Taming Complex Curves

Long before the days of computers and sophisticated CAD programs, boat builders used-and still do today-a flexible spline to draw the long, graceful curves of a hull. To hold the spline in place on the drafting board, they used a series of lead weights called "spline ducks."

We borrowed this idea to make a template for drawing the curves on the leaves of the drop-leaf table in WOOD® magazine issue 181 (December/January 2007/2008). In this case, a drawing like the one at insert directional shows the beginning and end points of the curve, plus several intermediate points.

Or, when designing your own project, mark where the curve begins and ends and then simply compose a pleasing shape between the two points. Using the drop-leaf table example, here's a simple method that serves in both situations: Cut a template blank longer and wider than the finished curve, and a spline from thin, flexible material. (We used a ¹⁄₄ x12x36" piece of hardboard for the template and a 1¹⁄₂ x36" strip of plastic laminate for the spline.)

Then cut enough ducks-simple scrap wood blocks-to space one every 6" along the spline. For precise adjustment of the curve ends, leave square ends on the two end ducks. To prevent flat spots where the ducks contact the spline at the intermediate points, make 45° cuts to form points on the others.

Drill shank holes through the spline at 6" intervals and centered in the width, and mating pilot holes into the ducks. To help keep the ducks from sliding around on the template blank, adhere sandpaper to the bottoms. Now draw the curve by following the five steps.

Draw a vertical line across the template blank at each "horizontal" dimension shown on the drawing.

At each line, mark the "vertical" dimension. A combination square makes a good marking gauge.

Screw the ducks to the spline. Tightly fasten the end ducks but leave the pointed ducks slightly loose.

Curves: Steps 4 - 6

Slide the ducks to push or pull the spline so it connects all the marked points. Bricks on the ducks keep them in place.

Trace the curve onto the template blank. Now you're ready to bandsaw or jigsaw and sand the template.

Because the ducks are fastened to the spline, you can push and pull it to draw curves that bend inward and outward.