Winter has become spring, so it’s time for an update on the carbon fiber hot tub side-project.  You see, it only looks like there’s been no progress, whereas in fact I’ve made many an attempt to cover the wood mold stays with an appropriate material that makes nice smooth curves that conform to the model’s compound shape.  I tried lots of experiments with foam and metal lath before ending up with slats of fiberboard tacked on with a pneumatic nailer.

Hot tub mold with fiberboard slats

As you can see it’s not quite done but well on its way.  The next step is to cover it with a peel ply mold release then fiberglass.  The peel ply prevents the fiberglass and epoxy from sticking to the fiberboard or wood.  Now the trick is how can I use Rhino, the 3D CAD modeling program to come up with an easy to cut fabric pattern that will properly fit the compound shape.  And as usual, Erik Walker had the answer.

I’ll break down the steps he showed me.  I figure I can manage a piece of fabric about two feet wide, so with an 8′ diameter bowl that makes about 12 pieces.  So first create 12 radials on the hot tub surface using a Polar Array with 12 pieces swept through 360.

Next two neighboring radials are projected on to the hot tub surface.  Then, before being able to unroll the projection, you have to create a new surface.  The trick is to keep it planar, so stay away from Curve Network and use Surface from Edge Curves.  Then Unroll Developable Surface to get the pattern that will be cut out.

In order to create some overlap, I selected the border curves then used Extend Curves to add an inch on all sides (two inch overlap between pieces).  Finally, add a chamfer to round the corners.  The final shape is exported as a DXF.  Since I’m not good at drawing patterns, I’ll use a CNC vinyl cutter then trace that on the fiberglass.

If you’ve followed any of our hot tub antics in the past, then you already know about the inflatable hot tub party on Ocean Beach in San Francisco. A few years ago we built a heat exchanger that would use a bonfire to heat up salty ocean water.

This winter I’m taking on a new project, following our work on Carbon Garden: to build a hot tub entirely out of carbon fiber.  Since fiberglass hot tubs are common, the same techniques should apply to building the carbon fiber tub.  Step one is to come up with a pleasing design, and for this I’m using Rhino, a CAD software tool.

The 3D shape is sliced into multiple flat pieces along the black lines, which are then cut out of 1/2″ plywood on a shopbot CNC router.  I’ve decided it will be easier to build the positive shape, so the mold will be more  of a hump than a dish.

Here’s a video of the shopbot going at it too

Next the mold stays are assembled by cutting correct length support pieces (according to the CAD model) and screwing the whole thing together. Here’s an image of the mold partially assembled.  In this shot the mold is upside down since it’s easier to put together that way.  Eventually it gets flipped over and it will look more like a hump.

Once the mold stays are assembled, I’ll lay sheets of paperboard across it to complete the bowl shape, then like drywall, apply plaster and sand it perfectly smooth (don’t want any bumps on the interior of the hot tub of course).  When the mold is complete it will be time to lay up the carbon fiber.  But that’s for another rainy winter day…