Making a light box.
I make boxes, and recently, a new customer presented a unique challenge for me. The item she required was a “circular light box” – some two feet in diameter – designed to hold a circular stained glass work of art. The prospective owner wanted self-contained lighting to softly backlight the stained glass. She also wanted the box to be made of oak to fit in with the décor of her home, where it was to be wall hung. As mentioned, this presented some interesting design challenges.
First, the normal air humidity on the west coast of British Columbia, where the box would reside, is a lot greater than that of the dry Canadian prairies of Alberta, where it would be made. How to overcome the moisture differences and the resulting movement of the wood? Second, in order to backlight the stained glass the interior void in the box must provide for a minimum of one inch clear space to house the “rope lighting” to be installed – a string of small light bulbs housed in a flexible clear plastic tube.
The prospect of building this crosssection in a circle out of solid oak, or any solid wood for that matter, given the concern over humidity conditions, was not appealing. Also, the prospect of cutting the circular frame from solid stock (albeit glued up) almost 2 inches thick, or from jointed and joined segments of solid wood arcs, was even less appealing. Instead I proposed using MDF (medium density fibreboard) as the base material for the box frame and cladding the entire frame in oak veneer. During construction these ¼” thick circles would be much more readily cut using ¼” MDF than solid wood stock. The veneered MDF design would remain stable when moved to a more humid climate, and also provide a more economical and easily fabricated box. With this design I could also provide a top cap ring on the frame that would allow the stained glass to be removed for service if necessary; and a removable back for access to the rope light. Fortunately, the client agreed to the change in materials and the project was begun. As with many creative projects, new challenges came up as I proceeded.
The light source needed to be softened or diffused so as not to appear too bright behind the stained glass piece, which was somewhat clear in places. This called for a diffuser plate of translucent white plastic to be mounted behind the stained glass itself, requiring another circle cut, this one in thin, brittle, white plastic (from a fluorescent light cover). This required smooth edge circle cuts. While all other circle cuts, inside and out, could be made using a standard circle guide and a router, I came up with a circle cutting jig for the diffuser plate which is safe and easy to use on the table saw
Making the Circle Jig
Let’s start with the jig, which is simple to build. Use a 28-inch square sheet of MDF with a plastic runner screwed to the underside. Rest the runner in the groove of the table saw. Then, screw a cleat to the underside of the MDF sheet and position it so that the sheet hugs the surface of the saw’s table. Make sure the inboard edge of the MDF sheet just clears the saw blade as it is raised above the table. To cut a circle in stock using this jig, measure back from the saw blade, a radius of the circle required. Drill a ⅛” hole vertically at that point. Now drill a hole in the centre of the square stock on which the desired circle has been laid out. Use the drill bit as a centre pin and insert it through the stock into the hole in the jig. Make sure that the centre line of the jig and stock is precisely at 90 degrees to and aligned with the centre, or highest point on the saw blade. Now, with the saw blade below the underside of the stock, start the saw and raise the blade until it just enters the stock. Rotate the stock counterclockwise, against the direction that the saw blade is moving, about the centre pin in the jig. When a full 360-degree rotation of the stock is complete, raise the saw blade by about 1/16” inch (about one full turn of the raising handle on my saw) and continue rotating the stock another 360 degrees. Continue this process until the saw blade comes through the top surface of the stock, all the way around and the cut is complete.
You will be amazed at how smooth a cut is achieved by the saw blade. Very little sanding, if any, is required to achieve a perfect, vertical, finished circular surface.
Cut five circles for this box on the table saw (fig. 1). Two of 1” and ⅝” thick MDF stock for the frame body, one of ¼” MDF for the cap ring, one in ¼” MDF stock for the back plate and one in 1/16” white plastic for the diffuser plate. Make the two inside rabbet cuts and interior circle cuts using a plunge router mounted on the circle cutting jig centred on the same base plate used for the table saw. Glue the two thicker rings together to achieve the depth needed for the box. Make sure all parts fit together with adequate clearance. Test fit the back and the cap ring with the diffuser plate and stained glass in place in the top rabbet. Clad the cap ring and main frame body of the box with oak veneer for the solid wood look.
Magnetic Fasteners: My original design of the box called for the cap ring to be screwed to the frame in order to lock in the stained glass art. However, I realized that, no matter how delicate, the little roundheaded brass screws would still be a distraction on the frame surface. In addition, not only was there was not enough thickness in the cap ring stock to allow the countersinking and the hiding of screw heads, but they also still had to be accessible. This presented a perfect application for rare earth magnets. You will need 16 of these powerful little magnets, in eight pairs. Embed them into the under side of the cap ring and top of the box frame, using epoxy. They are so strong that the stained glass circle is held in securely, yet they can be slid off their normal firmly mounted position by applying firm sideways pressure. However, in case the magnet mount fails, I inserted some safety clips to hold the work of art in the frame.
Finishing Touches: Drill two holes in the back plate, one to provide entrance for a sturdy hanging bolt to catch the inside lip of the main frame, the second at the bottom to provide egress (exit hole) for the rope light power cord. Drill the bottom hole to provide for flat, flush mounting of the frame on a wall.
Credits: Thanks to Zo Ann Morten for permission to photograph the stained glass artwork of her own design. Photo by Pat Morten, Norlynn Audio/Visual.