Giant Egg

Summary

This giant egg was built for La Plata and is to be installed in a community called "The Farm" located in Northern Colorado Springs. The community park is farm-themed, and the egg will provide a fun item for kids to play and interact with.

Modeling

The egg was modeled in Fusion 360 as a complete unit, hollowed out with a 4 inch wall thickness. The model was then split into 4 inch tall slices so it would be machinable on my large CNC. 

The majority of the slices needed to be machined on both sides to accommodate the inside geometry, so 3 indexing pins were located for each side of each slice. These indexing pins also provide alignment help during assembly, and possibly some minor strength gain (though industrial foam adhesive is pretty amazing, so it likely makes no difference).

Those pin locations were then all dropped down to a single plane to create the fixturing plate. This single plate contained holes for every single slice and its second side operation as well. A single origin located in the center of the fixture plate was used for every slice.

Gcode for all slices was created, and then they were all cut out in a blizzard of pink foam dust.

I also modeled and machined two aluminum mounting plates and HDPE caps to cover the bolt heads.

Assembly

As each slice was complete, it was glued to the stack using the alignment pins and holes with some weight on top. The glue-up was done in two parts just to keep things manageable and then the two sections were combined into a single egg. 

The outside was then sanded to remove the machining steps and the egg was cut in half along a cartoony jagged line using a long jigsaw blade (this went much better than I expected, jigsaws don't get enough credit.)

Then the inside of each half was sanded and the edges rounded with a large router bit on a trim router.

A pocket was cut for the mounting plates and holes were drilled for the bolts.

Fiberglass

Not a whole lot to say here, just a ton of epoxy, glass, part flipping, and patience. Using high flexibility weave glass was essential to getting the egg even remotely smooth.

After 3 layers of glass, the egg was sanded, voids were filed, the egg was sanded, voids were filled, the egg was sanded, voids were filled, the egg...

Paint

The egg was painted using Rustoleum Marine Topside Paint. Initially, I sprayed on the first couple of layers but wasn't liking the layer thickness and surface finish. Rolling on the paint turned out to be much better for this particular project. 

I suspect spraying on would work great for a large smooth surface like a boat or car. However, on something like this that still needed some pinhole filling and smoothing, the thicker layer from the roller worked better.

The outside color was mixed custom by eye and the speckles were applied manually, one by one, via my wife Julia's steady and patient hand.

Finishing Up

The aluminum mounting plates and plastic covers were hand fit to match the fiberglass pockets. They were matched to fit before fiberglass but keeping the glass exactly where it was expected to go proved to be too difficult. I sealed in the mounting plates using FlexSeal to give it a little cushion and damping.

All that remained was removing the blue tape separating the blue from the white and applying some minor touch-ups.

This was a challenging project, with a lot of problems solving at every step of the build. I'm pretty happy with the result and officially announce my retirement from making giant egg sculptures.