From time to time our work requires a curved piece of wood. We can use a large piece of wood and a band saw to cut this curved piece (often wasteful and hard to do) or we can bend the wood into the desired shape. Bending, once one learns a few basic procedures and technical requirements, is easy. When done properly, there should be less than 10 percent bending failures (mostly cracks and splits).
Hot and wet
To bend wood without breaking, it is critical that the wood be fairly elastic. We can increase the elasticity of wood by making it hot (180 to 212 degrees F) and wet (22 to 25 percent moisture content). Actually, it is important that the wood not be dried to a low moisture and then moisture added back when we are ready to bend it; once dried, wood becomes brittle and this brittleness is hard to eliminate.
Once bent, if we remove the pressure that is bending the wood, it will straighten. So, once bent, we need to dry and cool the wood while it is still being held bent. That way it will stay in its bent form forever.
To achieve the desired conditions in the wood, it is common to preheat the wood in a chamber that is full of steam. One common cited rule is that the wood is in the steaming chamber for one-half to one hour for every inch of thickness. My experience says one hour is a bit too long in most cases.
Compression and tension
Even when the wood is wet, hot and very pliable, when it is bent in a curve, the outer radius fibers are being stretched; that is, they are in tension. The inner radius fibers are being compressed. The more severe the bend, the greater the forces that are developed. If the tension or compression forces increase so much that they exceed the wood's strength, we will have a failure. The tension failure will be a split or crack which probably means that the piece is a reject. The compression failure will typically be a wrinkle and that doesn't really look too ugly.
To prevent the tension forces from getting too large, it is common practice to push on the ends of the piece being bent. This pushing, or compression force, will offset the tension forces. As a result, the entire piece being bent will be in compression, avoiding the fatal tension failures. For slight or gentle bends, where the radius of the bend is at least 20 times greater than the thickness of the piece being bent, often the end pressure can be eliminated as the wood is strong enough to withstand the slight amount of tension in this gentle bend.
A variety of devices and techniques have been used to develop this compression force, but the most common is a metal band on the outer radius that has metal blocks on the each end. This band fits snugly (and this is the key requirement) on the piece of wood before it is bent. As the wood is bent, the band, being on the outer radius automatically gets tighter and tighter, developing adequate compression to prevent the wood at the outer radius from going into tension. Simple indeed, but it works great.
Incidentally, with the entire piece in compression as a result of the metal band, it is possible to develop a few compression failures. However, hot and wet wood actually limits the risk of such failures, so they are not too common.
Sometimes, the bending jig is left on during subsequent drying. Other times a second jig is put on the bent wood to prevent the piece from springing back straight and then the first bending jig is removed.
We can help bending by paying attention to several wood factors. First, the grain of the wood should be parallel to the sides of the piece being bent and not at an angle. Even a small grain angle of 20 degrees can cut the wood's basic strength in half. In the old days, a log was first split before sawing bending stock. The split would follow the grain, and then the sawyer would saw parallel to the split. It is too bad that this procedure is seldom done by commercial sawyers today.
As the grain angle varies considerably around a knot, avoid knots and the surrounding wood.
Avoid pieces that have the pith (the exact center of the tree) as this wood is really weak.
Next, if possible the wood annual growth rings should be oriented so that they run from inner to outer radius (quartersawn) and not so that the rings run from edge to edge (or flatsawn).
Very high-density woods, including many imported woods, do not bend well.
Avoid any wood that is less dense than normal for the given species. Such low density wood, often called brash wood, is subject to failure.
Avoid decayed wood as it is also weaker than normal.
As mentioned, avoid low moisture, kiln dried wood. Rather, use air-dried wood no lower than 20 percent MC and preferably a few percent wetter.
Avoid trying to bend pieces with a lot of surface roughness. A planed surface is stronger than a rough-sawn surface.
Bend the wood slowly taking a minute or so for a severe bend. This allows the wood to better absorb the stresses without breaking.
Finally, do not use bending blanks that are thicker than required, as thicker wood is harder to bend than thinner wood.
Choose a species that is noted for easy bending. Some of the better species include hackberry, white oak, red oak, magnolia, pecan/hickory, black walnut and beech.
Once bent, the wood is dried, usually in a hot room (100 degrees F is common although temperatures up to 140 degrees F are used) rather than in a lumber dry kiln. Drying can be fairly rapid, but if it is too fast, cracks can result. Do not over-dry as machining will be poor. Do not under-dry, as some subsequent movement of the finished piece might occur. Generally, the drying target is 7 percent MC.
Due to shrinkage, it is common to find that the metal bending jigs will be loose and can be removed easily.
When bending oak and when using iron bending jigs, it is likely that iron tannate stain will develop and may go deeply into the wood. Switch to non-iron straps and blocks; the steamer may also need to be non-iron.
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