Kitmotter 0002: It’s made of wood!

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!

5 thoughts on “Kitmotter 0002: It’s made of wood!”

  1. Quick question: Am I reading your scope wrong? That screenshot says there’s a delta t between the two cursors of 200Hz, since that’s half the period that screenshot is at 100Hz. It also says there’s a delta V between the two cursors of 33.6V, so Vppk would be that. Cursor 1 is at 17v, and cursor 2 is at -16.6v.
    Maybe you meant Vamplitude is ~16.8v average?

  2. The scope always treats its channel ground as “0”. It took me a LONG time to realize what was happening.

    Say you had a positive voltage (e.g. 15v) on phase A and negative (e.g. -15v) on phase B. A voltmeter on AC mode will tell you that the delta-V is 30v, which is correct.

    But the scope will assume its ground probe to be 0v, so what it will display is “+30v” in that state. The motor is a fully floating measurement to it.

    Now say the motor enters a state where there is positive voltage on B and negative on A. The voltmeter will still say delta-V of 30 volts, but the scope will show -30v. The resultant waveform on the scope LOOKS like 60Vpp.

    The only way to get the scope to read 30Vpp is if I grounded it not to the phases, but to the star point (midpoint) of the windings.

  3. You’re entering truly Soviet engineering here. Should swap the ball bearings for bronze ones, since such extravagances are reserved for dirty capitalist pigs.

  4. oooh I know what you can do about the scope showing double! you can make a poor man’s differential probe. I saw this on the EEVBlog.

    Basically you use two probes, connect the two grounds together but to nothing else, and connect the two probe tips to the two points you want to measure. Then you set the prob to math mode, and display CH1-CH2 (or vice versa).

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