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.