Archive for the 'Project Build Reports' Category

 

This Week in Mikuvan

May 07, 2013 in mikuvan, Project Build Reports

Nope, it’s not Miku colored yet. Quit asking.

It’s hard to believe that I only went on the Great Van Escapade a week and some ago. Between then and now, I’ve done many hours of disassembly, testing, and debugging. I think I’ve finally rooted out the problem, but am waiting on some more advice and recommendations before proceeding. Why am I even buggering with fixing the ICE engine, with all its attendant pre-OBD-but-post-CARB Mitsubishi-only oddities, when I’m just going to unbolt it all and drop in an electric power system? Not quite sure, but some of it has to do with curiosity in figuring out exactly how much of a complexity nightmare ICE vehicles are, and others because I have 20 more days of temp plates left. Getting in some driving feel would be immensely helpful too.

Mikuvan lives in the enclosed underground parking garage under my apartment block, next to a Honda CR-V, a Volkswagen Golf, a Prius, and (among other cars) a 1963 Mercury Comet. Good, I’m not the only project car sitting in a pile of its own parts. Looking down the row of parked cars is amusing – all you see is hoods and headlights…and then there’s this.

At least it’s not on-street or outside. But the downsides of this arrangement include the total lack of AC outlet power nearby, poor lighting, and a lack of Wifi or cell reception. The nearest outlet is 75 feet away, necessitating some extension cord creativity. I have a 500W halogen work light to relieve the lighting issue, but it is still only one source. The latter issue means I often neglected to bring cameras or camera-enabled things with me into wrenching sessions. Hence, even though there were plenty of cool photo ops, this post will sadly be mostly text.

Hey, so is my air filter supposed to be furry?

The story of digging in around in the drivers seat engine bay is centered around consulting with people who know a thing or two about what cars are, then vaguely following their suggestions but ultimately falling back to the Official Strategy Guide / Shop Manual to figure out through its well-drawn but extremely narrow view diagrams where the parts in questions actually were.

It’s often said that you need 3.5 things to get an engine to work. Spark, fuel, compression, and when-does-the-spark-fire (i.e. timing the spark, the 0.5 part). I basically began by checking the ones that were easy: spark and compression. To check the timing properly would have involved exposing the timing pulleys, which, as far as I could tell necessitated removing the radiator and cooling fan shroud, then also removing the distributor cap which was more accessible. I did not feel like attempting this in the dark with limited tools. In Pennsylvania, we already verified compression, so I started by checking the spark plug lines.

I bought 2 of these in-line plug checker lights from Harbor Freight (not sure why I just didn’t go ahead and get 4). The firing order of the engine is 1 – 3 – 4 – 2, so I started by putting the lights on 1 and 3 to verify the order, then 3 and 4, and so on. Basically to make sure that 1) there was spark even if it may not be the correct timing, and that 2) the  cables weren’t switched around or something.

The sparking order checked out fine, so I began reading up on fuel injector testing and cleaning. My suspicion at this point moved to the injectors, since they were really the only element left. I highly doubted it was a timing issue in that somehow the timing belt (which is in great condition as far as I can see – it must have been replaced fairly recently) skipped 1 tooth or the distributor cap rotated enough such that I got completely inconclusive cranking – even a late spark would give me some kind of ‘puff’ and an early spark would cause premature detonation and horrible noises.

But I couldn’t help but think that all 4 injectors failing or clogging at once was extremely unlikely. In my experiences with watching friends tell stories of problem cars and from a few trouble vehicles my family has owned, engines don’t just suddenly stop working unless either something

  1. catastophically failed on the mechanical side, which I would certainly know by now, or
  2. a single electrical point of failure such as a sensor is preventing the ECU from running the engine properly

My money was moving towards some stupid sensor failure. For instance, if the crankshaft position sensor, used for fuel injection timing and electronic spark timing (the ECU fires the ignition coil when it feels like) is out, then the ECU won’t know when to do either of those things. If the throttle position sensor, which is potentiometer based, was broken or worn, it could be reporting a completely nonsensical value, though this seemed less likely since you’re never supposed to step on the throttle while starting, unless you know exactly why you have to. There’s other sensors involved too, like the mass air flow sensor which the ECU uses to determine how much fuel is metered into the cylinder.

With all of these things having to work in synchrony, I’m amazed cars function at all.

Here’s the scene of the crime, lit up by the aforementioned 500 watt halogen light. It kept the area reasonably warm, as the rest of the garage is unheated and basically settles to its own temperature by thermal inertia alone (surely it will get unbearably hot during the summer).  At this point, to access the fuel injectors and high pressure fuel rail, I have the passenger seat slung up, the driver seat removed, and the underframe of the driver seat also detached but just shuffled out of the way a bit since it has the parking brake lever, fuel door lever, seat belt anchor, and a host of other stuff on it I don’t feel like dropping into the engine.

Here’s the whole mess from the other side. The shop manual has been my reading material of choice for the past week. It’s extremely informative, but at the same time I can tell it was written by mechanics for other mechanics. I assume that the unlabeled detail shots require some background in wrenching to understand where to insert the thingimadoodle and how many degrees to turn the whatchamadoosit. There’s other info missing such as sensor pinouts right after it tells me what voltage this or that sensor should read…

…While the engine is running. How about a little help for the other case here, guys?

Before taking even more things apart to get to the injectors, I decided to see if it could tell me what was wrong.

Okay, now I’m seeing something familiar. My van debugs like a Kelly controller or Hobbyking controller!

It predates OBD (“OBD-1″), so it has multiple means of debugging available. You could buy the $500+ “multi-use tool” which is like a form of proto-OBD scanner, or you can debug with a voltmeter. Not a digital one – an analog one. It puts out little pulses of voltage so you can see the needle move (digital meters do too much time-averaging to see this effect). If I added an LED to the circuit, I literally could have watched it blink. It probably would have said “FREQUENT RESET” or something, knowing the average Kelly controller.

So an analog voltmeter it is. It took me a while of digging in MITERS to even find one of our crufty analog voltmeters, and I ended up having to make hardwire leads for it anyway.

But it worked! The key has to be turned to ON (not start) for a few seconds for the ECU to start putting out pulses. The result is:

Normal State!

Oh, come on.

My guess is that since the vehicle has not been started since my new battery was connected, the ECU doesn’t know what’s good or bad. The engine must run, no matter how crappily, for a while before the ECU can recognize something is out of range or nonresponsive. My mission now was to try and get the thing started no matter what. If the injectors were clogged, then I’d have to unclog them.

One thing I was told to try was to drop carb/throttle body cleaner (i.e. vicious, surely carcinogenic,  and highly volatile solvent cleaner) directly into the fuel rail, mixed in with the gasoline, to try and dissolve anything which might be causing injector blockage on the spot. Basically you cycle the injectors bathed in disgusting solvents and let it sit for a while, then try again. Rinse and repeat. I bought a little can of Seafoam on recommendation from friends, which appears to the most disgusting of the disgusting solvents since it claims to clean everything. Seems legit, right?

The procedure was to disconnect the high pressure fuel line from the rail, get most of the fuel in there out, and replace the rest with Disgusting Solvent #81289. I wicked fuel using a few shop towels, which were promptly lit on fire for my own amusement (this process does not have photos associated), and mixed in Seafoam about 50/50 into the rail. Next, I gave the engine about 10 seconds of crank to get the new mixed drink into the injectors. During this time, the engine sputtered a few times.

Promising.

An hour later, I came back to give the engine another spin. 10 more seconds of excited cranking and sputtering later, it took off.

It was shaking like crazy and white smoke was everywhere (allegedly a sign of the cleaner doing its thing), and the revs were unsteady for the first few seconds of run. It seemed to settle into an idle, though I was both too excited and scared shitless to check the tachometer for functionality. Something was happening.

I was under the impression based on checking the dipstick in Pennsylvania that the engine was very low on oil. It was also running with zero coolant. Fearing causing damage due to lack of lubrication, I shut the engine off after about 20(ish) seconds of running.

it did something

Unfortunately, that was the only run I got out of it that night. I regrouped thoughts for a bit to formulate the plan of attack if it started and ran more than once. On the next shot, there was some more sputtering, but no consistent behavior. I gave the air intake a dose of starting fluid to no avail. By the next few tries, the battery was wearing down too low to crank effectively. I’d have to bring in my charger and top it off before trying again, so I cleaned up for the night.

That was when I noticed that the air flow sensor wasn’t connected at all.  Remember the air filter shot? I opened the air cleaner box to remove and replace it with a fresh one, but neglected to reconnect the airflow sensor.  So, the engine running must have been pure luck – or the cleaner/solvent making for such a volatile mixture that any small amount was sufficient for it to keep turning over. The air flow sensor is a “hot grid” type sensor (looks like this) used for air mass calculations. If “disconnected” also means “off scale low”, it means the ECU would think that there was no airflow. No airflow means no air mass to calculate fuel injection quantity with. And no fuel means no combustion except if you’re basically mainlining Seafoam. My exhaust system is probably really clean right now.

Yesterday evening, I tried re-adding some cleaner to the fuel rail (in lesser quantity) to try and confirm this theory. I got the engine to sputter some times, but no starting and running was observed.  I also noticed that the ECU code had finally changed to:

Air flow sensor.

It was definitely connected. I even abraded the pins a little and recrimped the socket to increase contact pressure just to make sure it had connectivity. I couldn’t tell if the element was damaged (it looked good, even clean) or the entire sensor had just stopped working or what. I cleaned the grid element with some rubbing alcohol and let it dry under the halogen lamp for a while. No obvious changes were noted, nor were any starts effected. Maybe “disconnected” is a totally different signal from “porked”.

A new-used MAF sensor costs about $120 on eBay, so I went ahead and ordered one. Even if it’s not the problem, I now have a debugging chain to follow instead of shooting in the dark.  The airflow sensor being problematic would corroborate my theory that some critical sensor failing is causing the ECU to not control air, fuel, or spark properly. We’ll see how this goes.

With these new developments, I decided to do some staging and preparation. First, I wanted to get the disgusting sludge oil leftovers out of the engine and put in something fresh. On the same Harbor Freight trip earlier in the week, I anticipated needing to do this eventually so I got an oil filter strap wrench and a waste oil container, the kind with the integrated drip pan. I ordered a new oil filter off eBay (the best auto parts store!) last week already.

The oil drain plug and filter were clearly designed to be accessed from an auto lift. I didn’t have this, so luckily the thing has a massive front nose cavity…

The plug and filter aren’t visible in this picture, but they’re right behind the front suspension arms. The radiator to the left is the A/C condenser – it’s the first thing to hit if you drive over a tall curb or something.

I also noticed while I was under there that the transmission oil pan is basically the first thing to hit the ground if I go over an enthusiastic speed bump. I’m not sure how they expected this to navigate the rough streets of the U.S. while loaded with seven U.S. sized adults. Maybe everything was smaller back in 1989…

It was black. ALL BLACK.

Around 5 quarts of entirely black oil poured out of the crankcase. Like, this stuff was basically the color of the filter. So it did have oil after all! “Oil”, anyway. I think we must have read the dipstick wrong in PA, since we swore it had very low oil.

Always a good thing to find in the drip pan – little metal particles. And chunks of sludge.

I let both filter and drain plug ports drip for over an hour (while waiting for the battery to charge) before refilling it with some new 5W-30 from the gas station. I didn’t bust money on premium oil since I figure it wasn’t going to stay in the car for too long anyway.

Oh, also, the oil filter had basically no torque on it. I didn’t even need the strap wrench – just the torque of my hand trying to engage the strap loosened it. No wonder there is a thin sheen of oil all over the underside – it must have just been leaking forever. I made sure to crank it down when I installed the new filter.

The game right now is to wait for the new airflow sensor and see what happens from there. I’ve pledged to give an honest debugging effort to this thing even if I’m not keeping the engine for long, and I’m willing to spend some money on it.  I’ll make sure to take more pictures of everything in the future.

This is also the first post in the new Mikuvan build thread. Oh boy, I’m in deep now…

A Preview of 2.00Gokart and Finishing BurnoutChibi

Apr 26, 2013 in Chibikart, Electric Vehicle Design, MIT, Bostoncaster, Cambridgeshire

With the semester winding down (or, perhaps, finally ramping up!), many of the 2.00gokarts are in the process of being wired up and tested. The final product is due next week, and our competition (last year’s video)  is on May 5th!

Some of the students have been industrious and scheduled their checkoffs and inspections early. Here’s a preview of the action that will unfold in a much larger space next week:

Because conventional controls and riding postures are for wussies, apparently. I’m both amused and somewhat terrified at the prospect of there being three (out of eight) karts in which you ride head first. As it was my stated mission to not interfere much with the design and construction of the karts to let students experience as much of the design process, I might have to start padding BurnoutChibi and run interception for wayward karts.

Speaking of which…

Here’s a picture of the aftermath of BurnoutChibi’s motor detonation. As I would later find out, the sparks seen in the video were not the magnets grinding on the can, but rather them cutting up the phase wires.

Here’s a better picture of the ownage. The red wire, in particular, was cut almost all the way through. The annoying thing about this is that the wires were so close to the stator. If they were further out, patching would be a simpler job. I’d have to loosen the epoxy holding the wire stubs in place and also trim the heat shrink selectively.

While I await better motors, i decided to try and repair these. First step was to pop them open. There is a front retaining ring that comes out, then 2 set screws loosen up to free the shaft from the rotor. Then it’s a matter of pushing the shaft out to the right in the picture – this step was done on an arbor press.

Ouch. In total, five magnets broke loose. I figure this must have been a chain reaction where one magnet ditched first, and the resultant imbalance caused can deformations which broke the rest loose.

This is why I recommend motors that have “rotor bearings” or “skirt bearings” to everyone who asks me about them for vehicle apps. Even though it adds a little drag, the distal end of the can is properly supported on its own bearing. The only exception is if the motor is very short, like a more “pancake” style design.

I mixed up a generous dose of long-cure epoxy with glass microsphere (microballoon) filler, to slightly under nutella-like consistency. The offending magnets were pried out, the mating surfaces cleaned, then this epoxy smeared into the new joint. I replaced the magnets and used as much of the remaining epoxy as possible to completely fill in the gaps between them.

Evidently, I didn’t add enough microballoons, as the mixture did sag a little. To keep the cure symmetric, I actually chucked this thing into Tinylathe and ran it on a very low speed for several hours.

After the mixture was firm (but not cured), I set it on a radiator to cure with heat. Luckily for me, the radiators in the building were still on; they were switched off successively as recently as 2 days ago!

I didn’t get a good picture of the wiring repair before, but it basically involved exactly what I described before – carefully scraping away the heat shrink tubing to expose as much wire as possible. The wire was actually all magnet wire, so it would have been difficult to solder. To combat this, I “frayed” each lead as much as possible to expose the maximum amount of magnet wire surface area. Then I cranked the 80W soldering iron up all the way to 850 fahrenheit and literally burned away the enamel by embedding the frayed ends in a big ball of solder for heat transfer.

I think I managed to get back 75% of the red lead. The rest were patched similarly, but did not need as drastic soldering measures.

After the real epoxy fully cured, I reassembled the motor and crammed it back into the left side transmission.

I have yet to ditch a single magnet. Though I figure it’s only a matter of time before the right side lets go…

And with that, BurnoutChibi is ready to lasso its rogue… brethren? Bastard children? Offspring conceived via assistive reproduction technologies? Something. The only thing it does not do very well, sadly, is burnouts! Because the rider weight is basically square in the frame, and is up so high, it really just like to drag the front wheels along even if I’m holding the brakes. The same reason contributes to its severe power understeer (and associated lift-off oversteer!) behavior. Oh well…

Finishing BurnoutChibi: Transmission & Drivetrain, Controller Mounts, and Wiring

Apr 19, 2013 in Chibikart, Project Build Reports

In the previous week of work on BurnoutChibi, I’ve fully completed the vehicle but have yet to get it out to really test. This thing really is too damned fast for our indoor.. uhh, test track. A motor quality issue also prevented me from blasting it around in our usual outdoor venue (for very long, anyway). These issues have since been addressed, so it’s almost time for more test video!

As previously discussed, BurnoutChibi is a refit of the derelict Chibikart1 frame into something a little more hair-raising, as if Chibikart 1 wasn’t bad enough already. Since the last update where I had just finished reconnecting the steering, I’ve finished mounting the braking system, the transmission shifter cables and linkages, and also completed electrical hookup. At the behest of some of my students, I completed it in time for CPW last weekend, though the aforementioned motor problem meant it was not out scaring parents and wide-eyed potential freshmen.

Here’s the story in the pictorial form.

I began with a little aside in order to solve the problem of how to mount the two “Sand Castle” controllers. They have no mounting flanges and both sides are made of heat sinks, so just gluing it to a plate would make for some pretty poor thermal design. I decided to come up with a “cradle” that held the two controllers right under a fan for some forced convection  cooling. The fan I selected was out of my plentiful stock of 80mm LED case fans.

This design was an exercise in designing a snap fit for 3d printing. While I could have made the base a little wider and added some through-holes to hold the two halves together, I decided to get creative and dovetail each corner post together. The angle is extremely steep – about 85 degrees – so the whole assembly could be pulled out with force, but otherwise snaps into place cleanly.

…and it’s printed out of PLA.

Yeah, so what if it’s going to melt at about 60 celsius? It’ll just smell like delicious waffles while the ESCs burn.

I decided to try the “translucent light blue” PLA which is sold commonly, and I must say it’s my favorite PLA color so far. It’s not the vaguely jaundiced-rainwater color of natural PLA, and I also don’t like solid color PLA. A tinge of blue helps, but is not overwhelming and makes me think it’s some real plastic.

Putting together some of the electrical deck and testing the fit of the ESCs. Result: pretty perfect!

I set aside the e-deck for a while to return to the transmission and drivetrain.

First order of business is to attach the sprockets to the wheels. This basically entailed making four standoffs which acted as the lug nuts (M6 thread) on one side, and regular 1/4″-20 on the other side. The standoffs hold the sprockets a set distance from the wheel so the chain clears the tires, and also holds them concentric.

Or so I hoped.

There is practically nothing concentric or wobble-free about these shitty caster wheels. I had picked them up since they’re $10 each, but I swear not even Harbor Freight wheels are this bad. While the sprocket seemed to have minimal runout (radial misalignment), the wobble from the poorly stamped wheel rims was incredible.

I literally had to take a dial indicator to the sprocket and hammer on the wheel rims to bend them around. I got most of the axial wobble out of the sprocket this way, but this meant it all ended up in the wheels themselves, which now are a bit “googly-eyed” as a result. It will look hilarious when running.

With all wheels mounted, the frame could finally support weight. It’s definitely lost the Chibikart look a little since it’s so far off the ground (in comparison…). I have an incredible 2.5″ of ground clearance now.

The brake pedal hookup was the exact same as for DPRC. This pedal design doesn’t have a spring return on the pedal side since it is handled by the built-in spring elements in the brakes themselves.

Which, as it turned out, weren’t quite strong enough, so the pedal felt quite mushy and also did not return all the way. I added a long compression spring on each side between the cable stops and the brake lever, and this made the pedal feel much more positive. The brake cables sit in barrel adjusters so the balance could be finely tuned.

Shifting to the back again, I’ve appended the Vex sprockets to the Vex transmission’s VHex output shafts. The Vex sprockets didn’t come with any set screws or other means of axial retention, so for a quick fix, I drilled and threaded three #10-32 screws 120 degrees apart. The three set screws will offer way more retaining power than just one. I decided to forego any other spacers and shaft end-tap screws for now.

 

Here’s a view of the shifter linkage. The mechanism is a spring-balanced cable setup where I provide the pull to shift into 2nd gear, and the spring pushes the shifter back into first.

This was simple enough, but I chose springs which were way too strong initially. I figured “10 pounds of force” at max deflection was enough, but that translated through the cable into the shift lever, times two, meant it was just too hard to throw!

I went to a hardware store and bought several sizes of springs in roughly the same length that were much ‘softer’. The replacement spring is about half the spring rate, and was also too long in that it could not compress enough. The solution to that was to really quickly dremel a few loops off the spring, just  like a good ricer. The shifter now has a positive click as the ball detents lock into place.

Once that affair was taken care of, I routed the chain and moved the gearbox up to tension it (the “goalposts” having slotted mounting holes for this reason). To lock the gearbox in place, I simply tightened the…

… Oh, I can’t reach those bottom socket screws.

Must have bought those hex headed screws for a reason! I was wondering briefly where they were supposed to go on this thing. With the hex heads accessible with a regular wrench, now I could actually tighten the drive up.

With both transmissions hooked up, I spent some time getting pushed around synchronizing the cables. I put another set of barrel adjusters on the shifter cables so they could be adjusted as needed.

What I (not surprisingly) discovered during this push testing is that the brake shimmy is pretty severe. This is caused by combination of factors, two of which include my “kinematically suboptimal” rotor retention method (two screws across a diameter) as well as the complete non-concentricity of the wheels. To reduce the severity of the effect, I had to dial the cables to different tensions. The braking is still effective, but it definitely feels like it’s trying to jerk all over the place.

Ultimately, I’m likely to ditch these drums and go to a disk brake setup with its own guide bearing on the front spindles to maintain concentricity. But for now…

…back to the electronics deck. Here’s the wiring mostly in place with batteries mounted. The batteries are my old 5Ah, 10S sticks. Two of them.

The batteries are secured by Velcro ties and sandwiched between two rigid plastic panels (the baseplate on one side, a 1/4″ thick polycarbonate strip on the other). A 1/8″ silicone rubber pad sits below each battery for shock absorption and more impact protection. Combined, this ought to ensure the batteries don’t move anywhere.

The ESC power leads directly into a 150A fuse junction, and ground has its own big brass distribution block also. Overall, this is the beefiest power system I’ve built since probably LOLrioKart.

At the point, the frame was flipped over for installation of the power electronics deck. The rest of the wiring, including connections to the motors and to the main switch, happened in-place after the installation.

The long run to the power switch is doubled-up 12 gauge wire in each direction.

The only other power side wiring was to make one motor extension cable. With main power wiring completed, I quickly hooked up a HV BEC to provide 5V and a servo tester to convert the foot pedal’s analog 1 to 4 volt output to servo pulses. These two components were heat shrunk and sealed, then attached with Velcro to the top of one of the battery pack plates. The signal electronics for this thing are extremely basic – no fancy signal processing is occurring.  One thing that could happen with this system in the future is converting to electronic shifting, such as with solenoids, upon which I think a system which cuts throttle before the shift and slowly brings it back in would be helpful.

After confirming the functionality of the ESCs and calibrating the controllers, the whole rig is put together.

Here is BurnoutChibi posed next to DPRC! The wheelbases for both vehicles are the same, but BC has a slightly wider track because of the pneumatic wheels. Otherwise, they handle alike and are mututally just as difficult to sit in.

testing

The first few test runs of BurnoutChibi were done indoors, in our Conveniently Circular Building hallway. Due to the extreme acceleration ability of the vehicle, I couldn’t really test it any faster than DPRC or original Chibikart, so we decided to not take video. More testing commenced in an underground garage, then our usual spiral parking garage haunting ground. Unfortunately, I really only got a minute or two of hard driving in before the left motor threw several magnets.

The high speed of the motor caused some serious sparking as the loose magnets scraped the stator and also cut up the motor leads. Unfortunately, the only video that was taken was not focused properly…

The accomplice vehicle is the (still unnamed) tricycle.

Since that test, I’ve reglued the magnets and repaired the wiring, and BC is currently operational. I am currently waiting for a day in Boston / Cambridge when all hell is not breaking loose (in fact, as I write this) to test in the garage again. These pictures and videos will be uploaded when they are taken.

BurnoutChibi’s Steering and Braking

Apr 06, 2013 in Chibikart, Project Build Reports

In the past week, I’ve been managing to intersperse bits of BurnoutChibi work between hosting extra hours for the 2.00gokart students as they edge ever more towards completion. On Wednesday, the “Milestone 7″ mechanical inspection occurred, where everyone had to demonstrate their rolling frames with steering and braking. The next steps for the students from here are focused entirely on assembling their electrical system. In fact, two teams have already blitzed their vehicles to completion, and more are surely to follow (parading them around during CPW is a huge motivator). I’m going to make a separate post about the progress of the class later – all I can say for right now is that this year’s competition is going to be awesome.

The first thing I had to do to build a new Chibikart is to disassemble the old Chibikart. Here’s the scene of the crime:

This work left me with a pile of redundant electricals – namely 4 more Jasontrollers and the massive A123 B456 battery. Needless to say, these will probably find their way into some other silly rideable thing.

The plan for BurnoutChibi’s electrical system is actually to use my left over 10S 5Ah lithium polymer packs, instead of making a custom pack or keeping the A123 pack. I decided to this mostly for the power and energy density of the lithium polymer packs (Chibikart 1 weighed 53 pounds because the big A123 bus battery module weighs almost 20!)  as well as the simple fact that said lipo packs have been sitting for almost 2 years, and I really don’t want to see them go to waste. The lipos themselves are from the erstwhile Deathcopter, so BurnoutChibi will surely be the health and well being hazard I envisioned it to be.

The first appendage to the old frame is the new style brake pedal. At this point, I haven’t even removed the old steering linkage yet, but I wanted to see if it would interfere with the new position of said linkage.

I started from the rear with fitting the Vex Ball-shitter transmissions onto the “goalpost” mounts. This whole ‘rebuild’ is essentially replacing Chibikart 1 frame plates with specially crafted DPRC ones. The only difference between this rear corner and DPRC’s is the goalposts!

I focused on getting the motors mounted and the rear end together. Here, I’ve mounted the NTM motors to my NTM-to-CIM converter plates. Eliminating units, the result of this evaluation is something which is basically like a CIM, but 4 times more power dense.

There’s only one problem. The NTM shafts need to have a 2mm keyway cut into them so I can easily used the keyed bore supplied with most FIRST OEM parts such as the Vex transmissions (The fact that I can say “FIRST OEM” is unsettling).

As it turned out, these shafts are casehardened. Wow, Hobbyking, you’re classy now – what this meant was I could not use my single HSS 2mm endmill to machine the slot. Instead, I went on eBay a few weeks ago and bought some 2mm solid carbide endmills. I recommend keeping a set of carbide cutters around for dealing with troublesome materials; the downside, of course, is that they are more brittle and need a stiffer machine setup.

I faced the slight issue of the endmill being too short and the Bridgeport spindle being too fat to reach the nether regions of the  motor. So I did what any self-professed machinist wouldn’t do, and chucked it up in a drill chuck. In my defense, I bought this integral-shank keyless chuck just to do dumb things like this.

I cut the keyway just a little short of actual dimensions because the NTM shafts were not long enough to use the included retaining ring with the gears. So I had to press the key in,and will need some creative gear pulling if I ever wanted to remove these gears.

And here they are mounted. I found the sheer number of hexagonal sockets on the gearcases a bit confusing at first, but now appreciate how versatile they can be.  Chain tension is adjustable using the slightly slotted mounting holes. I inserted locknuts (nylocks) into the opposite side hex sockets, so torque retention will be positive.

Notice how the seat mounts have been turned around. This was necessary because of how big the gearcases were. The seat mounting centers, and overall position, will remain unchanged.

Crawling up the side of the vehicle, I reached this build’s star attraction: The gear shifter. This came together amazingly well, and the feel of the ball detent plungers is extremely satisfying.

Heading up front, I popped out these new steering knuckles. In keeping with the tradition of doing the least possible work, these were specifically designed as drilling operations in a 1″ aluminum square barstock. The four flange holes will be where the drum brake mounts.

Continuing work on the front end, the drum brake mount has been attached and the new narrower steering…ears? are mounted. I’m not sure what to call them on Chibikart. They’re too short to be A-arms or wishbones.

Recall the new steering linkage arrangement – the crank arms are basically socket wrenches that fit over the hex head bolts. Motion is transmitted via giant set screw in the steering knuckle. To ensure positive engagement, I machined a deep flat into the hex head bolt shanks and picked flat-bottom set screws to maximize the contact area. To retain the crank arms, I center drilled a hole and threaded it for a retainment bolt. Otherwise, the crank arm is thinner than the bolt head and will be free to float about 1/16″ or so.

I moved on to chopping up the 90mm drum brake to fit up front. The mounting method I ended up devising would have been fine with keeping the giant torque arm, but the design would be cleaner without.

To maintain the cleanest possible lines, I brutally slashed the housing with a Dremel cutting wheel.

To attach the drum brake itself to its mount, I first had to machine the little round spacer which adapted the 14mm bore of the brake housing to my 1/2″ bolt wheel spindles. I sandwiched the brake housing between the mounting bracket and the spacer so it was reasonably centered. Next, it was a quick drill press job using the mounting bracket holes as a drill template. The steel housing on these brakes is just thick enough to hold a few threads of #10-32, so a socket cap screw was screwed directly into it through a standoff.

The mounting bracket itself involve one sheet metal bend to create a spot which will eventually anchor the brake cable. Well, I managed to bend it the wrong way the first time. Heating up the aluminum with a torch and carefully bending it back the other way worked, but the metal still cracked on one side. I had a buddy on MIT FSAE lay a quick TIG bead across it (see the irregular texture where the sheet metal arm bends left).

The brake drum mounting itself is what I’d call “kinematically suboptimal” very nicely. Basically I squished the slightly tapered stamping flat on a hydraulic press to get a flush mounting face on the bottom side. Then, two standoffs which each have a small shoulder that is precisely fitted to a mounting hole keep the drum attached to the wheel. On the top side, the standoffs have a 1/4″20 thread so I can use already available button head screws to retain the rotor. On the other side, the standoff is tapped M6 X 1 to interface with the original wheel lugs bolts.

The concentricity, needless to say, is less than stellar, but turned out way better than I had anticipated. I’m likely to replace this whole rig with a custom machined aluminum dish that has M6 x 1 holes tapped into it so I can just dismount the whole tire without causing loss of alignment. The brake does scrub, but only slightly and intermittently, and works very well otherwise. I have no doubt that this thing can lock up and skid.

And the front end is basically together.

Work now will move to the rear again with assembling the drive wheels and sprockets. I have an order of brake cables and associated parts coming, so I hope hooking up the whole drivetrain and shifter this week is a possibility.

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.