Bending technology for joinery wood. How to bend wood - bending wood How to bend a pine wood block

In the woodworking industry, curved parts are produced in large quantities. Curved parts are manufactured in two ways: by sawing out of boards or slabs and bending straight bars (solid-bent parts) or layers of wood with simultaneous gluing (bent glued parts).

Technological process of bending wood. The technological process of bending solid wood bars includes the following operations: material preparation for bending, hydrothermal treatment, bending and drying.

Procurement of material for bending. Blanks for bending are obtained from unedged boards by cutting them on circular saws. The following requirements are imposed on blanks for bending.

The oblique layer should not exceed 10 °. With conventional bending methods, knots are absolutely not allowed in the workpieces. In blanks with simultaneous pressing, knots are allowed within wide limits, which sharply increases the output of blanks. The blanks should be cut out taking into account the allowances for the subsequent processing of parts. When bending with simultaneous pressing, in addition to the allowance for processing, there must be an allowance for pressing the wood across the fibers and an increased allowance along the length of the workpiece. In order to increase the yield of blanks for bending, it is recommended to cut boards after preliminary marking.

At small enterprises, the method of obtaining billets for bending by splitting blocks has been preserved. The chipped workpiece does not have a cross-layer, therefore, when bent, it gives a lower percentage of scrap. However, this method is very laborious, since it is performed manually and gives 20-25% lower output of blanks from the log than when sawing it.

After cutting (or splitting) workpieces for circular section parts are processed on turning-copying or circular-rod lathes, and blanks for rectangular section details - on longitudinal milling machines. You can also bend non-planed workpieces, but in this case, the boards are cut with planing saws, which give a clean and accurate cut.

Hydrothermal treatment. Hydrothermal treatment of wood before bending is carried out in order to increase the plasticity of wood. Optimum plasticity of wood is achieved when it is heated in a wet state. This is due to the fact that when heated, some of the substances that make up the cells pass into a colloidal state.

As a result, the ability of cells and all wood to deform is increased. When drying deformed (bent) wood, colloidal substances solidify and retain the shape given to the workpiece.

Hydrothermal treatment of wood before bending is carried out by boiling in hot water or steaming. For boiling, use wooden vats or metal baths and tanks. The water in baths and vats is heated by steam.

The temperature of the water is maintained at 90-95 ° C, without bringing it to a boil. The cooking time depends on the initial moisture content, size and type of wood.

When boiling, it is difficult to obtain a uniform temperature and humidity of the entire workpiece, the outer layers are oversaturated with water. Therefore, boiling in hot water is used only in cases where steaming is technically difficult.

The most widely used in production is the steaming of wood in an atmosphere of saturated steam. Steaming allows you to heat the wood to the desired temperature (70-80 ° C), regulate the moisture content of the wood and get it always close to optimal for bending, i.e. about 25-30%.

For steaming, low-pressure saturated steam (0.02-0.05 MPa) is used, which corresponds to a temperature of 102-105 ° C. Wood steaming is carried out in hermetically sealed metal drum boilers or concrete chambers. The capacity of boilers and chambers is small, designed for laying bars in the amount of 30-40 pcs.

Boilers are located at each bending machine and are connected by a steam line to each other to form a battery. Bars are placed in boilers and chambers on gaskets in order to ensure better washing with steam.

The duration of steaming depends on the initial moisture content and temperature of the wood, the size of the blocks and the steam pressure in the boiler. The steaming time is determined according to a special diagram. For example, for workpieces with a thickness of 40 mm with an initial moisture content of 30% and a steam pressure in a steaming boiler of 0.03-0.05 MPa, the duration of steaming is 12-13 minutes, and for workpieces with a thickness of 80 mm - 65 minutes.

Plywood in the case of bending to small radii of curvature can also be subjected to hydrothermal treatment. Plywood glued with synthetic adhesives is boiled, and glued with casein or albumin glue is only steamed.

The workpieces removed from the steamer or brewing tank must be bent immediately. Cooling of the outer layers of wood, which experience the greatest stresses during bending, must not be allowed.

Bending of wood and equipment. Wood bending machines are divided into two types: cold and hot forms.

Machine tools of the first type (Fig. 4.13) are used for bending into a closed loop. The bars are bent around a removable unheated rotating template 6. Template with tire 2 fits on vertical shaft 8 , which is driven in rotary motion by an electric motor through a gearbox 7.

The free end of the bar is fixed in the carriage 4, sliding on guides 3. Block 5 is laid between the template 6 and bus 2 and is fixed with a movable stop. Then the electric motor turns on, while the shaft turns 8 with a template put on it and the bar is bent together with the tire.

In the place of the bend, a roller / is installed, tightly pressing the bar to the template. The rear end of the tire is secured with a clip to the template. The template with a bar and a tire is removed from the machine and sent to the dryer, and a new template is put on the machine, and the operation is repeated.

Rice. 4.13.

7 - pressure roller; 2 - tire; 3 - guide; 4 - bar; 5 - blank;

b - template; 7 - reducer; 8 - shaft

Rice. 4.14.

7 - hook; 2 - template; 3 - emphasis; 4 - tire; 5 - blank

Roll-forming machines with hot forms are called bending-drying machines, they can be with two- and one-sided heating. Machines with double-sided heating are a hydraulic or pneumatic press with heated profile template plates, between which bending bars are clamped. In these machines, the bars are held in a clamped state until the shape is completely fixed and the workpieces are dried.

In machines with one-sided heating (Fig. 4.14), blanks 5 are laid between the hot template 2, heated steam, and a tire 4 and are fastened with an emphasis 3. Curved blanks 5 together with tires are fixed on the template with special hooks /. The workpieces remain in the machine until the shape is fixed.

This is achieved by drying the wood to approximately 15% moisture content, which takes 90-180 minutes. To increase the productivity of the bending-drying machines, the workpieces are recommended to be dried to 20% moisture before bending, kept in the machine to a moisture content of 12-15%, and the final drying of the workpieces removed from the machine to the production humidity in drying chambers.

Bending of plywood is carried out in templates consisting of two parts: a matrix and a punch, between which plywood is laid and folded. This uses special tools, screws, pneumatic and hydraulic presses.

Bending with simultaneous pressing consists in the fact that the wood is bent around a template provided with a notch, and in the process of bending from the outside of the workpiece it is pressed against the template through a tire by a pressing roller.

The billet is rolled. The thickness of the workpiece decreases, the layers of wood on the concave side of the workpiece take a wavy shape from the indentation of the notch of the template, the outer layers are compacted. This contributes to an increase in the compression resistance of the concave layers in the wood and the tension of the outer ones.

Bending with simultaneous pressing significantly improves the bending ability of wood, allows bending wood with large knots located on the outside of the workpiece. It is used for bending softwood and softwood.

Drying of workpieces after bending. The bent blanks are dried in drying chambers to operating humidity, and the blanks are placed in the chamber together with the templates and the tires covering them. The design of the drying chambers is similar to those used for drying sawn timber.

The dried workpieces are unloaded from the chambers and sent to the cooling compartment, where they are kept for at least 48 hours to equalize internal stresses. Only after that, the workpieces are freed from templates and tires and sent to the machining workshop.

The sequence and principles of machining bent blanks on machine tools, i.e. giving them final dimensions and a clean surface do not fundamentally differ from the processing of straight workpieces.

Manufacturing of bent glued parts. To obtain bent-glued parts, hydrothermal treatment of wood before bending and drying after bending are not required. Bent parts are made of rotary cut veneer or plywood. The technological process for producing bent-glued parts consists of preparing raw materials (veneer, plywood or thin strips), applying an adhesive solution to the surfaces to be glued, gluing the blanks with simultaneous bending in molds or in templates, and holding the parts after pressing to equalize moisture and stress.

Gluing is done either in blocks or in separate parts. Pressing is carried out in hydraulic presses with molds or templates. One of three types of heating of the pressed package is used: electrical contact, steam or high-frequency currents (HFC). The most progressive heating of HDTV. This method requires less pressing time and a more even temperature distribution over the cross section of the bag.

Adhesives based on carbamide resins of high concentration and increased curing speed are used as a binder in the manufacture of bent-glued parts. The consumption of such adhesives per 1 m 2 of the surface to be spread is 110-120 g.

Many owners of private houses are not satisfied with the traditional form of roofs and standard sizes of buildings. Wood provides a very wide range of construction opportunities, and one of them is the erection of domed structures using curved structures, which allow you to create the most original and unusual outlines of buildings using curved surfaces. In addition, bent glued timber has many more additional advantages.

Areas of application of bent wood elements in construction

This material is used not only in residential construction, but also in the construction of economic and industrial structures.

The curved beam expands horizons in the field of design and implementation of structures, allowing you to solve the following tasks:

The bent beam allows you to overlap large distances between the walls, since beams, trusses and other types of structures for roofing are made from it. This allows the construction of warehouses, buildings for keeping animals in large farms and other structures that require a large area;
The tree is resistant to various factors of influence, therefore, structures made of bent-glued wood are used for the construction of warehouses;

Important! Since wood, impregnated with special compounds, does not rot and does not corrode at all, it becomes even more reliable than metal.

This material is used in the construction of greenhouses and greenhouses, since it can be used to create strong and stable structures of a convenient shape and large area;
The most common way of application: bent laminated veneer lumber - a material for roofs of an unusual shape and domes;

Bent glued laminated timber makes it possible to build small architectural structures: with a domed roof they will be an excellent decoration for the site, they look original and unusual.

The domed roof structure is not only unusual and beautiful from the aesthetic point of view, it is beneficial for other reasons as well. Snow will not accumulate on such a roof, it heats up very little from the sun and does not create a large load on the walls. It is no coincidence that this type of roof has been used since ancient times.

Manufacturing of bent structures

How to bend the timber so that the wood does not crack, and the material itself takes the required geometric shape? They are engaged in the manufacture of bent beams in an industrial environment, but on the other hand, you can try to do this work with your own hands.

Bending wood is a rather complex technological process in which wood is exposed to several factors at once. This work requires a certain skill and strict adherence to technology, otherwise the product will be very fragile.

Usually hardwoods are used to make bent forms, as they are more plastic and easier to process. The main species are oak, alder, maple, ash, etc.

Conifers are rarely used for this. Most of the bent blanks for furniture and other products are made of birch veneer; this material makes up about 60% of all bent glued blanks. An example instruction for bending wood is as follows:

Basics of manufacturing technology for bent elements

The basis in bending wood becomes hydrothermal treatment, that is, the simultaneous exposure to steam and high temperature.

Such processing increases the plastic properties of wood, as a result of which it acquires increased flexibility, and it is possible to change its shape without causing damage to the very structure of the material.

Wood acquires the highest plastic ability when its moisture content reaches 30%, and the temperature in the core of the workpiece is 100 degrees. It is not easy to achieve such indicators at home, especially since steaming must be long. A bar with a cross section of 3.5 cm must be steamed for at least 1.5 hours to become flexible.

The scheme of manufacturing a curved beam

The workpiece is placed on the tire and carefully attached, after which it is steamed for the required time. Billets are supplied with special tapered or rectangular cuts, which will allow the wood to bend;

The prepared material is bent to achieve the required curvature using a tire and a press;
The workpiece, still attached to the tire, is sent to the dryer, where it acquires moisture suitable for construction work.

As a result, the workpiece acquires the required radius of curvature, and it can be used both for construction and for the manufacture of wooden interior elements. Glued laminated timber is made by combining two processes: the lamellas are bent and immediately glued into a single block.

Ultimately, the material retains all the positive, while acquiring a special shape. The technology allows you to create a wide variety of shapes with different bending radii.

If you decide to decorate a room with wood or start creating beautiful furniture in a classic style, then you will need to make curved details. Fortunately, wood is a unique substance as it allows the experienced craftsman to play with the shape a little. It is not as difficult as it sounds, but also not as easy as we would like it to be.

Earlier on the site there was already a publication on plywood bending. In this article we will understand the principles of bending a solid board and a bar, we will learn how it is done in production. And also we will give useful tips from professionals that will be useful for a home craftsman.

Why bending is better than sawing

A curved wooden part 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 manufacturing 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 curved wooden product, it is necessary to use a workpiece that is obviously too large, and a lot of valuable material will be irretrievably lost as waste.

But the main issue is the performance of the parts received. When cutting a curved piece made of ordinary edged lumber, the wood fibers do not change their direction.
As a result, transverse cuts fall into the radius zone, which not only worsen the appearance, but also significantly complicate the subsequent finishing of the product, for example, its milling or fine grinding. In addition, in the rounded sections that are most vulnerable to mechanical stress, 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 timber or board, the "end" cuts of the fibers do not come out, therefore, subsequently such workpieces can be processed without restrictions, using all standard operations.

What happens in wood when bending

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

The layers of the wood 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"). Critical deformation breaks the fibers at the outer radius, while the inner radius usually forms "folds", which are a fairly common defect when bending softwood. Fibers of plastic deciduous or coniferous species 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 of bending (without breaking), the more important indicator will be the limit of the relative elongation of the stretched layer. It directly depends on the thickness of the part and determines the radius that needs to be obtained. The thicker the workpiece and the smaller the radius, the greater the relative elongation along the fibers. Having data on the physical properties of popular types of wood, it is possible for each of them to formulate the maximum possible ratio of the thickness and radius of parts. In numbers it will look like this:

Bending using a steel bar

Bending without using a bus

These data suggest that softwood lumber is less adaptable to free bending than dense deciduous species. To work with sawn timber on aggressive radii, it is imperative to use combined methods of preliminary preparation of parts and mechanical protection.

The tire as an effective way to avoid the destruction of wood when bending

Since the main problem is the breaking of the fibers on the side of the outer radius, it is this surface of the workpiece that needs to be stabilized somehow. One of the most common methods is to use a patch bus. 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 bends 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 humidification and heating, the permissible bending radius is reduced significantly.

In parallel with the use of a steel bar 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 form in such a device is often endowed with 3 mm teeth (with a pitch of about 0.5 cm), oriented towards the direction of the workpiece.

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

Pressing with a tire allows you to bend coniferous and soft deciduous wood bars and boards with a minimum percentage of rejects. Note that parts made from relatively hard rocks become about ten to twelve percent thinner when bent and pressed, and pine and spruce blanks are 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 decrease in the requirements for the presence of defects and defects in wood blanks.

How to improve the plasticity of wood

In a normal state, lumber has elasticity, significant spatial rigidity and resistance to compression. Wood gets these valuable properties from lignin - a natural "reticulated" polymer, which gives plants their stable shape and strength. Lignin is located in the intercellular space and in the cell walls, connecting cellulose fibers. Coniferous wood contains about 23-38 percent, deciduous trees - up to 25 percent.

In fact, lignin is a kind of glue. We can soften it and turn it into a "colloidal solution" if the lumber is heated by steaming, boiling, high-frequency current (for small parts, a household microwave is also applicable). After the lignin has melted, the workpiece is bent and fixed - while cooling down, the molten lignin solidifies and does not allow the wood to return 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 temperature exposure will depend on how massive the part is. The thicker the part, the longer it will take to heat it up. For example, if you use steaming to prepare for bending a 25 mm strip (with a moisture content of about 28-32%), then on average it takes about 60 minutes. It is noteworthy that the holding time under steam for parts of similar dimensions for any rocks is approximately the same.

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

The boiling method is not often used, since the workpiece is strongly and unevenly moistened, and such water-saturated fibers and cells can break during bending, at least with the formation of pile. After cooking, the parts have to be dried for too long afterwards. But this method shows itself well if only part of the workpiece needs to be processed for bending.

Steaming allows you to warm up the workpiece evenly, and its humidity at the exit tends to approach the optimum. The most suitable moisture content for achieving maximum plasticity of sawn timber is considered to be a range of 26-35 percent (the moment of saturation of the wood grain).

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

Chemical methods of plasticizing wood

It is also known that it is possible to make lumber more pliable by using different impregnation compounds. 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 work very simply - they increase the workpiece's ability to absorb water and help retain moisture in the fibers.

Thin products such as veneers are sprayed, but the chemistry impregnation of normal lumber is usually done by full immersion. It takes time for the working substances to get inside the bar or rail, usually from 3-5 hours to several days (although heating helps to reduce the wait).

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

Tips for bending lumber using hydrothermal conditioning

Select very carefully the quality of the workpiece for bending. It is better not to use material with cracks, knots (even alive and accrete), inclined fibers. If there are no options for this, then orient the part in the bending device (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 busbar bending method.
When selecting a blank, 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 volume of work is small, then it is better not to cut out the blanks, but to prick them from the chocks. In this way, it is possible to avoid cutting the fibers and, as a result, defects during bending.
For bending, it is advisable to use lumber with natural moisture. If you use dry blanks, then preference should be given to those that were not processed in the drying chamber, but dried under a canopy - in an atmospheric way.
After steaming, work on the softened wood very quickly, as the lignin begins to harden almost immediately, especially in the most vulnerable outer layers of the 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 the surfaces facing the outer radius are easily exposed to the steam jet.
To save time, many carpenters refuse to use clamping templates. Instead, they use metal staples and wedges or bumpers 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 accuracy 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 the part has cooled in the mold, let it stand still. Some experienced furniture makers prefer to mature for 5-7 days. The tire is usually left attached to the part for the entire time.

Despite all its strength and strength, wooden parts can be easily and simply bent, if suddenly during the construction process you need some special and original oval or round parts. With proper processing, a tree can easily change its shape, and you can carry out this procedure on your own, without resorting to the help of professionals.

Types of work

There are two main ways to bend a tree to the desired shape, and one of them is cold, the other is hot. As the name implies, the methods differ only in the use of hot temperatures, in terms of efficiency, both of these methods are completely the same, it's just that the hot method of fixing the tree is much faster. For each method, you will need glue, pva or wallpaper, depending on what is at hand, you do not need to buy anything special. And with the help of metal parts, you can create a kind of press or frame that will hold the tree in the desired shape. In order to bend the timber, it is necessary to grease it with glue, strongly and thoroughly, without fear that the tree will become wet from this. In fact, under the influence of the glue solution, all excess moisture will leave the tree, and it will become even more durable and strong, which is extremely important. After the timber has been greased with glue, it must be fixed with tools in the desired shape, and in the case of a cold work process, simply leave it fixed to dry. If you choose hot, then the timber should be covered with a regular film so that it dries faster and all the moisture evaporates.

Durability

Whichever method is chosen, both are effective and work in the same way. The bar is completely fixed in its new form, and will no longer return to the old one. You can bend a wet bar from the glue as much as possible without fear that it will break. And as a result, you will receive an original and beautiful piece of furniture or detail to create an even more attractive interior of the house or its facade. The timber bent with glue will not even need to be processed for durability, which is extremely convenient. Moisture will not soak through the glue solution, and insects will not encroach on such a tree, in which there will be much more glue. That is why this method is the most optimal and practical if you urgently need to bend a wooden beam.

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