Melonscooter 2 Epic Post

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Much work has been done on Melonscooter 2 since the last week’s update. In fact, it’s so much that I’m wondering if I should split it up into two posts or not. Since that would be against the tradition of my style of build reports, here; have like 40 pictures. Melonscooter2 is in a state where I essentially just have to put together the electrical system – assembling the battery pack, mounting the controllers, and then wiring everything up.

Last time I left off with the new rear “stern deck” just having been cut from 12 gauge steel and origami’d into shape. It was time to hit up the welders once more.

First step is to atone for my sheet metal sins via clamps. I’ll admit my sheet metal technique isn’t perfect, and that the equipment (an old enormous 10 gauge capacity box and pan type) is pretty clapped out. So, to make the tabs fit into the slots, out come the bar clamps.

After tacking the corners, I removed the clamps and proceeded to scientifically draw metal slugs using a MIG welder.

Next was rigging up the deck on the scooter frame. I had to make absolutely sure it was welded on straight – there’s of course no turning back after a certain point. I again used almost all the clamps in the shop and adjusted things little by little, albeit still visually, for alignment. The rear fork plates are actually not parallel (which could arise from manufacturing or the thing having been crashed at least once in its past life), so I was mostly relying on the existing tube frame.

After initial adjustments and tacking, it was time for some more steel loogies. I welded entirely around the outside as well as on the two sides on the interior.

Next, I turned the frame over and removed the existing starter battery box. It isn’t anywhere big enough to house an actual traction battery, so I would have to devise a custom solution.

The finished stern deck with wheel mounted once again. I had intended on “filling” the fold slots with weld material, but determined that it was pretty unnecessary.

I have a roughly 2.5″ tall space to put a battery box in before the ground clearance becomes too low to be worthwhile. Melonscooter’s lowest point was about 1.25″ fully loaded, which I used as a benchmark. I spent a little while thinking of the approach I wanted to take – custom folded steel box, use an existing steel chassis or some sort, or Landbearshark style waterjet-cut polycarbonate box. Speaking of batteries, I haven’t introduced them yet, have I?

This is a veritable cluterfuck of A123 B456 what letters are they on now?! 32157 type automotive-grade cells. They’re 9Ah apiece and will be arranged 12S, for basically the same capacity as Melonscooter’s former 12S4P A123 pack made of the 26650 type cells. I can’t reuse the Melonscooter pack, which is just fine and functional, because of the height of the cells impacting ground clearance. The cells will be arranged 12 in a row, necessitating quite a battery box that will almost be in line with where the deck ends with the front. I’m planning on equipping this row with some 1010b-compatible cell taps, but I’m also awaiting a shady BMS shipment from my new favorite sketchy e-bike parts store, eLifeBike. We’ll see how that works out first – if I can embed that board inside, then I can even obviate old Melonscooter’s once-per-semester cell checkup.

I decided to build a waterjeted polycarbonate box over folding up a new steel box. Initially, I had even looked around in area military surplus stores to see if any old ammo cases would fit my application. Unfortunately, the only boxes I could find were for 7.62 NATO rounds, which were too long (i.e. tall in the orientation I would need to use the box in). Whipping up a sheet steel box was hampered by the lack of reasonable steel sheets on-hand – I could go out and get several 22 and 24 gauge sheets from the likes of Home Depot, but that’s pretty thin. I’d also need to make a lip with a removable lid. Making up a custom RP’d box was in fact the path of least resistance since I had a spare plate of 1/4″ tinted polycarbonate left over from some robot project that never happened (or perhaps did not happen hard enough). I elected to skip this session of sheet metal lab for now.

In my How To Kinda-ish Maybe Build Everything Instructable, I have a page on “making boxes” – which is something done often with laser cutters and their ilk. So here’s how I went about making this box. Regarding “edge precedence” in the chapter/page, it’s which side I wanted to be able to remove/install the quickest. You could imagine (and be correct in doing so) that I would want to remove the bottom for servicing the battery. However, I decided that having a material-on-material interface for the top and bottom would be the best, since this thing will most likely be shaken violently up-and-down when it attacks the “meh, it’s not a sinkhole yet” roads of the local area, and that the only thing holding in the batteries being a few #4 scres would not be the best scenario.

So I made the two endcaps the “highest precedence” components – to remove the battery itself, I’d have to at least take off one endcap. It would make it less serviceable, but if I am removing the battery that often, something’s very wrong.

What’s that notch in the corner? It’s to clear the kickstand. I could have made a totally rectangular box for easiness, but I knew that other wiring components – distribution, charging ports, switches, etc. will have to go in too, lest they be haphazardly arranged external to this. So one side is about 2″ longer to hug the kickstand and house these parts.

Getting more together now. When I make these kinds of boxes, I make the solid shapes first, then decide which things to tab into each other. The cell models are now in, and I’ve also made a cutout for the Hella battery switch.

Continuing the box design, basically all the corners are accounted for.

And the last two pieces are in. The round hole will be where I mount an actual charging jack, probably made of an embedded Deans or XT-60 connector like on the other vehicles, in a small printed carrier. On old Melonscooter, I had to disconnect the battery itself to charge it. This is just one little layer of refinement.

I made a prototype out of laser-cut plywood just to test for dimensional sanity. Result: Satisfaction. The battery box stops at where the deck stops up front, and while it isn’t nice and curvy, at least it doesn’t stick way out. I moved a few things such as the charger jack around so it wasn’t obstructed by the kickstand as much. Four little ears pop up from the top of the side rails and keep the battery box sitting flat with respect to the frame. I made it this way so it’s easier to fixture the future welded mounts.

I tried to take one shortcut in making the battery box – trying to laser cut it from PETG plastic.

This resulted in what must have been  the most dismal failure I’ve ever generated on a laser cutter. PETG is often advertised as “halfway between acrylic and polycarbonate” – unlike polycarb, it can be laser cut, but not as cleanly as acrylic. And it’s not as shattery as acrylic, but not as strong as polycarb. Well, it also melts, smells like death, and turns yellow halfway as shitacularly as polycarbonate, and takes far more energy to melt than acrylic. It’s not that it wouldn’t laser cut – it just laser-cuts like total unshaven ass. And I suppose instead of smelling like death, it smells like terminal cancer or diabetes.

It doesn’t help that the Epilog 36EXT has an almost-useless gas assist system – instead of, say, a cone over the lens that focuses pressured air into a single stream, it just has a derpy little bent steel tube that kind of puffs on the cut. So, it couldn’t really clear the melted PETG material from its own cut. If I went slow enough that it cut through the first pass, then the melted kerf becomes enormous.

I ended up having to hammer a few pieces out anyway, before just totally writing off this sheet. Luckily, it was a leftover of a previous class run in the IDC space that I’ve been hiding, so I didn’t actually have to spend money on this wreck.

PETG. Not even once. (At least, not without a laser that has a high pressure gas nozzle….)

The battery box waterjet-cut from tinted 1/4″ polycarbonate. That’s much better!

To mount the battery box, I cut up some random steel strap things which were made of 1/8″ thick, roughly 1.25″ wide steel. I literally do not know what these were – they were found in a scraps bin at MITERS.

This was the pilot application of the all-new cold saw I commissioned for the IDC fabrication space.

The steel mounts will each have a hole drilled into them to mate with the battery, and the rest will be welded to the frame. The battery box will help jig up the mounts so all I need to do is tighten them vertically and clamp the whole assembly to the frame.

Here’s the battery box mount prepared after drilling and finishing.

And it’ll go like so!

Back to the welding room for some very quick beads. I clamped the box such that the plates were in position in order to tack them once. Then, to prevent melting the battery box, I’d remove it and finish the welds.

Tacked in place and battery box removed..

…and a fat MIG slug deposited onto each side. That does it for the mounts – this is all they are.

On the same waterjet run that yielded the battery box, I also took the chance to cut out new a drive pulley for the rear wheel. The X-Treme scooter came stock with a weird 8mm pitch chain that nobody uses anywhere except on derpy scooters. Favoring HTD timing belt drive, it was clear that I was going to have to replace this.

The bigger wheel is a double-ended drill bit, a modern cousin to the double edged sword. On the one hand, it’s easier to achieve a higher gear ratio for the same motor speed since the output stage pulley/sprocket/gear can be larger and not hit the ground while turning, but the fact that the wheel itself is larger mostly negates this – your ground speed is theoretically unaffected. What the bigger output stage pulley allows me to do if I wanted to keep the same gear ratio is to use a larger motor-side drive element. This has the upside of lessening the tension in the belt and bending it far less (the curve of a larger pulley is more gradual), and lessening the load per tooth since there are more teeth in contact in the angle of wrap.

Small pulleys and sprockets wear out their belts and chains much quicker because of the increased material flex and decreased tooth contact. Melonscooter was known for going through a drive belt every few months just from them becoming tattered and separating from their rubber backings and breaking the tension elements – these weren’t cheap unbranded belts we were talking about either, it was Gates belts straight from McMaster. I was using a 15 tooth pulley on the motor to transmit north of 1500W most of the time.

With this wheel, I should be able to retain my top speed while using a much larger 22 tooth motor pulley which will have nearly double the number of teeth in contact. I hope to get more belt life this time.  The motor side pulley will be a stock one I have sitting around from playing with gear ratios in the past.

The wheel side pulley, though, is something a little weirder. Notice that it’s made of chunks of pulleys. To save material, I split the outer profile of the pulley into 120 degree arcs, fastened to each other by a ridiculous number of bolts. This was a technique I tried out first last year on a Silly Media Lab Vehicle, and it worked very well. The nice thing about this method is you can quickly build up thick dished pulleys and other elements with rings, without going through 8 plates of metal and generating lots of thermally conductive round pot and dish coasters at the end (hence ruining the point of a coaster).

The critical part of doing this right is overlapping the segments on each successive layer such that there’s not a “parting line”, which would occur if the segments were just stacked one on top of another. That’s why there’s a million bolts around the edge – so I can shift each layer like 60 degrees. In the end, when everything is tight, all the material overlap will approximate a solid pulley to a degree more than what I set out to accomplish.

Here’s the pulley installed on the former sprocket perch.

I actually generated this pulley profile with a custom template part that I made in Autodesk Inventor because of two major reasons: first, nobody seems to make a CAD program that comes with a HTD belt generator. I can’ tell if it’s because HTD is a private brand or what (I, at least, use it to refer to every pulley that has rounded tooth profiles). Inventor has a “T” metric belt line which seems to be an ancient metric trapezoidal tooth profile. I even tried Solidworks (Oh boy, using the CAD program I’m supposed to teach to people…) and it, too, has tons of English belts but only metric T belts.

And second, nobody seems to commercially make a HTD pulley this big. The largest downloadable ones I found were about 70 teeth. My pulley is 108 teeth…

So I grabbed an image of the HTD belt cog profile and made a parametric part in Inventor. With a bit of nozzle offset magic, the belt wraps with no problems all the way around.

What’s left after welding everything that needs to be welded and making the wheel driveable again? Painting!

I’ve never been a big painter or finisher, but if I left the bare metal and welds untreated, sooner or later I’m just going to be riding a small hill of oxidation again. To paint over everything, I cleaned up all the surfaces and used a self-etching primer first on the bare spots. Unaffected paint spots near the welds were hit with a fine grit sandpaper befoerhand to encourage sticking.

Next up were a few coats of black acrylic enamel.

And finally a clearcoat. I have yet to master the art of spraypainting without the “orange peel effect” – a finely textured surface resulting from uneven spray thickness, droplet sizes, time-between-coats, etc. In this application, I don’t really care, but I would of course prefer not to generate it on the van.

It’s bad karma to paint indoors, so I did it the best way possible – right next to the 300CFM laser cutter ventilation fan. Outside that day was approaching 80% humidity – I felt like almost drowning just walking around outside, and my paint would have stayed wet for the next 3 years. The ventilator kept the funky smell from spreading to any other room.

After the paint fully cures, it’s time to start putting things together. First, I still need to assemble the battery pack itself, BMS or otherwise – given that I might not receive the BMS shipment for another week, I might just pitch together some JST connectors for my balance charger. Next, the slated controller, a KBS48121, needs to be mounted. Yes, this does entail putting sensors on the old Melonscooter C8085 melon motor (which I have since re-bearinged, so it should stop sounding like a sandblaster while running!)

 

DERPDrive: Structural Fabrication

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Continuing on the DERPDrive after a quick melon break, here’s what all happened to get DERPDrive to an almost ready-to-install (mechanical) state. Bear in mind that at this point, the thing’s been sitting on a handtruck for a week and a half, waiting for the weather to stop being incredibly humid and spontaneously rainy so I can go outside and sandblast and paint the whole thing. I got a little wimpy sandblasting gun from Harbor Freight the other day, so I can move to finishing it (and test fitting!) as soon as the weather window opens up.

Last time, the pile of parts was reaching critical mass, just waiting for a day when I can hide in the shop to put it all together. It coincided well with the welding work on Melonscooter2, so there will be an update on that soon too.

Step 1 was to section the large tubing sections into the proper lengths. To do that, I meandered down to the FSAE & Solar Car & Mexican Grill shop and used the 10″ coldsaw. This saw is on-and-off maintaned, and luckily it’s currently in an “on” period where the blade actually has teeth. Get a load of the color of that coolant! Machine coolant, especially the new vegetable-based biodegradable stuff, actually spoils pretty fast if left unused and unchilled. I was told it was changed “like a few months ago, I think”.

Whatever, it was still oily and didn’t smell like the local greasy Thai food place, so it ought to do something.

 

Tubing and rod stock sectioned to length and ready for the next step, drilling.

I designed this assembly to be thrown together quickly from square tubing with holes drilled in it, so there’s no fancy fishmouthing or angled round tubemancing here. Fine positioning was accomplished on the venerable MITERS Bridgeport.

I bought the two sizes of hole saw I’d need to cut the larger holes. These Home Depot class hole saws are really designed for wood only, and these few holes completely destroyed them. That “Bimetal” must be “horseshit” and “pot castings”.

Drilled, sanded, and deburred. There’s only one thing left to do…

Time to join metal. This post should really be entitled “How to work in 4 shops at once”, because that’s what happened. No one space I was working in had the right combination of everything to do all the jobs needed. Up in the IDC, I really have no heavy equipment at all, but a universe of hand tools and a laser cutter, so I can do the assembly work. In MITERS, there’s everything but welding and sheet metal equipment, and the hand tools are in ass condition. And finally in the FSAE/Solar Car/Pastries shop, there’s welding, big machines, and sheet metal tools, but everything’s just barely maintained and there are no welding jigging and setup tools anywhere.

That’s one thing which buggers me about MIT shopdom in general – everyone would rather have their own spheres of influence and fiefdoms than one well-manned, well-equipped place.

Anyways, here I am invading the D-Lab where they have a very high end welding setup with actual clamps and whatnot, for rigging creations using very high end third-world bicycle frames.

I began with the TIG to join the swingarm sections together. This went well enough – I would actually show my product in public in front of people who, like, know how to weld. But there was one thing which kept me from finishing the job with TIG – it wasn’t fast and dirty enough. Yeah, sure, TIG can let me weld an aluminum can onto a fairy-sized airliner…

…but for something like this where I’m beasting into thick walled steel tubes with no real need for pretty or even incredibly strength, the ability to draw a huge loogie of metal in 10 seconds and be done with it was far more appealing. The MIG welder in the space was much, much larger than the little dinky one that was in MITERS, and the feel was a world of difference. This translated to some very nice looking loogies.

Above is my setup to put the frame tubes together after having finished the swingarm. I used almost all the available clamps for maximum rigidity in trying to prevent warping. Overall, everything came out pretty square.

Next up was attaching the motor mounting plate to the swingarm. This was once again a dance of clamps, using the trunion tube and the folded flanges of the 12 gauge sheet (the same sheet that Melonscooter’s bits came from!) as fixturing spacers.

Here’s a mockup of the assembly after the major welds were done.

During this mockup, I discovered that I welded on the back rail completely backwards. Like, utterly backwards. Both upside-down *and* facing the wrong way. Phenomenal.

A trip back to the mill to grind through the remains of my 3/4″ hole saw, which by this point was cutting more like .800″ polygons of constant width, solved this.

With the frame done, it was time to finish the things which attached to it. To make the leadscrew nut trunion assembly, I took the 3/4″ Acme hex nut from Surplus Center and machined it down to 1 1/8″ OD most of the way, then stuffed it into the hole.

The nut was then welded in place. This joint is of questionable metallurgy, since the nuts are made of 12L14 steel. 12L14 is well known in machinist circles for parts that need to 1. sink and 2. be magnetic – it’s not very strong, and the (very trace) lead content technically makes it impossible to weld because it forms big globules and makes the weld porous. However, opinions seem to differ – some say it can be welded just fine if the material is preheated (which I did with a propane torch for the additional reason of the section thicknesses being very different), others say it cracks and destroys itself immediately.

It seemed to go down just fine with preheating. I wouldn’t, say, put it in space or something, but no matter how starship-like Mikuvan looks, it should, unless the circumstances were most unusual, stay firmly planted to the ground.

To attach the endcaps, which are 1/4″ waterjet-cut donuts, I just MIG welded a huge bead around the perimeter…

…and finish-machined it on the 19″ LeBlond, the only machine with a chuck big enough to swallow the protruding Acme nut.

With the trunions complete, I next turned to the jack, the floating half of the frame which would be pushing against the van ladder frame.

This thing is made of a few chunks of threaded rods and 2 standoffs, which I machined in the same session as the trunion endcaps. The standoffs shown are actually made from chunks of leftover 3/4″ shafting from the same order. They serve to align the jack in the stationary frame. The long threaded rods to either side are what will be providing the force.

The other part of the jack is made from some plain steel tubes that the threaded rods insert into. Aligning this whole setup for welding was therefore simple: put it together like it’s supposed to go, then weld it. The base of the tubing was welded from both the outside and inside of the frame, since by welding the back rail incorrectly the first time and being forced to redrill, I’ve opened up a way to get at it from the other side. Strength and concentration-of-stresswise, this is probably for the better.

Here’s the entire frame completed.

Moving on, the last link in the system – literally, since the frame is one and the swingarm another – is the leadscrew. I needed to put a hex or other drivable shape on the end of the leadscrew so I can crank on it with a power drill or ratchet to raise and lower the assembly (automatic electronic raise and lower would have been funny, but overboard and unnecessary). To start, I machined the leadscrew down to something which was fully round.

Other machined parts include that chunk of 3/4″ steel hex which will be the driving end, and the preload spring retainer on the left, made from a leftover chunk of 1.25″ shafting.

I began by welding the hex onto the end of the leadscrew. For this precision operation, I went back to TIG.

Next, I threw this on a drill press and drilled a few shallow radial holes. Then the holes were filled with plug welds to fuse the material together in those spots like inserted pins would do the same.

The excess weld plug was ground off and the end of the screw machined for prettyiness and consistency. I might have overdone it on the plug welding a little, judging by the deformed hex, but it still fits a deep 3/4″ socket easily.

Here is the finished leadscrew assembly. The J shaped piece is responsible for lifting the assembly back up. In case it’s still hard to see, imagine the tube fixed and the leadscrew being slowly pulled back away from the camera. The spring would compress and cause the hook of the J piece to move along with the leadscrew. This compression is what forces the 5th wheel into the ground to give it traction.

To lift the assembly back up, the leadscrew is cranked back towards the camera, the spring relaxes, and then the force is transmitted into the J piece which now hooks the tube from behind. Because the swingarm is only going to weigh about 75 pounds, the return mechanism doesn’t have to be as hardcore.

The J was made first by bending in discrete “facets” on the big sheet metal brake, then heating it up with a torch and beating it over the tube until it was a little rounder. Recalling the CAD model, it has a big slot where a round hole to pass the screw would otherwise be, since “beat on with hammer” is not considered a precision operation by me at this time (but wait until I start doing bodywork…)

The observant will notice the tiny thrust bearings (by tiny I mean 3/4″ bore) which provide for free movement of the leadscrew relatively to The J while still transmitting force into it. The whole sandwich is retained by a giant E-clip, which can’t be seen from this angle.

Next chapter: Sanding and painting this thing in a fashion which would reflect what I need to do to properly repair the body rust after patching it. That’s why I’m even taking steps at all to make this thing not a rust ball on its own – I figure if one little chunk of the project would help me practice for others, so much the better.  The same sort of thing has to happen on Melonscooter’s frame too.