Styrofoam Shaping Fixture

I built this fixture because I needed to develop a way to shape a big block of Styrofoam when I built The Happy Birthday Party Napkin Rocket of the Apocalypse. Most of the the photos here are from that project, but I've utilized this concept in the construction of various nose cones and transitions.

This page will explain how to make a fixture that will enable you to accurately sculpt a very complex symmetrical shape. Rocket builders can use this fixture for building airframes, nose cones, boat tails, and transitions. Or, if you prefer, a 200x upscale of Cindy Crawford's mole.

First, here's a quick refresher for everyone who slept through Geometry class in 10th grade: The illustration at left shows that a circular shape can be created by drawing a series of intersecting lines, each line being tangent to the circle defined inside the figure. This figure could be drawn either by repositioning the straightedge or by keeping the straightedge in a fixed position rotating the page around the center point. But just imagine that the page is really a sheet of Styrofoam, and each line is a piece of hot wire (from the Hot Wire Foam Cutter) -- this illustration then represents a horizontal cross section of the carved airframe.

The shaping fixture allows you to convert this two-dimensional drawing into a three-dimensional sculpture. As shown in the line-circle illustration, the symmetry is achieved by rotating the Styrofoam around a central axis. The profile is sculpted using templates cut to match the silhouette of the airframe.

The idea for this fixture didn't really originate with me. I stole the idea from the kitchen of a Lebanese deli. One day I was standing in line waiting for my gyro and watching the server slice off chunks of Shawarma as she rotated the large loaf of meat on a vertical spit. The Styrofoam shaping fixture works a lot like that, but with more precision and less garlic.

The fixture described below would fit a sculpted piece up to 34 inches long and 16 inches in diameter: Yours will probably be different, so add or subtract to the length and width accordingly.

Parts List

• 1/8-inch Masonite
• Two (2) 38-inch lengths of 2x4
• Four (4) 20-inch lengths of 2x4
• One (1) 48-inch x 1/2-inch diameter steel rod
• Construction screws
• Metal corner braces

Fixture Assembly
Draw a line on your Masonite 2-inches from and parallel to one edge. This line will align with the centerline of your airframe. Draw one-half of the profile of your airframe onto the Masonite using the centerline as a guide. Cut out this piece and use it to trace a second piece just like it. Stack them and sand the edges smooth and even.

Select two of the 20-inch long pieces of 2x4 and drill holes in the exact center of each for the 1/2-inch steel rod to pass through. Add the 38-inch pieces to make a square frame and reinforce the corners with the metal braces. These corner braces are available in hardware stores -- they add a lot to the rigidity of the frame. Complete the frame by adding the two other 20-inch 2x4 pieces perpendicular to the bottom of the frame so you will have a way to make it stand upright. Mount the Masonite templates to the inside of the frame, carefully aligning the centerlines with the holes you drilled for the steel rod.

Foam
The composition of the piece you will be carving will be different depending on the type of project, the shape of the object, the availability of different types of foam, etc. I'll try to describe what I've used, and you can adapt it for your own purposes.

I typically buy a sheet of white 2-inch Styrofoam insulation and cut it into squares. For my larger projects I've cut the squares smaller for the top and bottom of the stack in order to conserve on material -- you usually only need a few of the full-size squares. For making airframes or boat tails, it is necessary to run a tube through the center of the foam stack. A hole saw works well for cutting a hole through the center of the foam -- just find one that fits the tube you will be using. I've used a hole saw on foam for tubes with an outside diameter as large as 4 inches. (I've actually cut center holes for tubes much larger -- but I'll save the description of that technique for another article.)

Inside the center tube you will need a pair of centering rings. The inside diameter of the ring will be 1/2-inch, just wide enough for the steel rod to pass through. To install the foam stack onto the fixture, align the holes on the centering rings with the hole on the fixture frame and feed the rod down through the top.

Carving
This is the fun part -- and it is so easy that my kids can do it. Fire up your hot wire cutter and drag it down along the sides of your templates, keeping the wire in light contact with the Masonite (this is why sanding the template edges is important). Rotate the block after each pass and repeat until you have eliminated all the ridges. You will eventually find that the wire cutter is no longer removing foam, so switch to 100-grit sandpaper. The final shape, (at this point about 95% complete) can be moved to the motorized Airframe Tube Fiber Glassing Rotisserie Stand for final sanding, smoothing, and fiberglassing.

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