Furniture Maker Tai-Workshop

Aso-Gun Kumamoto Japan

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Contents

The wood bending

The beginning

Wood bending is one of the techniques used in the wood processing from a long time ago. Even it is particularly special, but actually not a difficult technique (especially when using a pure material). Some of the classic examples are boat, barrel and container types. The native Indian of the American traditionally bends a wood to make a snowshoes, sledge, luge and canoe. In Europe, this technique already been used for a long time in the making of chair such as the slat back chair and the ladder back chair that already existed in the middle ages (1100-1450) and often could be seen in their wood block print and the copperplate print. Here we could imagine how old is the origin of the technique. In northeast Japan, this technique is used by the old community in the making of their traditional snowshoes called as "kanjiki". Their method of bending the wood is to place the material directly on the pot, put a cover on it, and after steaming it for a while they just press it to the desired shape. Basically, we can say the wood bending technique is an easy thing to do.

Nowadays, there are various designs for chair with a value-added element, and to make it as a product, the need to learn the manufacturing technique and its application becomes very important. From now on, wood bending is one of the techniques that is necessary in the furniture making and also in its design strategy, make us able to make and design furniture accompanied with a new value-added elements.
This time, I would like to focus on a bending process of a solid wood using the Tohnet's method.

Type of bending process

Bending process such as a heating method, depends on the different of the method used, could be classified as below. These are simple explanation for every bending process.

Wood
Bending
Laminated Bending
Solid
Wood
Bending
Kerfing
Bending process
using a chemical treatment
Ammonia treatment
Alkali treatment
Compression bending
Cold bending
Hot
bending
Internal heating (microwave heating)
External
bending
Direct heating method
Hot plate method
Boiling method
Steaming method High pressure method
Low pressure method

1.Laminated Bending

Jig-2Jig-1

The method used in laminated bending is to press the lamination of veneers glued at the former made from metal, plywood etc. With the former, this method is comparatively easy to make the bend component. It is widely used in the making of chair component for the mass production. In Japan Tendo-Mokko Co., Ltd. (http://www.tendo-mokko.co.jp/) is well known for their specialty in laminated bending.

In laminated bending the thin layer is chosen so that it would be easy to bend (between 1mm to 5mm) depends on its curvature. For the mass production, the ready-made veneer is widely used. Regarding the former, it is better to make both the male former and the female former, but for a trial making or a partial component making, we can make only the one side former and do the clamping.

prototype-1

In America, we can see the laminated bending technique using a vacuum type mold device. Here, the molding range is limited, but of course one side mold is already enough.
Different from a pure bending technique, the laminated bending technique keeps the 'R' shape of the product from straightening out when it is removed from the mold. It has the strength.
One of the merit is there is almost no spring back occurred to the material. Keeping a fixed shape of the 'R', and to get a same quality of the components means a big merit for the maker side. Moreover, since the laminated bending using a large quantity of adhesive material causes a rise in the moisture content, it is important to fully dry it before removing it from the former. Basically, if the drying process is done carefully after applying the adhesive to the material, there should be no spring back problem. But, if it is removed from the mold too quickly, spring back will occur. prototype-2 Moreover, the problem in the manufacturing of the veneer, small crack along the grain of the material after the molding caused by its moisture content are some of the things that might occur. In the case where a gap occurs between the veneer layers, wood dust is applied to the place and super glue is used to fix it (use a low adhesive type used for metal use, high adhesive type for wood material use is not good enough to be used). More, the efficiency would increase if we use a spray type primer.
The reference figures show the bending process work and prototype. Figure at the previous page shows the laminated veneers with a thickness of 1.5mm. Since it is just for modeling, only one side mold is made.

Ref figures : Lounge Chair 1,2
Design/making (top figure) : Kohji Katsuragi , Akira Sugawara / 1986
(bottom figure) : Kohji Katsuragi , Koki Nikaidoh / 1987

2.Kerfing

Illustration of Kerfing

Generally, the method used is to make a saw kerf (cut) on the inner surface of the material. The formula for this kerf bending is the relation between the thickness of the material, the curve 'R' (outer side) and the quantity of the saw cut. Here we figure out how many saw cut can be made with in the R/4 or ? of the circle.

Quantity of saw cut = 1.75 x thickness of wood / thickness of saw blade

In the above formula, the quantity of the saw cut has no relation with the value of 'R'. For example, for the cuts with the thickness of 2mm, to get a 300mm radius of the outer 'R', we need to make 15.7 (about 16 cuts) to the material with the length 471mm (1.57x300). Here, make about 1 ~ 2mm of the outer side of the material remain uncut. Since the 'R' is not included in the formula, even the quantity of the cuts made decided from the formula, there will be a case where we could not get the desired 'R'. Here we need to make a little adjustment, by adding more cuts or by crushing the angle of the saw cut (right figure : above).
The surface where the saw cut is made becomes the inner side, and glue is applied so that the saw cuts are filled. Then bend the material so that the saw cuts close. By this way, we can see the cuts from the end of the material but not from both surface (right figure : below).

3.Chemical softening used in bending process

This type of method is to apply a chemical substance to the material in order to soften it before the bending process is done. Some of the common examples of chemical softening in bending (plasticizing) are ammonia treatment (liquid ammonia, ammonia water, ammonia gas), alkali treatment and etc. Below is an explanation regarding the alkali softening.

3-1.Alkali softening

Natrium hydroxide (caustic soda), caustic potash and etc are the chemicals used to plasticize the material. All these chemicals are used in a solution form. By this way, the process can be done in a normal temperature and normal pressure without any unpleasant smell, and there is no need to provide a special device and a heat source. Moreover, once the material is applied with the chemicals, the softness (ductility) of the material is maintained as long as the water (moisture) is there. Even, when it becomes dry, adding water would make the material to become soft again. The weakness of this method is that the chemical changes the color of the material. The thickness of material suitable for this method is between 10-20mm, means that the range of its use is limited.
The reason for the material to plasticize (becomes ductile) after the chemical is applied is that the fiber (cellulose) that is straight in the normal condition shrinks, cause the structure to slack, thus making the material becomes easier to bend. This has a different in term of physical and the chemical aspect, but it is same with the compression bending (will be discussed later). Moreover, the effect of shrink occurs only when the concentration of the chemical is between 5-10% for caustic soda and 10-20% for caustic potash.

3-2.Preparing the solution

It is practical to use the caustic soda since it is cheap and easy to get. Firstly, make a solution of the caustic soda with a concentration between 10-15%. Dissolve 10-15 parts of caustic soda to 90-85 parts of water in weight proportion. (In the case of caustic potash, make the concentration of the solution to about 10-20%)

  1. Immerse the material into the solution to penetrate the chemical.
  2. Bleach.
  3. Take it out from the alkali solution, put it into the 3-5% hydrogen peroxide solution and bleach (could be skipped).
  4. Wash it with water.
  5. Start the bending until the drying process.

4.Compression bending

This is a new technique in bending process. Firstly, the material is pressed along its length direction (along the grain) before it is bended. Means, the structure of the fiber becomes like a bellow, making it easier to bend. However, compare to other method that usually use a heat (Tohnet's method : will be discussed later), it is weaker in term of the strength.
The method used is to use a device that hold both the ends of the material firmly, then it is inserted into a casing (like a pipe etc) that just fit its size, and using a hydraulic cylinder the material is pressed along its length for about 10%, and then it is returned back to about 5%.
The material used is the one with moisture content of about 20-30%. After it is pressed, it is bended using a former and dried. During the bending, it is better to use the strap so that the material sticks to the former properly. This method is same as Tohnet's except for the pressing process. After the material is processed, if we could maintain the content of the moisture, then we do not have to do the bending process right away. This is the merit of this method.
Even it seems very simple in term of the theory, this method is still not a common thing. 'Compwood' Machine Company from Denmark is one of the little companies involves in the making and sale of the machinery equipment used for cold bending.
In this patent system, material that already dried naturally to 20-25% moisture content and processed using a planer is put to the autoclave and heated using a steam. Next, the process is complete when the steamed material is pressed along its grain with the hydraulic pressing device.
By applying a pressure, a pleat occurs at the wall surface of the fiber (cellulose), and leaving it in several minutes would make it easy to bend even in the cold condition. Then, by dry it in its bended condition, the material would harden. Since, the material that is processed using this compressing method is suitable for storage, it is possible for an effective production control in the making of partial component.

5.Cold bending

Cold bending

Bending method used after the material is boiled or steamed usually called as a hot bending. Cold bending is a method where water is applied to the material before it is put into the mold and bended. The left figure shows a bending process for a slat of the backrest of a chair. The material becomes soft when it is applied with water, so in case of a big 'R' bending, it could be bended sufficiently. However, this method needs to use a good material with no shake compare to the one used in hot bending. Usually, bending with a small curvature has a high possibility of failure. Component with a big 'R' such as the slat of the ladder back chair and a spindles of the Windsor chair etc have a high possibility to bend using this cold bending method. After it is bended, apply a heat treatment would stabilize the form of the material. In Japan, this method is widely used and is not something very particular, where water is applied to the bended material and then it is bended in reverse and hold to fix it again.

6.Internal heating / Microwave heating

Microwave heating

This is a method of using a microwave to do the heating. Microwave is a wave with a frequency about 300Hz to 300kMHz, wavelength of about 1m to 1mm range. In this range we have UHF (Ultra-high Frequency, wavelength 10-1cm), SHF (Super-high Frequency, wavelength 10-1cm) and EHF (Extremely-high Frequency, wavelength 1-0.1cm). The heating process is done using these waves. Microwave, like an AC current repeatedly irradiates a plus and minus charge to the material. At the other side, the molecules constructing the material itself, has a plus and minus characteristic. These molecules respond to the direction of the electric field comes from the external source, but when the frequency becomes higher, they become more independent resulting a friction between the vibrating and rotating molecules. The friction generates heat to the material.

Moreover, water is very ideal substance in term of heating, because its absorption and loss of energy is big (generates heat easily). At the frequency of 10GHz, 915MHz and 2450MHz, it shows a big loss of energy. For that reason, material contains water (moisture) generates heat more easily. The frequency of microwave in the microwave oven used at home is about 2450MHz (245 millions cycles per seconds). By the way, microwave permeates through a ceramic, and reflected by a metal.
Some of the merits of using a microwave are as below.

  1. Heating process can be done to the internal side in a short time.
  2. Shortening the time needed for heating process.
  3. Energy efficiency is high.

For that reason, this method of bending is widely used during softening process, or during a molding process where the heat is applied at the same time (above figure), and the use scope is expanding. The device itself is simple, where the heating part consists of the microwave oscillator, heating room, wave guide assembly, antenna etc. Moreover, since it is possible to bring the microwave to some distance, it is possible to use it for a partial heating (there is a noise problem and there is a case of a maker using it for partial bonding).

7.External heating

7-1.Hot plate method

This type of method is to apply a fire to heat the plate (former), where the material is placed and bended on it. It also can be used to make the bended material becomes straight again. The material is placed and fixed on the plate until it is cold. However, this method is almost not used anymore in the industry. Other method such as steaming or microwave method already took its place.

7-2.Boiling

This type of method is to apply a heat and moisture to the material by boiling it in a hot water. Except for the boiling process, compare to the steaming method, it is all the same in term of material consideration and bending process. However, compare to the steaming method, more time is needed for the drying process after the bending is done. This method is suitable for a partial bending, or a small component such as the spindle.
When we want to bend just some part from the whole material, we just boil it in the hot water. Small component such as a spindle usually just need a short time in about 30 minutes, the drying process also do not take too much time and not much problem is expected.

7-3.Steaming

Steamer

This type of bending is to apply a moisture and heat to the material using the steaming method. Here, steaming method itself can be divided into 2 type, low pressure and high-pressure method. The concept of device for the high-pressure method is same as a pressure pot used at home. Pressure between 3 to 7 atmosphere is applied during the steaming process. Since the materials is steamed in a high temperature and high pressure condition, the process finish in a short time, and becomes easy to bend. For the low-pressure method (at normal pressure), the method is same as a method of cooking using a steamer. The device used for low pressure steaming is simple and very suitable for a small quantity bending. Moreover, it has a high safety.

Tohnet's bending method

Applying a heat to get a ductility - general technique of bending.
This is the method of bending that we are going to do in the course. Tohnet's method use a pure material, where using a heat to ductile the material, then bending is done using a bending strap and a former. It's popular among other technique for bending. Here I will explain about the procedures, points of notice and problems regarding this method.

Tohnet's bending

Tohnet Side Chair #14

The person that responsible to transfer this bending technique so that it could be used in the mass production was an Austrian Michael Tohnet (1796-1871). He started to develop this bent wood technique in 1930.
At the beginning, the method designed by him was to use a glue to bond thin boards together before bending it (a method that now known as laminate; a basic technique to make a cast plywood), and released a chair that using that method. After that, he designed another method of casting, using a bending strap to establish the technique that could be used for a mass production known as "Michael's Method". By using this technique, the pure material could be bended effectively. Here the wood material has to go through the steaming or the boiling process to improve its ductility, before placing it on the former along with the strap and force it to bend. Then the material will be hold in that condition. If the material is dried in this condition, it will keep its bended shape after that.
His bent wood chair was very epoch-making, not only in the aspect of making method but also in the aspect of design. This is because, his chair was completely different compare to the others, very light, strong, the components were assembled using a screw that was also made from a wood, make it lot easier to fix it when the chair is broken. Moreover, there were no excessive decoration, with most of the components in its structure took a curve shape that make it looks simple and couldn't be found in other chairs at that time, its functions that closed to the daily life and the practicality it has, all these things had made it to become the forerunner for the modern chair. After that, his chair received lots of recognition in the expo and exhibition, made his light and modern bent wood chair to become very popular. Since it was released in 1859 (above figure), more than 200 millions had been sold.

Bending and ductility of wood material

The structure of the wood material

Wood material, like the raw wood will be easier to bend if it contains a lot of moisture. The structure of the material is composed of a slender cell with a hollow in the middle of it. The structure of the cell could be described as concrete with a reinforcing bar. Lies at the reinforcing bar is the cellulose (high molecules that tied to each other like a chain) that crystallize to form the microfibrillar that is filled with a lignin attached to the concrete. In order to improve the intimacy between the reinforcing bar and the concrete, the hemicellulose (a high molecules with a non-crystal characteristic) exists on the surface of the reinforcing bar. Since the microfibrillar is a crystal substance, it takes almost no effect of the temperature and moisture, but the lignin and the hemicellulose soften, causes the wall to slacken, thus making the microfibrillar to move easily.
From the above facts, we know that by using the method of steaming, boiling or the microwave, it will increases the moisture content and the temperature of the material, and the bending process could be done easily.
Moreover, much more transformation could be done to the wood materials using a press method compare to the pulling method. In a room temperature, the wood material with a moisture content of 17% breaks when the warp is about 1% when it is pulled or about 5% when it is pressed. However in the temperature of 100°C with 33% moisture, it breaks when the warp is about 2% when it is pulled, but do not break even the warp is more than 35% when it is pressed. This suggests if we could hold using a pulling technique and only use a press to transform the material, then it is possible to bend it until to a small diameter. There, for a small radius bending, if we put a strap with a suitable thickness to the expanding side of the material, means we join the strap and the wood material together during the bending so that the pulling force would be absorbed by the strap (note : the strap functions as an absorber of the force caused by the pulling action at the outer side), then we should be able to bend it without breaking it. This method is called "Tohnet's Method" and is widely used now.

Note 1) In a rough calculation, based on the Young index of both the hardwood and the strap, we assume that the force produced by both materials are same at the contact surface, then we could calculate the thickness of the strap to be about 1/10 of the thickness of the bended wood material. However, this value is just for reference because same wood material is not equal at all, and the expansion quantity of the strap in case of mild steel and steel is different.

The suitable material for bending

Below we discuss about the condition of material that is suitable with low possibility of failure to be used in bending.

  1. Material with good grain condition (with a little shake)
  2. High ductility in characteristic.
  3. Use materials that are free of defects such as knots and checks.

A bad grain (pattern made by the line of fibers in wood) condition and a knot on the surface of the material are the factors that might cause the material to break. In term of material type, the broadleaf tree is more suitable to use than the conifer, and the tropical wood is hard to bend. Generally, a flexible material is easy to bend. Moreover, round shaped material makes a line contact with the bending strap cause a crack to occur easily. Elm, hackberry, oak, beech, ash, hickory, and walnut are some of the examples of materials that are easy to bend. For a tropical wood, rubber, taun, teak etc are comparatively easy to bend. Moreover, in some cases, same materials are different in the characteristic depends on their source, region etc. For that reason, there are many cases where we would not be able to decide whether the materials are easy to bend until we actually try to bend it. This different comes as a different at the curvature of the material. Moreover, it is said that the different among the same materials in term of weight and growth rate has a connection with the different in the aspect of the easiness to bend and the finishing 'R' condition of the material.

Equipment used for bending process

1.Steamer

Steamer 1

Here I introduce a simple type of steamer that could be used in the prototype making or to be used in a workshop and education institution. This time we are also going to use this type of steamer. It is the same as the one that used in England to make a traditional Windsor chair. The material from a wood like the water-resistant plywood is the best material to make the steamer. The reasons are as below.

  1. A good heat-retaining characteristic, make it hard to condense in the cold region.
  2. It can be made easily to any size.
  3. Cheap.
Steamer 2

Dimension of steamer
225(h) x 260(w) x 1625(l)(mm)

The using method is to place the steamer directly on the pot to steam the material. Hose is used to guide the steam into the steamer, and it is the easiest method.
There is no need to provide a ceiling at the body of the steamer. The steam might come out but it would not be a problem. The steam might escape moderately, but there is no need to hold it from coming out. In side the steamer, a sticker is fitted to float the material.By this way the steam circulate thoroughly. When there are many materials needs to be steamed in one time, more stickers are fitted into the steamer in several levels like baking a cookie in an oven.
Steamer 3 The size of the hole used by the steam to enter the steamer usually same as the size of the pot.
In order to make the steamer cover could be opened to up direction, rubber strap attached at its body is snagged to the cover part and locked. When the steaming continues for a long period like a several days, even the concrete panel get warped. Rubber is used because a flexible rubber strap adapts to the warp.

2.Bending strap

The function of the bending strap during the bending process is to absorb the stress caused by the action of pulling. Needless to say, it is the most important part in the bending and its material characteristic decides the success and failure of the bending.
The general steel or a high speed steel are not strong enough, because the expansion of the steel would cause a failure. It is good to use a strap that comparatively easy to get, like the band sawing blade with 1mm thickness that is made from the carbon steel. Stainless steel does not get rusty, and does not make the wood material dirty, make it a good material to be used, but the material characteristic need to be equal with a carbon steel. There are various types of materials that could be used to make the stainless steel, so it is important to carefully choose the material to avoid a failure caused by the lack of strength. The width of the strap depends on the width of the wood material we want to bend. As a notice, the bolt used to fix the strap must be the highly strong high tension bolt.
The strap needed depends on the bending done in one time. Basically, after the material is bended, it could not be removed until the material is fully dried.
Except for the stainless type, other type of strap made from steel reacts with the wood material where usually the contact surface becomes black in color. So there is a need to place a veneer between the strap and the material, or by sticking something like the vinyl sheet to the strap using an adhesive.

3.Former

Former is one of the very important equipment during the bending process, and usually different former is needed for every different 'R' of the bended part. Steamed material is bended along the former. The former itself is made using the plywood, or a metal in the case of mass production.
When making the former, the factors of the negative spring back and positive spring back is considered, so the 'R' of it is made a little bit big or a little bit small from the actual finishing 'R'. The factors that decide the quantity of spring back are the material, finishing 'R', moisture content and drying method. In case of the traditional bending method used this time, only the opening section is pressed to the inner side about 10mm from the actual finishing. By this way, we could get about an acceptable good 'R', but in case where we could not get the desired one, the overall is made to be a little bit small including the halfway 'R'. Since there is no actual right one for this, just bend it and at the same time try to verify the quantity of the spring back. Moreover, the spring back quantity also changes depends on the drying process after the bending. At the company etc, they are using the portable drying furnace with in some period to make a suitable shape for the former. Regarding the thickness of the former, there is no problem to make it same as the thickness of the material, but give a little bit leeway to its thickness.

In the case to get a uniform bending between the left and right side, there is a need to make a mark at the center of both the former and the material. By this way, the bending material is fixed at the center, and the material bends uniformly to the left and right.

4.Shim

Shim is inserted between the material and the end block of the bending handle, functioning to adjust the gap. Usually, the material needs to be cut a little bit shorter. After the material is steamed, usually expansion occurs along the material make it longer than before. So if it is cut exactly to the desired size, there is a possibility it could not be set into the bending handle. Depends on the material, the expansion quantity is different, so prepare a shim with a size of 2-5mm. There is a matter needs to be considered regarding the clearance between the material and the end block, depends on the thickness of the shim to be used (will be discussed later).

Bending procedures

1.Softening process

Before starting the bending, the material needs to be softened first. As explained earlier, this is to improve the ductility of the material, thus lessening the possibility of the material to break. Here, boiling and steaming are the general method used. We can think boiling and steaming as a process of applying a heat and moisture to the material. Important points need to be considered during steaming process are the material's moisture content, processing temperature and processing period.
Regarding the period of the softening process, the practical standard used is for the air-dried timber, for every inch of the scantling material, 1 hour is needed to steam (or boil) at normal pressure (used in many cases). From my experience, both the air dry or the kiln dry timber could be bended without any problem after it is steamed for 1 to 2 hours.
To be more specific, it is good to use a material at its saturation point (moisture content of 30%). For that reason, depends on the material used, there is also a bending method where the moisture content is increased to its saturation point before it is bend. A testing was done to know the time needed when using a boiling method to reach the saturation point (30% moisture content). The result was that japanese beech, jonkong and ramin needs 30 minutes, japanese ash, red lauan (shorea spp), red seraya (shorea spp) and natu (plaqium spp) needs 1 hour and lastly the maple, kapur and bintangor needs 2 hours.

(Material size : 20x90x580mm/ Study report 1968/ Hokkaido Forest Products Research Institute)

The table below shows the measurement result of the moisture content and the temperature change when using a steaming method. Caused by the material that is thick, more time is needed to increase the moisture content.

(Nobuyoshi Ishii/ 1993 Study report/ Hita Industrial Art research Division)

Moreover, since it takes time to increase the moisture content of the material, so by applying a heat at the same time penetrate a moisture to the material under the decompression condition, we could get a material with a high ductility in a short time, thus making it easy to bend. There is now a research (Japan Patent Acquisition Authority) regarding this thing (Shigeru Yoshiwara 1981).

Material Change of moisture content %
Before process After 1 hour After 2 hour After 3 hour
Red Oak 12 15 17 18
Elm 12 17 18 20
White Ash 12 15 16 16
Beech 12 17 19 21
Hackberry 12 17 19 22
Size of material : 50x100x600
Material Change of internal temperature C°
Before process After 1 hour After 2 hour After 3 hour
Red Oak 15 97 91 96
Elm 15 96 91 97
White Ash 15 92 88 93
Beech 15 89 92 97
Hackberry 15 97 92 98
Size of material : 50x100x600

2.Bending procedure

Bending procedure

The former must be fixed firmly with a bolt to the workbench so that it would not move. Do not forget to wear a cotton glove to prevent burn during the bending work.

  1. After the material is taken out from the steamer, set it right away to the bending handle and insert the shim with a suitable thickness.
  2. Make sure the center marking made in advance at the material and the former meets with each other and fix it using a wedge.
  3. Start to bend the material promptly, same to both right and left side, and clamp it so that it would not straightening out by itself.
  4. Together with the former, take them off from the workbench and dry it. Or together with the bending handle, take them off from the former and dry it (depends on the case).
Fix in the former

After taking out the material from the steamer, bend it as soon as possible. This is the key to make it successful. When taking out the materials from the steamer, take out the thin and the slim one first. It is better to steam the thick one much more longer.

Figures at below show a former and its using method used by the maker specialized in wood bending. The former is made from steel.

3.Drying

In the bending process, after the material is taken out from the former, it is important to make sure the material has a little spring back and a little varying in property. For that reason, after the bending is finish, the material is dried in a high temperature without taking it out from the former. During the drying process, we need to relief the internal stress of the material, stabilize the moisture content condition and to fix the radius of the curvature as planned before.
At one company, the method used is to circulate a 50-60°C wind for about 12 hours to dry the material. In order to prevent a crack from occurring at the surface of the material, the drying is done in a low temperature condition. By this way we could assume that the material somehow is in a negative spring back condition.
As a reference, here I carry out a result for one drying test done. Every type of hardwood, after it was bended, using the electrical drying machine (fixed temperature), drying test was done under the condition as below.

  1. Temperature 80°2 hours
  2. Temperature 100°2 hours
  3. Temperature 100°4 hours

After the drying process, the material is leaved in the air (room temperature 20°C, humidity 50%) for 1 hour. After that, the material is removed from the former and after 1 hour the spring back quantity is measured.

Drying condition Spring back Beech Oak Maple
80°C : 2hours % 10.0 4.3 2.9
80°C : 2hours % 1.4 0.0 2.9
80°C : 2hours % -1.4 -1.4 -1.4

In the condition of 100°C / 4 hours crack at one part and a spring back occurred. In the condition of 100°C / 3 hours, the moisture content of the material became 10% with the smallest quantity of spring back. However, the data for the spring back quantity is taken after 1 hour, and the quantity is expected to increase with the change of time (related data will be given later).

(Study report 1968/ Hokkaido Forest Products research Institute)

Problems during the bending process

The some examples of the failure

1.The some examples of the failure

Illustration below shows some examples of the failure occur during the bending process. Most of the problems are around these types.

Collapse of compression side (above figure)
Factors : Softening process that is not enough, high moisture content, the use of material with holes and the use of material that is not flexible.

Split of draw side (Middle and bottom figure)
Factors : Expansion caused by the bending strap that is not strong enough, softening process that is not enough and small curvature of the material.

2.The spring back

A problem regarding a spring back is something very hard to be explained roughly with so many difficult parts. There are 2 type of spring back. The first one is after the bending is done and the drying is already finished, the spring back usually already occurred here at the moment we take off the material from the former. The other one occurs after that while it is leaved in the open air caused by the surrounding environment (temperature, humidity).
There is also a different depends on the material used. Especially the material that is lack in flexibility, the spring back is bigger. Drying condition and temperature also makes different. Spring back also occurs when the material is not dried enough and the moisture content is high. By the way, spring back quantity is small when microwave is used for the drying process. Depends on the drying method used, humidity in the environment causes a spring back to the material. The table below shows an example in data.

Springback of the bended material
Leave under a natural environment(temperature 18-20°C : humidity 70-80%)
between 24-120 hours
Sample material : hackberry (30x50x1200mm)
Hour 24 48 72 96 120
Hot air drying (%)
(Electrical drying machine under 40-45°C condition, 8hours/day of intermittent operation)
7 8 8 9 10
High frequency wave drying (%) 1 2 3 3 3

(Nobuyoshi Ishii/ 1993 study report/ Hita Industrial Art Research Division)

'R' of the bending material changes depends on the environment condition. In the case where the material is kept in a place with a high humidity, spring back occurs. Bended material, after it is dried must be processed promptly, or fix to prevent the spring back from occurring. Other than that, it must be stored in the environment with adjusted humidity. In reverse, under a low humidity condition, the 'R' of the bended material becomes smaller with the drying process.

3.Clearance

Measurement of stress from pressing action
Average at every clearance taken from 3 samples.
Sample material : hackberry (30x50x1200mm)
Clearance (mm) Stress (kgf/cm2)
0 533
1 429
2 436
3 340
4 329
5 223
The some examples of the failure

Looking at Tohnet's method, bending strap is used to hold the expansion of the material, both ends of the material are fixed and it is bended by pressing it. Depends on the clearance between the material and the end block, the stress produced by the pressing action is different. This would cause a problem as below.
It is different depends on the material, but if the clearance is too big then a crack would occur easily. Moreover, when the clearance is below 0-2mm, this would cause a crush and back ring, and a big force would be needed to do the bending. Normally, clearance is set in the range of 0-3mm. At below is a data regarding the measurement for the stress force produced by the press action (Nobuyoshi Ishii, Hita Industrial Art Research Division).

As shown in the figure below, hydraulic cylinder is hold, and the pressure gauge is used to measure the load produced at the end of the material.

References
Norimoto Misato : Wood bending/ Wood Research Institute,Kyoto University
Nobuyoshi Ishii : Study report 1993/ Hita Industrial Art Research Division
Norimoto Misato : Wood bending by Microwaves/ Wood machine No.104
Study report 1968/ Hokkaido Forest Products Research Institute

(Feb 2002 / kohji Katsuragi)


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