Making a Cold Bending Form
Pretty simple, huh? The only real trick to cold bending is in making a form that will apply an even clamping pressure all along the laminated assembly. Traditional bending forms consist of two parts, the form (the positive shape) and the press (the negative shape). Both of these parts are normally cut from the same stock. Begin by drawing the curve you want on the face of the stock. Cut the curve with a band saw, separating the stock into two parts. On the negative part, mark the thickness of the bent wood part. (Tip: Use a compass like a calipers, set it to the desired thickness. Follow the curve with the point of the compass, marking the thickness with the scribe.) Cut away the thickness on the band saw — this will create the press.
The trouble with this traditional bending form is that the press doesn’t compensate for small variations in the thickness of the laminated stock or a band saw blade that wanders a hair off the line. Consequently when you apply the clamps, the clamping pressure may not be completely even all along the form. This may result in weak laminations or even gaps between the laminations when the glue dries.
To ensure that this didn’t happen to our glider ribs, I designed a compensating press. After cutting away the thickness of the bent wood part, use the compass to mark yet another curve on the negative part, this one 1″ larger in radius than the curve you just cut. Saw this curve then cut the 1″-thick piece into 3″-long segments. Adhere the segments back to the negative part temporarily with double-face carpet tape. Glue a strip of canvas to the inside curve of the segments and cover the canvas with 6-mil plastic.
When you separate the segments from the negative part and discard the tape, they should be held together by the canvas like the tambours of a rolltop desk. This is your press. When you squeeze the laminated stock to the form, arrange the clamps in the middle of each segment; this will compensate for any variation in stock thickness or inaccuracies in the bending form and keep the clamping pressure relatively even.
Note: The plastic on the press will keep any glue that squeezes out from between the laminations from sticking to the canvas. To prevent the squeeze-out from sticking to the form, apply paste wax to the form before each glue-up.
Spreading the Glue
Just as uneven clamping pressure will reduce the strength of the lamination, so will an uneven application of glue. You must spread it as evenly as possible, and I’ve got just the ticket. This little trick was shown to me by the good folks at Franklin International (makers of Titebond glue). Get rid of your glue brushes and spread the glue with the teeth of a 3/8″ x 32 threaded rod. The threads spread the glue to just the right thickness (about 0.005″) for a strong joint with a minimum of squeeze-out. For this particular project, I mounted a short length of threaded rod in a wooden handle. Between glue-ups, I keep the rod submersed in water to prevent the glue from drying on the threads. PW
Nick Engler is a contributing editor for Popular Woodworking.