Chibi-Mikuvan: Adventures in Foam-Core Sandwich Composites

Sunday, April 13th, 2014 @ 3:02 | Chibi-mikuvan

I would never make anything out of composites. There’s too much making of the part that makes the part that makes the part. I prefer to just make the part.

- Charles some time in 2011, probably not on this website but at least once in person with someone

Okay, okay, you win, 2014-charles. If this exact time last year you’d told me I would be a composites nerd today, I’d have left the room in a huff.

However, you could also have the said the same thing about becoming a van mechanic, or becoming a shady back-alley auto body man.

Welcome to Big Chuck’s Mobile Garage & Auto Body!

Seriously, there’s enough equipment back there to do everything short of rebuilding the engine on the side of the road, and that’s only for lack of parts.

Hell, if you told 2011-charles he’d end up becoming the very thing he loathed the most back then: a machine shop instructor…

Anyways, last time I left off, the two halves of the foam core were drying independently. The reason I had to split the core into two halves was because I mistakenly welded the axles 1 inch too far apart, necessitating a 1″ sliver inserted into the middle to make up the difference.

I used the hot wire cutter that I built in MITERS back in 2011 in anticipation of making Chuckranoplan 0005 (do not tell me that I’ll become a combination naval architect and aerospace engineer next year) and a MDF template to slice some 1″ thick foam up into the cross section of the sides. By this point, I’d gotten my own shipment of microballoons and epoxy, so I applied this patch the “correct” way with microballoon putty. This stuff beats the colloidal silica slime I used before, because it’s actually sandable – fairly easily. I used the heavier duty stuff because a bucket of it was easily accessible, but I see why it’s only used for very high strength areas, edges, etc.

I did a practice piece in the back first where the geometry was easiest.

I generally followed the procedures outlined in The Burt Rutan Book, since I had gotten a copy of it a while ago. This book was what ultimately threw me into this direction – my gripes with composite materials originated from witnessing people spend tons of time making molds and plugs just to produce a single part. The foam method seemed like a way to get to the end quickly, and if it’s good enough to, like, get in and fly yourself it surely must be good enough for this!

For this practice piece, I took no shortcuts and went ‘by the book’ as much as I could. That wouldn’t last long.

I tackled one of the broad sides next. To keep things as uniform as possible, I kept the outside faces as one large sheet of fabric. I laid an oversized rough-cut sheet on the surface, anchored it in a few places with some epoxy blobs, then trimmed to rough shape.

The front was done in the same fashion.

One thing I had trouble with consistently while “Rutaning it” was controlling how the fabric stretched and deformed. The first few square inches of contact with wet epoxy essentially determine how the rest of it will go on, and I found that I couldn’t really push one area in the direction I needed afterwards without having to pull from across on the other side or slowly work a wrinkle back towards the center and out the other side. This method clearly requires patience and methodicalness to get right.

The ideal shape of the rear inside would have been having the black edges roughly parallel to the sides. The edges seen are the ends of a long strip that I wrapped around the backside – the exterior is continuous, the inside is messy and ragged.

…so I said goodbye to the Way of Rutan, realized once and for all that this is in fact not going into space or even in the air slightly, and began slathering. In fact, if it ends up in the air even a little bit, we can assume something went terribly wrong.

Now I was on the interior, so I could reuse the scraps and cutoffs from the other faces. My tactic became laying the fabric out dry so I could have control over its shape, dabbing a few spots to anchor it, and then begin pouring on the epoxy-microballoon slurry, working it in like I would do to non-filled epoxy.

This inside right face is made from about 7 or 8 pieces, most of which were spent on the little concave wheel cutout in the front end. For the other side, I smartened up a lil’bit and cut a semicircle shape out of the cloth stock.

The process went by much, much faster after this. Here, the whole body has been covered in at least 1 layer of fiberglass. I decided to stop here, instead of trying to add layer 2 everywhere. I reinforced the corners and front outside edges where it will most likely bump into things with another layer only in those areas.

This whole adventure actually occurred through several non-consecutive days, so in between sessions, the layer of glass cured fully. To ensure good bonding at the intersection of cured and fresh, I returned to the recommended Rutanistani procedure of roughing up the zone of overlap with 220 grit sandpaper.

After a day of settling, it was time to start on the smoothing and sanding.

To finish the inside curves and radius portions of the wheel cutouts, I bought one of these…. fuzzy abrasive foamy wheel things (poly-abrasize wheel) that attach to a drill. This allowed me to grind off the edges of the cloth and make the area generally smoother.

I didn’t spend too much time on this portion since nobody’s going to be staring at it, but wanted to get rid of the stuff that was sticking up.

I did a first round of “orbital sander low-pass filtering” reveal the low spots and knock down some of the more prominent high spots where the glass fabric stops. The plan is to just fill every location the sander didn’t hit with some Bondo and call it a day.

One thing I wanted to take care of before reducing this problem to a known solved one (“Van bodywork”) was to make the mounting facilities for the silly Japanese style emoticons. As you might be aware, the whole existence of this project hinges on these silly emoticons.

I decided to try and make them out of ‘refrigerator magnet stock’ and have them be rearrangeable on a thin steel surface. Flexible fridge magnets are made of iron oxide powder mixed into a rubber backing. The idea is that the laser cutter can melt through the rubber and leave me with an arbitrary magnetic shape (even though the heat might demagnetize a small portion of the whole thing).

The steel stock shown here is common galvanized steel roofing flashing, for weatherproofing household rooftop HVAC implements. It forms and cuts very easily and is extremely, extremely sharp. I used to use aluminum flashing extensively in 1 and 3lb battlebots back in the day [when such robots could win anything].

Using a pipe and gentle hand pressure (Like, seriously, this stuff is sharp) I formed the flashing into the curve of the lower front side. I made it just a little tighter of a curve such that clamping the flashing down to the surface will cause it to flatten out fully and conform to the curve.

Here’s the extremely ad-hoc clamping configuration for the upper portion. The lower portion was being adhered to what was basically a smooth and flat surface, but I didn’t have a good way to clamp it.

I therefore decided to make the first attachment using E6000 contact cement (c.f. Goop and other ‘construction adhesive’ type products; the shit that reeks of brain damage). I painted a thin layer onto the metal, then slapped it into position and quickly peeled it back off. This created a layer of the cement on both fastening surfaces. I then came back 5 minutes later and slapped it on once more.

These contact cements count on the evaporation of the solvent to create a super tacky, microscopically porous surface that then instantly fuses to itself. Using it between two nonpermeous surfaces, such as the cured fiberglass and steel, means the solvent has to dry out substantially first, or it’ll never set (Guess who tried making a composite E6000 and aluminum flashing laminated frame for some of his first bots, and took apart the frame after the bot was destroyed and found that the center of the sheet never cured!)

Time for Stage 1 of Bondo. Shown here is the right side, which was a bit of a disaster because I made a cutting mistake that left the outer surface slightly concave, and tried to make it up during glass application with dry microballoon putty, but made it too wet so it just flowed and blobbed everywhere. D’oh. The biggest splotches of filler are therefore on this side. Otherwise, the only other major use location is where the steel flashing is bonded, since there’s some thickness to be made up.

After letting this cure for a few hours, I did another “low pass” sanding run. Upon the completion of this round, only small low spots and divots remained, which I stuffed a little bit more Bondo into. While it was curing, I turned my attention to the fridge magnet stock:


This stuff machines like thermite. Like, actually. I was fearing for the life of my lens, but luckily the air assist kept the flying molten iron balls away from anything important. Because the material is iron powder mixed into rubber, it sort of explodes when cut – the laser vaporized the rubber, and the high temperatures melt the iron powder into little droplets:

This is the end result. Not the cleanest cut, of course, but all the little balls fall out when you pop the piece out. I found through making a handful of these test pieces that there is indeed a demagnetized heat-affected zone close to the cut, so the less heat put into the part the better.

I therefore left the laser on a setting where it would penetrate maybe 75% through the thickness, then flexed the rest of the material out.

So with this operation successful…

I can stop now, right? This is all I came to see.

I knocked out the last few filler smears (one can be observed on the front left corner) with primarily manual sanding. The most versatile and flexible sanding block is still you holding a wad of sandpaper. Power sanding at this stage would have been too risky because I might get too zealous in one spot and go right through the Bondo and fiberglass. Nope. That didn’t happen at all. I swear.

After I was satisfied with the outward appearance of the body, I moved onto bonding the body mounting brackets. These were 3D printed over one of the nights when everything was being left to cure. They’re fairly thick and bulky brackets which hopefully will have enough attachment area to handle some bumps and impacts. Now it’s time to let this fully cure and come back in the morning to verify fit.

Once this is done, I’ll move onto priming and paintwork.

In the mean time, let’s return to mechanicals:

Check out that handlebar. I obtained this for free from Cambridge Bicycle after showing them what the hell I was building (MITERS and Cambridge Bikes go way back, too, which helps). However, as awesome as it is, I might have to ditch it for a smaller handlebar because it’s too huge. I do like having the ability to attach standard bike/scooter handle accessories like brake handles and throttles, so if I do let this go, it’ll only be for a custom-made one.

I’ll have to do a full ergonomics test once the shell and myself are both installed in their final positions.

The steering linkage is hooked up with 5/16-24 threaded rod. Unfortunately, one of them ended up having to go through the frame – I had to cut a 1/2″ deep notch in one part of the frame. I chose to do this rather than lower the center steering link further because it’s already pretty low – it would become the first thing to hit something I go over (ahead of the battery pack, anyhow…).

I intend to install more mechanical parts while paint dries in the near future. On deck to be completed in this realm include the following tasks:

  • Machining the 12mm brushless motor adaptor shaft for the gearbox
  • Machining the sprocket to fit the angle grinder gearbox output shaft
  • Slightly modifying the gearbox mount because the angle grinder box is a little bigger than my rough model on one end
  • Hooking up the brakes to the handlebar

After this, I’ll need to turn back to electrical system work, starting with remaking the battery pack endcaps – I have not observed any “95% scale” weirdness with this shell, so I can only assume that it was a fluke, or someone changed the setting back between my machining periods a few days in between?!




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    3 Responses to “Chibi-Mikuvan: Adventures in Foam-Core Sandwich Composites”

    1. FrankTheCat Says:

      Jesus, and I thought carrying around TWO milk crates with oil, coolant, duct/silicone tape, RTV silicone, a screwdriver set, basic metric ratchet and wrench set, some visegrips, a bottle of tire goop and an air compressor was excessive.

    2. the chuxxor Says:

      The square rear hatch means 3 x 3 milkcrates is totally reasonable, but it would intrude on headroom for anyone sitting in the back. Best part is this leaves the passenger area totally clear and usable!

      There is a hydraulic floorjack tucked under the rear seat, too. Two of the crates have all my ‘body shop’ equipment in them – sander, grinder, abrasives, bondo, spare paint and primer, everything. I can literally pull over and do bodywork if I get bored on a trip or something.

    3. the chuxxor Says:

      Everything is in the back partially out of necessity (no real place to dump all of it) and for winter – the added Ass Mass (that’s a technical term) helps traction in the wet/snow.