How to Bend Wood with Steam
A new woodworking skill, right around the bend!
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Steam bending has long been employed to build boats, ships and wagons, even toboggans and snowshoes. A time-honored method of manipulating wood, steam bending will be a fun and rewarding addition to your woodworking skill set.
To begin your wood-bending experience, you’ll need a few basic items: an inexpensive steam generator, an easy-to-make plywood box and medium density fiberboard (MDF) to make a form. Bending wood takes patience and practice; a simple round mirror frame is the perfect place to start.
- Cordless drill
- Router (optional)
- Steam Generator
- Table Saw (or Circular Saw With Cutting Guide)
- 2'x4' 3/4" Plywood
- Bending Strap Clamp
- Quarter-sawn oak or walnut
- Round Mirror
Meet the Expert
Project editor Jay Cork has taught classes on bending wood with steam. Truly, no one is more full of hot air.
How Does Wood Bend?
Some wood species — ash or oak, even cherry or walnut — can be bent by softening their lignin with heat. Think of lignin as the glue holding the wood fibers together. As the lignin and fibers are heated, they become pliable for a short time and the wood can be bent around a form.
Air-Dried is Best, But …
It’s a common misconception that only air-dried lumber bends well. I have found this to be only partly true. Kiln-dried lumber has a bad reputation because during the drying process, the lignin cooks and becomes stiff. If this happens, no amount of heat will bring it back.
Despite this risk, I’ve found kiln-dried wood can indeed be bent if it’s steamed for a longer period.
Project step-by-step (25)
Build the Steam Chamber
Cut the box parts
Size the steam chamber to the parts being bent; any larger will waste heat energy. The box can be made from any wood — even PVC pipe!
- Pro tip: I chose CDX (exterior) plywood because it’s cheap, easy to work with and holds heat better than PVC pipe.
Drill holes for support dowels
It’s important to suspend the wood inside the steam chamber to allow the heat to fully penetrate the full circumference of the wood inside. I used 3/8-in. dowels for this and drilled 13 holes, four inches apart.
- Pro tip: Screw the two sides together while drilling so the holes align perfectly.
Assemble the box
Assemble the box with 1-1/2-in. screws.
- Note: Make sure the holes for the support dowels are oriented near the bottom of both sides of the box.
Insert the support dowels
Cut the dowels to the width of the steam chamber. With the chamber on its side, guide the dowels into the holes, securing them with CA glue.
Add the legs
Adding legs provided space for the steam generator to sit underneath the steam chamber. I cut legs at a slight angle and attached them with 1-1/4-in. screws.
Drill holes for thermometers
Installing thermometers isn’t absolutely necessary, but having them lets you monitor the internal temperature, which should be a constant 220 degrees. I used two grill thermometers; the threaded neck made it easy to install each in the plywood box. Drill 1/2-in. holes and thread in the thermometers.
Drill the hose port
After shortening the hose, I drilled a 5/8-in. hole in the side of the steam chamber to accept the smaller, inner hose.
Make a simple lid
A single hinge on a plywood lid will suffice. I used a wood screw to keep the lid closed.
Make the Bending Form
Rout the shape
The frame I’m bending for the mirror will form a partial circle. With a circle-cutting jig, cut the round shape with a plunge router.
- Pro tip: Cut this in multiple passes with a 1/4-in. solid carbide spiral bit, going about 1/4-in. deeper on each pass.
Glue the form
Once the form has been routed, trace the outline on another piece of MDF. Cut out this part close to the line. I used a band saw, but a jigsaw works just fine. Glue and screw the two parts together and trim the excess flush with a router.
- Pro tip: Once you glue the form together, drill evenly spaced holes around the circumference with a 2-in. Forstner bit. This provides space for the clamps needed to bend the wood. The edge of each hole should be 1-1/2-in. away from the edge of the form.
Attach the form to the base
When you’re attaching a bending form to an MDF base, there’s no such thing as too much glue or too many screws. Make it strong! After gluing the form to the base and allowing it to dry, I used 2-in. screws through the bottom to maximize its strength.
Add an anchor block
When bending wood, it’s important to anchor the wood firmly to the bending form. I prefer a lag bolt for this purpose.
Cut two lengths of hardwood to the same thickness as your workpiece plus the thickness of the backing strap. Drill a hole for the lag bolt in the longer piece and attach both to the form as shown using CA glue and 2-in. screws.
Bend the Wood
Drill anchor hole
With a 5/16-in. bit, drill a hole in one end of the bending stock to match the hole in the anchor block. When it comes time to bend, the lag bolt will go through this hole, anchoring the wood to the bending form.
Prepare the backing strap
I used a Veritas bending strap jig ($100 at leevalley.com). The strap that comes with the kit was 1-1/4-in. wide, but my stock was 1-1/2-in. wide, so I made my own backing strap. I used a rotary tool to cut it to length and drilled a hole in one to match the anchor hole in the wood.
Steam the wood
One rule of thumb says leave the wood in the steam chamber for one hour per inch of thickness. This assumes you’re using air-dried lumber with a moisture content of more than 20 percent. Kiln-dried lumber should be steamed for twice that long. This 1-1/4-in. thick, kiln-dried walnut steamed for two hours.
Once the wood comes out of the steam chamber, quickly get it on the form and start the bending process. You have less than two minutes before the lignin cools and starts to reset.
Clamp as you go
Here’s where a helper comes in handy! As you pull the wood around the form, have someone follow you, tightening clamps against the form as you go.
Complete the Mirror Frame
Cut the rabbet
On a router table, cut a 1/2-in. wide by 1/4-in. deep rabbet around the back of the curved frame. This will provide plenty of room for the thickness of the mirror and the adhesive to keep it in place.
Profile the frame
Here’s where you let your creative side take over. Profile the frame in any way you find appealing. I rounded over the inside edge with a 1/4-in. round-over bit and the outside edge with a “tabletop” bit — a slightly oblong round-over.
Apply a finish
Apply the stain and finish of your choice. I used iron oxide stain and shellac. The iron oxide will turn the wood almost black without obscuring the grain, and the shellac dries really fast.
Attach a hanger
Because I chose an acrylic mirror, not glass, I felt comfortable gluing the hanger right to the back with construction adhesive. It doesn’t even have to be perfectly centered. The frame is round, so there’s no perfect up or down.
Set the mirror
After the finish dries, set the frame face down on a workbench. Place the mirror in the rabbet and glue it in place with construction adhesive.
Shopping for materials
You can find steam generators at woodworking outlets like Rockler or Woodcraft, but big box stores sell them, too, as wallpaper steamers. (They’re the same thing.) I purchased my Wagner model at The Home Depot.
Figure B: The Bending Form
What is springback?
When wood is bent, it has a natural tendency to return to its original shape. It will never go back to straight, but it will “spring back” a little. To compensate for springback, undersize your form by roughly 10 percent. The mirror I chose for this project has a 17-in. diameter, so I cut my form with a diameter of 15-1/4-inches.
Leave it alone
In general, allow the wood to sit in the form for 12 hours before removing it. If you’re making many parts and want to speed things up, you can make a frame to hold the now-bent piece of wood. After letting it cool for an hour or so in the form, transfer it to the frame. Then your form is freed up, ready for another part to be bent.