A fascinating but simple toy for kids and adults that uses an item from an unexpected source: military surplus.
Tools: Bandsaw, disc or edge sander, square
Time: A few hours
Materials: Prism, wood stock, ½" dowels, chipboard or heavy paper, CA glue, sandpaper, wood filler, mirror adhesive or silicone
It was probably close to 50 years ago that I bought my first surplus tank prism taken from a periscope housing. I remember being amazed at the colorful spectrum it projected across a room when placed in direct sunlight. A few years ago I “rediscovered” some of these precisely ground pieces of glass and decided to build a functional cradle that allows for a more controlled aiming of the “rainbow.”
It is possible to use a prism by just standing it on end on a sunny windowsill, but you will get much better results by constructing a stand that will allow for horizontal mounting and making fine adjustments to compensate for the daily changes in the sun’s position in the sky.
Before we begin, there are a few things you should know about these surplus prisms. They are available in three configurations: mounted in the original metal housing, one side silvered (a mirror coating), and plain unsilvered. You want the plain ones. You can usually save a few bucks by purchasing one in the original housing, but remember these were built under contract from Uncle Sam for use in military combat vehicles. To say they are ruggedly mounted is an understatement. Trying to extract one may result in the invention of new swearwords and a broken prism.
You should also remember that these are takeouts from surplus military gear and may have some minor flaws such as little chips or fogging of the glass. If there are areas of the glass with a gray coating that glass cleaner won’t remove, try a solvent such as acetone or paint thinner with fine steel wool, which will usually do the trick.
These prisms are ground, not cast, glass, and are very fragile. If one is standing on end on your workbench and falls over, it can easily break in half. This will definitely have an effect on the performance of your rainbow thrower – unlike small chips, which will usually not cause any noticeable problems.
1. Much of the process for making the ends involves simple “eyeballing” rather than taking exact measurements.
2. Learn to be accurate in tracing or outlining by using a sharp pencil held at a consistent angle.
Choose your wood
Your first decision is what kind of wood to use. Unlike most woodworking projects, this one will intentionally be placed in direct sunlight (something we usually try to avoid). So a more stable wood will work better. Mahogany and teak are good choices, and if you prefer domestics, I’ve had good luck with walnut. Poplar would be a good choice for those of you who prefer working with softer woods; and, of course, the closer you can get to a true quartersawn board the better, regardless of your choice. I always like to mix different species, and this has presented no problems when the gluing process is done correctly.
These prisms sit in direct sunlight and can get quite warm, so woods with dramatically different expansion/contraction rates should be avoided. Don’t worry about your project becoming a fire hazard; unlike magnifying glasses, rainbow throwers do not concentrate sunlight.
Make the end pieces
Making the two end pieces that will hold the prism is the first step. Trace the outline of the prism end near the center of a piece of heavy paper or thin cardboard at least 3" x 3" square (Fig. 1). The location is not critical. I use single-ply chipboard for all my patterns. It is cheap, available at art stores, easy to cut with a sharp knife and durable, even for repeated use.
Cut some ¼" x ¼" stock, which you will use to make the triangular frames that secure the prism to the endpiece backing parts. The dimension of this stock isn’t critical either, but the thicker it is, the larger the area of the prism faces it will cover, decreasing the size of the projected rainbow. You can make it wider, but keep the thickness to about the recommended ¼". The smaller the finished cradle the better, as it will fit on more windowsills, and some of the newer sills are pretty skinny.
Place the prism back on the cardboard pattern where you previously marked its outline, and using a piece of the framing stock, mark another outline the width of the stock (Fig. 2). This will be the size of your backing parts. Cut out your new pattern.
3. The more you use CA glue for small parts, the more you will appreciate what it can do.
4. “Sanding to fit” makes it possible to remove much smaller amounts than with cutting tools.
Trace the shape onto your choice of stock, which should be at least ½" thick. You will need two for each rainbow thrower you are making. Cut out these pieces “fat” (outside the line). I just use the bandsaw, as the parts will be sanded to the line after the framing strips are added.
Cut six framing strips to length for each cradle. Four should be a bit longer than the sides of the backing parts and two can be a bit shorter, as they will fit inside the other two. Cut a few extra of these, as we are going to use the trial-and-error method for fitting them. Glue the two longer ones to the backing plates right inside the line you previously traced (Fig. 3). I use thick cyanoacrylate, or CA glue, more commonly known as superglue.
The next step is the only tricky part of this project, fitting the third framing piece between the two you have previously glued onto the backing parts. It’s time to get into angle cuts, or as I prefer, “freehand angle sanding.” You can calculate the length and angle of the cuts you will make on this piece, but for me, sanding gives me the needed control.
You will need either a disc or edge sander to make this part fit perfectly. Place the prism upright between the two previously glued framing strips. (Be careful here, this is where they can fall over and break.) Sand the ends and the angles of the third piece until you have a pretty good fit (Fig. 4).
5. Don’t use much pressure when sanding small parts to fit. Save your workpiece and your knuckles.
6. If you’re drilling a number of identical parts, set up a jig for support and positioning.
This fit should be a little “sloppy,” a bit oversize to allow for seasonal wood movement. Let the prism move about 1/64" in all directions. Glue this last strip in place, using wood filler (if necessary) to neaten it all up, and use your sander (Fig. 5) to finish the job. I have made a number of these, and on some I have squared off both ends of the long side of the triangle. See the finished sample shown on the first page of this article, and the exploded drawing on the next page.
The final step in completing these parts is drilling the ½" holes for the pivot dowels. These should be centered in the triangle defined by your framing parts. I suppose their exact locations could be calculated, but I'm satisfied to just eyeball it.
Design your base
Take a deep breath and relax; all the hard stuff has been done. Having framed the prism, the only step left is making the base, the design of which is pretty much up to you. Fitting and placement of the pivot dowels is important, but once again not critical. Let’s make the base and be done with this one.
The base unit consists of five parts: two uprights to support the prism assembly, two ½" dowels about 1½" long, and the baseplate itself.
The uprights are the first parts to make. They must be long enough to attach securely to the baseplate while allowing the prism assembly to rotate above it. Their exact length will depend on how large you made the backing plates and the thickness of your base. Keeping them as short as possible will add stability. They must be wide enough to drill a ½" hole through. Fabricate these parts and drill the holes (Fig. 6).
Select your dowels for a snug fit in the holes in both the uprights and backing plates. The rotating parts will wear over time, so I use thin CA glue to strengthen them. This also can be used to build up the dowel size and pivot hole in the backing plate if the fit is a bit sloppy. Just apply the CA to one end of the dowel up to about as far as it will fit in the backing plate and around the entire circumference of the hole. If you need multiple coats, use a CA accelerator to speed up the process. You don’t need to treat the other end of the dowel or the hole in the upright, as these will be permanently glued together after finishing.
The base can be as simple or complex as you like. The joint for attaching the uprights is your choice; it would be a perfect application for a dovetail, but to keep things simple, I just used a captured dado. Make the base long enough so when the uprights are fitted you have about ¼" clearance from the prism assembly on each end. Cut your joint, test everything with a dry fit (Fig. 7) and if it all works together, glue the uprights to the base.
Finish up
Apply a finish of your choice to all the parts before final assembly. When dry, the next step is fastening the end plates to the prism. Clean the prism thoroughly and use a small dab of mirror adhesive or silicone in each corner. Flexibility is necessary here to allow for seasonal wood changes.
Lay the base down on its side and push the dowels into the holes on the uprights. Move the prism assembly into position and push the dowels the rest of the way through until they seat against the prism itself (Fig. 8). Adjust their depths to get the prism centered between the two uprights. As the prism will pivot on the inside ends of the dowels, it’s time to glue the outer ends to the uprights. This is another great application for thin CA. Use a piece of wire or other pointed implement to touch a drop of CA to the joint between the dowel and the upright. It will wick right in through capillary action. Three or four drops is plenty.
William McDowell
William McDowell of Syracuse, N.Y., has been making high-quality, one-of-a-kind boxes and other items in his Art Tekno Deco series for more than 30 years. See more of his unique work at teknodeco.com.
