Kitmotter 0002: It’s made of wood!

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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!

RazEr REV2: Mostly there

chorl3 Comments

Over the past few days I’ve been mostly hanging out at the Georgia Tech Invention Studio. I was nominally there as “guest lecturer”, but I don’t quite think their own 2.00EV is organized yet to the point where I can feel comfortable with that title. All the ‘students’ are actually lab instructors (similar to our MITERS keyholders), so there wasn’t that much ‘teaching’ to do. I did hold some impromptu lecture-like things and generally advised people on their builds where needed (and fixed the waterjet?). Regardless, some… interesting products are coming out of it:


It’s literally twice as long as some of the other scooters.

I’m back now, and one of the first things on the agenda is getting the half-assembled repackage of RazEr up and running. I sort of left this in the middle of construction when I zipped off to Atlanta for the weekend. I had the frame ‘box’ assembled to test fits, but I pretty much had to take it apart again to actually install stuff. My direction was essentially assemble the major subassemblies first (make the fender, reinstall the motor, attach the front end) and then lob the electroncis back in as-is, since it worked fine before.

Here’s the fender in place with its leaf spring installed.The ‘sheet metal work’ was done on a vise, then fitted in place using just tightening screw pressure. 5052 aluminum bends very easily, especially in 1/16″ thickness, so I was literally just leaning on the part to get the bends I wanted. To do the large radius sweep at the top, I bent little by little in ‘facets’ which weren’t drastic enough to be seen as disrete (though you can kind of see it).

Now that I’m a little more comfortable with making sheet metal geometries compatible with other 3d solid parts, I might incorporate it into more builds in the future.

The fender is just mounted on a chunk of 1/4″ threaded rod. Nothing fancy at all this time – no spacers, even. The pressure of the leaf spring alone is enough to keep it in place reasonably.

More progress has been made on frame assembly, with the folding joint  reattached now. I traded the former front end for a new A3 type front that was part of the leftovers from my 2.00EV. It’s substantially less beat to shit and doesn’t wobble as much, and I swear it’s a little taller than the one I had before.

I had a left over new fork from building Straight RazEr (whose wreckage has since been donated to Kramniklabs) which I dug out for this build.

I forgot to take a picture of what’s going on with the 5″ colson wheel, but there is actually a type 1614 bearing bored into each stock Delrin bushing. The Colson comes with a 5/8″ bore bushing  that has a 30mm OD (which presses in to the 30mm bore of the wheel itself). I tried to find a > 30mm bearing that wasn’t of a ridiculous axle diameter so I could bore it into the wheel directly, but gave up and went the other direction instead.

A ‘stock check’ of bearings I had turned up some R8 type and 1614 type bearings. Both were 1 1/8″ outer diameter, which I could bore into the Delrin bushing, but I settled on the 1614 bearing since I easily located a stock 3/8″ bolt to serve as the axle pin.

The job itself was done on tinylathe, which is probably one of the handiest tools I’ve ever worked with.

Moving on to the electronics deck now, I put together the ‘switch panel’ which holds the charge and controller ports as well as the annoyingly bright blue LED endowed power switch. The idea is to have BAT and PWR jumped externally with a Deans ‘patch cable’ so I could jack in a flow-through measuremen device like a Wattmeter if needed. Else, the switch is to serve as the primary turn-on mechanism.

It’s better than the yank-the-battery-connector setup RazEr Rev has used since forever, but I’m wondering how long until this switch falls victim to no-precharge arcing damage like the very first switch arrangement.

The interesting part is on the back. Instead of connecting the switch’s built-in LED to ground directly, I threw a 100 ohm resistor on it. This should prevent the light from exploding right away, as it happens when you try running 12v rated switches on 36 volts… Otherwise, there are just a few select wire jumps which bridge the two Deans ports through the power switch. Note the back-to-back soldered Deans connectors on the right…

With the switch panel done, it’s time to load all the electronics back in. The same shell-less Jasontroller appears, bolted to the aluminum frame directly for some heat sinking. There’s a bit more space for batteries this time, since they can reach all the way under the folding joint, but unfortunately it isn’t enough to actually add more cells – just maybe some padding. If I wanted more battery energy about the only good option is moving to prismatic cells.

With everything wired back to the way it was, I shoved the 3d-printed front endcap on. This was one of those pieces made on the Lab Replicator™.

And the repackaged shot. Unfortunately I gave away my only other black Colson wheel, so it’s gray for now. When I get another one (or get it back), the bearings are transferrable.

This frame rides significantly lower than RazEr Rev – too low, actually. This is likely due to a difference in head tube length between the A3 I got like 5 years ago (for the original RazEr!) and now. RazEr Rev rode slightly ‘nose up’, but this one is definitely nose down. The clearance at the front is about 3/4″, decreasing to less than 1/2″ when I’m actually riding it and the rubber block is compressed.

Not going to work. I’ll compensate by making the two wheel fork sides a little longer. In the mean time, it is rideable, and handles just like it used to except with more stopping. And less exposed wires – check out the 3d printed wire guide at the lower right.