Melonscooter 2 Epic Post

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!)


6 thoughts on “Melonscooter 2 Epic Post”

  1. On painting: the #1 requirement for a good looking paint job is lots of patience. Use very thin layers and wait a day between coats.

    The #2 requirement is the right paint. Try using “Automotive Lacquer” instead of the cheaper spray enamel.

    On cutting PETG: I can vouch for the fact that higher wattage and a strong air assist will give you a decent cut. One pass through material up to 0.25″ thick. The edges will still turn brown though.

    Also, PETG doesn’t tend to catch on fire like acetal (Delrin) does in the laser, so it’s got that going for it.

  2. My vote is on thinner layers too. If it flows, you’re putting on too much at once.

  3. I’m liking this, back to your roots. Also, delightfully simple and straightforward.

  4. Another major factor to effect your paint outcome is the relative humidity. High humidity (even that outdoors) can cause strange effects in your paint, including cloudiness. And don’t even consider painting while it’s raining, unless you have a controlled environment like a painting booth.

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