazEr REV2 now have their own pages. I’ve added tinystar to the copters page too (at the bottom – what I haven’t figured out is how to make WordPress accept inline page sections using the # sign) and reorganized the EV page to reflect the current lineup. Next, the combat robots page has been updated to reflect that I took half my fleet apart a few weeks ago.

Finally, I’ve also gotten rid of the Future Projects page. I have almost never built something out of it – it was just functioning as a random musing dump. I didn’t delete it, so I can still have it as a random musing dump, just not publicly.

Hub Motors on Everything: Johnscooter, the Testbed for Kitmotter 0002

Jul 17, 2012 in Kitmotter!, Project Build Reports

This scooter might rank as both my fastest-built and shortest-lived vehicle. It was called into existence solely to test Kitmotter 0002 on the evening of the 14th of July, and lasted about an hour and a half. That should tell you that Kitmotter did not fare well under load testing, but it might not be for the reason you think.

The base of Johnscooter is…. well, a scooter frame left behind by someone named John in MITERS almost a year ago. Occasionally, people drop things off and swear they will come and fix it or upgrade it, but it ends up not happening because MIT eats them shortly thereafter, or they ran away to Nicaragua, or some other life event detrimental to finishing scooters occurs. John, if you want your scooter back, I’m done messing with it.

This frame is quite interesting. A little research led me to find that it was called the FX-1 EZRider, which is probably one of the more hardcore names I’ve ever heard given to something like this. They seemed to have been produced a decade ago and used nickel metal hydride batteries, and oddly enough, sold at Walmart. It’s very compact, with 6″ pneumatic wheels native (though I’ve been unable to find where John left his rear wheel), and weighs a bit south of 20 pounds.

I started tearing down the frame to extract the components inside, and I must say I am a big fan of the design. It’s not cheap and thin like a Razor scooter – there’s 5mm and 4mm aluminum all over the place. The frame comprises a single aluminum extruded tube about 3.5″ wide and 1.5″ tall, with the forks (made of 5mm bent aluminum plate) bolted through the sides. The same bolts retain all the internal electrical components. Up front, there is a solid endcap which seals the aluminum tube from the weather.

I’m also a fan of the combination fender brake. You can actuate it both with the brake lever on the handlebar or by stepping on it.

The batteries are in stick form and they slide right into the aluminum frame tube, surrounded by rubber. I was expecting these to be lithium, and therefore toast if they’ve sat for too long, but after dismantling a small portion of the heatshrink, I found that they’re actually 4.0Ah nickel metal hydride cells, of the 4/3 A size. That’s alot of battery. Each pack (there are 2) is made of 24 cells in two sticks, each made of a 3-long string of 4 cells in parallel. Then there’s two of them. That’s 230 watt hours of battery, a number that is only rivaled in this volume by using prismatic lithium polymer or LiFe packs. Seriously, I could not make an A123 pack this good.

As far as I can tell, each pack is only 14.4v nominal, and they are used in series to run at 28.8v. To my surprise, the whole pack charged right back up and peaked at 33 volts after sitting for Robot Jeebus Knows How Long.

Some times you just can’t beat a little bit of old school technology. Unfortunately, for me to actually but 230Wh of those cells is about $140 (nearest match) if I don’t mind something generic, and a cool $570 if I want Sanyo cells. Nickel is still classy, and now you understand why I consider the advent of Hobbyking batteries to be a pivotal moment for the creation of actually useful small vehicles by hobbyists.

Now, this is actually an aftermarket pack – John mentioned when he visited that he purchased this scooter from a builder in California, nicknamed Deafscooter. I’ve actually snooped around Deafscooter’s work on the EVAlbum back before I built my first EV in 2007. In fact, I am fairly certain it is this exact scooter, because the date of sale matches roughly with John’s description. Something about going full circle…

To mount Kitmotter, I decide to cheat a little and just use the axle hole that was already in the rear forks. This involved end-drilling and end-tapping the shaft. Another solution using only ‘garage tools’ is to drill out the forks’ axle holes to 5/8″, the shaft diameter, then drill and tap set screws perpendicularly. I decided to just go with easy since testing the motor was a higher priority.

I cleaned up the battery pack wiring a little (putting them permanently in series) and then stuffed the now-charged batteries right back into the tube from whence they came.

And on the Kitmotter goes! Very little engineering occurred here: Kitmotter had a 3″ width between shaft end faces, and the scooter fork natively spanned 2.6″. Solution: Mash the bent plate forks in a vise until they became unbent.

A view from the other side, showing the quite sophisticated brake mechanism – the little off center thing at the bottom actually rotates with the brake cable, giving the brake lever variable leverage as it is depressed. Did I mention I love this frame design?

For sheer simplicity and speed of setup, I plucked a spare Jasontroller off the table. Jasontrollers handle hub motors very well (almost like they were designed to do it or something…) and Kitmotter2 is indeed a hub motor – many poles, low speed, high resistance and inductance.The throttle that came with the scooter was a generic thumb lever type that was spliced right into the controller.

And here it is!

I replaced the front 6″ pneumatic wheel with a spare 125mm wheel because it otherwise caused the thing to look very strange, and ride ‘nose up’.

Ride testing of Johnscooter indoors revealed that Kitmotter’s hardboard sides seemed to work just fine. I took it through a few bunnyhops to make sure. The #2-56 studs and nuts seem to take a little time to ‘wear in’ the hardboard surface – more washers and Loctite may be necessary in the future. Even though the wheel was bored manually, it still came out reasonably on center because of the hole saw pilot bearing hack, and the wobble wasn’t noticeable in riding.

It was, however, a little underpowered because I never actually wound it to produce torque, being a demo motor and all. I could have definitely put another 2 strands of 28 gauge in parallel and lowered the resistance, or just put more turns on to attain a higher torque per phase amp. Or both. The stock Jasontroller puts out only 25 amps, limited by hardware, so it wasn’t too much.

Here’s some indoor ride testing of Johnscooter:

Next, we took Johnscooter to Ye Olde Silley Vehicule Proving Grounds, the garage (riding the whole way just to abuse the hardboard endcaps some more, just in case). Since it was comically slow, there’s no test video, but it finished our defined test interval in 120 seconds while consuming 10.9 Wh of battery, for an “action score” of 1308.

This is actually not bad at all given its non-optimized motor. For comparison, RazEr Rev finished the same test in 82 seconds while pulling 11.1Wh, in part because it generates so much more torque per Jasontroller-limited amp. Pneu Scooter offers a more fair comparison, having performed a 99 second @ 13Wh run previously (1287).

Unfortunately, Johnscooter only lasted for one run. After the first, the motor was very unhappy – the heat could clearly be felt through 1/4″ of wood, which is a pretty damn good insulator. After letting it cool for about 20 minutes, I tried a run, but only made it up 3 levels before it totally cut out. After that, the response seemed to be intermittent, which I initially blamed on using the aged battery too roughly. As some of the other guys were riding it around, the motor cut out again and this time it felt like it failed short.

Post mortem analysis once Johnscooter was back in the shop revealed…

…that Shapeways’ “White, Strong, Flexible” is definitely not “White, Strong, Flexible, and Heat Resistant Too”. Basically, it seems the nylon stator hub heated to the point of losing structural integrity, stripping its D-flat against the steel shaft keyway and causing the whole thing to shift. This pinched and shorted the wires against the shaft.

Additionally, melt lines can clearly be seen at the stator interface. This thing definitely got hot – though not hot enough to damage the magnet wire, as it was neither discolored nor smelled funny.

But it’s clear that I can no longer suggest 3d printing a stator hub as a viable solution for anything but the lightest duty motors. I’m thinking now of doing a wood (yes, more wood) laser-cut stacked hub that uses the 3-eared shape of the stator bore fully, and also grips the entire depth of the keyway. Yeah, sure, make fun of it for being more wood, but it wo…..uld have a higher temperature resistance, especially composite wood-like substances like hardboard.

I will put Johnscooter back together – it worked very nicely for what it is, and Kitmotter needs far more abuse heaping before I can confidently tell other people to copy and paste.

Kitmotter 0002: It’s made of wood!

Jul 15, 2012 in Kitmotter!, Project Build Reports

Two weeks ago I said that I would build the redesign of Kitmotter in the next 2 weeks. Well, like any good college student, I did it the day (and night) before it was due. So in another installment of the summer of short, one-day builds, I present the NEW AND IMPROVED Kitmotter, now with 99% more wheel.

Most of the design intent of Kitmotter is centered on making it accessible to people who do not have a shop full of machinery at their disposal, and this design was definitely a leap in the correct direction. It was finished without the use of a mill or lathe…or even a drill press for that matter, since all the holes were laser-cut and did not need finish drilling.  The stator was harvested from a HP Laserjet 8000 series main drive motor (copier motors spreadsheet here), and the wheel was cored manually with a hole saw that had a 5/16″ pilot drill swapped in place of the 1/4″ one. The shaft is a pre-cut chunk of 5/8″ keyed steel drive shaft that McMaster sells to you for one hell of a value-added markup, but at least it’s better than buying 6 feet of drive shaft to use 3 inches of it. The hub that mounts the stator onto the shaft was hired out to Shapeways to be produced in laser-sintered nylon plastic. And the rotor, the hardest part to make conventionally, was hired out to Big Blue Saw (for real this time) as a stack of steel plates based on the original “kitmotter principle” seen in Pneu Scooter and Kitmotter 1. Other than that, there’s no special hardware.

As I mentioned before, Kitmotter 2 is a prime candidate for my next DIY vehicle centered Instructable, but there are some unresolved issues with this version that I might try and take care of with another one. But in the mean time, here’s how it was built.

Over the past few days, my order to Big Blue Saw which consisted of both waterjet cutting in 1/4″ steel and laser cutting in…. wait, is that particleboard?! Yes, in the interest of not making it cost a billion dollars to try the idea out (the minimum charge for waterjetting is about $80, plus or minus), I elected to make the side plates out of wood. None of the plastics were really appealing in terms of mechanical strength, and wood is some times underappreciated as a material. I figured I would make it out of the most plastic-like wood, Masonite/hardboard, or high-density fiberboard. This stuff is pretty fantastic in compression because it’s basically a solid brick of cellulose.

Only downside, I suppose, is that this Kitmotter should never be operated in the rain…

The rotor rings were split in half and arranged together with “sprues” so they were one closed profile and could be cut without falling into the tank or wasting huge swaths of material in the center (one of the downsides of this kind of design). Because there are so many perimeter screws, alignment and concentricity shouldn’t be an issue.

The motor’s design constraints (namely the need to only increment the axial thickness in 1/8″ and 1/4″ steps) meant it had to be about 2″ wide, which is wider than they make #2-56 screws long. I had to make meta-bolts using #2-56 threaded rod chunks and locknuts. Which, by the way, McMaster will also sell to you.

I ‘laminated’ the wood endcap together with slow-setting CA glue in between the layers, using the bearing as a centering jig. Slow-settingness was critical to this build because of the extra time you have to push the parts together, and if needed, align them radially. Slow-setting adhesives also tend to be stronger, and the motor is made of wood.

Putting all of the endcap bolts in forced the wheel mounting flange to be ‘geometrically averaged’ so it was the least out-of-round.

The rotor is built up similarly. The screws are positioned such that they are internally tangent to the rotor’s outer circle. They should never be seeing any cyclic ‘wheel loading’, unlike my through-the-wheel bolting scheme in the original Razermotor.BBS’s waterjetting tolerances are fairly typical of standard waterjetting – one side is on dimension and the other side is usually 0.003″ to 0.005″ bigger. I pre-compensated for this in the size of the slots, so even with standard waterjetting the screws could still pass through.

The “taper free” waterjetting is more expensive but can produce true square ends to better than 0.003″ tolerances.

Notice the irregular spacing of the ‘seam’ between the two semicircles of the rotor. This is once again an exercise of geometric averaging – by rotating where the seam is throughout the stack, I not only make sure there’s not a single ‘weak spot’ in the whole rotor, but also average any inconsistencies the waterjet may introduce. While a professional shop is going to keep their machine running pretty tight, waterjetting is still fundamentally machining something with a wet noodle (especially noticeable on hard corners using non-high-quality settings).

Here’s the stator assembly. I used a RH7-5219 core this time, but that particular hub also fits the Laserjet 8150 motor (RH7-1260). There is a flat in the hub to grip the keyway of the shaft (but leaves the keyway itself open to run wire outside). The whole assembly is pressed together and sealed with CA glue in the middle. A thinner CA was used here so it wicked into the semi-porous top surfaces of the laser-sintered nylon.

The nylon parts are easy to press fit because their outer surface is not smooth and still a bit powdery. Not only does a disturbingly cocaine-like powder fall off them as you press, but the added compression hopefully helps part retention…

(say, can you laser sinter cocaine?)

I’m going to skip over the gory details of winding for now, since the process was the same for this as the Chibikart motors (an Instructable would go a little more in depth about how painful it is). I decided to just use the hex-28 gauge rig I put together for Chibikart, though these motors can definitely stand another strand or two in parallel, which would also cut my resistance. There are 30 turns per tooth (120 per phase).

I did a fairly standard hex-28 termination and ran the wires, heat-shrunk for insulation, straight out of the keyed slot. That’s why I chose this drastically oversized shaft – the stock keyway is enough to pass some real wires through. If I increase the wire size, though, this may no longer be true, so I’ll have to test it out anyway.

Now, how do I install this thing? 5/8″ is too big to grip inside a drill press chuck to safely lower it in. I actually have no good answer for this at the moment, but I know it is definitely not “grab it and try to hold on”.

The best I could do right now was to ‘angle’ it in using  a set of giant channel-lock pliers to grip the shaft, then wiggling it until the other side centered in the bearing and slipped through. Because this motor is a fairly low aspect ratio (pancakey), I could do that given the loose airgaps. Even this was a bit of an adventure.

Now with wheel installed. The other endcap doesn’t support the wheel in any way, but is larger in diameter than its bore, so it will at least stop the wheel from falling off.

And all closed up!

I decided that the little short wire stubs were unsatisfactory, so some 16 gauge noodle wire was used to extend it. The termination is 2mm bullet connectors, my new favorite after 4mm bullet connectors – 4mm is just a little ridiculous for 16 gauge wire.

Alright, now to dyno it so I can find out its properties:

Poor scope.

Basically, RazEr REV2 was acting purely as a speed source here, with the scope on averaging mode to get a cleaner reading. I’d run the motor up to speed for a few seconds, then hit stop to capture the waveform. Hey, this thing was impossible to “lathe-o-mometer”, so I had to think of a way around it. Here’s the resultant waveform:

V peak-to-peak of 16.8 volts at speed 100Hz – that works out to a Kt in V/(rad/s) of right around 0.191. Not bad, and it makes sense given the motor geometry. For comparison, Chibikart’s motor is the same stator height but  73% of the radius (50 vs. 68mm), about 50% more airgap (for a decrease in stator surface flux of 87%) and has 90% of the turns (27 vs. 30). Just direct scaling from these factors alone from this motor gets me (0.19 * 0.73 * 0.87 * 0.9) = 0.11 Nm/A….. which is exactly what I found.

There you have it, the beauty shot. Now I need to verify its durability by shoving it onto a vehicle and riding it, but for the time being, it spins quite nicely I SERIOUSLY PAID MONEY FOR THOSE BEARINGS? Seriously, watch the video and listen for the bearings.

They’re the cheapest, “not rated” grade of bearings found on McMaster. Now, I bought them because I figured if I was going to tell someone else what to buy, the more goods on McMaster the better. I’ve had good experiences with the “Not Rated” bearings before, but it looks like cost-cutting has taken their toll on these things, because holy shit they’re bad. Built-in radial backlash and tons of axial wobble. They’re lawn mower or handtruck wheel bearings, not electric motor bearings – hell, the handtruck wheels we get to harvest the tires from have better bearings.

Unfortunately, I was mostly after these bearings for their flange, which makes installation easy. They don’t seem to make many precision bearings in this size – only for said lawn mowers and handtrucks. I might have to just deal with that.

Stay tuned for the next episode, where I ride Kitmotter in the rain and the MDF sides melt!