There are three basic ways to bend wood to form curved parts: steaming solid stock to make it temporarily pliable; glue-laminating thin strips of wood around a form; or cutting closely spaced kerfs into the back face of solid stock, leaving a thin web of wood on the outside face holding together a series of narrow ribs on the inside, as shown on Drawing 1, below. Of these, kerf bending ranks as the simplest method, used primarily on parts whose edges do not show.
Kerf spacing determines the minimum bending radius for a piece of stock. (When the kerfs close on the inside, the piece won't bend anymore.) But bending the part until the kerfs close often produces visible flats on the outside surface, as shown on Drawing 2, below. Sanding these flats into a smooth curve may weaken or even break through the thin web. To avoid this, space the kerfs closer together than required by the "closed kerf" limit. The closer spacing, while not completely eliminating the flats, reduces them so they are easily sanded into a smooth curve. See Drawing 3, following for recommended smooth-curve kerf spacing for three different radii. Using the round table's 46"-diameter curved aprons as an example, here's how to kerf-bend parts.
First, make the form
Before you start kerfing the apron stock for our table, make the bending form, shown on Drawing 4, below. To do this, cut three 24×48" pieces of 3⁄4 " plywood, particleboard, or medium-density fiberboard (MDF). Draw a centerline across the 24" dimension of each piece. Mark a point on this line 13⁄4 " in from one edge. Using a beam compass, draw a 221⁄8 "-radius arc, centered at this point. Bandsaw just outside the line. Drill a hole at the arc centerpoint, and use a router trammel to trim the parts to the line.
Plane scrap stock to 5⁄8 " thick, and cut 20 spacers to 2×5". Position spacers between the three layers (seven just back from the curved edge and three along the straight edge), and glue and clamp the 31⁄2 "-thick form together, keeping the edges flush.
Next up, the aprons
For the round table's aprons, cut two 3⁄4 "-thick apron blanks to 31⁄2 ×755⁄8 ", and two 1⁄8 "-thick backer blanks to 31⁄2 ×73". (For self-supporting parts like the table aprons, gluing a backer to the kerfed part forms a rigid assembly that keeps its shape.) To save time cutting the kerfs, tape the two aprons together edge to edge. You can use either a tablesaw or radial-arm saw, as shown in Photos A and B, below.
Finally, the glue-up
Note: As in any unusual glue-up, first make a dry test run. This way you'll avoid surprises after you've applied the glue. (We did this, and found it helpful to add a couple of scrap blocks with beveled ends to the straight edge of the form, as shown in Photo D, above. This helped ease the band clamp around the form's corners.)
Mark centerlines on the edges of the kerfed apron and backer. Apply glue to one backer, and position it on a kerfed apron, as shown in Photo C, above. Clamp the parts to the form, as shown in Photo D. Leave the apron clamped to the form for 24 hours. Repeat with the second set of apron parts.