Wood bending. Wood bending technologies Is it possible to bend a plastic board along a plane

Bending wood with steam. Or how to bend a strong, unbending oak into the shape you need without any problems.

I have been working with flexible wood for 13 years now and during this time I have built many steaming chambers and tested them in action. different systems steam generation. What you are reading now is based on reading literature and personal practical experience. Even mostly experience. I usually worked with oak and mahogany (mahogany). Had a little deal with thin birch veneer. I have not tried other breeds, because I am engaged in the construction and repair of boats. Therefore, I cannot judge with authority about working with other species such as cedar, pine, poplar, etc. And since I didn’t do it myself, I can’t judge it. I write here only about what I personally experienced, and not just read in a book.

After this introduction, let's get down to business...

To begin with, there are a few basic rules that are always respected.

By steaming the wood to bend it, you soften the hemicellulose. Cellulose, on the other hand, is a polymer that behaves like resins - thermoplastics. (Thanks to John McKenzie for the last two suggestions).

To do this, you need heat and steam at the same time. I know that in Asia people bend wood and just over the fire, but that wood is definitely quite wet - usually freshly cut. Shipbuilders in ancient Scandinavia harvested materials for plating their ships and put them in a salt water swamp to keep them flexible until they needed to be put to work. However, it is not always possible for us to obtain freshly cut wood for this purpose, and excellent results can also be achieved using ordinary air-dried wood. It would be very good if, a few days before the operation itself, you immerse the workpieces in water so that they gain moisture - those Vikings knew what they were doing. You need warmth and you need moisture.

The main rule is about steaming time: one hour for every inch of wood thickness.

As I found out, along with the probability of understeaming the workpiece, there is also the probability of oversteaming it. If you have steamed an inch board for an hour and it cracked when you tried to bend it, you should not conclude that there was not enough time. There are other influencing factors that explain this, but we will return to them later. Longer steaming of the same workpiece will not give a positive result. It is not bad in such a situation to have a workpiece of the same thickness as that intended for bending and which is not a pity. Preferably from the same board. It is necessary to steam them together and after the supposedly necessary time to get a test sample and try to bend it into shape. If it crackles, then let the main workpiece steam for another ten minutes. But no more.

Wood:

Usually, the best option it will if you can find freshly cut wood. I understand that the carpenters-carpenters will shudder at these words. But the fact remains that fresh wood bends better than dry wood. I can take a two meter inch board from white oak, clamp one end of it in a workbench and bend it along any curvature I need - fresh wood is so malleable. However, of course, she will not remain in this state and she will still have to soar.

In shipbuilding, the main evil is rot. If you are concerned about this issue, then take note that the mere fact of steaming fresh wood eliminates its tendency to rot. Therefore, you don’t have to worry - the frames of the boats are usually made from fresh oak bent under steam and do not rot if cared for. It also means that at least blanks for the Windsor chair can be made in this way. However, I have also worked a lot with air-cured oak and the result was also excellent.

When selecting wood for bending, one thing should be avoided - the oblique layer. If you try to bend such a workpiece may burst.

Therefore, regarding the moisture content of wood, the rules are as follows:

• Fresh wood is best.
• Air-dried wood is a second good option.
• Wood after the dryer is the third and very far from the first two options.

If all you have is after the dryer and nothing else to get - well, then you have no choice. I dealt with this as well. But still, if you can get air-dried wood, it will be much better. Just this week I was bending 20mm thick walnut planks for the transom of my yacht. The blanks were dried for several years and their bending went absolutely smoothly.

Steam chambers.

It is absolutely useless, and even harmful for the bending result, to strive to make an absolutely sealed chamber. The steam must leave her. If you do not provide steam flow through the chamber, you will not be able to bend the workpiece and the result will be as if you had been steaming it for only five minutes. This is familiar to me after all my experiences.

Cameras can be the most different forms and sizes. It should be large enough so that the workpiece is, as it were, in a suspended state and steam flows around it on all sides. A good result will be obtained from pine boards with a section of the order of 50 x 200. One way to provide "hanging" the workpiece is to drill through holes in the side walls of the chamber and drive hardwood round rods into it. With their help, the workpiece will not touch the bottom and the area of ​​\u200b\u200bclosed wood will be minimal. However, you should not make a chamber of such a size that the amount of steam generated is not enough to fill its volume. The chamber should be such that it is humid inside and the steam rolls in waves. This means that the dimensions of the chamber must correspond to the capabilities of the steam generator (well, or vice versa).

When I needed to bend a five meter mahogany board with a section of about 200 x 20 for the new deckhouse of my yacht, I made a chamber from pine boards with a section of 50 x 300. A 20 liter metal tank acted as a steam generator. The power source was a propane burner. The thing is absolutely wonderful, because it is convenient and mobile. Performance 45000 BTU (1 BTU ~ 1 kJ). This is an aluminum cylinder on three legs and with one burner with a diameter of 200 mm.

I recently found a $50 160,000 BTU propane torch in West Marine's catalog and purchased that as well. With its help, I can bend the frames even for the "Constitution".

When I say "one hour of steaming per inch of thickness" I mean one hour of SERIOUS CONTINUOUS steaming. Therefore, the boiler must be designed to provide steam for the required time. I used a new 20-liter fuel tank for this purpose. It is possible to put blanks into the chamber only when the installation has reached full capacity and the chamber is completely filled with steam. It must be absolutely guaranteed that the water does not run out prematurely. If this happens and you have to add water, it is better to quit this business. Adding cold water will slow down the generation of steam.

One way to make the most of the water is to have the chamber at a slight slope so that the condensation water inside runs back into the boiler. But at the same time, it is necessary that the fitting through which steam enters is closer to the far wall. Another way is to make a siphon system that ensures the replenishment of its level as the water boils away.

Here is a photo of such a system:

In the picture you see a wooden chamber, slightly tilted. Directly below it is the boiler of the steam generator. They are connected to each other by means of a hose from the radiator. If you look closely, you can see an L-shaped pipe coming out at the base of the boiler on the left. This is hard to see in the photo, but its vertical part is actually translucent and thus we will know about the water level inside the boiler. To the left of the cauldron is a white bucket containing make-up water. Look closely and notice the brown tube connecting the bucket to the vertical part of the pipe - the level gauge. Since the bucket is on a rise, the siphon effect is observed: with a drop in the water level in the main boiler, water enters it from the bucket. It can be topped up from time to time, but do it very carefully so that it does not rush quickly into the boiler and cool it too much.

To minimize the need to add water during the steaming process, it is better to start work with a bucket filled to the top. I myself prefer to leave a small air gap in the boiler.

Many chambers have a door at the end, through which you can move the workpieces if necessary and remove them if necessary. For example, if you're in the business of making bent frames and you'd like to do it in as little as a day, you fire up the boiler and (when you're at full capacity) put the first blank inside. After 15 minutes, put the second. After another 15 - the third and so on. When it's time for the first one, you take it out and bend it. I assume this procedure will take less than 15 minutes. When she sits still, the second one is already on the way ... and so on. This allows you to do a great job and avoid oversteaming.

The door has another important function. It doesn't even have to be made of hard material - on my small camera, just a hanging rag serves for this purpose. I say "hanging" because the steam must come out from the end (since steam flow is needed). It must not be allowed that excess pressure will be created in the chamber, making it difficult for steam to enter inside. And besides, the picture itself wooden box, from which steam pours out in clubs, looks quite cool - passers-by just get stupefied. The second purpose of the door is to prevent cold air from entering the chamber from below the workpieces.

So, we will assume that the wood is being cooked (with a pleasant smell) and the templates are ready. Try to organize everything in such a way that the operation of removing the workpiece from the chamber and bending it is quick and smooth. The most important thing here is time. You have a matter of seconds for this. As soon as the wood is ready, quickly take it out and immediately bend it. How fast does human dexterity allow. If pressing against the template takes time, simply bend with your hands (if possible). For the frames of my yacht (which have a double curvature), I took the blanks out of the chamber, put one end into the clamp and bent this end and then the second one simply by hand. Try to provide more fold than is necessary for the template, but not by much. And then attach it to the template.

But once again I repeat - the curvature of the wood must be given immediately - within the first five seconds. With every second that wood cools, it becomes less pliable.

The length of the blanks and the curvature at the ends.

It is practically impossible to make blanks of the exact length and expect that it will be possible to provide a bend at the ends. You just don't have the power to do it. For this reason, if you need a workpiece a meter long, and its thickness is more than 6 mm, you'd better cut off a piece of two meters and bend it. I'm just starting from the assumption that you're not in the workshop. hydraulic press- I don't have one myself. When cutting a blank with a margin, remember that the shorter it is, the more difficult it will be to bend.
And if it is with a margin, then the end of the real part will have a large curvature - for an inch oak board, the last 150 mm are absolutely straight. Depending on the radius required at the end, it may be necessary to resort to wood carving in such places and take into account the required thickness when choosing a material.

Templates.

After steaming the workpiece and clamping it on the template, it is necessary to wait a day for complete cooling. When the clamps are removed from the workpiece, it straightens somewhat. The degree of this depends on the structure and type of wood - it is difficult to say in advance. If the workpiece already has some natural curve in the desired direction that can be exploited (I try to do this whenever possible), the degree of straightening will be less. Therefore, if you need a certain curvature of the final product, the template should have more curvature.

How much bigger?

Here we are dealing with pure black magic and I personally cannot give you any figures. One thing I know for sure: it is incomparably easier to unbend an excessively bent workpiece than to bend a cold, unbent one (provided that you do not have a giant lever).

Warning. If you are bending blanks for lamination, the template should be exactly the shape of the blank in the laminate - I have rarely had a lot of flex in well bent laminated wood.

There are endless options for bending templates. And it doesn't matter which one you choose if you happen to be the owner of a clamp factory - you can never have too many. If wood is bent with a thickness of more than 12 mm, the template must have a significant mechanical strength- he will experience quite high loads. You can see how it looks in the photo at the beginning of the article.
Quite often, when bending, people use a metal strip on the outside of the bend. This helps to evenly distribute stresses along the length of the workpiece and avoid cracks. This is especially true if the fibers are located outside at an angle to the surface.

Well, that's probably all my thoughts at the moment.

If there is a need to create a curved wooden element, then most likely you will encounter a number of difficulties. It may seem easier to saw the desired component in a curved shape, but in this case the wood fibers will be cut and weaken the strength of the part. In addition, during execution, a rather large overrun of material is obtained.

Stages of work on bending the board at home:

	Training. Choice suitable variety tree and familiarization with general principles work with him.
	Options for bending wood. Heating in a steam box, chemical impregnation, delamination, propyl.

Wood is cellulose fibers bound together by lignin. The arrangement of the fibers in a straight line affects the flexibility of the wood material.

Tip: reliable and durable wood material to create various products can only be obtained if the tree is well dried. However, the change in the shape of a dry wooden blank is a rather complicated process, since dry wood can break easily.

Having studied the technology of wood bending, including its main physical properties of wood, allowing to change its shape, it is quite possible to perform bending wooden material at home.

Features of working with wood

Bending of a wooden material is accompanied by its deformation, stretching of the outer layers and compression of the inner ones. It happens that the tensile force leads to rupture of the outer fibers. This can be prevented if a preliminary hydrothermal treatment is carried out.

It is possible to bend blanks of timber made of glued wood and solid wood. In addition, peeled and sliced ​​veneer is used to give the necessary shape. The most plastic is hardwood. Which includes beech, birch, hornbeam, ash, maple, oak, linden, poplar and alder. Glued bent blanks are best made from birch veneer. It should be noted that in the total volume of such blanks, about 60% falls on birch veneer.

According to the manufacturing technology of bent wood, when steaming a workpiece, its ability to compress significantly increases, namely by a third, while the possibility of stretching increases by only a few percent. Therefore, you can’t even think about bending a tree thicker than 2 cm.

How to bend a board at home: heating in a steam box

First you need to prepare a steam box, which can be DIY. Its main task is to hold the tree that needs to be bent. It must have a hole for steam to escape. Otherwise, under pressure, an explosion may occur.

This hole should be in the bottom of the box. In addition, a removable lid must be provided in the box, through which it will be possible to remove the bent wood after it has received desired shape. In order to hold the bent wood blank in the required shape, it is necessary to use special clamps. They can be made independently from wood or purchased at a hardware store.

Several round scraps are made from wood. Holes are drilled in them, offset from the center. After that, you should push the bolts through them, and then drill another one through the sides in order to push them tightly. Such simple crafts can perfectly serve as clips.

Now you can start steaming the tree. To do this, you need to close the wooden blank in the steam box and take care of the heat source. For every 2.5 cm of product thickness, the time spent on steaming is about an hour. After it expires, the tree should be removed from the box and given the desired shape by bending. The process should be carried out very quickly, and the bending itself should be soft and accurate.

Tip: due to varying degrees of elasticity, some types of wood will bend more easily than others. Different methods require different amounts of force to be applied.

As soon as the desired result is achieved, the bent workpiece must be fixed in this position. Fastening a tree is possible during the process of its formation new form, due to which it will become much easier to control the process.

How to bend a board at home using chemical impregnation

Since lignin is responsible for the durability of wood, its bonds with the fibers should be destroyed. This can be achieved by chemical means, and it is quite possible to do this at home. Ammonia is best suited for such purposes. The workpiece is soaked in 25% aqueous solution ammonia, which greatly increases its elasticity. Thus, it will be possible to bend, twist it or squeeze out any relief forms under pressure.

Tip: you should pay attention to the fact that ammonia is dangerous! Therefore, in the process of working with it, you must strictly adhere to all safety regulations. Soaking wood should be carried out in a tightly closed container, which is located in a well-ventilated area.

The longer the wood is soaked in an ammonia solution, the more plastic it will become later. After soaking the workpiece and forming its new shape, it should be left in a similar curved form. This is necessary not only for fixing the shape, but also for the evaporation of ammonia. However, you need to leave the bent tree in a ventilated area. Interestingly, when the ammonia has evaporated, the wood fibers will regain the same strength as before, allowing the workpiece to hold its shape!

How to bend a board at home: a layering method

First, you need to harvest wood, which will later be subject to bending. It is imperative that the boards are slightly longer than the length of the piece required. This is because the bend tames the lamellae. Before you start cutting, you will need to draw a diagonal line with a pencil. This must be done across the underside of the workpiece, which will make it possible, after moving the lamellas, to maintain their sequence.

The boards must be cut with a straight edge, and not with the right side. Thus, they can be put together with the least change. The cork layer is applied to the mold, which will help to avoid any irregularities in the shape of the saw and will make it possible to make a more even bend. In addition, the cork will hold the delamination in shape. After that, glue is applied to the upper side of one of the lamellas with a roller.

It is best to use a two-part urea-formaldehyde adhesive. He has high level clutch, but takes a long time to dry.

You can also use epoxy resin, but such a composition will be very expensive, and not everyone can afford it. The standard version of wood glue in this case will not work. Although it dries quickly, it is very soft, which in this case is not welcome at all.

The bent wood product must be placed into the mold as soon as possible. So, on a lamella smeared with glue, another one is laid. The process must be repeated until the bent workpiece reaches the desired thickness. The boards are attached together. After the glue has completely dried, it should be shortened to the required length.

How to bend a board at home: propyl

The prepared wooden piece must be sawn through. Cuts are calculated for 2/3 of the thickness of the workpiece. They should be located on the inside of the bend. You need to be extremely careful, as rough cuts can not easily deform the tree, but completely break it.

Tip: The key to success when cutting is to keep the distance between the cuts as even as possible. Perfect option 1.25 cm.

The cuts are made across the wood pattern. Then it is necessary to compress the edges of the workpiece, which will allow you to connect the resulting gaps into one. A similar shape and gets a bend at the end of the work. After that it is corrected.

In most cases outer side processed with veneer, less often with laminate. This action makes it possible to correct the bend and hide almost any defects made during the manufacturing process. Gaps in a bent tree are hidden very simply - for this, sawdust and glue are mixed, after which the gaps are filled with the mixture.

Regardless of the fold option, after the workpiece is taken out of the mold, the fold will relax a little. In view of this, it should be made a little larger in order to subsequently compensate this effect. The sawing method is used when bending a metal corner or part of a box.

So, using such recommendations, you can bend the tree with your own hands without any special problems. Bending is one of the methods for making beautiful and durable parts from wood, for example for furniture. Home master it is quite possible to master such a technology. A bent part is much stronger than a sawn one, less wood is spent on its manufacture, and one-butt and end cuts are obtained on the sawn surfaces, which complicate further processing and finishing of parts.

There are three ways of bending wood. One of them - the most famous one - is the preliminary steaming of wood, followed by giving it the required shape in powerful presses. This hot way bending is used mainly in mass production, for example, chairs.

Along with it, especially at home, two other methods of bending wood are practiced, but already in a cold state.

1. First - solid wood bending with preliminary incisions along the bend.
2. The second is bending, in which bent part obtained by pressure in molds from a blank, which is a package of several layers of thin strips of wood smeared with glue.
3. When bending in the second way - with notches - in the workpiece to a depth of 2/3-3/4 of its thickness, narrow grooves parallel to each other are sawn, after which the workpiece is given the desired shape.

The maximum bending radius depends on the depth of the cuts (and therefore on the thickness of the blanks), the distance between them and the flexibility of the wood. The incisions are made both parallel and perpendicular to the fibers. This work operation is carried out using a crosscut or manual circular saw with guide stop. If not special tool, an ordinary hacksaw for wood is also suitable. The main thing is that the depth of the cuts is the same.

GLUING WITH SIMULTANEOUS BENDING

At wood bending fibers on the inside are compressed, and on the outside they are stretched. Wood "tolerates" the compression of the fibers relatively easily, especially if it is pre-steamed. Stretching it is almost impossible.

Flexibility also depends on the type of wood and the thickness of the workpieces. For example, hard wood from temperate climatic zones - beech, oak, ash, elm - can be bent more easily than tropical tree species (mahogany, teak, sipo, etc.). Conifers are too tough for this.

The resistance value of the bent wood until its destruction is determined by the ratio 1:50, i.e. the bending radius must be at least 50 times the thickness of the workpiece. For example, a workpiece with a thickness of 25 mm requires a radius of at least 1250 mm. The thinner the wood, the easier it bends. Therefore, where possible, it is advisable to make a part of the appropriate shape by bending (Fig. 1).

With this method, individual strips of wood of the same thickness and width are glued, laid in several layers so that their fibers are parallel, and placed in a mold made of hardwood. The matrix and the punch of the mold are compressed with clamps and the package is left in this position until the glue dries.

The thickness of the strips glued to each other can vary between 1-6 mm, again depending on the required bending radius. Cold curing glue is suitable for gluing the layers. If the bent-glued blanks are intended for use in outdoor structures, it is best to use a waterproof adhesive.

BENDING WITH THE USE OF CLAMPING DEVICES AND PRESS FORMS

To determine the allowable thickness of veneer strips or planks to be bent (larger thicknesses may break the wood), it is necessary to know the smallest bending radius. Most wood is deformed on the inside of the bend. Therefore, it is always necessary to measure here.

As an auxiliary device, it is advisable to use a template that you can make yourself. To determine the bending radius, we take an ordinary school compass and draw several circles on tracing paper (with a slight increase in their radius) that have a common center. As a result, we get a template. We apply it to the surface of a bend, for example, a mold and move it until we find a suitable circle of the largest diameter. Its radius is measured on the template. Divide the resulting value by 50. The quotient of the division will be the maximum allowable thickness strips of plank or veneer.

When working with molds, bending on outside workpiece should be smoother than on the inside. In this case, we draw two circles from one center, the radii of which differ by the total thickness of the material of the strips.

The most difficult situation is when it is required to bend a part of a complex configuration with different bending radii. Here, bends for the inside or outside of the workpiece can be built freely if its shape is not tied to the contours of any piece of furniture.

The line for the second cut (the first - at the beginning of the bend) can be built in this case like this. We measure the total thickness of the layers to be glued with a compass, draw a circle with it on hard cardboard, cut out a circle and attach it in several places to the line of the first cut. At the same time, we apply a circle so that it is in contact with the first line, and draw its outline, respectively, on the opposite side. The second cut line will be a through connection between these auxiliary lines.

TECHNOLOGY OF BENDING WITH THE PERFORMANCE OF INSECTIONS ON THE WORKPIECES

When determining the number of notches cut on the workpiece for bending along a known radius (it also depends on the width of the groove and the type of wood), we use an auxiliary structure. To do this, we take a bar similar to the workpiece (Fig. 2). We cut out one single cut on it with a depth of 2/3-3/4 of the thickness of the bar. Draw a straight line on a sheet of paper and mark the point of the incision on it.

We put the bar on the paper so that its lower edge before the notch coincides with the drawn line and the marked point of the notch, and fasten the bar with a clamp to the desktop. We set aside the distance of the required radius b on the line and the bar and bend the bar until the upper edges of the notch close. The distance a between the end of the line and the mark on the bar will be the distance between the individual cuts that can be marked on the workpiece.

If the cuts need to be cut on the outside of the workpiece, the distance between them and, accordingly, their number is determined in the same way. We bend the workpiece as much as the elasticity of the wood allows. If the test piece of wood breaks, then this can be expected from the workpiece fixed in the mold.

According to the materials of the magazine "Do it yourself"

Nature does not like straight lines and it takes a lot of time and effort to make wood products straight. However, wood bending is also a rather laborious process. bent wooden elements often used in Scandinavian furniture in the form of beautiful, light and durable products with a unity of structure and form. In this article, we will look at several ways to bend wood: simple power bending, kerf bending, pair bending, and layer bending.

This is the simplest method and consists of manually bending the wood and attaching it to a mold to fix the bend. The thinner the wood, the easier it is to bend. It follows from this that the greater the bend, the thinner the wood should be. The veneer can acquire almost any bend, while a 2 cm thick oak blank is almost impossible to bend. It should also be taken into account that different breeds woods have different degrees of flexibility. So the ratio of the thickness of the workpiece and the minimum bending radius for some species will be: beech - 1/2.5; oak - 1/4; birch - 1/5.7; alder - 1/8; spruce - 1/10; pine - 1/11. That is, with a beech blank thickness of 10 mm, the minimum bending radius will be 25 mm.

As a rule, the workpiece is fixed with glue, since the use of nails or screws in the bends can lead to cracks and breaks. There is an unspoken rule - what can be fixed by hand can also be fixed with glue. When gluing, to reduce gaps, you need to move from the center to the edges, or from one edge to the other.

This method of bending wood is the least durable, since the wood is sawn through almost its entire thickness and there is very little strength left in it. Therefore, this method of bending wood is used only where there is no heavy load, such as rounded corners, etc.

The main material for the manufacture of bent sawn parts is plywood. For best results, cuts are best made on circular saw using a limit stop. Cuts should be made across the fibers at a distance of about 5-10 mm. from each other. The depth of cut should be about 1/5 of the thickness of the workpiece.

To fix the bend, you can use glue or wood filler. By gluing and fixing two curved slats with the cuts inward, interesting elements can be created in some designs, albeit with limited strength.

In terms of its physical properties, such wood is close to hydrothermally bent wood, and in terms of some indicators of dynamic loads, it has even better mechanical properties than solid wood.

The first step is to prepare the slats. The thickness of the lamellas is largely determined by the magnitude of the bend. The greater the bend, the thinner the lamellas should be and vice versa. As a rule, the thickness of the lamellas should not exceed 3.2 mm.

Then the cut layers of wood are smeared with glue and compressed in a template using grippers. To connect the lamellas, you cannot use ordinary carpentry glue. For these purposes, adhesives based on urea-formaldehyde or epoxy resins are best suited.

Before removing the part from the template, the glue should set for a day. It should also be borne in mind that after unclenching the mold, the workpiece may straighten a little. This effect can be minimized by reducing the thickness of the lamellas or creating a shape with a slight inflection.

Hydrothermal bending of wood gives the highest quality result, but requires a lot of labor and technical equipment.

Before starting the bending process, due attention should be paid to the choice of wood. Almost any type of wood can be bent, but hardwoods such as oak, beech, elm, birch, cherry, maple, walnut, ash have the best flexibility. It is not recommended to use coniferous and softwoods such as spruce, pine, cedar, alder. It is also important that the wood is free from cracks and knots at the intended bend.

Each cell of wood fibers is coated with lignin, a glue-like chemical that binds the fibers together at normal temperatures. The heat transferred from the steam softens the lignin, allowing the fibers to contract and stretch. As the lignin cools, it hardens and binds the fibers together again.

Freshly cut wood is best suited for bending. The best option atmospheric drying wood is also, since during chamber drying, lignin is noticeably strengthened, making it difficult to bend. Before steaming the wood chamber drying can be soaked overnight in water. Atmospheric wood and pre-soaked kiln-dried wood require the same processing time.

Wood acquires the best plastic properties at a moisture content of 25-30% and at a temperature of about 100°C. Steaming time depends on the thickness of the wood. So, for example, for steaming a workpiece with an initial moisture content of 30% and a thickness of 25 mm to reach a temperature in the center of the workpiece up to 100°C, it takes 1 hour, and about 2 hours with a thickness of 35 mm.

Before removing the workpiece, make sure that all tools are at hand, as the wood cools and hardens very quickly. Be sure to wear thick gloves to avoid getting burned by the steam. Remove the part and immediately fix it with grippers.

The removal of the product from the mold is usually accompanied by a slight extension. Therefore, the shape must have a bend a little more than required in order to maintain the desired curvature of the product. To minimize buckling, leave the piece fixed in the mold for at least a few days.

If you decide to decorate the room with wood or start creating beautiful furniture v classical style- then you will need to produce curved parts. Fortunately, wood is a unique substance, because it allows experienced master play around with the shape a bit. It's not as difficult as it seems, but it's not as easy as you'd like.

Previously, the site already had a publication on plywood bending. In this article, we will understand the principles of bending massive board and timber, we will find out how they do it in production. We will also bring helpful tips from professionals who will be useful to the home craftsman.

Why bending is better than sawing

curvilinear wooden detail can be obtained in two ways: by bending a flat workpiece, or by cutting out the required spatial shape. The so-called "cutting" method attracts users with its simplicity. For such manufacture of parts and structures, you do not need to use complex devices, you do not have to spend a lot of time and effort. However, in order to cut out a curvilinear wooden product, you have to use obviously too large a workpiece, and a lot of valuable material will be irretrievably lost as waste.

But main problem are the performance characteristics of the parts received. When cutting a curved part from ordinary edged lumber, the wood fibers do not change their direction.
As a result, transverse sections fall into the zone of radii, which not only worsen appearance, but also significantly complicate the subsequent refinement of the product, for example, its milling or fine grinding. In addition, in the rounded areas most vulnerable to mechanical impact, the fibers run across the section, which makes the part prone to breaking in this place.

Whereas when bending, the opposite picture is usually observed, when the wood only becomes stronger. On the edges of a curved beam or board do not go "end" sections of the fibers, so subsequently it is possible to process such blanks without restrictions, using all standard operations.

What happens in wood when it bends

The technology of bending is based on the ability of wood, while maintaining integrity, to change its shape within certain limits as force is applied, and then retain it after removal of mechanical stress. However, we all know that without preparatory activities lumber is elastic - that is, it returns to its original state. And if the applied forces are too great, then the beam or board simply breaks.

The layers of a wooden blank work differently when bent. Outside the radius, the material is stretched, inside it is compressed, and in the middle of the array, the fibers practically do not experience significant loads and have little resistance to the forces acting on the workpiece (this inner layer is called “neutral”). Under critical deformation, the fibers on the outer radius are broken, and on the inner radius, “folds” are usually formed, which are a fairly common defect in the bending of softwood. The fibers of plastic hardwood or softwood can shrink by 20 percent or more, while the stretch limit is about one to one and a half percent.

That is, to determine the possibility for bending (without breaking), the limit of relative elongation of the stretched layer will be a more important indicator. It directly depends on the thickness of the part and determines the radius to be obtained. The thicker the workpiece and the smaller the radius, the greater will be the relative elongation along the fibers. Having information about physical properties popular types of wood, it is possible to formulate for each of them the maximum possible ratio of the thickness and radius of the parts. In numbers it will look like this:

Bending using a steel bar

Bending without using a bar

These data indicate that softwood lumber, in comparison with dense hardwood, is less adapted to free bending. To work with lumber at aggressive radii, it is imperative to use combined methods pre-training parts and mechanical protection.

Tire as an effective way to avoid the destruction of wood during bending

Since the main problem is the breakage of the fibers from the outer radius, it is this surface of the workpiece that needs to be stabilized somehow. One of the most common methods is the use of an overhead tire. The tire is a steel strip with a thickness of half a millimeter to two millimeters, which covers a beam or board along the outer radius and is bent on a template along with the wood. The elastic strip absorbs part of the energy during stretching and at the same time redistributes the breaking load along the length of the workpiece. Thanks to this approach, coupled with moistening and heating, the allowable bending radius is significantly reduced.

In parallel with the use of a steel tire in bending devices and machines, mechanical compaction of wood is achieved. This is done using a pressing roller, which presses on the workpiece along the outer bending radius. In addition, the template mold in such a fixture is often endowed with 3 mm teeth (in increments of about 0.5 cm), oriented towards the workpiece travel.

The task of the jagged surface of the template is to prevent the workpiece from slipping, to prevent the mutual shift of the fibers in the solid wood, and also to create a small depressed corrugation in the concave radius of the part (the fibers are pressed into the array, therefore, problems with folds are solved).

Pressing with a tire allows you to bend bars and boards from softwood and soft hardwood with a minimum percentage of rejects. Please note that parts made of relatively hard rocks become about ten to twelve percent thinner when bending with pressing, and pine and spruce blanks become 20-30% thinner. But the positive aspects of this method include a significant increase in the strength characteristics of the finished product, as well as a significant reduction in the requirements for the presence of flaws and defects in wood blanks.

How to improve the plasticity of wood

In the normal state, lumber has elasticity, significant spatial rigidity and resistance to compression. Wood receives these valuable properties from lignin, a natural "network" polymer that gives plants a stable shape and strength. Lignin is located in the intercellular space and in cell walls, connecting cellulose fibers. In coniferous wood it contains about 23-38 percent, in hardwood - up to 25 percent.

Essentially, lignin is a kind of glue. We can soften it and turn it into a "colloidal solution" if we heat the lumber by steaming, boiling, treating with high frequency current (for small parts household microwave is also applicable). After the lignin is melted, the workpiece is bent and fixed - cooling down, the molten lignin hardens and prevents the wood from returning to its original shape.

Practice shows that optimal temperature for bending solid wood (bar, rail, board) will be 100 degrees Celsius. This temperature must be obtained not on the surface, but inside the workpiece. Therefore, in many respects, the time of thermal exposure will depend on how massive the part is. The thicker the part, the longer it will take to heat up. For example, if steaming is used to prepare a 25 mm thick strip (with a moisture content of about 28-32%) for bending, then on average it takes about 60 minutes. It is noteworthy that the exposure time under steam for parts of similar dimensions for any species is approximately the same.

By the way, it is believed that it is also impossible to overheat the part, since lignin after hardening can lose elasticity and become too brittle.

The boiling method is not often used, since the workpiece is strongly and unevenly moistened, and such water-saturated fibers and cells, when bent, can tear, at least with the formation of a pile. Parts after cooking have to dry for too long. But this method shows itself well if you need to process only part of the workpiece for bending.

Steaming allows you to heat the workpiece evenly, and its humidity at the exit tends to approach the optimum. The most suitable humidity for achieving maximum plasticity of lumber is considered to be in the range of 26-35 percent (the saturation point of the wood fibers).

To steam wood for bending at home, use home-made cylindrical chambers made of metal / polymer pipes or rectangular wooden boxes. Heating tanks act as a source of steam, electric kettles and other similar devices that can provide a temperature of about 105 degrees and a little pressure. This is always followed by the stage of drying the part (+ exposure of the fixed form) to about fifteen percent and its finishing.

Chemical methods of plasticizing wood

It is also known that it is possible to make lumber more pliable using impregnation with various compositions. There are ready-made impregnations that make wood cells more plastic, for example, Super-Soft 2. Some practitioners soak wood in so-called textile conditioners, with similar results.

But rather primitive “recipes” containing ammonia and ethyl alcohol, glycerin, alkalis, hydrogen peroxide, dissolved alum can also be used ... Many of them are extremely simple - they increase the ability of the workpiece to absorb water and help retain moisture in the fibers.

Thin products such as veneer are spray treated, but pre-treatment of normal lumber with chemicals is usually done by full immersion. It takes time for the working substances to get inside the bar or rail, usually it takes from 3-5 hours to several days (although heating helps to reduce the wait).

It is largely because of the duration of the processes that chemical plasticization is not often used, although there are other problems: the cost of chemistry, color change, the need to provide protection from harmful fumes, the increased tendency of such curved parts to straighten ...

Tips for bending lumber using hydrothermal preparation

• Very carefully select the quality of the workpiece for bending. It is better not to use material with cracks, knots (even live and intergrown), fiber inclination. If there are no options for this, then orient the part in the bending fixture (machine or template) so that the defects fall into the concave radius zone, and not into the tension zone on the outer radius. Give preference to the bending method with a bar.
• When selecting a workpiece, it is imperative to provide for a change in the size of the part after molding. For example, the thickness of a coniferous bar can be reduced by 30 percent if bending with pressing is performed.
• Even if you are planning an extensive finishing- do not leave too much material. The thinner the workpiece, the easier it bends without breaking.
• If the amount of work is small, then it is better not to cut out blanks, but to prick them from chocks. So it is possible to avoid cutting the fibers and, as a result, marriage during bending.
• For bending, it is desirable to use lumber with natural humidity. If dry blanks are used, then preference should be given to those that were not processed in a drying chamber, but dried under a canopy - in an atmospheric way.
• After steaming, work with softened wood very quickly, as lignin begins to harden almost immediately, especially in the most vulnerable outer layers of solid wood. Usually you need to focus on a margin of time from half an hour to 40 minutes, so there is no point in making large cameras if you simply do not have time to install all the material from which into templates.
• Position the material in the steam chamber so that it is the surfaces facing the outer radius that are unobstructed by the steam jets.
• To save time, many carpenters forgo the use of clamp templates. Instead, they use metal staples and wedges, or limit posts, on the templates.
• Keep in mind that a curved bar or rail will still tend to straighten. And this straightening always occurs by a few percent. So when required high accuracy in the manufacture of the part, it is necessary to conduct tests and, based on the results obtained, correct the shape of the template (reduce the radius).
• After cooling the part in the form, let it stand for a while. Some experienced furniture makers prefer to make an exposure of 5-7 days. The tire, as a rule, is left fixed to the part for all this time.