Chibikart’s New Unobtainium-Free Sibling

Monday, May 21st, 2012 @ 3:53 | D.P.R. Chibikart, Project Build Reports

Hi. I’m building another Chibikart.

But this one will be just a little different. Historically speaking, most of my projects have involved a little bit of “unobtainium” – or perhaps to put it more accurately, an Unobtaining Machine. As well documented as they tend to be, they are admittedly difficult to reproduce by anyone outside of a university or similarly well-equipped makerspace setting, the reason being that they tend to involve much manual or CNC machine work; waterjetting, laser cutting, precision boring, etc.. I’ve generally made a attempt to explain the processes involved in addition to investigating ways that things can be built by more people with different resources, but it isn’t usually the focus of a project.

I support the methodology of getting up and building something as an effective way to learn engineering. Part of my research here (if it could be qualified as that yet) is to engage more  students in learning engineering and design concepts using electric vehicles – it was the whole motivation for 2.00EV this past semester, an experiment that I consider to have been successful (and useful as a data point to work from for future iterations of the course). Working with a more life-sized system, in my view, makes you understand the mechanical engineering concepts commonly taught during the 2nd year such as bending/flexing, bearings and constraints, power and work, etc. more than building a small robot would. The numbers are in a range which make more sense to think about – you understand more what 10 pounds-force is like than 0.1 lbf, or why your vehicle can’t have 1/4″ aluminum axles. You can’t bend 1/2″ aluminum vehicle frame plates to suit your misaligned bearings, but 1/32″ sheet metal robot frames just don’t illustrate that well.

And that talking in the customary units system is fun and all, but Charles will make you write everything in SI because things make more sense that way.

While “2.00EV” engaged the students in learning the machining, designing, and analytical skills  that MIT typically expects out of sophomores, I’m wondering if a slightly detuned version would be accessible to more people – younger, older, different university, or just out to learn & build.

Through the activities of the Collegiate Silly Vehicle League, we have both discovered and created a sizeable array of resources and links for collecting knowledge about buildables. Many of the vehicles that we have built (and zip around on) have their own web pages and long, detailed build reports – so many that listing them here is going to be impractical, so feel free to click through the links in the left sidebar. Some of the principles and sources of parts have been amassed together – for instance, the ‘scooter instructable‘ and my very own ‘hub motor‘ instructable. These have all been guides more so than telling people how to build one specific thing in the spirit of Instructables, but both their 5 star ratings and the emails I get related to them speak alot. Some times, knowing the resources is all the push that someone needs to get started.

Despite pushing several billion dollars a year in revenue, very few people know what McMaster-Carr is, or that they deal directly with end users and customers. When they find out for the first time, it’s quite daunting because there’s everything there and how are you going to find the thing you need? Shopping on McMaster (and related enormous catalogs like DigiKey, for example) is a skill which is most definitely not taught here at MIT, at any level, and I’m definitely not okay with that. It’s something you have to pick up by going above and beyond the call of handing in problem sets. That’s why I made it a point for students this year to order things from McMaster that they found by themselves, but I digress.

Very few people anywhere also know that you can in fact have parts custom made for you by places like Big Blue Saw and ShapeWays, just to name two. There is an interesting “uncanny valley” of personal fabrication here – the general nontechnical public, of course, does not know about the existence of it and online rapid prototyping services. Yet the university and academia crowd also tends to not know about it, from my surveys, probably because if you needed an Unobtaining Machine it is usually right there. In the middle of that is the realm of makers and amateur engineers for whom these services are a boon. (The other big reason is that personal fabrication is still quite new, so it’s unfair to equate it to established systems of multi-shop quoting and contracting machining services.) Oddly enough, even as some people complain that infinite waterjet is the hard part of my projects to replicate, it is actually the milling and turning which will stump most people – there is as of yet no equivalent fast service for custom 3d-machined parts.

I also believe it’s important that you not immediately drop the textbook on people from the very outset. I sometimes betray a disapproval of ‘academic engineering’ when I talk to people, a fact that I used to be ashamed to admit in a place like the MIT Department of Mechanical Engineering. About 10 years ago (…. whoa…) I started out building things from the opposite end of the spectrum that academia is used to – I didn’t learn mechanical physics, structures, beam bending, electric motor dynamics, etc. first and then go out and build a robot – I started throwing junk together and then wondering why it didn’t work, then went to local robotics clubs and tournaments and asked people where they got parts from. Alot of reading online (and real books! Even in 2002 the Internet wasn’t as everywhere as it was today). While that sounds like alot of “Well it worked for me…”, I understand from personally knowing many people who also became interested in engineering early and then subsequently were driven to excel that relevant experience is of serious value even if you cannot explain fully to a professor why something works. You incrementally… build… both experience and knowledge, which are two important qualifiers of competence people conflate alot.

Building for your own satisfaction and learning is also discredited alot – both academia and the curiously onlooking public contribute to this effect. I’ve been asked countless times why I built something when it’s already on the market, or already been invented or done. I’ve been asked “how is it useful?” both facetiously and seriously. Other students I have known in the past years have told me they don’t really build projects because they don’t think they can invent anything new. It’s a hard mentality to change, especially in academia where cutting-edge research is going on all around you and you feel pressured to take as many advanced classes as possible to get to that level so you can contribute. With the growth of MITERS and awareness of projects on campus in general, I hope this is something that will improve in the future. No matter how much we joke about the MITERS “build fads” – robots, scooters, motor controllers, quadrotors, tesla coils, go-karts… (roughly in that order actually, from my recent memory) the fact is that one person starts out building something and their peers look on, go “hmm, I want one too”, and many people benefit from the learning experience in the end. Even though we’ve made literally 5 of the same thing like 4 times over. So what? There’s now 4 more people who can, for example, more confidently design a power converter or mixed-signal closed-loop controller than who could before.

Is it just me or do I insert a long rant or philosophical essay before every build post now?

So, finally getting to the point. What does “detuning” mean for this project? It will mean fully documenting online the build process of a small electric vehicle using commonly available but modern parts and a reasonable set of garage tools and hardware store purchases, while utilizing available digital/online/personal fabrication resources. It will not feature cutting-edge science nor attempt to teach principles of engineering simultaneously. The goal is a fun buildable project of more difficulty and exploration of the hidden underbelly of engineering than a trip to Home Depot will net someone. Again, some times all you need is the resources and the technique – I’m guessing individual sections of this documentation will be way more useful to more people than the whole project is.

Now, Chibikart is probably the worst possible project to do something like this with. First of, it’s not going to be cheap at all – it’s not a weld-together-a-steel-frame-insert-scrap-lawn-mower-engine-here kind of project. It’s also substantially more complicated than the aforementioned WTSFISLMEH go-kart. I decided to pursue it, though, both because it has novelty factor (while it may be impractical it’s fun as hell to ride around), is made of a pretty adjustable/extensible platform (80/20 extrusion), and because it is harder. It’s abundantly clear to me that not everyone will be able to pick up and do it, but in my opinion, to throw it out there is better than just relegating it to a build category in my sidebar that I will occasionally linkdump to.

Let’s set the ground rules:

  1. fully documenting online the build process“. I will use Instructables and create a step by step document that shows the vehicle’s construction from start to finish. This is a departure from my usual style, but I will try to keep things as open ended and customizable as possible.
  2. a small electric vehicle using commonly available but modern parts.” No old car batteries, galvanized frame tubing, and hacked wheelchair DC motors. This will be brushless, lithium ion, aluminum framed, and powerful & lightweight using parts that can be purchased new. Much McMaster, Hobbyking, and Surplus Center (with other minor vendors) will be involved.
  3. “a reasonable set of garage tools and hardware store purchases.” This will be the biggest constraint on technique. I will limit the use of tools to those which could reasonably be found in someone’s garage, or the average high school builder had access to. For me, it was a drill press, hand drill, Dremel tool, a fairly common 29-piece drill set (couldn’t afford the 115 number-letter-fraction set!), and a set of hand tools including some wrenches (not a whole set), pliers, cutters, and limited taps and dies.
  4. Finally, the kicker: “utilizing available digital/online/personal fabrication resources“. The limitations set forth above may make it sound difficult, but this is the part where I get to essentially pick and choose the fabrication exercises. The advent of on-demand waterjetting means that nothing substantial has to really change from Chibikart’s design. Like its conceptual predecessor tinyKart, Chibikart is a pile of 80/20 and waterjet-cut aluminum plates that I really didn’t have to do that much to in order to put together. I’ll publish the flat layout files publicly such that anyone could conceivably Big Blue Saw a copy.

the interesting part

I should be clear that this Chibikart is not like the current Chibikart. In fact, I took Chibikart’s Inventor assembly file and pretty much ripped everything out of it, and the first things to go were the hub motors. That’s right – no more hub motors, since as much as I would love to corner the market on small hub motors, I don’t have the time or patience yet.

We start with a redesign of the back end, now featuring indirect drive. The horror.

The motor is a Turnigy Sk3 5065. It’s pretty weird for an outrunner, since it’s long and skinny, but it’s compact. I chose this largely because it was left over from 2.00EV – I had purchased a “sample plate” of motors at the start of term for students to fondle. It should be more than powerful enough for this purpose.

The Jasontrollers are staying. Even though I’ve seen that they are not very good at starting low inductance and resistance loads like R/C outrunners, I’m interested to see how well they perform at this scale. My experience is with Melon-sized motors and 8″ tires, and the very small 4″ wheels (those are staying, because lol) will give a comparatively larger force at the ground. The reduction is also pretty high – 2.5:1 as designed, resulting in a max speed of 20mph as limited by the Jasontroller.

Note the “live axle” suspended in bearings – this is actually an outdated design that I scrapped in the interest of making people build as few live spinny things as possible. I couldn’t figure out a good way to couple sprocket to wheel except through risky bore-shimming and drilling of steel sprockets in precise alignment with the wheel bore.

The updated method takes a page from Jamison and uses a sprocket cut from 1/8″ aluminum. I hope to avoid needing as much “drill chamfering” since the material is closer to the native width of the #25 chains. The wheel is now a 1.25″ wide, 4″ diameter Colson wheel, of robot fame, because of their softness and pliability in the face of cutting tools. I picked the ball bearing version, and the bore of the sprocket hub is designed to be two soda can wall thicknesses larger such that a shim can be inserted for drilling the bolt holes, then removed for smooth rotation. This is risky, and I have to build one soon to make sure it can work.

Not being able to do this whole “precise right angle hole” thing means that I need to think of creative ways to retain a right angle dead axle. I used methods commonly seen in the DIY 3d printer universe here – trapping the hex head of a bolt against an internal pocket. Once the front wheels are cranked down with a nut, then the bolt head will have to strip the entire aluminum thickness around it to fail.

Because I only have control of material in Z resolution of 1/8″, I decided to make ‘frame thickness spacers’ such that I have a full 1.25″ between the uprights. Enough to put in a few bronze washers to space the wheel block away from the bearings so there can never be a case of my aluminum parts wiping off on eachother like I experienced with Chibikart’s first steering arrangement.

Up front, I’ve changed the big mill handle to a waterjet-cut yoke, since it’s actually quite hard to interface with a 3/4″ hex shaft. The lovely flange-bolt shaft collars make a return, and they will be the adjustable hubs which hold the steering components in place.

Working out the brakes took the whole weekend. Mostly because I wanted to avoid an excessively complex solution like Chibikart’s eggy cam thing. I was also kind of stuck on the mechanism to use, until I started staring at bike parts at Cambridge Bicycle across the street from MITERS. The result after some more thinking is this half-of-a-single-pivot-brake dealie that uses a stock road bike brake pad and stock brake cable mount. In this case, the cable sleeve actually moves too – usually I’m used to seeing fixed cable sleeves. The modeled spring thing is commonly found with scooter/bike brake part kits (I plucked a random one off McMaster to make it complete the look).

And the outrunner can is the brake drum. Isn’t this a wonderful idea? I decided to not do rubber (pad)-on-rubber (colson) because the engagement would have been less predictable, it would have eaten the colson pretty fast, and the motor can was just right there. This is not possible unless the motor has a “can bearing” – the brake pad is basically pressing right on top of it, so there’s no forces which can cause the can to become misaligned.

Oh, and during this process I found out what all of those random little brake parts are called. For future reference, the cross-drilled screw doohickey is a “Brake Cable Anchor Bolt” and this tubey thing is a “6mm Barrel Adjuster”. I went through so many iterations of “brake cable adjusting tubey thing with the nut on it” or similar search terms before finding the correct looking object.

On the other end is a custom brake pedal. The Jasontrollers’ don’t have regenerative braking anyway, and so having the left Hall sensor pedal on Chibikart was kind of silly. It’s also a bit fragile, and the pedal plate actually bends significantly since it’s made from some seriously bullshit steel. Having it as a mechanical brake pedal was therefore suboptimal, so I got rid of it in favor of a stronger aluminum one.

This design will use two of those cross-drilled-anchor-bolt-thingies and pull on both brake cables simultaneously. It does not have a ‘wiffletree’ or force splitting hinge, so some adjustment of the tension on either side (using the tubey-things-with-nuts) will be needed. For rear wheel braking, though, it’s not as bad because unbalanced brakes aren’t as likely to jerk the vehicle around as much as having unbalanced front brakes, which might cause it to pull suddenly in one direction.

One minor change to the steering – I’m reconfiguring the location of the collars and bushings a little because of the flange-bolt collars being annoying.

They use clearance holes for a #10-32 screw which are actually tapped holes for 1/4″-28. Well, nothing else on this design so far uses #10 screws, and it would have required a 3rd tap size (#10-32 or #10-24) since the collar’s holes are clearance (necessitating threads elsewhere). To avoid the need to do this, I elected to expand the holes in the linkages to be 1/4″-28 clearance so the tapped holes in the shaft collar can be used. McMaster conveniently sells 1/4″-28 screws for cheap in packs of 10.

The protruding screw heads necessitated a bit of rearrangement. I actually like this better, because currently the lower shaft collar is the lowest point on Chibikart and it is the first to hit anything.

So here’s where it’s at so far. It still looks vaguely like Chibikart, is still the same size (34 x 26 at the outer ends of the wheels), and the estimated weight is 28 pounds without hardware (so maybe 30-32 with hardware, wiring, duct tape, etc.)

Through a bit of late night Wikipedia jiggling, I’ve decided to christen this the Democratic People’s Republic of Chibikart. Because like North Korea, I swear this is for everyone to build and learn from but the Instructable will just mean it’s me telling everyone what to do anyway.

(Occupy Chibikart was briefly considered as a name before I realized that DC motors and lead-acid batteries are more 99%-ish than brushless and lithium)

More details to come. Parts are due to arrive in massive amounts this week, and I want it rolling by next week.

 

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    6 Responses to “Chibikart’s New Unobtainium-Free Sibling”

    1. Max Says:

      I’m sure many will appreciate the instructions. Just a tiny leetel suggestion if I may: please consider also hosting the project on some ‘non-instructables’ site (like http://makeprojects.com/), considering that since it boldly went ‘profit-oriented’ there are quite a number of people out there who hate the guts of instructables.com with a white hot rage compared to which even the fiery depths of Mount Doom would seem like a mere Bose-Einstein condensate.

    2. jafoca Says:

      I haven’t gotten past the first paragraph yet, but w00t!

      I am the one that was clamoring about how ‘we can’t source / make these parts!’ on hack a day, so this is a GREAT post to see!

    3. jafoca Says:

      Very cool. I have been trying to figure out how to build SOMETHING ride-able with a giant hobbyking outrunner attached to it (except not fankart style…) since seeing all of your posts.

      I will definitely be following closely, and depending on exactly how ‘not cheap’ it is I may try to build one! Thanks for taking pity on us deprived commoners! :P

    4. curtis Says:

      first, i agree with you 100% on your rant.
      second, firstcut(dot)com is a decent(but expensive) CNC milling operation. they are pretty quick on quotes too.

    5. Simon Arthur Says:

      Why not just buy pre-made sprockets?

    6. the chuxxor Says:

      The first live axle version did use a coupled stock sprocket from McMaster, but I decided to move away from it because 1/2″ bolts (and 3/8) are actually really undersize, so it would have been much harder to make it spin true. Asking people to drill through steel in a coordinated fashion was also one of the things I thought was extra risky.

      Plus, more things for you to waterjet!