Kitmotter

Tuesday, January 25th, 2011 @ 7:02 | Kitmotter!, Project Build Reports, Reference Posts, Stuff

Hey Charles, where do I  buy one of those motors you use on your scooters?

Hey Charles, where can I get parts for my own hub motors?

Hey Charles, how do you build a hub motor?

Hey Charles, can you make some motors for me?

Hey Charles, have you ever watched Air Gear?

Hey Charles, what does the third wire do?

OKAY ENOUGH ALREADY

Ever since the first RazEr motor was completed and validated, I’ve been getting various permutations of those questions. The operating mechanics of a hub motor vehicle are mysterious to many people not directly familiar with brushless DC motors. What I have some times found is that people (here, anyway) are willing to learn to understand said mechanics, and possibly build their own vehicles around them. Another thing I (and certain other scooter-fancying parties) have found is that building your own little EV, or even a motor, can be a decently well-rounded engineering learning experience. After all, even a simple EV has a chassis, a motor, a drivetrain, control electronics, and some kind of user interface. It’s an all-in-one package for learning the basics of mechatronic engineering, and yields a practical and usable thing at the end.

Unfortunately, the motors I’ve built are all handcrafted pieces of metal billetwork. RazEr rEVolution’s motor is crafted from a 4 inch aluminum pipe with 1/2″ thick walls and some 3.5″ diameter aluminum solid billet.

Such processes aren’t very “democratizable”, for lack of a better term. Few people will have regular access to well-equipped machine tools that they know how to use. I could also just mail one to a shady backwater motor shop in China and potentially make negative millions of dollars, but that’s not very fun and engaging now, is it? The material costs for such a motor like the DNIR alone are pretty hefty. Certainly the first batches for the early adopters (like all 3 of them?) wouldn’t fall under $1,000 USD each. And again, a shiny aluminum cylinder is very “black box” and doesn’t tell you much about what’s inside, and if the point is to make you see what’s inside, then it’s not very… err… productive. As the “decentralization” of manufacturing continues, so the accessibility of previously proprietary and industry-specialized technologies spreads. In general, that’s good for the likes of Makerdom. It also means people can stop bugging me about how to build these damn things.

Soon, anyway.

That is why for the past short while, Shane and I have been investigating how to manufacture these motors with now ubiquitous personal fabrication methods and short-run online fabrication services. Such methods include 3d printing parts from places like Shapeways or your very own machine, laser cutting components using services like Ponoko or Pololu, and abrasive waterjet machining by places like Big Blue Saw (HI SIMON). Other typical motor components like magnets and wire can be purchased readily from mail-order dealers right now. There’s still some kinks to work out, and some machine work is sort of unavoidable for most motors, but we think the majority of a motor can now be cobbled together using nothing but the willpower of the Internet.

Without further ado, I present Project Kitmotter.

Alright, so what’s going on here?

First off, the motor architecture is similar to the motors of BWD, Pneu Scooter, and the Jedboard. Shane & Company have piloted the “stacked plates” method of motor design, where…. well, stacked plates of material are aligned by pins and secured by through-bolts. The rotor is made of several stacked rings of waterjet-cut steel that have magnet-securing indents as part of the structure. The endcaps are formed by a spacing ring and flat plate with a center bearing bore. And out of my design book, the stator is secured on a (possibly machined) center shaft with a channel cut out on one side to route wires out.

The three motors above are identical topologically, but there are minor variations resulting from playing with the design.

The first motor uses a 63mm (or 2.5 inch) stator with a can that is 82mm (or 3.25″) in diameter. This is particularly interesting because 82mm is generally the largest you can core out a 125mm scooter wheel to. The 150mm variants such as this or the first three or four on this page are also corable to the same range.

The shaft, as-designed, is a single machined piece of aluminum that is double-stepped, 1″ in the center and 3/4″ on either side, with a milled channel and two end-tapped holes.. This motor was originally designed a little while ago as a prospective machining exercise for the introductory machine tools course here, and so contains a sampling of machined features.

Motor #2 is a 68mm (2.6″) stator’d design with a 3.75″ outer diameter. This thing has massive magnets – 0.25″ thick, for maximum airgap flux density. Given that these motors are designed with very wide tolerances (airgaps on the order of a whole millimeter or more, unlike my ultra-tight 0.5 to 0.3mm airgaps!), I think the fatter magnets are beneficial as the ratio of airgap to magnet thickness becomes smaller. The center shaft on this one is much the same as the first: single piece and machined.

I don’t know of a wheel that fits around this motor at the moment, but a 68mm stator was chosen for reasons I will detail shortly.

Here’s something a little different. This motor is essentially motor #2′s design – quarter inch magnets with the same rotor plates, but 3d-printable endcaps and center shaft. The shaft has been up-armored to 1 inch diameter (with accompanying absurd bearing) for  extra strength, since it would be 3d printed from plastic.

A plastic motor shaft might seem like a horrible idea, but it’s important to remember that in these motors, the shaft is stationary. Therefore, it can actually be considered part of vehicle structure. Furthermore, the anchoring points of the shaft on the vehicle are usually close enough to the bearing that the plastic is loaded for the most part in shear, not bending, which is one type of loading where the diameter increase helps substantially.. And if all else fails, just drive a 1/4″ bolt right through that center hole there and all the problems are solved. The vehicle loads will then be transmitted and taken up by the steel hardware, and the plastic shaft around it becomes more or less just a shaft diameter spacer for the bearing.

Printing a shaft that’s 3-4″ tall and only 1″ across at the base, though, is kind of problematic. Especially with the moving-bed designs typical of smaller and DIY fab-scale 3d printers, including mine. So I dropped in a quick way to print the shaft as two seprate, shorter prints.

The two halves are joined by a dovetail interface for alignment purposes. The idea is then to finish-drill the pilot holes to 1/4″ diameter after fusing the two halves together with epoxy or CA glue, then putting a bolt all the way through. While the shaft could be used by itself without metallic reinforcement, adding a bolt in the through-hole would make the whole thing more stable. I came up with a few other dovetail joint designs, including one that doesn’t have the mating faces handling bending loads (i.e one that is like the above but turned 90 degrees), but the idea is the same.

I printed some design iterations out on MaB to test for tolerances and fits. For the truly budget- and sanity-constrained, I think the 3DP shaft is perfectly fine as a solution.

ok, so where do i get one of those stator things?

As mentioned before in my electric hub motors writeup on Instructables, the stator is the most difficult part to get for a motor, usually. That’s because it’s a component typically produced in the thousands or tens of thousands at a time by automated machinery, so you can’t get just one. The stator is always a series of stacked, very thin, silicon-alloyed iron sheets that are insulated from eachother by an oxide, phosphate, lacquer, or epoxy layer.

No, cutting up sheets of Home Depot galvanized roof patching does not work. Nor does machining it from a solid piece of steel.

Citizen-obtainable stators include the stock factory-made ones found on places like Gobrushless to custom laser-cut laminations that you can get from places like Protolam (who supplied the rotor plates for BWD and its kin). However, the latter option tends to be rather expensive. A custom stator is, therefore, the trivial solution if you have money to throw at the problem. For Kitmotter, I’m taking the approach that I have historically used: by harvesting stators from existing sources. Generally the source has been large laser printers and copiers, which tend to have outrunner-style stepper motors to crank on the paper feed path or spin the transfer drum.Copiers and printers are copious here at MIT (and I would guess the same for any large institution with a paper-pusher army). However, what I never did was think about logging the part numbers or models of machines that my stators for RazEr, the skates, and the like came from.

A while back, I sleepily ordered a pile of different copier motors from Ebay, and therefore finally had the opportunity to do some cataloging.

I’ve saved these numbers in a spreadsheet here. Yeah, I know, there’s all of 3 different motors on there right now. If you come upon a reliable model, you should help me append the list.

Bottom line is, the 3.25″ kitmotter uses a stator from the Docuprint 2125, which seems to be rather rare, even on eBay. I only got 3 of those motors, and now can’t seem to find more. The stator was chosen for its good fit to a 3.25″ motor. Further “production” of the motor, such as for the machine tools course, would probably necessitate a custom-cut stator. The 3.75″ kitmotter uses the 68mm Phaser 4400 main drive motor, Xerox part number 127K35701 (That sounds so much like a McMaster part number. I wish…)

In fact, I use three of these things back-to-back-to-back in RazEr rEVolution’s motor. The Kitmotter only uses one stator for simplicity. They appear to be fairly common on Ebay, so I’ve temporarily designated the 3.75″ kitmotter as the “safe design”. What’s even more convenient is that these 68mm stators are the same type used in the Turnigy “Melon” motors. So, conceivably, you can just get a Turnigy motor for the sole purpose of harvesting the stator out, and making an ultra-wide motor. Or splitting the stator into several smaller segments. Trust me, you cannot find a custom stator that big for only $100.

where does the wheel go?

As it stands, the kitmotter designs have no wheel mounting provisions. For instance, I have settled on a threaded locking ring design, whereas Pneu Scooter from above actually secures the motor into the wheel using machine screws. The method used on BWD, which is wrapping polyurethane strips around the tire and attaching them with high-strength urethane glue, is currently the method of choice for Kitmotter. McMaster has urethane strip in many different hardnesses and thicknesses, so it’s not too difficult to wrap a custom tire. The tire could be sanded to fit a curved profile, or like BWD, just run straight and flat.

Alternatively, one rotor plate can be expanded into a flange shape and have circumferential radial holes waterjetted into the flange so a tire of arbitrary size can be bolted on. I have yet to find a tire that is amenable to this treatment, and that is of course a branch of investigation for the project as it progresses.

tl;dr

I’ve already ordered some of the parts needed to finish one kitmotter, the “#2″ 3.75″ diameter, 0.25″ thick magnet design. The rotor plates will be cut on-site here, along with the endcaps. I’m not going to release the designs quite yet, since they’re still unproven and may need some more dimensional adjustments. I’ll also look into tire-mounting provisions.

Regardless, there’s already more than enough information lurking in this post and others for you to boot up your favorite CAD program and fire off a design to Big Blue Saw.

 

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    3 Responses to “Kitmotter”

    1. Miss Outlier Says:

      Hey there!

      I can’t let a post like this go without saying that I totally <3 mfg.com for making my designs. I love machining, but if I just don't have the time I turn immediately to mfg.com. Any SolidWorks model, or open source CAD file, or blueprint, can be uploaded and manufactured. I've done everything from basic milling jobs to getting 3/4" steel parts plasma cut – the manufacturing possibilities are endless. Waterjetting and laser cutting too (although those I do myself, since we do have access to that here at World's Best School). And mfg.com is free to set up an account. And China RULES on getting me excellent, cheap parts – I've never had a quality problem.

      P.S. You actually know me IRL – we took Mechatronics last fall. I'll have to pass along the video of the professor on your scooter… :)

    2. the chuxxor Says:

      Yeah, I have a MFG.com account. But I’ve never done anything with it since I’ve just made everything here.

      Maybe it’s time to take a second look.

    3. Jed Says:

      Nice. It’s about time one of us combined all the best elements of the hub motors (rotor plates, through shaft wiring, see-through hubcaps, copier stators, etc).

      I dig the tracked skateboard btw…you and shane need to race.