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Part 1


PART 1: Bending Solid Wood with Steam and Compressive Force


There are three basic requirements for the successful bending of solid wood using steam.
  1. The wood must be plasticized. Although wood can be plasticized chemically or even by microwaves when in a green state, the most convenient way to plasticize wood is with steam.

Wood cells are held together by a naturally occurring substance in the wood called lignin. Imagine the wood fibers to be a bundle of rods with the space between them filled with lignin. The strength of this lignin bond between the rods can be decreased by subjecting the wood to steam. With unpressurized steam at 212? Fahrenheit, steaming for one hour per inch of thickness (regardless of the width) will soften the bond enough for bending. Substantial oversteaming may cause the wood to wrinkle on the concave face as the bend progresses.

  1. Only air-dried wood of an appropriate species should be used.

Kiln-dried wood must not be used; the lignin in the wood has been permanently set during the hot, dry kilning process. No amount of steaming or soaking will weaken the lignin bond sufficiently for successful bending. The same applies to air-dried wood that has been allowed to dry and stabilize below 10% moisture content; the lignin will only partially plasticize with steam, not enough for successful bending of anything beyond a shallow curve.

  1. Wood must be kept under compression during the bending process.

Because of this third requirement, the Veritas line of clamping equipment for steam bending was developed. Wood fibers will stretch only a small amount before they fail, usually less than 1/2 of 1%. If you think of bending a stick over your knee, as the wood bends, it is the fibers on the outside of the bend that start to separate and break first. The drier the wood, the easier it will break. However, when well plasticized, wood will compress to an amazing degree. It is these two properties of wood that we avoid and exploit respectively in the steam-bending process.

To stop the wood from stretching on the outside face during the bending process, it must be restrained at either end by stops securely attached to a metal backing strap. The wood face in contact with the strap is not allowed to stretch as the bend progresses; however, the wood face against the form is subject to compression exerted by the end stops.

For example, a straight piece of wood 1" thick and 18" long bent to 90? around a 4" radius will remain 18" along the outside (immediately next to the strap), but will have the inside dimension reduced to almost 16". Nearly two inches have virtually disappeared through compression along the inside face!

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Species for Steam Bending

To avoid a lengthy description of wood cell structure here, we are providing general guidelines regarding appropriate woods for compressive steam bending using the Veritas Steam-Bending System.

Two rules of thumb are:

  1. Exotic woods do not bend well.
  2. Softwoods do not bend well and should be avoided.

Some common domestic hardwoods will bend with great success. Based on air-dried 1" thick stock at 25% moisture content going into the steam box, the smallest bend radius you can achieve without risk of failure is shown in the table below. You can get tighter radii but at higher risks.

Species
Smallest Radii
Oak (red and white)
2"
Hickory
2"
Elm
2"
Walnut
3"
Ash
4.5"
Cherry*
6"
Maple**
8"

* Requires some experience to bend flawlessly. It is prone to compression wrinkles on the inside face. These can often be removed during shaping and sanding once the bend is complete and cured.

** Can be difficult to bend. It requires more leverage to put the blank into compression.

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Bending Blanks

The stock itself should be the straightest grained, knot-free material you can find. The ends should be square-cut and all sides planed smooth. Generally, we recommend bending pieces only slightly larger in cross section than the finished piece needed, just enough to square up, sand, or scrape the surface down to clean wood. The exception is if you are bending a wood that is notoriously difficult or if you are bending to a very tight radius. For example, cherry is prone to compression failure so bend a piece 1/8" or 1/4" thicker than needed and then bandsaw away the inside face marks. If the exact curve is critical, you have to remember to accommodate this difference on the bending form.

The discoloration caused by the metal strap in contact with the wood may also require additional thickness. Woods high in tannic acid such as red oak, white oak and walnut will develop a 1/16" deep purple stain if the strap is left on as the piece dries on the form. Most woods (ash, cherry, hickory, maple) do not develop this stain to the same degree. Light stains can be removed by sanding or scraping. Deep stains may be more easily removed by bandsawing a thin slice off the stain area, followed by sanding.

When bending tight radii, some reduction in stock thickness will occur as a result of the wood being compressed between the strap and the form, particularly right at the corner of an "L" or "U" bend. Bending a piece of 1" thick ash around a 2" radius form may reduce the thickness as much as 3/16". Backing off the end stop one turn, half way around the bend, will decrease this amount.

The grain of the blanks should be relatively straight. The grain should not "run out" in

less than 15" along the blank, and preferably not where the bend is to take place (see Figure 2). Knots should be avoided. However, if there are small grain irregularities, then they should be put next to the strap side of the blank; the wood remains relatively neutral there, neither in tension nor compression. Our tests have not revealed any advantage in bending wood with the growth rings of the blank's plane, rift or quarter-sawn face against the form face. Never attempt to bend a piece that is thicker than its width, as it is prone to collapsing. To obtain such narrow pieces (e.g., a boomerang), bend a square cross section, and then bandsaw it to the required width once it is dry (see Figure 3).

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When to Steam Bend

The ability to bend solid wood can give your projects both a structural advantage and an aesthetic appeal. For example, a sweeping curve on the back leg of a chair can be very weak if it is cut from a wide solid board. A portion of the leg will inevitably be short grain and prone to failure if it is subject to any stress. That same leg, if steam bent, will retain virtually all of the strength of the original straight piece of wood. The grain will also follow the curve and visually reinforce the shape you have created.

Questions that you might want to ask yourself before deciding to steam bend a curved part of your project are:

  • Is the curved part structurally important?
  • Is it visually important that the grain follow the curved shape?

If both answers are "No", maybe a bandsawn shape will do. From a design point of view, try to avoid steam-bent parts that have unanchored ends that are not fastened down. Humidity changes can cause the extended piece to "wave" a bit.

  • Will I be shaping the piece afterwards?

Laminating a curve from thin strips of wood may cause problems when applying the finish. Any glue that is exposed during shaping will not accept the final finish in the same way as the wood. Also, the laminates are always under stress and, if some are cut away during shaping, the curvature may change. Steam-bent parts, on the other hand, have no memory of ever being any other shape, unless immersed in water.

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Making Forms for Wood Bending

The best bending forms are made of plywood stacked slightly higher than the width of the blank to be formed. Particleboard can also be used, but is lower in tensile strength, so larger cross sections are necessary. When bending to a tight radius (e.g., less than 4"), it is best to use plywood for most of the form and insert a hardwood nose.

If bending on a bench, screw the form down to a larger piece of particleboard and in turn secure that to your work surface. If you are using a bending table, you will have to drill holes in the form to match table holes. Whenever possible, the back of the bending form should be cut parallel to the face; it makes it much easier to apply clamps as the bend progresses.

There are some exceptions to this practice. To minimize clamp congestion, large holes (e.g., 1-1/4") can be drilled in the form to accommodate clamp heads (see Figure 5).

If you are using particleboard forms, it may be necessary to use more holes because of the greater width of the form. It might even be simpler to leave the form in its basic rectangular shape and rely completely on clamp holes and bar clamps.

The work surface should be attached to the floor (or anchored to the wall or a post) if the blank is thicker than 3/4". Whenever possible, cut the back of the bending form parallel to the face (see Figure 6 and Figure 14) so that it is easy to clamp the part as the bend progresses.

When bending a piece narrower than the strap, it is best to still have the part run down the center line of the strap. This helps to keep the forces on the strap and end stops balanced, which in turn reduce the possibility of the workpiece skewing perpendicular to the bending plane. Block the table up with spacer blocks 2" apart along the face of the form so that the part can be tapped down snugly to the blocks to keep it parallel to the table during the bending process.

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Springback, Close-In and Failures

It is difficult to give hard and fast rules in wood bending because of the variability of the material. Not only do you have to deal with differences between species but even within species. There is even variation within a single board due to the uneven grain. As with many things, your own experience becomes your best guide. But there are general precautions you can take that will avoid many problems.

If all the main variables are under control: air-dried domestic hardwood at the appropriate moisture content, temperature in the box, steam time, bending radius not too tight, correct end pressure (not so compressive as to create wrinkles on the concave face nor so loose that fiber separation occurs on the outside of the bend) and the form is cut to the exact shape you want, there are still a few things to keep in mind.

Springback

Springback can occur when the curve is so shallow that the lignin does not shift enough to hold the new shape, the part is too dry (either from original moisture content too low before steaming or insufficient steaming time), the end pressure was not adequate, the part was not left on the bending form until fully cooled, the part was allowed to straighten while being transferred to the drying jig, or the part was not allowed to dry thoroughly (less than 10% M.C.).

Close-In

Close-in can occur for several reasons. If the moisture content of the wood is too high when bending (in excess of 30%), the displaced and compressed wood fibers continue to contract as the bent wood dries out.

If the accumulated end pressure is too great during the bending process, the overcompressed inner face of the blank will contract as the wood dries out. On a "U" shape, it is advisable to back off the thrust screw a couple of turns half way around the "U". (See Figure 19.) If a part is bent to a radius smaller than the limiting radius for that species, again the overcompressed face will contract on drying.

Failures

Most bending failures happen when you step outside the guidelines (e.g., you might be trying to bend kiln-dried wood, or you forget to pre-tension the strap). But if you follow the guidelines, you will have very few failures. When you do, you will often find that there was a pin knot at a critical spot in the wood that you did not notice or another abnormality at a point of high compression. If you follow recommended practices, your only bending failures will occur at some point of inconsistency in the wood.

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The Drying Process and Setting the Shape

The drying rack should be exactly the same shape as the original bending form because, as the part dries, it takes the shape of the drying rack. It is best to support the part being dried over most of its length. For example, placing a clamp diagonally across the legs of a 90? "L" bend may stop the shape from opening, but it won't stop the legs of the "L" from bowing (see Figure 7). It is better to cut "L"s out of plywood and clamp the steam-bent "L" shape to the drying rack at several points.

For one-offs, simply leave it on the bending form but remember to take the strap off within an hour; otherwise, the wood will discolor. For very small quantities of relatively narrow stock, cut 3/4" plywood drying racks. Be sure to clamp the drying rack on the center line of the bent part, as shown in Figure 8, otherwise lateral twisting may occur.

For wide bent parts or large quantities of narrow parts, constructed drying racks are better. A bulkhead every 12" will stop the cross members from deflecting. The cross members should be closer together at the curved portion of the part.

There is an accumulation of stress as the parts are loaded onto the drying rack; it must be strong enough to withstand this. Whatever the drying rack used, it is absolutely essential that the parts be moved quickly to the drying rack from the bending form. Unless the parts are left on the bending form for 8 hours before moving to the drying rack, the parts will immediately start to open up as you move them. Typically, the parts are removed from the bending form after 20 to 40 minutes and transferred quickly to the drying rack. This time can be reduced to as little as 5 or 10 minutes if compressed air is blown across the part to cool it quickly and thus start the resetting of the lignin bond. Speed is of the essence; otherwise, breakage will occur as the part is rebent on the drying rack.

The ideal drying condition involves passage of lots of dry, warm air over the parts. Pieces at 25%, 1" thick will dry to 8% in as little as 96 hours. A danger in drying too fast is surface checking and/or developing a case-hardened outer shell on the blank. Once the parts are mounted on the drying rack, put it aside for 12 hours covered with a fabric blanket. This allows the surface moisture from the steaming to evaporate slowly, thus avoiding checks. Checking is particularly prevalent with woods like oak with strong medullary rays. They tend to open on the plane-sawn surface alongside the ray.

Once the blanket has been removed, blow air across the parts with a household electric fan on a timer, one hour on, one hour off. This draws the moisture out of the wood by osmosis without undue damage to the parts. The time lapse from 25% to 8% is approximately 5 to 7 days for 1" stock.

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Moisture Content

Domestic hardwood species that are suitable for bending typically have a moisture content of about 70% when fresh sawn. Wood bends best between 20% and 30%. It is best to catch this "on the way down" rather than to try to remoisturize the wood once it has passed this point. The proper moisture content is important if you are bending to tight radii and less important when bending shallow curves. Once the wood has air dried to 6% or 8% it may not be possible to bend it to small radii. The lignin bond is only partially reversible at this moisture content, particularly if the wood has been sitting for a year or more in this dry state. It is not possible to gain all the original elasticity back by remoisturizing the wood, even by excessive steaming or soaking in water for extended periods (days or weeks).

Moderate bends may still be possible with wood at 10% to 20% moisture content, 10" to 12" radius on 1" stock in ash, oak, etc., but not very tight radii of 1" or 2". Remember, too much moisture (i.e., over 30%) extends your drying time and may contribute to erratic curves piece to piece once the parts are dry. At one end of the scale, wood bent "green" using the compression method may fail due to hydraulic rupturing of the wood cells. Too little moisture makes the wood difficult to bend and you run the risk of breaking your bending equipment.

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Plasticizing the Wood with Steam

The general rule is one hour per inch of thickness (regardless of width). Wood at 30% will require a little less time, wood at 15% a little more. Oversteaming is not recommended, as it may cause compression wrinkles to develop as the bend progresses around the form. Experience is the best teacher.

Try to get as close to 212?F (boiling water at standard atmospheric pressure) as possible inside the steam chamber. We recommend drilling a few drain holes in the bottom of the chamber, particularly at the extremities. Drilling a hole at the top will allow you to insert a thermometer to check for the correct temperature, if you are concerned about the temperature. This hole can be plugged with a cork, but leave the drain holes open. If the temperature at the extremities in the chamber falls below 200?F (93?C), insulate the outside of the box with fiberglass (similar to what is used to wrap water heaters).

BE VERY CAREFUL WHEN WORKING AROUND STEAM — it will scald skin on contact. When you open the steam box a cloud of steam will escape. Keep your face and any other bare skin away. Use tongs to pull out the blank. Wear work gloves when handling the steamed bending blanks.

Under no circumstances should the steam chamber be pressurized or allowed to become pressurized should, for example, the drain holes become clogged. It is actually detrimental to successful wood bending and it is extremely dangerous.

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The Steam Chamber

"Keep it simple" is the rule here. Consider the size of the blanks you will be bending. For walking sticks, shelf brackets, small chair parts, etc., a chamber made from commonly available ABS pipe will be sufficient (Figure 10). However, a chamber made from plywood in a straightforward manner will accommodate most needs. There are a few points to consider when constructing the box.

  1. Use 3/4" exterior-grade fir plywood; the glue is steam resistant. (The glue line should be very dark brown.) Do not attempt to use interior-grade poplar ply.
  2. Tongue and groove the corners, silicone the joints and use brass or stainless-steel screws every 6" along the corners.
  3. Do not paint or seal the box in any way. The steam will eventually penetrate the finish and rot the box. It is best to let the plywood absorb the steam and then thoroughly dry out between uses.
  4. An interior end dimension of 6" square seems to work well. With the use of our kettle, making the length modular is also a good idea. Start with a 4' section, then add 2' sections as necessary. A dried bead of silicone makes an adequate gasket between the sections. If only localized steaming is necessary, then the open ends of the box can be fitted with specially made baffles or simply stuffed with rags. If the box is 6' or more, then two steam sources should be used.
  5. Install hinged doors on both ends of the 4' box and one door on the shorter sections. The door should be operable in as simple a way as possible. A rubber band stretched over protruding screw heads is fine. The screws on frequently used latches tend to work loose in the hot, damp plywood.
  6. The blanks should be supported off the bottom of the box. Brass or copper rods or even 1/2" dowels through the sides are all effective. Do not pack the blanks in the box tightly. Steam should be free to circulate around all surfaces of each blank.
  7. When set up, the box should slope to a drain hole in one end to prevent condensation from collecting. Do not reuse this water because it will have contaminants that may damage your steam source.

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Generating Steam

For blanks of smaller cross sections, up to 1" by 2", an electric kettle is an excellent source of wet steam. The kettle offered by Veritas comes with a spout that can be inserted directly into the steam chamber.

For volumes of steam beyond the capacity of a kettle, commercial steam generators are available. Wallpaper steamers can be purchased or rented. A simple source of steam can be fashioned using an inexpensive electric hot plate and a 1 or 2 gallon (4 to 8 litre) pot with a 1-1/2" diameter hose from the lid to the chamber. If your source of heat is an open flame, then it must be used outdoors.

CAUTION: Be very careful when working around boiling water and steam. It will scald skin on contact. Under no circumstances should the steam source be allowed to become pressurized.

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The Steam-Bending Table

If you are going to be doing a large amount of steam bending or bending pieces with large cross sections (greater than 2-1/2 square inches), you should construct a steam-bending table. A bending table can be built according to the sketch provided, using plywood and 2×4s. Clamped to the top of your existing bench, this table is solid enough to counteract the bending forces exerted. A substantial amount of weight should be put on the base to counteract the force of bending your blanks, or secure your bench to the floor if necessary.

Because all Veritas bench accessories (brass Bench Dogs, Pups, Wonder Dog clamps, and Hold-Down) are based on 0.740" diameter stock (slightly less than 3/4"), we strongly suggest drilling 3/4" holes in your steam-bending table to accommodate all these tools. Spacing these holes in a matrix will also add to the versatility of the table. If you know exactly where all the holes in the table are before you make your forms, you can mark and drill your restraining holes in your forms accordingly. Any 3/4" steel stock (e.g., 3/4" bolts) can also be used to hold your forms in place. If you are bending small stock, under 1 square inch in cross section, 3/4" hardwood dowels could also be used to hold your forms in place. You may want to hold your forms down with several wood screws or bolts during heavy bending so the forms do not lift off the surface of your bending table.

Materials required: two 4' x 4' x 3/4" thick plywood sheets, 24' of 2 x 4s, glue and #10 flat-head screws 2-1/2" long
 

 

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The Block and Tackle

When bending wood with large cross sections (greater than 2-1/2 square inches), you may need the block and tackle to help bend the blank around the form. As the cross-sectional area increases, so does the force required to bend the part. When using the block and tackle, it is best to have it attached directly to your bending table, and not to a structure nearby; otherwise, the pull of the block and tackle will cause you to pull your work around the shop. As shown in Figure 13, try to maintain a 90? angle between the block and tackle and the lever arm as the bend progresses. This exerts the maximum pull on the lever arm at all times.

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Part 1


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