Wood bending technology. Correct bending of wood and its technology. Standards for the strength of wood that is bent Why is bent wood straightened in pairs?
If you decide to decorate the room with wood or start creating beautiful furniture V classic style- then you will need to make curved parts. Fortunately, wood is a unique substance because it allows an experienced craftsman to play with the form a little. It's not as difficult as it seems, but not as easy as we would like.
Previously, there was already a publication on the site on bending plywood. In this article we will understand the principles of bending solid board and timber, we’ll find out how they do it in production. We will also provide useful tips from professionals that will be useful to the home craftsman.
Why bending is better than sawing
A curved wooden part can be obtained by two methods: by bending a flat piece, or by cutting out the required spatial shape. The so-called “sawing” method attracts users with its simplicity. For such production 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 a curved wooden product, you have to use a workpiece that is obviously too large, and a lot of valuable material will be irretrievably lost as waste.
But main problem are the performance characteristics of the parts obtained. When cutting a curved part from ordinary edged lumber, the wood fibers do not change their direction.
As a result, cross sections fall into the radius zone, which not only worsen appearance, but also significantly complicate the subsequent finishing of the product, for example, its milling or fine grinding. In addition, on those most vulnerable to mechanical impact In rounded areas, the fibers run across the section, which makes the part prone to fracture in this place.
Whereas when bending, the opposite picture is usually observed, when the wood only becomes stronger. The edges of a curved beam or board do not have “end” cuts of fibers, so subsequently such workpieces can be processed without restrictions, using all standard operations.
What happens in wood when it bends?
Bending technology is based on the ability of wood, while maintaining its integrity, to change its shape within certain limits as force is applied, and then retain it after the mechanical stress is removed. 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.
Layers of a wooden workpiece do not work equally when bent. Outside the radius, the material is stretched, inside it is compressed, and in the middle of the array, the fibers experience virtually no significant loads and have little resistance to the forces acting on the workpiece (this inner layer is called “neutral”). With critical deformation, the fibers at the outer radius break, and at the inner radius, “folds” usually form, which are a fairly common defect during bending. soft wood. The fibers of plastic hardwood or softwood can compress by 20 percent or more, while the tensile limit is about one to one and a half percent.
That is, to determine the possibility of bending (without destruction) more important indicator there will be a limit to 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 will be the relative elongation along the fibers. Having data on the physical properties of popular wood species, it is possible for each of them to formulate the maximum possible ratio of thickness and radius of parts. In numbers it will look like this:
Bending using a steel bar
Bending without using a tire
These data indicate that softwood lumber, compared to dense hardwoods, is less suitable for free bending. To work with lumber at aggressive radii, it is imperative to use combined methods preliminary preparation of parts and mechanical protection.
Tire as an effective way to avoid wood destruction during bending
Since the main problem is fiber breakage on the outer radius side, it is this surface of the workpiece that needs to be stabilized somehow. One of the most common methods is to use an overhead splint. The tire is a steel strip with a thickness of half a millimeter to two millimeters, which covers the beam or board along the outer radius and bends on the template along with the wood. The elastic strip absorbs part of the energy when stretched and at the same time redistributes the destructive 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 steel bars 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 equipped with 3 mm teeth (in increments of about 0.5 cm), oriented towards the movement of the workpiece.
The purpose of the jagged surface of the template is to prevent the workpiece from slipping, to prevent mutual shift of the fibers in solid wood, and also create a small depressed corrugation in the concave radius of the part (the fibers are pressed inside the array, therefore, problems with folds are solved).
Pressing with a tire allows you to bend bars and boards made of coniferous and soft deciduous wood with a minimum percentage of defects. Please note that parts made of relatively hard wood when bent with pressing become approximately ten to twelve percent thinner, and pine and spruce blanks become 20-30% thinner. But the positive aspects of this method include a significant increase in strength characteristics finished product, as well as a significant reduction in requirements for the presence of defects in wood blanks.
How to improve the plasticity of wood
IN in good condition lumber has elasticity, significant spatial rigidity and resistance to compression. Wood receives these valuable properties from lignin, a natural “mesh” polymer that gives plants a stable shape and strength. Lignin is located in the intercellular space and in cell walls, connecting cellulose fibers. Coniferous wood contains about 23-38 percent of it, and deciduous wood contains 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, or treating with high-frequency current (a household microwave is also suitable for small parts). After the lignin has melted, the workpiece is bent and fixed - as it cools, the molten lignin hardens and prevents the wood from returning to its original shape.
Practice shows that the optimal temperature for bending solid wood (block, strip, board) is 100 degrees Celsius. This temperature must be obtained not on the surface, but inside the workpiece. Therefore, the time of temperature exposure will largely depend on how massive the part is. The thicker the part, the longer it will have to be heated. For example, if you use steaming to prepare for bending a 25 mm thick rail (with a humidity of about 28-32%), then on average it takes about 60 minutes. It is noteworthy that the steam exposure time 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 may lose its elasticity and become too brittle.
The boiling method is not often used, since the workpiece is heavily and unevenly moistened, and such water-saturated fibers and cells can tear when bent, at least with the formation of lint. After cooking, the parts have to dry for too long. But this method works well if you need to process only part of the workpiece for bending.
Steaming allows the workpiece to be heated evenly, and its output humidity tends to approach the optimum. The most suitable humidity for achieving maximum ductility of lumber is considered to be in the range of 26-35 percent (the moment of saturation of wood fibers).
To steam wood for bending at home, use homemade 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 low pressure. This is always followed by the stage of drying the part (+ holding the fixed shape) to about fifteen percent and finishing it.
Chemical methods for plasticizing wood
It is also known that it is possible to make lumber more pliable using impregnation 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, obtaining a similar result.
But quite primitive “recipes” can also be used containing ammonia and ethyl alcohol, glycerin, alkalis, hydrogen peroxide, dissolved alum... Many of them act extremely simply - they increase the ability of the workpiece to absorb water and help retain moisture in the fibers.
Thin products such as veneer are processed by spraying, but preparatory chemical impregnation of normal lumber is usually carried out using the full immersion method. It takes time for the working substances to get inside the bar or slats, usually from 3-5 hours to several days (although heating helps reduce the wait).
Largely because of the length of the process, chemical plasticization is not often used, although there are other problems: the cost of chemicals, changes 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 preparation
- Select the quality of the blanks for bending very carefully. It is better not to use material with cracks, knots (even live and fused ones), or sloping 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 at the outer radius. Give preference to the splint bending method.
- When selecting a workpiece, it is necessary to provide for a change in the size of the part after molding. For example, the thickness of a coniferous beam 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 the workpieces, but to prick them from lumps. 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 workpieces, then preference should be given to those that have not been processed in drying chamber, and were 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 time reserve of 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 them into templates.
- Place the material in the steaming chamber so that the surfaces facing the outer radius are freely exposed to the steam jets.
- To save time, many carpenters refuse to use templates with clamps. Instead, they use metal brackets and wedges or stop 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 in the manufacture of a part is required, it is necessary to conduct tests and, based on the results obtained, adjust the shape of the template (reduce the radius).
- After the part has cooled in the mold, let it stand some more. Some experienced furniture makers prefer to cure for 5-7 days. The tire, as a rule, is left attached to the part during this entire time.
When bending wood, you need to take into account many points: fresh wood is best suited for this task, wood needs to be steamed a certain amount time, which depends on its thickness.
Processes for changing the properties of wood to meet requests: technology and properties Wood is a natural polymer composite material that changes its properties under mechanical and chemical influence. Knowing the patterns of material changes, you can create them purposefully, imparting the qualities required by the consumer. This is called the wood modification process. It is necessary when chipboard production, MDF, OSB, WPC and other wood materials, where crushed wood mixed with a polymer binder is pressed to produce a homogeneous material of standard sizes.
The proposed wood modification technology changes the properties of wood in the massif, that is, to the entire depth of the processed material, without resorting to grinding it. This is achieved by the fact that the molecules of the modifier, i.e., a substance that helps change the properties of wood, are comparable in size to the molecules of the wood substance and less than the size of the intercellular spaces in it. Therefore, by diffusion or forced impregnation under pressure, the modifier penetrates the entire thickness of the impregnated product, and then, under the influence of temperature and pressure, reacts with natural chemicals found in the wood substance.
Thus, the technology makes it possible not to chop wood, not to use expensive polymer binders, and to achieve the same effect that was achieved in the production of MDF, for example, but in a cheaper way. At the same time, it retains all its positive properties, the texture stands out brighter, you can change the color (lamination is not required). 
So, the modifier must penetrate into the cells in a dissolved state, be chemically active for the components that make up the woody substance, and, reacting with these components, purposefully change the physical and operational properties material. The most suitable substance for this is urea, because in the previously mentioned MDF or OSB, the most applicable binders are urea. Urea is soluble in water, including that contained in a bound state in wood, which means that by saturating the wood with an aqueous solution of urea, we, paradoxically, “dry” it, “taking” part of the wood moisture onto hydrophilic urea. Urea or urea actively reacts with components of wood substances such as lignin, hemicelluloses, and extractives. 
And since the polycondensation reaction occurs in the macromolecules of the wood substance, the solid wood acquires new characteristics specified by the manufacturer useful qualities, preserving the positive old ones. The urea solution is not harmful, chemically neutral, moreover, grade A urea according to GOST 6691-77 is used as a feed additive for livestock. Urea-modified wood is certified (GOST 24329-80) and is mainly used under the Destam or Lignoferum trademarks in the production of bearing shells. In the production of construction and joinery products, thermally modified wood is currently also used, the technology of which is similar to that proposed, except that the chemical modification of the wood substance is carried out in the absence of urea due to the polycondensation of decomposition products of lignin, hemicelluloses, extractives and xylans. 
Due to thermal degradation, the physical and mechanical properties of thermally modified wood are partially reduced. The technological process for producing mechanochemically modified wood consists of impregnating the original wood of any species and any moisture content with a modifier solution. Impregnation can be carried out using the “hot-cold bath” method - diffusion or in an autoclave - forced. Then drying is carried out, if necessary - with compaction (pressing), and heat treatment, which fixes the new properties of the wood. It should be noted that it is more economical to use low-value rocks, since their performance properties after modification exceed the properties of expensive rocks
How to bend wood correctly and in what ways?
Currently manufacturers wooden products they prefer to do without this operation, and if they use bent elements, then they are made of plywood. It's easier to bend plywood. It should be noted, however, that furniture makers natural wood They stopped pampering the buyer a long time ago. All furniture is made from wood board or fiberboard. Products from bent wood, be it a chair or something else, is without a doubt stronger, lighter and more elegant.
Wood selection
The success of bending largely depends on the type of wood chosen. Almost any species can be bent, but elm, oak, beech, etc. have the best flexibility. If carefully dried wood is needed for carpentry work, then in our case it is better to use freshly harvested wood. You should not use old (aged) wood. The younger the tree, the more flexible it is. From the desired rock you need to select pieces without cracks or knots. At the very least, there should be no knots in the area of the intended bend. It is important that the wood is straight-grained, without strands, cross-layers and “screws”. It is best to prepare not sawn boards and beams, but solid round timber.
Making a blank 
Blanks for bending wooden elements are best obtained not by sawing, but by splitting round timber. The direction of splitting should be along the chords of the circle to eliminate the core, which is fragile and unsuitable for bending. Prepared this way wooden blocks and the planks will not flake during bending. Future detail marked so that the direction of the bend coincides with the radius of the round timber from which the workpiece was split off, and the outer side of the bend coincides with the outer part of the former round timber. The split blanks are processed with planes to the required dimensions with a small allowance for final finishing.
Steaming the workpiece
To give the workpiece the best plasticity, it must be steamed. To do this, you will need a metal container of a certain size. The whole workpiece will be “steamed” in it or only at the bend. The second is preferable, since it is more convenient to take the workpiece simply with your hands (without tools), which cannot be done if the workpiece is all steamed.
If this type of work is intended to be put on stream, then a special metal container can be made with a sealed lid and two holes for placing the bendable part inside the “steam room”. This entire simple structure must be tightly closed to reduce the escape of steam to the outside. Place a rubber gasket under the cover. Do not screw it tightly; it may swell or even explode under steam pressure. A fairly heavy lid will ensure a tight seal and still work safety valve with excessive pressure increase.
It is difficult to indicate the time for complete steaming. It depends on the type of wood, the cross-section of the workpieces, and the degree of dryness of the workpieces. You just need to take the workpiece out from time to time and test it for bending. The readiness of the workpiece can be felt immediately by its flexibility in bending.
Workpiece bending
It is best to bend the workpiece using a template. A blank bent and dried in a template will provide the part configuration we need. Moreover, if you need not one, but several completely identical parts.
With a certain skill, you can do as they do a shower for horse harness - the steamed workpiece is bent and the ends are tied with a rope. Leave it in this form until completely dry. Bent parts should be dried in a ventilated place protected from the sun. Trying to artificially speed up drying by heating can lead to cracking of the wood.
It should be noted that after removing the part from the template, it “gives up” a little, i.e. straightens up. Taking into account this property, the workpieces must be bent a little “steeper” so that upon release the desired shape is obtained. How “cooler” is a matter of experience. Much depends on the cross-section of the workpiece, the type of wood, and the degree of steaming before bending.
Press for bending wood materials
Manufacturer ORMA, Italy
Purpose
This equipment is intended for bending (bending) wood materials. Before bending, the workpieces are steamed in specialized chambers. Stabilization of the workpiece is carried out by high frequency current.
This equipment found wide application in the manufacture of chairs, sleds, school furniture.
Specifications: 
Complete bending kit includes
- Steaming chamber - a reservoir for moistening workpieces with a condensate collector, complete with a steam generator (a separate generator for each autoclave)
- Pre-bending press (necessary depending on tasks and productivity)
- The bending and stabilizing press (selected depending on the tasks and productivity), depending on the complexity of the product, can be equipped with additional side cylinders. The possible total force varies from 30 to 120 tons. Specific pressure up to 7.5 kg/cm2
- Electronic frequency generator – with the ability to work on two presses for bending and stabilization
Standards and structural strength of bent wood
In addition to the traditional use of bent, today construction materials are increasingly used structural elements made in this way. Use of load-bearing elements from bent wood allows you to create new interesting types of architectural solutions, which, combined with the optimal economic indicators of such structures, explains the increased interest in them from outside practical application not only in industrial, but also in private housing construction.
There are two ways to make a curved structure from bent wood: sawing it out of boards, bending timber (solid bent products) or layers of wood and simultaneously gluing them together (bent-glued products). The wood bending process is based on its ability to certain conditions under the influence external loads change its shape and retain it in the future. 
It is clear that cutting a product from a board large sizes and curvature is almost impossible, therefore, in order to make a bent board or beam at home for the construction of a beautiful or dome crowning a decorative turret of a house, you should prepare everything necessary for bending wood. Just as website optimization allows you to increase the ranking of an Internet resource, so does the choice quality material for bending improves its result. An unedged board or timber without knots, with a cross-layer of no more than 10% of the surface area, is selected as blanks. The best varieties woods with increased plasticity are hornbeam, maple, beech, oak, ash and elm.
After the material has been selected, you can begin the bending process, the main stages of which are: hydrothermal treatment, bending the workpiece and drying the product. Optimal parameters conditions at which bending takes place with the highest quality are the wood moisture content in the range of 25-30% and the temperature in the center of the workpiece from 80 to 90°C.
Professional promotion of sites dedicated to the intricacies of wood bending technology will certainly arouse the interest of a wide audience, since the simplicity of this process is incomparable with the result obtained. Hydrothermal involves steaming or boiling the workpiece in hot water.
Steaming is technically more complex, so at home it is easier to organize the boiling of wood in a cooking tank of a suitable size. The workpiece removed from the cooking tank should be immediately secured to the tire using clamps while the wood is still warm. Otherwise, stress will arise in its outer layers, leading to cracks.
Flexible plywood and its application
Flexible plywood (bending plywood) is now in great demand due to the fact that it is a convenient material for the manufacture of structures that require rounding. The use of such bending plywood is effective and expedient, since it can take any necessary shape. Its flexibility allows you to embody the wildest fantasies of designers and produce the most fashionable and modern furniture, be it a uniquely designed cabinet for your living room, cute shelves for your kitchen or modern and comfortable office furniture. 
Such plywood is made from tropical trees, mainly from CEIBA wood, but sometimes flexible plywood is made from other woods: Parika, Keruing. Flexible (bending) plywood is, as a rule, a 3-layer board that is glued together in the transverse or longitudinal design of the shirts.
Flexible plywood, can be used for all types of bends, even with very small radii. No need to heat or treat with water. The self-supporting design of bending plywood makes the use of structural and special supports unnecessary. Unique designer models, rounded designs and complex shapes with multiple radii, which cannot be created from traditional materials, are produced easily and quickly. Flexible plywood fulfills almost all thickness requirements by increasing the number of layers of material (for example, increasing the thickness to 10mm, 15mm, 16mm, 18mm, 20mm, etc.). Greater sheet thickness can be obtained by gluing several sheets of thinner bending plywood together.
High quality tropical plywood is a combination modern technologies and traditional materials. A product created to satisfy the most sophisticated needs modern manufacturers furniture and carpentry. Flexible plywood (bending plywood) is cheaper than pre-made wood forms. Significant time savings, less labor intensity and greater profitability are its advantages over any other method of changing the shape of plywood.
Except flexible plywood Our company offers another unique product - this is Ultra-light plywood. The range of use of this plywood is also quite wide: this is the production of door panels, the manufacture of cabinet furniture, sofas, armchairs, shelves. Ultra-light plywood is a new product on our market; it is 1.8 times lighter than birch. This plywood can be veneered well, finished with films and varnishes, and most importantly, it can significantly reduce the weight of the finished product!
Specifications
Bend direction Across the grain: along the width
Composition Hot-pressed tropical wood with heat-set adhesive
Density 300-400 kg/cub.m.
Thickness 5 mm, 8 mm, etc.
Dimensions 2500/2440 mm x 1220 mm, etc. by department. order
Bending radius For 5 mm thick, minimum 7 cm for 8 mm thick, minimum 10 cm
Elasticity
Perpendicular to grain: 210 N/mm2
Parallel to grain: 6300 N/mm2
(For 5mm panel at 10% humidity)
Store panels horizontally in a clean, shaded, dry place.
Apply glue to the panels, fixing the desired shape. After the glue dries, the panel will retain its shape. H.P.L. or the plywood can be glued both during the initial molding and at a separate, final stage.
You can use any wood glue.
The panels must be transported on a hard, flat surface. Individual panels can be rolled up, but they cannot be stored in this position for a long time.
Bending is widely used in industries such as shipbuilding. To begin with, there are a few basic rules that are always followed.
By steaming wood to bend it, you soften the hemicellulose. Cellulose is a polymer that behaves like thermoplastic resins. (Thanks to John MacKenzie for the last two suggestions).
To do this, you need heat and steam at the same time. In Asia, people bend wood over fire, but that wood is definitely quite wet - usually freshly cut. Shipbuilders in ancient Scandinavia prepared hull materials for their ships and placed them in a saltwater swamp so that they would remain flexible until they were ready for use. However, we are not always able to obtain freshly harvested wood for these purposes and excellent results can be achieved by using ordinary wood air drying. 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 concerns steaming time: one hour for every inch of wood thickness.
Know that along with the probability of understeaming the workpiece, there is also the probability of oversteaming it. If you hovered an inch board for an hour and when you tried to bend it, it cracked, do not conclude that the time was not enough. There are other influencing factors that explain this, but we'll get to those later. Longer steaming of the same workpiece will not give a positive result. In such a situation, it’s a good idea to have a workpiece of the same thickness as that intended for bending and which you don’t mind. Preferably from the same board. They need to be steamed together and after the presumably required time, take out a test sample and try to bend it into shape. If it cracks, then let the main workpiece steam for another ten minutes. But no more.
Wood:
As a rule, the best option it will if you can find freshly cut wood. I understand that the cabinet makers will shudder at these words. But the fact remains that fresh wood bends better than dry wood. You can take a two-meter inch board of white oak, clamp one end of it in a workbench and bend it to any desired curvature - fresh wood is so malleable. However, naturally, it will not remain in this state and you will still have to float it.
In shipbuilding, the main evil is rot. If you are concerned about this issue, then please note that the very fact of steaming fresh wood eliminates its tendency to rot. Therefore, you don’t have to worry - the frames of boats are usually made from fresh steam-bent oak and do not rot if it is taken care of. This also means that in this way it is possible to make at least blanks for the Windsor Chair. But air-dried oak also gives excellent results.
When selecting wood for bending, one thing to avoid is cross-layering. When trying to bend such a workpiece may burst.
Therefore, regarding wood moisture content, the rules are as follows:
1.Fresh wood is best.
2. Air-dried wood is a second good option.
3. - the third and very far from the first two option.
If all you have is from the dryer and you can’t get anything else - well, then you have no choice. But still, if you can get air-dried wood, it will be much better.
If there is a need to manufacture a curved wooden element, then at first glance it may seem easier to cut out required element V curved form, but in this case the fibers wood material will be cut, thus weakening the strength of the part, and as a result, the entire product. In addition, when sawing, a large waste of material is obtained, which cannot be said about the method when the wooden blank is simply bent.
Wood is cellulose fibers bound together by a chemical called lignin. The flexibility of the tree depends on the arrangement of the fibers.
Pay attention! Only well-dried wood will be reliable and durable source material for production various products. However, changing the shape of a dry wooden blank is a difficult process, because dry wood can break, which is very undesirable.
Having studied the technology of how to bend wood, as well as the basic physical properties wood, which allow you to change its shape and subsequently preserve it, it is quite possible to take up bending wood at home.
Some features of working with wood
Bending of wood is accompanied by its deformation, as well as compression of the inner layers and stretching of the outer ones. It happens that tensile forces lead to rupture of the outer fibers. This can be prevented by carrying out preliminary hydrothermal treatment.
So, you can bend blanks of timber made from solid and laminated wood. In addition, planed and peeled veneer is used for bending. The most plastic are hardwoods. These include beech, ash, birch, hornbeam, maple, oak, poplar, linden and alder. Bent glued blanks are best made from birch veneer. It is worth noting that in the total volume of bent-glued blanks, birch veneer occupies approximately 60%.
When steaming the workpiece, the compressibility increases significantly, namely by a third, while the tensile ability increases by only a few percent. This means that it is not a priori worth thinking about whether it is possible to bend wood thicker than 2 cm.
Steam box heating
First you need to prepare the steam box. It can be made by yourself. Its main task is to hold the tree that needs to be bent. There should be a hole in it to allow the steam pressure to escape. Otherwise it will explode.
The steam outlet should be located in the bottom of the box. In addition, the box should have a removable lid through which you can pull out the bent wood after it has acquired the desired shape. To hold the wooden bent part in the desired shape, clamps should be used. You can make them yourself from wood or buy them at a specialty store.
Round cuttings should be made from wood - several pieces. Off-center holes are drilled in them. After this, you need to push the bolts through them, and then drill another hole through the sides to push them in tightly. Such simple crafts can become excellent clips.
Now it’s time to steam the wood; to do this, you should take care of the heat source and close the wood piece in the steam box. For every 2.5 cm of thickness of the workpiece, the product needs to be steamed for about an hour. After time has passed, the tree must be removed from the box and given the required shape. The process must be completed very quickly. The workpiece bends neatly and softly.
Pay attention! Some types of wood bend more easily than others due to different elasticity. Different ways require the application of force of varying magnitude.
Once the desired result is achieved, the bent tree must be fixed in this position. You can secure the tree while shaping it. This makes it easier to control the process.
Using chemical impregnation
To destroy lignin bonds between fibers, you can act on wood chemicals, and it is quite possible to do this at home. Ammonia is ideal for this. The workpiece is soaked in a 25% aqueous ammonia solution. After which it becomes very obedient and elastic, which allows you to bend, twist it and squeeze out relief shapes in it under pressure.
Pay attention! Ammonia is dangerous! Therefore, when working with it, you should follow all safety rules. Soaking the workpiece should be carried out in a tightly closed container located in a well-ventilated room.
Ammonia aqueous solutionAmmonia aqueous solution
The longer the wood is in the ammonia solution, the more plastic it becomes. After soaking the workpiece and giving it a shape, you need to leave it in this curved form. This is necessary to fix the shape, and also for the ammonia to evaporate. Again, bent wood should be left in a ventilated area. Interestingly, after the ammonia evaporates, the wood fibers will regain their former strength, and this will allow the workpiece to retain its shape!
First you need to make a piece of wood that will be bent. The boards should be slightly longer than the length of the finished part. This is because bending will shorten the slats. Before you start cutting, you should draw a diagonal line with a pencil. This needs to be done across the underside of the board. This will help maintain the sequence of the slats after they have been moved.
The boards are cut with a straight-layer edge, in no case with the front side. So, they can be added together with the least change. A layer of cork is applied to the mold. This will help avoid any unevenness in the shape of the saw, allowing for a cleaner bend. In addition, the cork will keep the delamination in shape. Now glue is applied to the top side of one of the wooden slats.
The glue is applied to the lamellas with a roller. It is best to use urea-formaldehyde glue, consisting of 2 parts. He has high level clutch, but takes a long time to dry. Can also be used epoxy resin, but such a composition is very expensive, and not everyone can afford it. Standard wood glue cannot be used in this case. It dries quickly, but is very soft, which is not welcome in this situation.
Boards are held together after gluing. Boards are held together after gluing.
The bentwood blank should be placed into the mold as quickly as possible. So, another lamella is placed on top of the lamella coated with glue. The process is repeated until the bent piece reaches the desired thickness. The boards are fastened together. After the glue has completely dried, you should shorten it to the desired length.
I drank it as a method
The prepared piece of wood needs to be sawn through. The cuts are made 2/3 of the thickness of the workpiece. They should be on the inside of the bend. You should be extremely careful, because rough cuts can break the tree.
Pay attention! The key to success when cutting kerfs is to keep the spacing between cuts as even as possible. Ideally 1.25 cm.
Defects can be hidden with veneerDefects can be hidden with veneer
The cuts are made across the grain of the wood. Next, you should squeeze the edges of the workpiece so as to connect the resulting gaps together. This is the shape the bend will take upon completion of the work. Then the bend is corrected. Most often, the outer side is treated with veneer, or in some cases with laminate. This action allows you to correct the bend and hide any defects made during the manufacturing process. Spaces between bent tree hidden simply - for this, glue and sawdust are mixed, and then the gaps are filled with this mixture.
The cuts are made across the grain of the wood. The cuts are made across the wood grain.
Regardless of the bending method, once the wood is removed from the mold, the bend will relax slightly. In view of this, it needs to be made a little more in order to subsequently compensate for this effect. The sawing method can be used when bending part of the box or metal corner.
So, using these simple recommendations You can bend a tree with your own hands without much effort.
One of the ways to process carpentry blanks is bending. Treated with hot steam wooden blanks are able to bend and retain their shape after drying. Such process is not particularly difficult, but some features of how to bend wood should be taken into account. You may also be interested in pine stairs, which you can order on the website http://mirdereva.ru/.
Wood fibers are held together by a special substance - lignin, which, under the influence high temperature softens, and after cooling again binds the fibers. The process of bending blanks is based on this. Please note that wood different breeds bendable in its own way. For bent products, it is best to use oak, beech, birch, yew, cherry, and elm. But pine, spruce, cedar, and alder should not be used for these purposes.
Work on bent parts begins with the choice of material. The workpieces must be straight-grained; the use of wood with curved fibers is not allowed. The prepared material is dried in natural conditions, under canopies, to a moisture content of no more than 20%. But artificially dried wood should not be used for bending, since it is less amenable to such processing. If you have to use such material, then before bending it must be soaked in water (at least a week). Soaking is also necessary for wood hard rocks trees such as oak, ash, beech.
To heat the workpieces before bending, it is best to use a steam chamber. It is easy to make such a camera at home using plastic pipe suitable sizes and a regular kettle. The parts are placed in a pipe, and steam is supplied from the kettle. The exposure time in the chamber depends on the size of the part and is determined experimentally. In this case, you can be guided by the fact that for 1 cm of the thickness of the workpiece, 30-40 minutes of steaming the wood are required.
In places of bending on parts, if the design of the product allows it, you can slightly reduce the thickness of the material and remove the chamfers. This will make the bending process easier. Thin workpieces, in the absence of a steam chamber, can be heated over an electric or gas stove.
Before you start bending wood, you need to prepare a form on which the part will be fixed, and clamps for fixation. It should be borne in mind that after heating the wood, there will be very little time to fix the workpiece, no more than 5 minutes. Therefore, everything needs to be done quickly, but if the part begins to cool, then it should be heated again. Otherwise, the workpiece may break.
Therefore, it is important to provide a design of molds and clamps that would allow the workpiece to be quickly fixed in in the right position. If the molds are made of wood, they should not be covered with any protective compounds, paint, varnish. Firstly, they deteriorate from heating, and secondly, they will interfere with the drying of the workpieces.
Short blanks are bent on mandrels of a larger radius, and then secured in a mold. This pre-bending reduces the likelihood that the part will break as the bend is formed. You need to keep the parts in shape until they dry completely so that they do not bend back. This usually takes 6 to 9 days and is determined empirically.
After freeing the workpiece from the clamps, it must be put aside for a day, and only then begin processing and finishing. This is necessary in order to relieve residual extension stresses. The tips are simple, but they will allow you to easily master the process of bending wood.
Bending is one of the methods for making beautiful and durable wood parts, for example, for furniture. For the home handyman It is quite possible to master such technology. A bent part is much stronger than a sawn part, less wood is consumed in its production, and the sawn surfaces produce half-end and end cuts, which complicate further processing and finishing of parts.There are three ways to bend wood. One of them, the most famous, involves preliminary steaming of wood and then giving it the required shape in powerful presses. This hot way bending is used mainly in mass production, for example, of chairs.
Along with it, especially at home, two other methods of bending wood are practiced, but in a cold state.
- First - solid wood bending with preliminary cuts along the bend.
- The second is bending, in which a bent part is produced by pressure in molds from a workpiece, which is a package of several layers of thin strips of wood coated with glue.
- When bending in the second way - with cuts - narrow grooves parallel to each other are sawn into the workpiece to a depth of 2/3-3/4 of its thickness, after which the workpiece is given the desired shape.
The maximum bending radius depends on the depth of the cuts (and, accordingly, on the thickness of the workpieces), the distance between them and the flexibility of the wood. The cuts are made both parallel and perpendicular to the fibers. This work operation is performed using a crosscut or manual circular saw with guide stop. If you don’t have a special tool, a regular wood saw will do. The main thing is that the depth of the cuts is the same.
BONDING WITH SIMULTANEOUS BENDING
At wood bending fibers on inside compress, and on the outside - stretch. Wood “tolerates” fiber compression relatively easily, especially if it is pre-steamed. It is almost impossible to stretch it.
Flexibility also depends on the type of wood and the thickness of the workpieces. For example, hardwood from temperate climatic zones- beech, oak, ash, elm - can be bent more easily than tropical wood species (mahogany, teak, sipo, etc.). Conifers are too tough for this.
The resistance value of bent wood until it breaks 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).
In this method, individual strips of wood of the same thickness and width are glued, laid in several layers so that their grains are parallel, and placed in a mold made of hardwood. The matrix and punch of the mold are compressed with clamps and the bag 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 layers. If bent-glued blanks are intended for use in external structures, it is best to use waterproof glue.
BENDING USING CLAMPING DEVICES AND PRESS FORMS
To determine the permissible thickness of the veneer strips or planks to be bent (with a larger thickness, the wood may break), you need to know the smallest bending radius. The wood is most deformed on the inside of the bend. Therefore, it is always necessary to measure here.
As assistive 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 the bend, for example, a mold, and move it until we find a suitable circle of the largest diameter. We measure its radius on a template. We divide the resulting value by 50. The quotient of the division will be the maximum allowable thickness of the strip of plank or veneer.
When working with molds, the bend on the outside of the 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 strip material.
The most difficult situation is when it is necessary to bend a part of a complex configuration with different bending radii. Here, curves for the inside or outside of the workpiece can be constructed freely if its shape is not tied to the contours of any piece of furniture.
In this case, the line for the second cut (the first one is at the beginning of the bend) can be constructed like this. Using a compass, measure the total thickness of the layers to be glued, draw a circle with it on hard cardboard, cut out a circle and apply it in several places to the line of the first cut. At the same time, we apply the circle so that it is in contact with the first line, and draw its outline accordingly on the opposite side. The second cut line will be the end-to-end connection between these auxiliary lines.
BENDING TECHNOLOGY WITH MAKING NOTCHES ON BLANKS
When determining the number of cuts to be made on a workpiece for bending along a known radius (it also depends on the width of the groove and the type of wood), we use auxiliary structure. To do this, we take a block 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 block. Draw a straight line on a sheet of paper and mark the cut point on it.
We place the block on the paper so that its lower edge before the cut coincides with the drawn line and the marked point of the cut, and fasten the block to the work table with a clamp. We set aside the distance of the required radius b on the line and the block and bend the block until the upper edges of the cut meet. The distance a between the end of the line and the mark on the block will be the distance between the individual cuts that can be marked on the workpiece.
If cuts need to be filed on the outside of the workpiece, the distance between them and, accordingly, their number are 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.
Based on materials from the magazine "Do It Yourself"
