Shaping Battery Box

Hey, working on my efoil build recently. I have a cnc board on the way but I was hoping for some ideas about shaping the hole in the box for the battery/esc. I have dremels to do the outline, think a belt sander will work for bulk material removal in the recess? Any other ideas? I don’t want to use a router as that will be a pain I think and any bits long enough to reach the bottom will cost an arm and a leg. Other than that, a hotwire setup seems like alot of extra equipment. Basically has anyone used a belt sander to do it?
Thanks in advance.

A belt sander will cause a lot of dust. Hot wire is the cleanest and cheap to build, use nichrome wire and an old notebook psu:
Nichrome Wire Dia 0.5mm Cr20Ni80 Heating Wire 24 Gauge AWG Roll 5.551Ohm/m Resistance Wire for Heating Elements
With hot wire you need to create a through hole, cut the botom of the box and glue it in again.
If you use a router, you can use an 8mm drill bit instead of an expensive long router bit, works good enough for foam. Many people built the cavity like that.

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If i was to do it again id make a mould for the battery hatch area (3d print or mdf) then carbonfiber it, demould and glue the demoulded part into the blank that way the dimensions etc will be perfect and you wont have to fuckaround trying to glass wierd angels. You can then cut the hole in the blank with a haksaw blade roughly and fill the gaps between the moulded part and the blank with pour foam then glass the whole blank.


Thanks for the idea for a 8mm drill bit, Thats the route Im gonna be going. Not much work for the bit anyways. Maybe a hotwire next time.

Yeah that seems to be a common theme. I think I will do a version of that where i make a box, lay fiber in the hole and set the wood box in over it to help push the fiber into corners. With a demolding spray.

Cutting box holes can be done in a lot of ways, I like to cut with a hotwire, if you haven’t got one then a soldering iron works (just exchange the soldering tip with a nail or metal pin) or even a torch and any old knife. Cutting like this is less messy than routing it all out with a bit.

I’ve also successfully used a hacksaw, cutting in a 2-3cm crisscross pattern and breaking off the small towers that are created.

Bottom parts and walls are then cleaned up / shaped with a sander regardless of the method to remove the meat of the material.

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Alright, gonna follow up on this for anyone else who needs solutions.

I ended up doing a mix of methods, I carefully drew out where the cutout needed to be, and then I used a router with a very very cheap endmill. It wasn’t long enough to cut all the way through so I ended up routing the edges out with it, to about 4" deep. Then I took a hacksaw and cut through the bottom of the foam fully. After smoothing out those edges with a belt sander and lots of patience I was able to get the hatch to fit. I bought some 1/2" birch plywood (it’s pretty light and very stiff). Cut it out and glued it down to the foam with about 2" overlap on all the edges over the cutout. I used cabosil and milled glass fiber to make a thick paste… After sanding the edges down flush with the foam for efficiency in the water, I just glassed over it like normal. A little heavy but Incredibly stiff and strong. — Was too distracted to take pics, Ill attach a few later to hopefully show how it went together.

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