home > Construction > How to bend a board - the simplest technology. How to bend a wooden beam. Reaction of wood to external influences

How to bend a board - the simplest technology. How to bend a wooden beam. Reaction of wood to external influences

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 cut the required component in a curved form, but in this case the wood fibers will be cut and weaken the strength of the part. In addition, the execution results in a fairly large waste of material.

Stages of performing work on bending boards at home:

	Preparation. Choice suitable variety trees and familiarization with general principles work with him.
	Wood bending options. Heating in a steam box, chemical impregnation, delamination, cutting.

Wood is cellulose fibers bound together by lignin. The alignment of the fibers affects the flexibility of the wood material.

Tip: reliable and durable wood material for creating various products This can only happen if the wood is well dried. However, changing the shape of a dry piece of wood is a rather difficult process, since dry wood can easily break.

Having studied the technology of bending wood, including its main physical properties of wood, which allow you 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 by pre-hydrothermal treatment.

You can bend blanks of timber made of laminated wood and solid wood. In addition, peeled and sliced veneer is used to give the required 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 technology for manufacturing bent wood, when the workpiece is steamed, its ability to compress significantly increases, namely by a third, while the ability to stretch increases by only a few percent. Therefore, you cannot even think about bending wood 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 done by yourself. Its main task is to hold the tree that needs to be bent. It must have a hole for steam to escape. Otherwise, an explosion may occur under pressure.

This hole should be in the bottom of the box. In addition, the box must have a removable lid through which the bent wood can be removed after it has received the desired shape. In order to hold the bent wood workpiece in the required shape, it is necessary to use special clamps. You can make them yourself from wood or purchase them 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 in tightly. Such simple crafts can perfectly serve as clamps.

Now you can start steaming the wood. To do this, you need to close the wooden blank in a 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 its expiration, the tree should be removed from the box and given the required shape by bending it. The process should be carried out very quickly, and the bending itself should be gentle and careful.

Tip: due to different 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 the tree is possible during the process of forming its new shape, 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 must be broken. This can be achieved chemically, and it is quite possible to do this at home. Ammonia is best suited for such purposes. The workpiece is soaked in a 25% aqueous ammonia solution, which significantly increases its elasticity. This will make it possible to bend, twist it, or squeeze out any relief shapes under pressure.

Tip: Please note that ammonia is dangerous! Therefore, when working with it, you must strictly adhere to all safety regulations. Soaking of 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, you should leave it in a similar curved form. This is necessary not only to fix the shape, but also to evaporate ammonia. However, bent wood should be left in a ventilated area. Interestingly, when the ammonia evaporates, the wood fibers will regain the same strength as before, allowing the workpiece to retain its shape!

How to bend a board at home: layering method

First, it is necessary to harvest wood, which will subsequently be subject to bending. It is extremely important that the boards are slightly longer than the length of the required part. This is explained by the fact that the bending tames the lamellas. Before you start cutting, you will need to draw a diagonal straight line with a pencil. This must be done across the bottom side of the workpiece, which will make it possible to maintain their sequence after moving the lamellas.

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

It is best to use urea-formaldehyde glue, consisting of two parts. 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. Standard option Wood glue will not work in this case. Although it dries quickly, it is very soft, which in this case is not welcome.

The bentwood product must be placed into the mold as soon as possible. So, another one is placed on the lamella coated with glue. The process must be repeated until the bent workpiece reaches the desired thickness. The boards are fastened together. After the glue has completely dried, you should shorten it to the required length.

How to bend a board at home: cut

The prepared piece of wood must be sawn through. The cuts are calculated at 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 only deform the tree, but even break it completely.

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

The cuts are made across the grain of the wood. Then you need to compress the edges of the workpiece, which will allow you to connect the resulting gaps into one. This is the shape it gets bent at the end of the work. After that it is corrected.

In most cases, the outer side is treated 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 bent wood are hidden very simply - for this, sawdust and glue are mixed, after which the gaps are filled with the mixture.

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

So, using these recommendations, you can bend a tree with your own hands without any problems.

It is difficult to produce curved furniture designs frames, and large curves cut from straight sections of wood will require complex manufacturing techniques to avoid the weakness of short fiber and large, uneconomical waste. However, using dry or wet bending techniques, complex curved shapes can be produced quite economically, and since the fibers will run along the bend rather than across it, the finished product will be stronger. Dry bending involves first dividing the wood into thin sections, but thicker pieces can be bent by soaking or steaming.

Bent Cafe Chairs and Rocking Chairs by Michael Toneti classic example bent furniture made using steaming, and in the thirties of the 20th century, furniture made from glued laminated materials became an element of high fashion, after the invention industrial methods production different varieties plywood. Both steam bending and layer bending can be accomplished in the home workshop, and both methods continue to be used in the antique furniture industry and by skilled craftsmen and designers.

Steamed wood can bend with a relatively large bending steepness. The steam softens the wood fibers enough to bend them and mold them into the desired shape. It may require significant bending force, but this is quite achievable in a home workshop using basic equipment. You will need to make a template, a clamping clamp and a steam chamber. Wood bending is not a precise procedure. There are many options, and often trial and error is the only way to possible ways obtaining the required result.

Thin wooden pieces do not require preliminary preparation. The minimum radius to which it can be bent will depend on the thickness and natural properties of the wood species. Thin wood, in the absence of deformation limiters (and in the form of, for example, a template), when bent freely, will take the shape of a ring if the ends of the workpiece are brought together. To obtain a greater bending steepness, the wood must be steamed and “kept” fixed on the template so that, having taken the desired shape, it is stabilized in this position due to internal residual deformation. When thick wood is bent, it is necessary to limit the stretching of the outer layers to prevent them from peeling off or breaking. The method described here is intended for bending relatively thick pieces of wood.

Wood preparation

For bending, choose straight-grained wood without knots or cracks. Any defect or flaw is potentially a weak point, so certain failures are possible. There are dozens of types of wood that can be successfully steam-bent, and many of them are hardwoods. In the table below you can find a short list of suitable types of material for bending. You can bend well-dried wood, but freshly cut wood is easier to process. Atmospheric-seasoned wood bends better than chamber- or oven-dried wood. If the wood is too dry and difficult to work with, you can soak it for several hours before steaming.

Depending on the type of workpiece, you can cut it to size in advance or do it with a saw, plow or stapler after bending. The latter method is often used in the production of bent furniture such as Windsor chairs and armchairs. Wood with an even and smooth surface finish is less susceptible to delamination and will make final finishing the whole product is lighter. Green wood shrinks more than seasoned wood, and when turned on a lathe before bending, it tends to take on an oval cross-section when drying. Regardless of the shape and size, make the length of the workpiece longer than the length finished product approximately 100 mm. Then, in case of delamination or splitting of the ends after bending, it will be possible to trim off the damaged areas.

To calculate the length, make a drawing of the bend shape on a scale of 1:1. Measure the outside of the curved piece to get the correct length. This will prevent unnecessary stretching of the external fibers, which could lead to cracking under the influence of internal stresses. The softened inner fibers will be able to shrink enough to take the shape of a smaller inner bend.

The key to making a tight bend is to use a flexible clamp. Make a clamp from mild steel, 2mm thick and at least as wide as the workpiece you bend. This will work for almost any job you might need to do. To avoid possible contamination of the surface of the part as a result of interaction reactions chemical elements wood, metal and environment, make the clamp from stainless steel or galvanized steel, or use a polyethylene gasket.

Install end stops or stops on the clamp to secure the ends of the workpiece, thereby preventing the fibers on the outside of the curved part from stretching and delaminating. These stops must be strong enough to withstand significant pressure on them, and be of sufficient size so that the end of the workpiece can rest against the stop with its entire surface. You can make them from thick angle metal or from solid wood, which is usually easier to do.

To equip the clamp with reliable end stops, install metal strips at the ends wooden blocks approximately 225 mm long. Along the center lines of each block, drill two holes with a diameter of 9 mm at a distance of about 150 mm from each other. Mark and drill the clamp strip for the end stop mounting bolts. The distance between the stops should be equal to length workpieces, including allowance. To ensure that the clamp has a lever action that is convenient for operation, attach sufficiently strong wooden blocks to the ends of the strip on its back side using long bolts for fastening the stops.

Making a template

Steamed wood is bent into a pattern that defines the shape of the bend and provides support for the fibers of the interior of the curved piece. The template must be very strong and have a width that is at least equal to the width of the part being bent. It must provide certain possibilities for fixing the workpiece on it using clamps or other clamps.

You can make a template from thick wood, setting the mold on a base made of artificial wood materials, or use sheets of plywood glued together. Because bent wood tends to straighten after the clamp is released, it is necessary to adjust the shape of the template to account for the straightening of the part. To do this, you will have to use the most reliable, although not always pleasant, method - trial and error to determine the parameters of such a correction.

Types of wood for bending with steaming

Ash
Fraxinus excelsior
Fraxinus amcricana
Beech
Fagus grandifolia
Fagus sylvatica
Birch
Betula pendula
Benda alleghaniensis
Elm
Ulmus americana
Ulmus procera
Ulmus liollandica
Ulmus thomasii
Hickory Cartja spp.
Oak
Quercus rubra
Quercus petraea
Nut
Juglans nigra
Juglans regia
Yew
Taxus baccara

Making a steam chamber

Make the steam chamber from plywood for outdoor use or use plastic or metal pipe boxes. Plywood allows you to make using glue and screws simple design exactly according to your specific requirements. This type of chamber is ideal if you plan to steam entire batches of wood. A chamber made of plastic or metal pipe limits the size range, but is quite suitable for small workpieces.

Cut a piece of pipe to the required length in accordance with the dimensions of the workpiece. The length of 1 m is a convenient size that allows you to process entire workpieces or even parts of increased length if you need to bend only its end part. Make removable push-in end flaps from plywood for outdoor use. Drill a hole in one of them for the steam pipe and use a plane to plan the lower part of the edge of the other damper to create ventilation and a drainage hole. Make special “open” flaps with a hole for long workpieces. Place wooden supports inside the corpses so that the workpiece does not touch the bottom of the chamber. Insulate the chamber using polystyrene foam or wooden blocks, securing them to the chamber with wire. Place the camera on stands with a slight inclination to allow condensation to drain out. Provide a receptacle for any leaking water.

Steam can be obtained using a small electric evaporator or a factory-made steam generator, or you can make your own evaporator from a 20-25 liter metal tank with a removable lid or cap. Connect one end of a short rubber hose to a pipe or valve (valve) soldered into the tank, and insert the other into the hole in the chamber damper. To heat the water, you can use a heating device, such as an electric or gas stove. Fill the tank halfway with water and heat it to 100 °C. to ensure a constant flow of steam. As a guide, wood should be steamed for 1 hour for every 25 mm of thickness. Longer steaming will not necessarily improve the ductility of the wood, but may destroy its internal structure.

Wood bending

You will only have a few minutes to secure the workpiece into the template before it begins to cool and stabilize. Prepare your work area in advance. Have a sufficient number of clamps available and, in the case of processing very thick wood, negotiate with a friend for help.

Turn off the steam supply and turn off the steam generator. Remove the workpiece from the chamber and place it in a pre-sized and heated clamp. Install it all together on the template. Secure the center by placing a piece of wood between the clamp and the clamp. Tightly screw the workpiece onto the template and clamp securely in place with several clamps. Allow the part to stabilize for at least 15 minutes before transferring it to a uniformly shaped drying mandrel or template. You can leave the blank on the first template. In any case, aging of the material should take from 1 to 7 days.

Security measures

When bending with steaming, observe the following rules:

Do not over-tighten the steam generator cap or plug.
Provide ventilation for the steam chamber.
Do not operate the steam generator without water.
Do not stand or lean over the steam generator or steam chamber when opening them.
Wear thick gloves or mittens when handling heated workpieces and steaming equipment.
The source of steam must be removed at a considerable distance from flammable objects and materials.

electrical connection

The layers are carefully lubricated with glue, placed in a template and pressed into place. Bent glued units made from veneer, from hardwood and softwood boards, from plywood. In bent-laminated veneer elements, the direction of the fibers in the veneer layers can be either mutually perpendicular or identical.

When manufacturing bent-profile units with longitudinal cuts, it is necessary to take into account the dependence of the thickness of the bent elements on the type of wood and the thickness of the bent part.

As the bending radius of the slab increases, the distance between the cuts decreases, as can be seen in the figure above. That is, the width of the cut directly depends on the bending radius of the slab and the number of cuts.

Now let's look at the theoretical aspects of bending

Curved solid wood parts can be made in two basic ways:

cutting out curved workpieces and giving a straight bar a curved shape by bending it on a template. Both methods are used in practice and have their advantages and disadvantages.

Sawing curved blanks It is characterized by simplicity of technology and does not require special equipment. However, when sawing, the wood fibers are inevitably cut, and this weakens the strength so much that parts with large curvature and a closed contour have to be made up of several elements by gluing. On curved surfaces, half-end and end cut surfaces are obtained and, in connection with this, the processing conditions on milling machines and finishing. In addition, cutting produces a large amount of waste. Manufacturing curved parts The bending method requires a more complex technological process and equipment compared to sawing. However, when bending, the strength of the parts is completely preserved and even in some cases increases; no end surfaces are created on their faces, and the modes of subsequent processing of bent parts do not differ from the modes of processing straight parts.

Element bending
A- nature of the workpiece deformation during bending;
6 - bending the workpiece with the tire according to the template:
1 - template; 2 - notches; 3 - pressing roller; 4 - tire

When the workpiece is bent within the limits of elastic deformations, stresses normal to the cross section arise: tensile on the convex side and compressive on the concave side. Between the zones of tension and compression there is a neutral layer, the normal stresses in which are small. Since the value normal stress changes across the cross-section, shear stresses arise, tending to move some layers of the part relative to others. Since this shift is impossible, bending is accompanied by stretching of the material on the convex side of the part and compression on the concave side.

The magnitude of the resulting tensile and compressive deformations depends on the thickness of the bar and the bending radius. Let us assume that the block rectangular section bent in a circular arc and that the deformations in the bar are directly proportional to the stresses, and the neutral layer is located in the middle of the bar.

Let us denote the thickness of the bar H, its initial length through Lo, bend radius along the neutral line through R(Fig. 60, a). The length of the block along the neutral line when bending will remain unchanged and is equal to Lo = p R ( j /180) , (84) where p is the number pi(3, 14...), j - bend angle in degrees.
The outer stretched layer will receive elongation D L (delta L). The total length of the stretched part of the bar is determined from the expression Lo+ D L= p (R + H/2) j /180 (85)
Subtracting the previous one from this equation, we obtain the absolute elongation
D L= p (H/2)( j /180). (86)
Relative extension Er will be equal to D L/Lo = H/2R, i.e. relative extension when bending D Ll/Lo depends on the ratio of the thickness of the bar to the bending radius; the thicker the block, the larger it is H and the smaller the bend radius R. A similar relationship for the value of relative compression during bending can be obtained in a similar way.
Let's assume that around the pattern R" bent block with initial length Lo and at the same time maximum compressive and tensile deformations are achieved. Designated by E szh the value of permissible compressive deformation of wood along the fibers, and through E grow the value of the permissible tensile strain along the fibers, we can write a relationship for the stretched side
L = Lo(1 + Erast)= p (R" + H) j /180 (87)
From here R" + H = / p ( j /180) .
For the compressed (concave) side there will be L 2 = Lo (1 - Eczh) = p R"(j/180)
or R" = / p ( j /180 ). (88)
Subtracting the second from the first expression, we get
H = )

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