Steam Bending

Curves such as those of the Standup Desk (below)add visual interest. And steam bending creates attractive curved parts.

Every wood species, and even individual workpieces of the same species, can react differently to steam bending. Start with these best practices and then let experience be your guide. Keep notes on species, thickness, bend radius, and steaming time, and always bend at least one more part than you need.

Here's how steam bending stacks up against other methods for making curved parts:

Steam-bent from solid wood
+ Finished part has best appearance with continuous end-to-end grain
+ No glue-up with visible glue lines
+ Strong; no short cross-grain structural weakness
– Bending and cooling forms required
– Steam box and boiler required
– Ultimate curve shape may be somewhat unpredictable

Laminated from thin strips
+ Cold process, no special equipment required
+ Strong
– Bending form required
– Lots of strips to cut and keep in order
– Difficult to control final part thickness
– Messy glue-up, strips want to slip sideways when bent
– Dried glue squeeze-out on part must be removed before further processing.
– Glue lines often visible

Cut from solid wood
+ No form or clamps required, no apparatus to build, just apply a pattern or lay out part directly on workpiece
– Board must be wide enough to accommodate the curve, often wasting material
– Wood grain does not follow curve of part giving an unnatural appearance
– Structural weakness where wood grain takes a short path across the part width (think broken rocking chair rockers)

Bending stretches wood along the outer side of the bend, and compresses it along the inner side, producing stress that wants to bring the bent piece back to its original shape, a tendency called "springback." Steaming softens the lignin to release this stress. When cooled, the lignin hardens, fixing the wood in the new shape.

Steamed wood compresses considerably but stretches little. That's why successful steam bending compresses the wood on the inside of the bend while restraining stretching along the outside. For tight bends (less than 4" radius), a steel tension strap with attached end blocks applied to the outside of the bend minimizes stretching [Drawing below]. Mild bends, with minimal stretching along the outside, do not need strapping.

Parts incorporating convex and concave curves in the same plane require a two-part form to sandwich the workpiece [Drawing below] or special tension-strap hardware [Sources]. Tension-strap hardware also is available to form parts with bends in two planes.

For best results, use air-dried lumber, with dehumidification-dried lumber a second choice. The heat of kiln drying makes lignin less susceptible to softening by steaming. Use kiln-dried lumber only for gentle bends.

All temperate-zone hardwoods (chart below) steam-bend well and, in general, bend better than softwoods. Steam-bending other temperate-zone species is possible, but limit their use to gentle bends. Among tropical hardwoods, mahogany gives acceptable results. The brittleness of highly resinous tropical exotics make them unsuitable for bending.

Regardless of the species you use, careful board selection increases chances of successful bends. Choose straight-grained lumber without knots or other defects and avoid decay (even slight spalting). The grain should run parallel to the edges of the workpiece or "run off" the edge at a shallow angle (maximum of 1" slope to 15" length). The greater the run-off angle, the more likely the piece will break when bent. Splitting wood from a larger straight-grained billet guarantees straight, parallel grain but is not always practical.

Straight stock runs through a planer or jointer easier than bent pieces, so do as much sawing, surfacing, or shaping as possible before bending. Surface irregularities can cause splintering, so remove rough saw marks. Holes or mortises distort or they cause the part to collapse or split; perform these operations after bending.

Leave extra length at both ends for trimming. Where a bend is near the end of the part, extra length also provides the leverage needed to anchor the bend. Cut stock for parts requiring tight bends so the annual rings lay flatwise against the surface of the bending form [Drawing delow]. For mild bends, grain orientation is less important.

Steaming takes about one hour per inch of workpiece thickness and parts must stay on the bending form for one hour, so unless you make more than one form, the steam box only has to accommodate one part at a time. For efficient use of the steam boiler [Sources], a steam box should be only large enough to hold the part blanks with room all around for good circulation. For tight bends on thick parts, increase box size to accommodate resteaming a partially bent piece. The steam box shown in [Drawing below] holds two leg blanks for the desk (shown above)and accommodates resteaming of a partially bent leg.

Make bending forms from particleboard or plywood laminated to a thickness equal to the width of the bending blank. Bending a workpiece puts considerable stress on the form, so when in doubt, more form width beats less every time. You'll never be disappointed by a tendency to overbuild.

Bent parts retain most of their shape after cooling but to minimize springback, must be held to a form until completely dry. To avoid tying up the the bending form, make lightweight drying forms from a single thickness of ³⁄₄ "-thick material. The forms shown [Drawing below] are for the desk, shown in photo above. (We made six drying forms to accommodate four leg blanks plus two extras.)

.Securely clamp the bending form to a sturdy workbench. Have all clamps handy. Bending should be accomplished within five minutes after removing the blank from the steam box so you may want to rehearse your procedure.

Fire up the boiler. When steam steadily flows out of the drain hole, slide your blank into the steam box. Steam blanks for one hour per inch of thickness (regardless of the width). Remove the part from the steam box and bend it [Photos below].

Safety note!
Steam scalds skin on contact. Opening the steam box releases a cloud of steam, so keep your face and any other bare skin away. Wear heavy leather gloves when handling steamed blanks. Make sure the drain hole remains open. Do not pressurize the steam chamber: Pressurized steam is detrimental to successful wood bending and extremely dangerous.

Steam-box Supplies:³⁄₄ ×48×96" exterior plywood, #8×1¹⁄₄ " flathead screws, ¹⁄₂ " dowel 48" long, 3" gate hook (2), 4" strap hinges (4), ³⁄₄ ×5⁄16×72" self-adhesive rubber-foam weather stripping.
Source:
Boiler: Steam-bending kit no. 42826, Rockler, 800-279-4441, rockler.com.
Bending- and Drying-form Supplies: ³⁄₄ ×48×96" particleboard, ³⁄₄ ×24×48" particleboard, #10×2¹⁄₂ " flathead screws (8), ¹⁄₂ " dowel 12" long.
Sources:
Spring steel strip, .094×1¹⁄₂ ×36" no. 9074K186; ¹⁄₄ " cobalt-steel drill bit no. 3069A25; ¹⁄₂ " cobalt-steel countersink bit no. 3285A352, McMaster-Carr, 630-833-0300, mcmaster.com.
Tension-strap hardware for complex curves: S-bend unit no. 05F12.01; change-of-plane bend unit no. 05F12.02. Lee Valley Hardware, 800-871-8158, leevalley.com.

• USDA Forest Products Lab publication Bending Solid Wood to Form
• Lee Valley Hardware publication Veritas Steam-Bending Instruction Booklet