Archive for March, 2013

 

A Little Messing with the Modela MDX-20

Mar 31, 2013 in Project Build Reports

My day to day task of what is essentially making sure ducklings don’t fall into storm drains (but in an engineering  instruction capacity) means that my free time is generally more fragmented. Students being about to come in and out of the shop at will means I can be interrupted by questions at any time. So, I’ve taken the past few weeks to fill in some knowledge gaps that I’ve not paid much attention to before, but have always been nagging in the back of my mind otherwise. Since they’re not extensive build projects, I find it easier to fill the voids while supervising the class. These little exercises include more experimentation with CNC subtractive fabrication (read: machining) and making things in CAD that aren’t straight lines for once.

cnc… sort of

One of my darkest and most personal secrets, which I guess I don’t really try to keep but everyone just seems to assume otherwise, is that I don’t know how to CNC things. There, I said it. You can judge me now. By “CNC things” I mean using traditional CNC  3+ axis milling and turning tools. I’ve done some simple “2.5 axis” things using the many EZ-Trak type machines on campus, but haven’t ever gone through the whole design-part-import-into-CAM-software-generate-toolpath-postprocess-zero-the-machine-and-go process once completely. I may or may not have done a few of those things disparately, or taken over for someone / handed a job off to someone else. But never once through.

I think the primary reason behind this is just that I caught onto rapid prototyping machines and processes early on – waterjet cutting and laser cutting in particular – and basically started designing everything around the much easier availability of them at the time. Another turn-off to “independent practice” later on was that the 2.5 and 3 axis machines I am around the most often are also heavily trafficked, and they’re often left in states which were way different than the last time I saw them, or all the tools have changed.

Really what I need to do is just spend some damn time going through the whole process, machining random objects. You learn just by using something so damn often, which is what I did initially with manual machine tools and the waterjet (and then 3D printing through building my own machine). To do this, ideally I’d have a machine which isn’t used often and so I can deteminately track the state of for the first little while.

Or I could start messing with something so simple it doesn’t have any states to mess up. I have one of these little things in the shop:

These contraptions seem to be used frequently by model makers, and they are the choice of machine in the MAS.863 class How to Make a Mess out of Almost Anything, which I helped TA last fall in said shop. They were also the staple tool of this guy, whose site I ran upon a few months ago for the first time, then recently once again, and now view as some sort of god. His Guerrilla Guide to CNC is a helpful read for the uninitiated (while I found the machining knowledge mostly nothing new, it was still an enjoyable read and I consider it an excellent resource if I also get into resin-casting). The most recent time, it was shown to me by one of my friends, upon which I went “Oh! Yeah, I’ve seen that.” and immediately went to hide in a corner afterwards. Reading it thoroughly was pretty much punted me into starting to mess with this bugger.

The best part is that now that the class is not running, the Modela has been sitting mostly idle. In the class, its primary duty was routing small circuit boards from copper-clad PCB stock, and it ran from a Python GUI running a C++ backend (which I am told is new – last year, it was run straight from the command line), which was entirely coded by the professor and his students. But it also has a proprietary Windows software, Modela Player, which is basically a simplified graphical CAM software. Nifty.

Let’s begin. I modeled an appropriate test part in Inventor and exported it as an STL file. Modela Player can import IGES 3d files or STLs. Based on my failure to convince it to read my IGES outputs, it seems to like STLs much better. Hey, it’s like a 3D printer, except it does the opposite of print!

Yeah. Hey, it has flat regions, internal radii, external curves (the o_O is made of filleted cylinders) and some hard to reach inner corners. Perfect! This is what the MP interface looks like. All of the usual CAM-like buttons are there – stock size, faces to machine, and adding machining processes.

For reference, I mostly followed this tutorial. I gave it a read-through, then tried exploring as many of the features as possible without it. Overall, I can say that the software is set up very intuitively and there is a definitely workflow, though the names and labels of functions could have been better translated. Since the software was originally Japanese, I’ll give it a pass for being a little Engrishy.

This is a roughing pass that I generated. MP is interesting in that the feeds and speeds are completely wrapped up in the tool settings. It comes with several built-in tool definitions, and you can add your own. Each tool has a certain feed rate, spindle speed, cut depth, stepover (basically density of those horizontal lines), etc. for each material. So, all you have to do is literally select a tool and indicate your material (which can also  be added custom). I elected to add a 1/16″ carbide ball nose endmill and 1/16″ square ended carbide endmill, using the settings derived from a built-in 1.5mm cutter.

MP also comes with a cute graphic visualizer of what your cut will look like. This is a preview of the roughing cut.

I discovered that there’s not really a way to “zero” anything, like on the corner of a stock. It seems like this machine is intended to cut shallow depressions into a block of material that is of indeterminate size, which – go figure – is what you’d do for a molding and casting scenario. The software even has built in draft angle capabilities and “margin” adding – it will automatically add a ring of full depth cut empty space around your parts.

The machine appears to the computer as a printer. The first time I got to this stage, it would output everything at once, but nothing would happen on the machine side. Some investigations concluded that the Windows side drivers were completely messed up. I had to uninstall everything related to the machine (including its strange USB-to-DB25 cable adapter) and reinstall it in the manufacturer’s recommended order before I could get the machine to perk up. This is what the output screen looks like if you have multiple operations involving different tools. You hit Continue and the machine will run its cycle, then pause at the end and move to a tool change location.

Or, at least, it’s supposed to. I tried a few different settings which may or may not have been the tool change location, but none of them were convincing enough for the machine to follow, it seems. While I had instructed it to go to an unmachined location so I can zero off the next tool, it just stopped at the end of its last machining motion on the roughing pass, which happened to be directly over the sinkhole in the middle of the first O.

Gee, thanks. And I have yet to discover how to jog the machine yet, if it even has that function.

Oh well. Onto pictures of the process.

The stock of choice is a little brick of machinable wax left over from the class which I sawed into essentially the right size. You line up the stock visually, on the white grid, and double sided tape is the official work fixturing solution. For the limited capabilities of this machine (which can move at a blistering 15mm/s maximum), that’s perfectly fine. Again, no way to jog and zero or touch off the stock that I’ve noticed.

Z axis zeroing just entails driving the spindle down towards the top of the piece, sticking in the tool, then letting it sort of fall under its own weight onto the material. You’re *supposed* to hit the Tool Up or Tool Down buttons with the tool already in the spindle, but that only increments in 0.1mm as far as I understand.

After the roughing pass completed, I changed the tool in the machine by moving the tool up until it was well clear of the material. Unfortunately, as mentioned before, I have yet to discover how to make it go to my big unmachined margin to the right so I can properly touch off the finishing tool, a 1/16″ ball endmill. So I eyeballed it.

It was pretty good eyeballing – the tool was too low by about 1/2mm. Either way, modeling wax is very, very permissive about how it is to be machined.

The finishing stage took approximately 6 or 8 hours, and ran overnight. The finish in the end was very, very clean. Like so clean. Way better than any 3d print can ever get me, for sure!

I decided to try something a little niftier, closer to a part I’d design and then export as an STL. I downloaded a spiral bevel gear from Thingiverse since it looked pretty machinable and was much more complicated.

This time, the path generation was a little more complicated. There’s no way in the software to say “skip this feature since your tool is too short to machine it”. Instead, you define specific rectangular regions to machine – the default is the whole part – that get pathed independently.

To avoid the through-hole in the middle which would have been too deep for the finishing ball endmill, I therefore had to make 4 rectangular regions which very carefully avoided the hole. There’s no “boolean difference” allowed in this operation.

Here’s the piece during one of the finishing passes. I ran the cutters faster this time, so some of the non-rigidity of the machine is visible in the gear teeth. I also set the margins to be very small and mismeasured my wax chunk, so it machined off most of its own sides anyway. I again couldn’t get the machine to go to a specific spot for a tool change. I must be misinterpreting what it means by “Tool Movement Location”…

I intend to keep experimenting with the machine in the coming days. The machine, sadly, does not talk in G code. Rather, it uses RML, which is Roland’s own little language. There seems to be an avid community of modders that replace the controller inside with a custom board that can interface with commercially available CNC driver software.

I  see how this machine can be very helpful and intuitive for model makers and designers who don’t have an engineering background, and I definitely see how it would be useful in making super fine molds for casting plastic parts. What I’d like to get squared away in the next few days is how to persuade it to go to a known spot for a tool change, something which the Media Lab tutorial I linked to at the start seems to hand wave. Once that’s done, then I will definitely consider trying a few actual molds. Maybe it’s time to stock up on that high density polyurethane board stuff…

However, I definitely should play with the EZ-Traks some more. I think my preferred realm still leans towards using a machine with more cast iron.

 

The Secret of BurnoutChibi

Mar 27, 2013 in Chibikart, Project Build Reports

Continuing on my last post, I’ll talk a bit about how Chibikart is getting a radical design departure from my usual silly vehicles, and later on some about what the hell I’ve been up to for the past week or so. These days, like last spring, my time during the day is heavily  devoted towards herding 2.007 students in some way, including my own “victory garden” of 2.00gokart students who have seemingly become accustomed (read: spoiled) to work late into the night like I do. I’m clearly being the best of influences for my impressionable undergraduates. With my spare time being more fragmented and less conducive to sustained building, I’ve been revisiting some things which I’ve mentally tucked away for better days.

Anyways, to recap, BurnoutChibi is my proposed refitting of the Chibikart 1 frame, which is currently sitting derelict waiting for motors I will never regain the patience to rewind, to a form which offers some more excitement. The reason for the upgrade is twofold – first, I’d like to bring it up to “Chibi 2″ standards, but more importantly, to up the power and show how an inexpensive sensorless drivetrain should be executed. The last time I showed the design, it was a simple one-stage design that was basically the DPRC with more power, and pneumatic wheels.

As I was designing the drivetrain reduction ratios, I began thinking more and more that having 1 speed, even in an electric vehicle, is just suboptimal. With the power levels that the selected drives were capable of, I could reach about 18mph and smoke some rear tires. But that would be it – the fastest it would ever go would be said 18mph. Yet just power-for-power alone – power dissipated by air resistance at speed versus the mechanical power produced by the motors at 50% load – the drives have enough punch to get me to nearly 40 miles an hour. If I were to gear for that speed, it would accelerate very… gently. And that’s only in the ideal case of the motors being able to sustain the high current draws needed to produce enough torque to get there.  Regular permanent magnet brushless motors are really quite limited in the ranges of speeds and torques they can achieve alone.

I’ve wanted to build a multi-speed vehicle for a while, because it still just seems like a better idea. You can shift the peak power and efficiency speeds around depending on your load requirements. Recalling Ben’s successful über-trike build pushed me even more towards that direction – that thing actually has 8 speeds using a bicycle hub gearset as the transmission. I was interested in even having two – burnout mode, and do-more-interesting-things mode.

It was easy enough for me to cook up a custom “shifter” design, but what better opportunity again to test out new parts on the market? Once again, I turned to Vex Pro for the answer. I’ve been eyeing their “ball shifter” (which sounds somewhat painful) transmissions since they were put up on the website.

Many a drivetrain in FIRST have been built using the classic AndyMark shifter transmissions, ever since the drill transmissions went out of style, and AM was the first place I went to when I was looking for COTS 2-speed options. What I didn’t like about them (both now, and years ago in FIRST) was how huge they were. The large, open frame sheet metal design is easy to manufacture, I’m sure, but there was no way I would have stuffed one onto this frame.

The new VEX transmissions seemed to be better packaged, and used a much more compact shifting mechanism. Inside its hollow shaft is another shaft with a round lobe on it that can slide axially. It pushes out on one of two sets of steel balls seated in the outer shaft, overall resembling a rachet wrench’s ball detent. So, depending on where the lobed shaft is pulled, a different set of balls is pushed outwards, acting like a 3-point spline. The output gears have little cavities that the balls lock into. If the lobe is not present under that set of gears, the rotation of the gear on the shaft will naturally shove the balls back towards the center. Pretty nifty.

Seeing no obvious complaints on the FIRST grapevine about them, I’m assuming they’re working pretty well for the competition and are robust enough to shuffle some 120 pound wimpy robots around. But can they stand up to moving a Chibikart at inadvisable speeds, while transmitting enough torque to break traction on asphalt?

I had my doubts – the ball shifting solution seemed like a great alternative to the AM Shifters’ dog clutches, but I trusted the metal-on-metal pushing of the dog clutches far more. Primarily because the dog clutches transmit torque at a greater radius – such that the stresses in the materials are much lower for the same level of torque transmission; and that the engagement is extremely binary – either engaged, or not. The ball shifters seemed to have a much greater potential of being caught “between gears” if one or more balls don’t release.  But maybe that is just a problem if I push so much power through them the ball detents deform.  I was already aware that shifting under power was basically out of the question – there’s no synchromesh devices on any of these things, and even in a real manual transmission car you wouldn’t hold the throttle down while shifting gears anyway.

So, with this many questions about whether the part would be worth anything, it was clear that I was going to have to get myself a set of Vexboxen. Because if nobody in FIRST is going to break them, I might as well.

Or, hold on… time to get myself another set of Vexboxen. I already have a VersaPlanetary that I’ve dissected and taken pictures of and am still waiting on a reason to do a Beyond Unboxing post on – they’re quire nice pieces of kit.

I downloaded the CAD model of the whole gearbox off the Vex website and immediately started cutting it up. First off, nothing was constrained – the solids just floated around, the constraints being broken by the export to a generic format. So I spent half an hour “reassembling” all the gearbox parts.

Next, I removed everything I didn’t care about – namely, all the hardware and the encoder mounting stuff. And the pneumatic cylinder that is supposed to run the shifting shaft. I was intending to cook up a mechanical linkage of some sort, since I didn’t want an air system on this build, no matter how air actuated brakes would have been.

I replaced the CIM motor with my NTM 5060 model using an adapter plate that will be machined. Both motors have an 8mm shaft, so interfacing with the supplied gears shouldn’t be an issue. I took the opportunity to raid eBay of some 2mm endmills to make the keyway in the NTM motors.

A quick fit test to the frame and… My goodness these things are huge. Luckily, when mounted upside down, the resultant sprocket spacing was acceptable.

Notice how these gearboxes are all designed for 2 CIM motors. I have no qualms, if this experiment is successful, of dropping 4 NTMs on this thing and upping my motor sprocket size for even more ludicrous performance.

I flowed some virtual metal around the mounting points to generate these two-pronged mounting adapters. The big gap in the middle means I can still access all the important motor mounting screws. Else, these gearboxes did not seem to require any other additional mounting – even the Vex website just recommended using their stock L-bracket mount.

The output shaft being a Hex of Convenience and Marketing Exclusivity, I decided to just get a 22 tooth hex-bore sprocket from Vex. Okay, Vex, you win this time.

The final drive ratios for each speed, then, are 18.17:1 in low gear and 8:1 in high, for a resultant speed of only 14mph in low gear and 31mph in high. I would have preferred a speed spread of less than 2.0x with a wider low gear, but hey, that’s what I get for not designing the thing. But the extra-tall low gear will definitely prove my hypothesis that sensorless drives can be successful if they are highly geared.

To actuate the gearbox, I needed a mechanical hookup. I was concerned with how much force the shifter shaft needed to operate – the installed shifting mechanism is a small pneumatic cylinder that is capable of nearly 30 pounds of static pull. Clearly this was not required for operation, because that would be ridiculous. After studying the mechanism in CAD more, I determined that it really should not take much force to actuate if I am not applying power at the same time. The actuation method would just need to be very fast to prevent the balls from skipping slot to slot.

I briefly entertained electronic shifting using big solenoids. The required travel was only 1/2″, which is well within the range of big open frame solenoids I could find for cheap. What drove me away from that was finding said solenoid that had a continuous power dissipation rating. Generally, industrial solenoids are rated for only 10% or 20% duty cycle – in other words, 1 minute on then 9 minutes off is 10% duty cycle. More frequently, the “maximum on time” is also specified, usually also 1 minute. Even though electric shifting would likely have been quicker and only required a button instead of running linkages or cables, I was more into the visceral mechanical solution and not particularly interested in cooking solenoids.

I elected to use a spring-balanced cable sort of mechanism with 10 pounds of return force to shove the shifter back into the starting gear. Luckily for me, the lowest gear was associated with the innermost position of the shifter shaft. So, it was easy to find a spring on McMaster-Carr which qualified for the needed return force at the needed stroke (about 1/2″).

What I could not do was find one which also worked with the dimensions of their included shifter coupler. The black object in the center of the gearbox between itself and the standoff-mounted plate is my own quickly whipped up coupler design, which let me fit a spring (not shown in image) between it and the rightmost black flanged doobob. This spring will try to keep the gearbox in 1st gear, so long as I am not tugging on the cable to keep it in second gear.

Some cable adjustment will be needed for sure to ensure synchronization of shifting.

On the other end of things, I put together a simple lever using mostly McMasterables. Did you know McMaster sold random knobs and levers? Now you do. Oh, and little ball detents already loaded into threaded bodies. Super simple instant two-click gear shifter!

This and many other reason are why, this coming Dragon*Con, I am hosting a panel session on how to shop on McMaster, among other places.

Time to mince some metal soon! I ordered this stuff last week. I have yet to assemble the gearboxes, but they seem legit. The casing is heavy fiber reinforced nylon (or fiber reinforced something or other), and the gears are… sticky. Seriously, whatever magic coating they put on these things, it straight up sticks to my hand. Repeatedly, even. I can flat-palm a big gear and lift it straight up off the table every time. Hey, aren’t you supposed to make gears slippery?

The picture quality is so lacking compared to my usual ones because after 5 years of nonstop service and over 11,000 pictures, my free-to-me Fuji S9100 has finally bit it. Cause of death? The USB port just straight up fell off inside and likely shorted something. So, back to the phone camera.

Oh, the thing was secondhand, too, so in a prior life it probably took even more pictures. This thing has been such a brick that I am actually eyeing its slightly newer successor, the S100FS, or even the current generation X-S1.

The Boom and Bust Cycle of Chibikart: BurnoutChibi

Mar 18, 2013 in Chibikart, Project Build Reports

All of my projects go through periods of ups and downs. I either end up demoing them too hard, or they get “temporarily” parted out for some valuable component only to never get put back together, or the weather stops being terrible. Given that the winter is letting up and the sun has emerged above the horizon, it’s time to think about the small silly electric rideables once again.

Almost a year after its construction, the story of Chibikart has played out not unlike many Hollywood celebrities and pop icons. After a meteoric rise to fame based on a single hit. which spawned a wannabe or two, Chibikart bungles a live performance but pulls it together just enough for the next show (At the bottom… though I still stand by everything I said in that dribbling polemic). Afterwards, however, Chibikart is never really the same, and sinks into a life of decadence and smoking magnet wire enamel. With my desire to never rewind those damned hub motors again, it seemed like Chibikart was doomed to live under a table forever:


…with his washed up co-stars…

But another season of 2.00silly-rideable-thing dawns, and as such, I must have an Instructor Vehicle. The Instructor Vehicle is totally out of spec and over budget, because Instructor Privilege. I had to decide what to do with Chibikart. It was easy enough to start on another bunny-brained scheme since I have more than enough parts to conceive any rideable, drivable Eldritch abomination, but when I had the semi-working hulk of one of my projects sitting right there taking up space, I found it hard to justify anything else.

So what do I do with it? This was an equally hard decision. I could just rewind the hub motors one last time and be done with it. Chibikart is actually quite fun in hub motor form. I could make, instead, 4 kitmotters to demonstrate the concept in real life even better – one of the ideas on my Infinite Time*Money Back Burner is to make a to-spec Power Racing entry using Kitmotters, since machine time and labor are not budgetable items.

One of my favored ideas from the beginning was to show how to do a sensorless drivetrain “right”. I spent much time in 2.00gokart scaring students away from using R/C style controllers because of their notorious unreliability and lack of current limiting and start-from-zero predictability. Yet at the same time, I wrote an entire section on this in the latest Scooter Power Systems instructable.

The way I see most people use R/C motors with sensorless ESCs in vehicles is in direct contrast to the guidelines mentioned in the Instructable. They’re generally the biggest (or bigger than necessary) motors you can buy, hooked up to a low or moderate gear ratio calculated from load necessity or cruising speed. While this is perfectly fine for DC motor drivetrain design and also sensored BLDC/AC motors, with R/C controllers it’s just asking for trouble. Huge current bursts will be drawn during the starting regime because the motor might not “start” in the right direction, or if it does, needs to expend tons of power to produce the torque needed to accelerate. More often than not, the first thing that goes is the motor controller, since they’re invariably small and unprotected.  It’s been my assertion for a while that a 50mm class R/C motor (typically 1500 – 2000 watt) is more than enough power, if used correctly, to move a person, but usually I see people gunning for the bigger 63mm and 80mm motors. Hell, that’s even what I run on Melonscooter. And precisely like I warned against, I go through motor controllers like crazy and really shouldn’t try using all 6000+ watts of power – it’s literally enough to get me to highway speeds and I have enough fun already flying by parked cars at 20-25mph.

So, perhaps a more noble goal is to troll everyone by making a properly proportioned sensorless R/C power system with typical hobby parts. I’d pair a 50mm motor with an oversized (for overhead) R/C controller, but instead of gearing sensibly and picking the slowest (highest torque per ampere of current) motor, I’d head the opposite direction: An unreasonably fast motor coupled with a double-digit gear ratio. Remember, R/C controllers love to spin things which are going really fast. In the combat robot world, this technique has a nickname: “Gene-ing” it, based on the habits of one particular builder to overvolt small outrunner motors dramatically and gear them down very low to get tremendous power into his weapons. The whole idea would be to divide down my apparently inertia to the motor so far that even the jerk the controller gives it is enough to move me a little.

I next spent some time on two sites. The first was Hobbyking, shopping for potential motors in the 50 to 60mm range. The second was the still-very-useful Torque and Amp-Hour Calculator whipped up for roughing out Battlebot drivetrains over a decade ago. I was aiming for a few goals, in no particular order:

  • Top speed of about 20mph at my desired system voltage of 37v, to try and conform to at least one of my own rules!
  • Gear ratio in the 10 to 20:1 range. Something easy to accomplish with  two relatively simple stages – such as 3:1 and 4:1.
  • Wheelspin current of about 100 amps or less. This was important – I wanted the wheels to actually break traction at a current which I could find an R/C controller to run ‘continuously’. I mentally derate these things by a factor of 2 or more when juggling numbers. This means that the wheel should never just fully stall out

What this came down to is if I apply a step throttle input to 100%, the motors will have a high enough mechanical advantage to start consistently each time, and as soon as they do, they have enough torque to just break traction.

That‘s my current limit. Doing a burnout. That’s how it should be.

Hence, this project was named BurnoutChibi.

motors

At the end of the night, I arrived upon my component choices:

On the right, the NTM 5060/380kv. This was pretty much the only thing available that satisfied my requirements – any slower and I was going to go back into undergeared startup nightmare world, and the few motors that ranked higher in RPM/V in this size were very expensive (by comparison) “competition” motors. The cost of the motor? $36.

I’ll be amazed if it stays together.

On the left is a Castle Creations Hobbyking “160A HV” controller. Gee, it sure looks an awful lot like an older Castle controller – I’ll call it the Sand Castle!

These things have piqued my curiosity for a while, but I’ve never really bought one to chop up. It turns out they are thermal epoxied together anyway, so it’s not like I can chop it up without permanently breaking something. I blame Apple.

While I could have gone with my usual trusty Sentilons (which I know the operating characteristics of well, have decent startup ability, and enough FETs per bridge to let me sleep at night), I decided to take this opportunity to try something new. If the Sand Castles fail, I have 4 left over Sentilon ESCs from the late and great Deathcopter project. The reason I’m running 37v (10S lithium polymer cells) is because I have two giant 5Ah 10S “sticks” left over from the same project. These Lipos are big enough to beat someone senseless with and probably light them on fire at the same time. You R/C aircraft people are nuts.

Two of those packs ought to be plenty to feed two NTMs flying at top speed. Oh, I’m sorry – did I mention I got 2 sets of the above?

tires and brakes

To dump multiple thousands of mechanical watts onto a Chibikart drivetrain would result in the tiny, stubby caster wheels being ground into a pile of elastomeric powder. I’d need something vulcanized and beefy to stand the increased power, but I also didn’t want to go to 8″ pneumatic tires which are the most common size for small EVs. It just wouldn’t fit the character. Chibikarts have to have small wheels.

I decided to try and get some 6″ pneumatic caster wheels. The 6″ pneumatic wheel is something which should exist, but is difficult to find; when it is found, it’s generally expensive and part of a caster already. Harbor Freight doesn’t sell any, and McMaster is generally unhelpful and ambiguous about which one of their 6″ casters is actually pneumatic. For the record, I have a sample of 2717T41, which became the basis for my hunt. I figured if McMaster sold it, there was a cheap and generic verison of it somewhere.

The answer came from Northern Tool, a  Harbor Freight function-alike of somewhat higher market segment. They have a 6″ pneumatic caster wheel replacement for Somewhat Less Price. I went ahead and snagged 4 to see if they were identical.

Also pictured above is a 90mm drum brake I got from Monster Scooter Parts. One of the missions I set out right away to complete was to find a good front wheel brake solution. While both Chibikarts have had rear brakes, this basically precludes the chance of doing any burnout whatsoever. The last time I could do a brake standing burnout was LOLrioKart, and I kind of miss that.

I started out trying to find a small enough disc brake, since I prefer those when I can fit them in. The smallest disc brake setup available stock is 120mm – a bit under 5″, which would have been too close to the ground; I’d be riding on brake rotors if the tire ever went flat or someone bigger than me got on this thing. While I could have just machined a custom brake disc, I decided to be adventurous and see what a cheap scooter drum brake is like. I have plenty of experience with cheap scooter band brakes on front wheels from LOLrioKart’s first front brake attempt, which ended with me discovering what the Capstan Equation meant in real life (Band brakes grip extremely asymmetrically with respect to direction!)

I settled on these and got some samples because I haven’t ever seen them before and was interested in exploring a new part For Make Better Glorious Nation of Silly Go-Karts. It’s my understanding that drum brakes also grip asymmetrically, though to a much lesser degree than band brakes.

As luck would have it, the two spanner wrench holes on the drum line up pretty much perfectly with the hub lug nuts on the wheel. Well then – that pretty much seals the deal.

What I’ll probably do for attachment is make two standoffs that replace the lug nuts with posts I can screw the brake rotor in from the other side with. Two of them will have a pilot “lip” to seat in those holes. I’ll shim up the bore to be concentric, then drill the other two holes using the wheel itself as a template. I’m sure two standoffs might work, but using all four possible locations is just more robust.

This is what the assembly would look like in real life. Instead of using the long built-in torque arm, I’m going to cut it off for compactness and instead mount the brake body itself with a circular bolt pattern. There appears to be enough space between the stamped housing and the brake shoes to drop some #10 or 1/4″ button-head screws into.

I went ahead and made a representative model of this assembly in Inventor. The bore of the brake housing is 14.5mm, so to use with a 1/2″ axle, I’d have to make another spacer with a seating lip.

The next thing to do was to take Chibikart 1′s CAD file and rip everything the hell out of it. Structurewise I was going to turn this into a DPRC, which I engineered a more robust brake pedal for.

I started adding in the new wheels and testing steering geometry, but soon ran into the curious issue of Chibikart being wider than it was long. The pneumatic wheels and brake combination were threatening to push the width out to 30″, because they were just so much wider than the skate wheels!

Some innovative compacting on the steering linkage was going to be needed.

I pushed in the “ears” on the frame a half inch or so, such that the flanged bearings were almost against the 80/20 side rails. Instead of a DPRC-style spindle which is a bolt embedded in a block made of stacked layers, I went for ultimate compactness and designed the spindle blocks to be made from 1″ aluminum square. Like the current arrangement on Chibikart 1, the ‘axle” is a bolt which is tightened into this block.

The hard part was the steering linkage itself. Part of the reason why I made the ‘ears’ so far out in the first place was so I could get enough linkage travel before the ball joint hit the frame. With scooting the steering axis inwards so far, I would only have gotten maybe 15 degrees of wheel travel in either direction. Besides seriously messing with the steering linkage geometry, the other solution was to move the linkage out-of-plane with the frame. I decided to try this first. Basically, the spindle block is solidly connected to the kingpin (which is now a hex head bolt), and on the other side, a big crank arm with a hex bore cut out of it sits like a wrench over this hex head, retained by an axial screw (not shown). This way, the linkage is moved fully under the frame and I can once again have proper steering geometry without  compromising angle.

This configuration might in fact offer too much angle – there’s not a physical hard stop in the system any more, so I might actually add something else later to prevent linkage overextension.

Back on top, I spent a while editing this sheet metal part in-assembly to get the lineup of the cable anchors correct. This plate is essentially an extended version of the “endcap” found on DPRC’s front wheel spindles, with a bolt pattern that engages the spindle block. The mounting bolt pattern for the brake housing, not yet shown, will also be put into this part.

I whipped up a quick 4.16:1 gearbox using VexPro gear models. Vex has gained my interest immensely after debuting their new line of “bigger bot” parts this Spring for the 2013 FIRST season, ostensibly to compete with the likes of AndyMark because FIRST is now big enough to sustain two parts houses. I actually have a VersaPlanetary box bought completely for the purposes of dissection, and intend on writing it up for Beyond Unboxing one of these days – they’re nice, I must say.

The NTM motors have convenient 8mm shafts already, so any gear that can go on a FIRST CIM motor can be fiddled onto this one.

The new rear end of BurnoutChibi nee Chibikart 1 is basically a DPRC back end with a different motor adapter plate. This time, the 4.16:1 gearbox hovers above the main 3:1 chain drive, with the usual chain tensioning method. At this 12.5:1 ratio, the top speed is right at 20mph and the per-wheel burnout current is something like 80 amps. Not bad, but not exactly good either.

So right now, Burnoutchibi is essentially DPRC on some serious roids, with pneumatic tires. By itself, no matter what, it’s not that exciting any more – I’ve built enough things like this already. And by this point, it’s maybe a little hackneyed. The next post will be about what I intend to do about that…

The Return of the Collegiate Silly Vehicle Brigade: 2.00gokart

Mar 09, 2013 in Electric Vehicle Design, MIT, Bostoncaster, Cambridgeshire

A year ago, I embarked on a project, nicknamed at the time “Project Seedling“, to create a class at MIT dedicated to silly electric vehicle design. The backstory of it was basically as follows: Since 2010, I had aided the MIT EVT in running a small section of the 2.007 class in which 4-5 students built a small EV, in years past having been scooter or bike-like contraptions. In 2012, the EVT members who had run that section had other obligations, and so could not take on the responsibility.

By that time, I was sufficiently interested in how small EV building had seemingly infected a sizeable portion of MITERS and even people beyond, and decided to take on that role as organizer and also (lecturer & shop guy & design advisor) in order to make the problems even worse. In doing so, I pushed for expansion of the section to 10 students, almost a full 2.007 lab section, but ended up taking 11. The class was scooter-centric with a focus on practical usage, but I allowed the students significant design leeway, so creations as varied as melonkart and… these things.

Taking what I learned from that year, I decided to do it again this year, with a twist. While giving students complete design freedom was beneficial to the few who took advantage of it, it was also frustrating to those who didn’t quite “know” the subject matter already (as quite a few of the students had seen our work prior to the class or had built their own projects already) as well as an astronomical amount of variety for me to keep track of.

Using Melonkart’s two-person effort as a starting foundation, I drafted right then and there at the bottom of the post what the project I codenamed “2.00gokart” would entail for 2013. I spent much of the fall semester and January thinking about and balancing those requirements and desires with how many hours I would have to put into it playing lecturer (since 2.007 mainstream would of course not cover EV-specific topics like how motor controllers work internally), design advisor (because when there’s 8 different combinations of parts that will hit your goal, I have to teach the concept of pick-one-and-try or compromising aspects of your design to those who may have never done it before), and shop guy since I de-facto am still the lab manager for the International Design Center tinylathe’s lair.

Over the course of late fall semester and January, I initiated a build trial for a “2.00gokart Prototype” of sorts, handing the job off to Banks. The end product was SmartKart:


You see where the priorities lie…

So called because it was awkwardly tall and short wheelbased, like a Smart car. Through the build, a few pitfalls of my original assumptions were revealed, such as the fact that no, even with the amount of free stuff included, a $300 per-team budget was going to be impractical. It COULD happen, but you’ll basically be running 150W scooter motors with relays and using Pink Harbor Freight Wheels. In the end, most of the ideas stayed the same. Some were removed, and other requirements added. Basically the final version of the rules goes something like:

  • You get 3 6-foot sticks of 80/20 1010-size T-slot extrusion and 4 square feet of both 1/4″ (6mm-ish) and 1/8″ (3mm-ish) aluminum for starters
  • Also included in the pile of free stuff is, optionally, 1 8″ pneumatic tire/wheel without a drive sprocket or pulley (i.e. a wheel with just bearings)
  • You may use up to 3 A123 bricks for no cost, and a fourth at $75.00 off your budget. This was one of the hardest choices for me to make, since if I straight up allowed 4 batteries, then there would be a clear obvious best solution. I also wanted to avoid allowing 48v electrical systems due primarily to the added danger of working with those voltages for people who were for the most part inexperienced in electrical systems.
  • The per-team budget is $500, since this was what the final cost was closer to.
  • Minor hardware is free so long as the cost of each individual screw (or random small part) is less than 50 cents each. So, I encouraged students to spec and purchase the hardware they will need (teaching them in the process that McMaster will only sell you a bucket of nuts at one time), a difference from last year where I ordered a hardware medley and encouraged people to design around it. I decided this was worth the extra overhead cost because one of the important details of building things that people always miss out on is the hardware and little piddly parts!
  • I scrubbed the part about having a preset range of parts to pick from, because come on. One of my Core Tenets of the Church of the Silly Electric Vehicle is to teach people how to shop and hunt for resources.
  • The “kart style” ruled stayed with the following wording: The vehicle must be statically stable in the powered-off condition without the rider present and must have at least 3 rolling points of contact with the ground. This excluded, for instance, scooters with landing skis. Unfortunately it also excluded things like Batpods (since a wheel of such width would ensure its stability anyway) because your Batpod would be required to have an awkward dong made of a scooter wheel or something, or monowheels. These were legit idea submissions during the early design phase. I shed a tear in solidarity with those would-have-been designs.
  • I also took out the size range, and let material use and cost determine how large the vehicles would be. This enabled such interesting designs such as “Supermankart” and “Lugekart” to spring forth. You can imagine for now what that all entails – I promise pictures will be forthcoming.

The challenges will remain a 50 meter drag race followed by that thing we keep doing in the videos of everything, driving all the way up one our our campus parking structures. I look forward to this year’s challenge because you can take turns so much harder with a kart style vehicle than a scooter or anything else. Furthermore, all karts will basically hit something at the same level, so instead of putting up catch fencing, I’m just going to pile bricks of fluffy insulation around columns, doorways, etc. to appease the Gods of Health & Safety. Last year, I piled a few bricks of that stuff around specific problem areas in the garage. While none of it actually ended up being used in the intended sense (which is good, mind you), I think a row of them will certainly beat having to roll up an eighth mile of little orange fences at the end. This enthusiasm might be damped once I calculate how many semis it will take to equip the walls properly.

So what’s currently happening on the front lines?

The first event of the semester was what I’ll basically call “hoonage night”, in which interested students could come take SmartKart and other vehicles, including DPRC for a joyride or two, to get an idea of what they might be building. My section had to change to applications-only this year, because in the span between last year and now, something like half the Mechanical Engineering class of 2015 had heard about it. I ended up having an acceptance ratio of something like 40% of the people who applied.

I also put out a “petting zoo” of parts left over from last year as well as samples purchased this year. Part of my agenda was to encourage more people to use DC motors for simplicity reasons, since wiring up a brushless motor with sensors is more electrical legwork. Last year, you were also hard pressed to fit a good controller into the budget along with a brushless motor, so some of the vehicles were somewhat over-motored for the amount of amps they could actually use. In the “petting zoo” was wheels, motors, brake parts, etc. so people could start modeling parts up right away if they saw something they liked.

Around week 3, students had to give a brief talk about their design and what parts they were planning on using. The design shown above will prove to be quite exciting…

Before this time, I used about an hour of each lab section per week meet to lecture about EV-specific subjects such as drivetrain/motor calculations, electrical systems, and mechanical parts and joinery methods.

Now we’re getting into Week 4, and things are picking up. First orders have been sent out for parts, and designs are reaching the point of being finalized.

With the fabrication period really picking up now, I rearranged the room such that each team is going to get 1.0 tables of space to work on everything, and keep their parts. These tables are on wheels and can be rolled into the shop spaces for easy access to tools.

The short term future promises to be very exciting as the teams are facing the infamous “Milestone 7″ rolling frame inspection in only about 3.5 weeks time! How many shall remain afterwards?