How to bend wood at home. Manufacturing technology of wood bending with steaming

Instruction

For the manufacture of curved wood parts, two main methods are used: sawing according to a template and bending pre-steamed wood using a template on special machine. The first method leads to cutting the fibers and reducing the strength of the parts. Bending also provides a high percentage of useful output of parts and significant strength. Bent parts can be finished with high quality and subjected to various machining (profiling, forming spikes, eyes, etc.).

Ability wooden board to bending is determined by the plasticity of wood and depends on the type of wood. Beech, birch, oak, pine and spruce have the highest plasticity. But it is possible to control the plastic properties of the workpiece by carrying out its hydro heat treatment.

At a temperature of 100°C and a humidity of 30%, some of the substances that make up the cells of the material go into a gel state, while the cell walls and wood fibers become elastic and soft. This wood bends easily. After drying, the bent board retains its shape, as the colloidal substances harden.

The processing of the workpiece consists in boiling it in hot water or steaming with low pressure saturated steam. Steaming is more common, because during this treatment the wood warms up more evenly, there is no excessive moisture in the wood.

When a board treated in this way is bent on a pattern or in a special clamping device, internal stresses arise in the material. Wood stretches on the convex side and contracts on the concave side. In the middle neutral layer, the stresses are zero.

The outer layer under the action of tensile stresses will receive elongation, and the inner one will become shorter. The amount of deformation depends on the thickness of the dock and the radius of the bend. To limit the elongation of wood fibers and prevent their possible rupture, a special tire made of steel up to 2.5 mm thick is placed on the convex side of the bent part. The workpiece is bent together with the tire. In this case, the neutral line extends towards the stretched fibers beyond the board, and the bending occurs only due to compression.

When making a wide variety of joinery, the craftsman often has to use curved parts. It is not always possible to obtain the required shape by sawing, since it is important here to ensure the strength of the material and its economy. In such cases, you have to bend the wood at different angles.

You will need

• - hot water;
• - open fire;
• - sample;
• - steel strip;
• - ammonia water.

Instruction

Use steaming or heat treatment for bending. If you endure wood within a few hours under the influence of high temperature and moisture, it is possible to achieve a change in the plasticity of the material and bend the workpiece at the required angle.

Carefully select which you will subsequently bend. Use boards sawn lengthwise for this processing. Avoid defective and kinky boards, as well as those blanks that have knots. If materials with damaged fibers are used, the workpiece may crack at the location of the defect.

If the original workpiece is made from freshly cut material, heat it over an open fire or in a closed oven. At the same time, do not allow overdrying of wood and its charring. To do this, it is enough to moisten periodically abundantly wood

If there is a need to create a curved wooden element you are likely to run into some difficulties. It may seem easier to saw the required 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. Choosing the right type of wood and familiarizing yourself 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: a reliable and durable wood material for creating various products can only be obtained if the wood is well dried. However, the change in the form of dry wooden blank- a rather complicated process, since a dry tree can easily break.

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 the 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 ways require the application of different amounts of force.

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 a 25% aqueous solution of 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 in no way front 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, 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 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 metal corner or part of a box.

So, using such recommendations, you can bend the tree with your own hands without any special problems.

If you decide to decorate the room with wood or start creating beautiful furniture in a classic style - then you will need to make curvilinear parts. Luckily, wood is a unique substance, as it allows the experienced craftsman to play 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 it is done 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 the curved beam or board do not go "end" sections of the fibers, so subsequently you can process such workpieces 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 data on the physical properties of popular wood species, 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 of preliminary preparation of 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 from relatively hard rocks become about ten to twelve percent thinner when bending with pressing, and pine and spruce blanks become 20-30% thinner. But to positive moments This method should 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 it is also applicable household microwave). 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 the optimum 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 changes, 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 plan on extensive finishing, don't 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 moisture. 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 - by atmospheric means.
• 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. Therefore, when high precision is required in the manufacture of a 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.

The layers are carefully lubricated with glue, laid in a template and pressed in. Bent glued knots produced from veneer, from plates of hardwood and coniferous species, from plywood. In curved glued veneer elements, the direction of the fibers in the veneer layers can be either mutually perpendicular or the same.

In the manufacture of 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.

With an increase in the bending radius of the plate, the distance between the cuts is reduced, as can be seen in the figure above. That is, the width of the cut directly depends on the bending radius of the plate and the number of cuts.

Now consider the theoretical aspects of bending

Curved solid wood parts can be produced in two fundamental ways:

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

Sawing curved blanks is simple 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 of large curvature and a closed contour have to be assembled from several elements by gluing. On curved surfaces, one-butt and end surfaces of cuts are obtained, and in connection with this, processing conditions worsen on milling machines and finishes. In addition, when cutting, it turns out a large number of a large amount of waste. The manufacture of curved parts by bending requires, in comparison with sawing out, a more complex technological process and equipment. However, when bending, the strength of the parts is completely preserved and even in some cases increases; end surfaces are not created on their faces, and the modes of subsequent processing of bent parts do not differ from the modes of processing of straight parts.

Element Bend
a- the nature of the deformation of the workpiece during bending;
6 - bending a workpiece with a tire according to a 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 stresses on the convex side and compressive stresses on the concave side. Between the zones of tension and compression there is a neutral layer, in which the normal stresses are small. Since the value of normal stresses varies over the cross section, shear stresses arise, tending to shift 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 strains depends on the thickness of the bar and the bending radius. Let us assume that a rectangular bar is bent along an arc of a circle and that the deformations in the bar are directly proportional to the stresses, and the neutral layer is in the middle of the bar.

Denote the thickness of the bar H, its initial length through Lo, bending radius along the neutral line through R(Fig. 60, a). The length of the bar along the neutral line during 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 an 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 equation from this equation, we obtain the absolute elongation
D L= p (H/2)( j /180). (86)
Relative extension yer will be equal to D L/Lo=H/2R, i.e. bending elongation D Ll/Lo depends on the ratio of the thickness of the bar to the radius of the bend; it is the larger, the thicker the bar H and the smaller the bending radius R. A similar ratio for the amount of relative compression in bending can be obtained in a similar way.
Suppose around the pattern R" bent bar with initial length Lo and at the same time, maximum compressive and tensile deformations are achieved. Denoting through E compress the value of the allowable compressive deformation of wood along the fibers, and through E the value of the allowable tensile strain along the fibers grows, we can write the ratio 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 = )