Making complex bends from a ‘simple’ design

Have you ever considered a project that would bend and twist and require near-perfect grain orientation? One that tapered along its length at varying rates and then reversed its taper? Or connected several specific locations at different angles? Flexible as an archer’s bow in some places, and stiff as a hammer handle in others.

The main bend of my “Simple Chair” meets those requirements. If you would like to work with those possibilities, here’s how to approach the project and avoid some of the pitfalls.

The chair’s main bend is a lamination of veneer assemblies, each comprised of pieces sawn and scarfed together, then machined to .001" accuracy at varying and reversing rates of taper.

This is part one of a two-part article on the construction of the author’s paradoxically named “Simple Chair.” Part two will appear in the February 2009 issue.

The first challenge to such a project is the drawings. At first, I couldn’t even make a satisfactory sketch of what I imagined. What helped to improve them was to trace and refine the shapes with empty hands, seeking fluidity and paying attention to the roll of surfaces as the angles changed. The sketches led to full-scale drawings, which are essential.

Full-scale drawings should be made on a 1/2" grid. No doubt you know some of the key dimensions. For my chair, they included the heights and angles of the back, arms and seat, and the distance between the legs. The evolving drawings suggested feet of 1-5/8" square, a crest 11/16" thick, and at the wrist, where the shape twists and wraps around a 3" radius, 7/16".

Beech veneers .039" thick could reliably negotiate that bend and 11 of them would be 7/16". For the feet, 1-5/8" feet divided by 11 meant .149" veneers, so I sawed the veneer stock blanks to a scant 3/16".

Full-scale drawings also helped me mock up a bending jig. From it, I could make a veneer pattern. That pattern suggests the shape of the “grinding” jig and, once enlarged, becomes the actual veneer pattern.

If you do a project like this, you must take the time to refine the lines of your drawings. It is a mistake to think you can resolve clunky lines by whittling away at your lamination.

Mocking up
By referring to each of your full-scale drawings, you can mock up the bending jig. The object is a stable framework where pieces of poster board can arch across several lines of contact. You can use MDF and hot-melt glue working off a center line and grid. Consider where bending stresses will accumulate and how to clamp the entire lamination, especially at the inside curves. If your lamination is to be symmetrical, begin by striving for symmetry here.

Glue and clamp any splices you have to make to the poster board. This first pattern ought to be just the length, width and shape you want in the finished lamination. If it should be symmetrical, fold it to see. Make half the correction to each side of the preliminary pattern, and then make an oversize pattern. A shape bent radically as the chair, considering its thickness, demands an extra inch of length at each end of the veneer and an extra 3/8" all around. This isn’t to cover for sloppy work. It’s for the inevitable slippage that occurs as each veneer wraps around or into changing radii.

You may be surprised by the shape of your veneer pattern. For my chair, it is a broad “C”, 100" long that fits into a 3' x 6' rectangle. Five pieces of veneer make that “C”. In aircraft construction, good practice calls for scarf joints cut at a proportion of 15 units of run for each unit of thickness, which translates to a joint nearly 2-7/8" long to splice a 3/16" veneer.

Lateral curves in flat veneer are induced by cutting the scarfs at an angle, rather than perpendicular, to the length of the veneer.

Cut your scarfs on an inclined jig in a planer or sander, holding the veneer with double-sided (2-S) tape. Thin veneers hold together more reliably in a sander and, in my shop, the sander’s greater width and longer bed simplify the process since I work alone. Straighter, thicker laminations and less complex tapers favor the use of a planer.

Regardless of which machine you use, if you’re going to vary the rate of taper, you’ll have to hold the work with 2-S tape and use a two-part jig to carry the laminate.

The two parts are a torsion box base and plywood top plate veneer carrier separated by stacks of shims that regulate the depth of cut. Both are roughly the shape of the veneer pattern and 8" wide to reduce the tendency of the boomerang shape to tip entering and exiting the machine.

Assembling the jig
My torsion box is framed in pine, 5/8" x 1-1/4", with crosspieces every 4". They not only stiffen the box, but they also provide solid seats for the shims and serve as anchors for the screws that hold the two parts of the jig together. The torsion box is skinned in 1/8" Baltic birch (BB) plywood. The top plate is cut from 1/2" BB ply to the same shape as the torsion box. Both are kiss-dressed to ensure parallelism.

The shim packs use identically cut and hole-punched strips of manila file folder stock about 3/4" x 6", clear packing tape, and thin Scotch tape in combinations specific to .001".

To assemble the jig, calculate the height of the shim pack at each crosspiece and tape it to the torsion box at its location. Drill pilot holes through the torsion box at each of the punched holes, clamp the top plate in place, flip the whole works and drill back through the veneer carrier. Flip it again. Use consistent torque as you screw the jig together.

Outline the veneer pattern and scarf locations on the top plate. Narrow strips of 2-S tape cross the outline every few inches. Mark those locations, too. I use yellow glue for the scarfs. In warm, dry weather they’re cured enough to grind in 20 minutes.

Since I have a 22" wide sander, finessing a 38" wide veneer carrier requires some side-slipping. To prevent breaking the veneers or tearing them loose from the tape, spray a little cooking spray on a towel and wipe the machine’s pinch rollers so the veneer can slide.

For my chair, more than 1/8" is ground off each veneer assembly at the wrists. Several passes are necessary to do that. A fresh, sharp, 60-grit belt minimizes deflection of the jig’s top plate and cross-sectional rounding of the veneer during the grind.

Measure one end of the first veneer assembly by peeling free the 2-S tape. When you’ve reached the target thickness at one end, the thickness will be right everywhere if the shim packs are right. For repeatability, mark the machine with tape to indicate where the grind begins, where it ends, and the number of turns of the machine’s depth-of-cut handwheel between them.

Compare your first veneer to the mocked-up bending jig. Survey the fit from all angles. If need be, modify the height or angles of the lines. Modify the veneer pattern, too, if need be. That’s done by altering the angles of the scarfs or the distance between them.