Wood & Steam. Charlie Whinney
the edge of the board because that part of the tree was not straight or there were branches there). For most woodworkers these subtle changes do not matter and give wood its character, but for us it is vital information. The hot wood will offer least resistance where there is most run-out, and we must notice and act to support and clamp the wood during the bending process to prevent the weakest parts of the plank developing kinks, which can then develop into cracks. We can also plan ahead to employ the straightest grain over the tightest part of the bend.
You may also want to use a steel compression strap if the bend is too tight to achieve safely, for example any bend tighter than 1:10 thickness to radius ratio, or if there is any doubt about the quality of the wood. Results of about 1:5 thickness to radius ratio is the limit I normally recommend; however, with very straight-grained ash, oak, elm, or yew, results of 1:2 thickness to radius ratio, or even tighter, are possible with the right setup.
Working with wood in this way is an art as well as a science, and I find the process addictively interesting. In my workshop if wood breaks this is rarely a bad thing, as amid the drama there is always an opportunity to learn something, and it is the continual learning process as well as the opportunity to make wonderful things that keeps me hooked.
GLOSSARY
Here’s a glossary of useful terms and equipment for steam bending that will be referred to throughout the book.
CLAMP
This is something to secure the wood into place while you are working. We will be focusing on using wedges to do this task where possible as they are powerful, fast, and inexpensive; however, metal clamps are also invaluable. G-clamps, F-clamps, J-clamps, and good-quality Quick-clamps are all suitable for steam bending. We have two types of metal clamps in our workshop; “good clamps” and “cheap clamps.” The “good clamps” (heavy-duty square-threaded F-clamps, often very expensive) are used for the actual bending/working tasks. The “cheap clamps” (budget G-clamps or Quick-clamps) are used for holding the wood in place only or more light-duty tasks.
COMPRESSION FAILURE
When the wood buckles and shears on the inside of the bend. If using a compression strap, often tiny little crinkles are visible on the inside of the bend. These can be sanded or machined off to leave a smooth surface. This is not a failure, as the overall structural integrity of the wood is intact. A failure is when part of the wood has actually sheared off the rest, often resulting in the bend not having the desired shape. The solution to compression failure is normally using different wood, the addition of clamps across the width of the wood prior to bending, reducing the tightness of the strap during the bend, or making the bend more gentle in your design.
GRAIN
The longitudinal arrangement of wood fibers or the pattern that results from this. For easy steam bending you want the grain as straight as possible, so the lines you can see on the surface of the wood should be as parallel to the edge of the board as possible. If the grain is not parallel to the edge of the board, this is known as runout.
JIG
An object that does some useful work, for example something to bend the hot wood over to give you a particular sized bend. In steam bending, a wide variety of familiar objects (e.g., cups, car tires, pots, pillars, etc.) can temporarily become “jigs” to bend wood over, and wooden and metal forms are often specially made for specific tasks. If you can imagine a piece of finished work which “could” exist, the design of the jig (or what you use to help you make the work happen) is a result of working backwards to do whatever is needed to make this possible. Jigs come in all shapes and sizes. They are used for bending, twisting, and clamping wood in steam bending. Some trial and error is normally required to get it right!
RUN-OUT
Run-out is a term used to describe when the angles of the wood fibers are not parallel to the edge of the board. Almost every plank will have some run-out. “Lots of run-out” is when a lot of the plank has the grain running off at an angle (the plank is not cut correctly for steam bending or that part of the tree was curved, twisted, or has branches or knots). “Really bad run-out” is when the angle of grain is over about 20 degrees, which will make it harder to bend without it breaking, limiting the tightness of the bend possible.
STEAM CHAMBER
A box/pipe/container which can be filled with steam to heat wood. There are a wide variety of steam chambers used. Materials include foam panels, plastic sheet, plywood, stainless steel, aluminium, and almost every type of insulation is suitable. If the wood can’t fit into your chamber, a “steam bag” can be made to heat large planks, ready-bent awkward shapes, or even just sections of the wood. We will use a variety of steam chambers in this book; boiling in a pan or using a microwave works too. As long as the wood inside gets to boiling point, your steam chamber is good.
STEAMER
This is a steam generator, a device to boil water to make steam for your steam chamber. Pans of water, pressure cookers, teapots, and camping kettles can all be adapted, and an electric wallpaper stripper is a ready-made solution that is very convenient although not necessarily the most efficient. Adjustable steam generators (e.g., a pressure cooker over a gas stove) offer great savings of energy as a well-insulated steam chamber does not need very much energy and steam to keep up to temperature between bends; however, when you open the lid you will need to turn up the power to replace the steam and heat that is lost.
STRAP
This is a compression strap, which is normally a thin piece of steel fixed to the “outer radius side” of the wood before bending, stopping the wood fibers from stretching during the bend. All the straps in this book use 1/32″ (1mm) thick stainless steel that we buy ready-cut from our steel supplier (you normally have to buy a whole or half sheet and they will cut it into your desired-width strips with a guillotine, or you can buy online). Spring steel, mild steel, and galvanized steel are all good too, in thicknesses of up to about ⅛″ (3mm), and thin composite straps of fiberglass or woven stainless steel mesh are good for thinner bends. The purpose of the compression strap is to force the wood fibers to squash and compress during the bend, so materials that can stretch, even a tiny bit, are not suitable. Do not use nylon rachet straps or aluminium as these can stretch. There are several ways to secure the wood into the strap, using wedges, clamps, threaded “adjustable end-stops,” and hydraulic methods. In this book we will be using variations on the wedged compression strap.
TENSION FAILURE
When the wood breaks on the outside of the bend, in tension where the wood is being stretched. The solution for tension failure is normally using cleaner wood, a compression strap, making your existing compression strap tighter, or making the bend gentler in your design.
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