How to bend wood - bending wood. Wood bending Steam chamber for wood bending

In the woodworking industry, curved parts are produced in large quantities. The manufacture of curved parts is carried out in two ways: cutting from 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: procurement of material for bending, hydrothermal treatment, bending and drying.

Preparation of material for bending. Blanks for bending are obtained from unedged boards by cutting them into circular saws. The following requirements apply to blanks for bending.

The cross-layer should not exceed 10°. With conventional bending methods, knots are absolutely not allowed in the workpieces. In workpieces with simultaneous pressing, knots are allowed within large limits, which sharply increases the yield of workpieces. Workpieces should be cut out taking into account allowances for subsequent processing of parts. When bending with simultaneous pressing, in addition to the processing allowance, an allowance must be provided 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 the boards after preliminary marking.

In small enterprises, the method of obtaining blanks for bending by splitting blocks has been preserved. The split billet does not have a cross-layer, therefore, when bent, it produces a lower percentage of rejects. However, this method is very labor-intensive, since it is done manually and gives a 20-25% lower yield of blanks from the ridge than when sawing it.

After cutting (or splitting) the blanks for parts round section are processed on turning-copying or round-bar machines, and blanks for parts rectangular section- on longitudinal milling machines. You can also bend unplaned 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. Optimal plasticity of wood is achieved when it is heated while wet. This is explained by the fact that when heated, some of the substances that make up the cells transform into a colloidal state.

As a result, the ability of cells and the entire wood to deform increases. When drying deformed (bent) wood, colloidal substances harden 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 water temperature is maintained at 90-95°C, without bringing it to a boil. The duration of boiling depends on the initial humidity, size and type of wood.

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

Most wide application In production, wood was steamed 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 always obtain it close to optimal for bending, i.e. about 25-30%.

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

The boilers are located at each bending machine and are connected to each other by a steam line into a battery. The bars in boilers and chambers are placed on gaskets to ensure better washing with steam.

The duration of steaming depends on the initial humidity and temperature of the wood, the size of the bars 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 at an initial humidity of 30% and a steam pressure in the steaming boiler of 0.03-0.05 MPa, the steaming duration is 12-13 minutes, and for workpieces with a thickness of 80 mm - 65 minutes.

Plywood, when bent to small radii of curvature, can also be subjected to hydrothermal treatment. Plywood glued with synthetic glues is boiled, but plywood glued with casein or albumin glue is only steamed.

The workpieces removed from the steaming boiler or cooking tank must be bent immediately. The outer layers of wood, which experience the greatest stress when bending, should not be allowed to cool.

Wood bending and equipment. Wood bending machines are divided into two types: cold And hot forms.

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

The free end of the tire is fixed in the carriage 4, sliding along guides 3. Bar 5 is placed between the template 6 and tire 2 and is secured with a movable stop. Then the electric motor turns on, and the shaft rotates 8 with a template placed on it and the block bends along with the tire.

At the bend there is a roller / that tightly presses the block to the template. The rear end of the tire is secured with a bracket to the template. The template with the bar and the tire are removed from the machine and sent to drying, 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 - block; 5 - workpiece;

b - template; 7 - gearbox; 8 - shaft

Rice. 4.14.

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

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

In machines with one-sided heating (Fig. 4.14), workpieces 5 are placed between a hot template 2, heated steam, and tire 4 and are secured with a stop 3. The curved blanks 5 together with the tires are secured to the template with special hooks /. The workpieces remain in the machine until the shape given to them is fixed.

This is achieved by drying the wood to approximately 15% humidity, which takes 90-180 minutes. To increase the productivity of bending-drying machines, it is recommended to dry the workpieces before bending to 20% humidity, keep them in the machine until the humidity is 12-15%, and final drying to industrial humidity Produce workpieces removed from the machine in drying chambers.

Bending of plywood is carried out in templates consisting of two parts: a matrix and a punch, between which the plywood is laid and bent. In this case, special devices, screws, pneumatic and hydraulic presses are used.

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

The workpiece is rolled. The thickness of the workpiece decreases, the layers of wood on the concave side of the workpiece take on a wave-like shape due to the pressing of the template notch, and the outer layers are compacted. This helps to increase the compressive resistance of the concave layers in the wood and the tensile strength of the outer ones.

Bending with simultaneous pressing significantly improves the bending ability of wood and allows you to bend wood with large knots located on the outside of the workpiece. It is used for bending coniferous and soft hardwood wood.

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

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 this the workpieces are freed from templates and tires and sent to the machining shop.

The sequence and principles of mechanical processing of bent workpieces on machines, i.e. Giving them final dimensions and a clean surface is not fundamentally different from processing straight workpieces.

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

Bonding 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: electric contact, steam or high frequency currents (HF). HDTV heating is the most progressive. This method requires less pressing time and the temperature is distributed more evenly across the cross section of the bag.

Adhesives based on urea 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 m2 of surface to be spread is 110-120 g.

If there is a need to create a curved wooden element, then most likely you will encounter a number of difficulties. It may seem that cutting out the required component in curved form it will be easier, but in this case the wood fibers will be cut and weaken the strength of the part. In addition, the execution results in a fairly large waste of material.

Stages of performing work on bending boards at home:

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

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

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

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

Features of working with wood

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

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

According to the technology for manufacturing bent wood, when the workpiece is steamed, its ability to compress significantly increases, namely by a third, while the ability to stretch increases by only a few percent. Therefore, you cannot even think about bending wood thicker than 2 cm.

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

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

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

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

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

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

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

How to bend a board at home using chemical impregnation

Since lignin is responsible for the durability of wood, its bonds with the fibers must be broken. This can be achieved chemically, and it is quite possible to do this at home. Ammonia is best suited for such purposes. The workpiece is soaked in a 25% aqueous solution of ammonia, which significantly increases its elasticity. This will make it possible to bend, twist it, or squeeze out any relief shapes under pressure.

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

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

How to bend a board at home: layering method

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

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

It is best to use urea-formaldehyde glue, consisting of two parts. He has high level clutch, but takes a long time to dry.

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

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

How to bend a board at home: cut

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

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

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

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

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

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

When bending wood, you need to take into account many points: fresh wood is best suited for this task; you need to steam the wood for a certain amount of time, which depends on its thickness.

Processes for changing the properties of wood to meet demands: 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 in the production of chipboard, MDF, OSB, WPC and others wood materials, where shredded 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 grind 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 wood substance, and, reacting with these components, purposefully change the physical and operational properties of the 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 to 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 with natural wood have long stopped pampering the buyer. All furniture is made from wood board or fiberboard. Products made from bent wood, be it a chair or something else, are 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 (age) wood. How younger 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 It is 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 sufficiently heavy lid will ensure a tight seal and at the same time act as a safety valve if the pressure increases excessively.

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 has found wide application in the manufacture of chairs, sleds, and 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 building structural elements made using this method are increasingly used. The use of load-bearing elements made of bent wood makes it possible 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 practical applications not only in industrial, but also in private residential 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 process of bending wood is based on its ability, under certain conditions, under the influence of external loads, to change its shape and maintain 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 types of wood with increased ductility 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. The optimal parameters 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 comfortable 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 closet with original design for your living room, cute shelves for the 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 several radii, which cannot be created from traditional materials, are manufactured 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 of 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.

In addition to flexible plywood, our company offers another unique product - 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 This can also be achieved by using conventional air-dried wood. It would be very good if, a few days before the operation itself, you immerse the workpieces in water so that they gain moisture - those Vikings knew what they were doing. You need warmth and you need moisture.

The main rule 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:

Generally, the best option is 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.

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

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

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

To begin with, there are a few basic rules that are always 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. I know that 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 conventional air-dried wood. It would be very good if, a few days before the operation itself, you immerse the workpieces in water so that they gain moisture - those Vikings knew what they were doing. You need warmth and you need moisture.

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

As I discovered, 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, you should 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:

Generally, the best option is 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. I can take a six-foot-long plank of white oak, clamp one end of it in the workbench, and bend it to whatever curvature I need—that's how malleable the fresh wood is. However, naturally, it will not remain in this state and you will still have to float it.

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:

Fresh wood is best.
Air-dried wood is a second good option.
Wood after drying is 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. I dealt with this too. But still, if you can get air-dried wood, it will be much better. Just last week I bent 20mm thick walnut boards for the transom of my yacht. The blanks were dried for several years and their bending went completely smoothly.

Steaming chambers.

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

Cameras can be the most different forms and sizes. It should be large enough so that the workpiece seems to be suspended and steam flows around all sides of it. A good result will be obtained from pine boards with a cross-section of about 50 x 200. One way to ensure the “hanging” of the workpiece is to drill through holes in the side walls of the chamber and drive round wooden rods from deciduous wood. With their help, the workpiece will not touch the bottom and the area of closed wood will be minimal. However, you should not make the chamber so large that the amount of steam generated is not enough to fill its volume. The chamber should be such that it is humid inside and the steam rolls in waves. This means that the dimensions of the chamber must correspond to the capabilities of the steam generator (or vice versa).

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

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

When I say one hour of steaming per inch of thickness, I mean one hour of SERIOUS CONTINUOUS steaming. Therefore, the boiler must be designed to provide steam for the required time. I used a new 20-liter fuel container for these purposes. Workpieces can only be placed into the chamber when the installation has reached full capacity and the chamber is completely filled with steam. We must absolutely ensure that the water does not run out prematurely. If this happens and you have to add water, it is better to give it up. Adding cold water will slow down steam generation.

One way to make the most of the water is to position the chamber at a slight angle so that the condensed water inside flows back into the boiler. But at the same time, it is necessary that the fitting through which steam enters inside is closer to the far wall. Another way is to make a siphon system to ensure that its level is replenished as the water boils away.

Here's what a photo of such a system looks like:

In the picture you see a wooden camera located slightly at an angle. Directly below it is the steam generator boiler. They are connected to each other via a hose from the radiator. If you look closely, you can see an L-shaped pipe coming out of the base of the boiler on the left. It’s hard to see in the photo, but its vertical part is actually translucent and this way we will know about the water level inside the boiler. To the left of the boiler you can see a white bucket containing water for make-up. Take a closer look and you will notice a brown tube connecting the bucket to the vertical part of the pipe - the level gauge. Since the bucket is located on a hill, a siphon effect is observed: as the water level in the main boiler drops, water enters it from the bucket. You can add it from time to time, but do this extremely carefully so that it does not rush quickly into the boiler and cool it too much.

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

Many chambers have a door at the end through which workpieces can be moved if necessary and removed if necessary. For example, if you are engaged in the manufacture of bent frames and you would like to complete this as much as possible in a day, you melt the boiler and (when reaching full power) put the first workpiece inside. After 15 minutes, add the second one. After another 15 - the third and so on. When it's time for the first one, you take it out and bend it. I'm assuming this procedure will take less than 15 minutes. When she sits still, the second one is already on the way... etc. This allows you to get a lot of work done and avoid over-steaming.

The door has another important function. It doesn't even have to be made of hard material - on my small camera, just a hanging rag serves for this purpose. I say "hanging" because the steam must emanate outward from the end (since a stream of steam is needed). We must not allow what will happen in the cell overpressure, making it difficult for steam to enter. And besides, the picture itself wooden box, from which steam pours out in clouds, looks quite cool - passers-by are simply dumbfounded. The second purpose of the door is to prevent cold air from entering the chamber from below the workpieces.

So, we will assume that our wood is boiling (with a pleasant smell) and the templates are ready. Try to organize everything in such a way that the operation of removing the workpiece from the chamber and bending it goes quickly and smoothly. Time is of the essence here. You have only a few seconds to do this. As soon as the wood is ready, quickly take it out and immediately bend it. As fast as human dexterity allows. If pressing to the template takes time, just bend by hand (if possible). For the frames of my yacht (which have a double curvature), I took the blanks out of the chamber, stuck one end into the clamp and bent this end and then the other just by hand. Try to provide more bend than is necessary for the template, but not much. And only then attach it to the template.

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

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

It is practically impossible to produce blanks of exact length and expect to be able to achieve a bend at the ends. You simply don't have the strength to do it. For this reason, if you need a workpiece one meter long, but its thickness is more than 6 mm, you are better off cutting a piece about two meters long and bending it. I'm just going by the assumption that you don't have a hydraulic press in your workshop - I certainly don't have one myself. When cutting out a workpiece with a margin, remember that the shorter it is, the more difficult it will be to bend.
And if it is with a reserve, then the end of the real part will have a large curvature - the inch oak boards the last 150 mm are absolutely straight. Depending on the radius required at the end, it may be necessary to resort to wood carving in such areas and to take into account the required thickness when choosing the material.

Templates.

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

How much bigger?

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

Warning. If you bend workpieces for lamination, the template must be exactly the shape of the workpiece in the laminate - I have rarely had cases of large bending of well-bent laminated wood.

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

Well, that’s probably all my thoughts for now.

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

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

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

Now let's consider theoretical aspects bending

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

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

Sawing curved blanks is characterized by simplicity of technology and does not require special equipment. However, when sawing, the wood fibers are inevitably cut, and this weakens the strength so much that parts with large curvature and a closed contour have to be made up of several elements by gluing. On curved surfaces, half-end and end cut surfaces are obtained and, as a result, processing conditions on milling machines and finishing worsen. In addition, cutting produces a large amount of waste. The production of curved parts by bending requires a more complex technological process and equipment compared to sawing. However, when bending, the strength of the parts is completely preserved and even in some cases increases; 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 straight parts.

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

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

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

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