If there is a need to develop a curved element made of wood, then most likely you will encounter a number of difficulties. At first glance, it will seem that it will be easier to cut out the required element in a curved form, but in this option, the wood fibers will be cut and weaken the reliability of the part. Plus, when executed it turns out to be very large high consumption material.

Stages of work on bending boards at home:

Wood is cellulose fibers bound together by lignin. Placing the fibers in a straight line affects the flexibility of the wood material.

Advice: durable and reliable wood material for the manufacture of all kinds of products can only be produced if the wood is perfectly dried. However, changing the shape of dry wood is a rather labor-intensive process, since dry wood can easily break.

Having studied the technology of bending wood, including its basic physical properties wood, allowing you to change its shape, it is quite possible to bend the wood material in artisanal conditions.

Specifics of working with wood

Bending of wood 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 carrying out preliminary hydrothermal finishing.

Can bend timber blanks made from laminated and solid wood. Plus, to give the desired shape, peeled and planed veneer is used. Hardwood is considered the most ductile. 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.

By production technology bent wood, when the workpiece is steamed, its ability to compress significantly increases, specifically by a third, while the ability to stretch increases by only a few percent. Thanks to this, you can’t even think about bend tree thicker than 2 cm.

How bend board in artisanal conditions: heating in steam box

First of all, you need to prepare a steam box, which can be made with my own hands. Its main task is to restrain the tree, which is required bend. There must be a hole in it that is designed for steam to escape. Otherwise, an explosion may occur under pressure.

This hole should be located in the bottom of the box. Plus, in the box you need to design a removable lid through which you can remove the bent wood as soon as it receives the required shape. In order to hold the bent wood blank in the required shape, it is necessary to use special clamps. They can be done without outside help made of wood or purchased at a building materials store.

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

Now you can start steaming the wood. To do this you need to close wood preparation in the steam box and worry about the heat source. For every 2.5 cm of product thickness, the time spent on steaming will be approximately 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 performed quickly enough, and the bending itself should be done gently and carefully.

Tip: due to varying degrees of flexibility, some types of wood will bend more easily than others. Any method requires the application of varying amounts of force.

As soon as the result you want is achieved, bent workpiece must be secured in this position. Fastening the tree is possible during the development of its new form, making it easier to control the process.

How bend board in artisanal conditions using chemical impregnation

Since lignin is responsible for the stability of wood, its bonds with the fibers must be broken. This is achieved chemically, and it is quite possible to do this in artisanal conditions. Best suited for such purposes ammonia. The workpiece is soaked in a 25% aqueous solution of ammonia, which significantly increases its plasticity. Likewise there will be an opportunity bend, twist it or squeeze out some relief shapes under pressure.

Advice: you should look at the fact that ammonia is dangerous! Due to this, while working with it, it is necessary to strictly adhere to all safety regulations. Soaking of wood must 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 developing its new shape, it is necessary to leave it in a similar curved form. This is necessary not only to fix the shape, but also to evaporate the ammonia. However, bent wood must be left in a ventilated area. It is noteworthy that when the ammonia has weathered, the wood fibers will acquire the same reliability as before, which will enable the workpiece to retain its own shape!

How bend board in artisanal conditions: layering method

First you need to harvest wood, which will subsequently be prone to bending. It is very, very important that the boards are slightly longer than the length of the required part. This can be explained by the fact that the bending tames the lamellas. Before you start cutting, you will need to draw a straight line diagonally with a pencil. This must be done across the underside of the workpiece, which will make it possible to preserve their order after moving the lamellas.

The boards must be cut with a straight-layered edge, and not with the right side at all. Likewise, they can be put together with the least amount of change. A layer of cork is applied to the mold, which can help avoid any unevenness in the shape of the saw and will provide the opportunity to make a more even bend. Plus, the cork will prevent delamination in the mold. After that, 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. It has a high level of adhesion, but takes a long time to dry. You can also use epoxy-based resin, but such a composition will cost a lot of money, and absolutely not everyone can afford it. The standard version of wood glue is not suitable for this type of application. Although it will dry quickly, it is considered too soft, which is not at all welcome in this option.

A bentwood product must be placed into the mold as quickly as possible. So, another one is placed on the lamella coated with glue. The process must be repeated until the bent workpiece receives required thickness. The boards are fixed together. After the glue has completely dried, it is necessary to shorten it to the required length.

How bend board in artisanal conditions: cut through

The prepared piece of wood needs to be sawn through. The cuts are calculated at 2/3 of the thickness of the workpiece. They must be located inside the bend. You need to be very careful, as rough cuts can easily deform the tree and even completely break it.

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

The cuts are made across the grain of the wood. Afterwards you need to compress the edges of the workpiece, which will make it possible to combine the resulting cracks into a single whole. This is the shape the bend takes upon completion of the work. After which he is corrected. In many cases, the outer side is finished with veneer, less often with laminate. This impact will give you the opportunity to correct the bend and hide almost any defects made during manufacturing. Gaps in bent wood are hidden quite easily - to do this, sawdust and glue are mixed, after which the gaps are filled with the mixture.

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

So, using such tips you can without big problems bend wood with your own hands.

If there is a need to manufacture a curved wooden element, then at first glance it may seem easier to cut out required element in a curved form, but in this case the fibers of the wood material will be cut, thus weakening the strength of the part, and as a consequence, 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 a reliable and durable source material for the production of various products. However, the change in shape is dry wooden blank the process is complex, 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 of wood, which allow you to change its shape and subsequently preserve it, it is quite possible to start 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 flexible 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. Her main task- 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 V in the required form, 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.
As soon as desired result reached, 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 the lignin bonds between fibers, you can treat the wood with chemicals, and this can be done 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.
Aqueous solution ammoniaAqueous solution of ammonia
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 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 grip, 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 glue-coated lamella. 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 must be with inside bending 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 - to do 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 a box or metal corner.
So, using these simple recommendations You can bend a tree with your own hands without much effort.

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 them in action. different systems steam generation. 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 a 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, 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. I can take a two meter inch board from white oak, clamp one end of it in a workbench and bend it along any curvature I need - fresh wood is so pliable. 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. However, I also worked a lot with air-dried oak and the result was also excellent.

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 come in a variety of shapes 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 come 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 hardwood into it. 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 the 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 comes 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 piece 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 - an 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 distribute stress evenly 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.

Often during the process repair work There is a need to obtain curved surfaces of products made from wood. How to bend a board so that the bend is strong and does not crack during the bending process? Well, if you have already decided to do major renovation with your own hands, then you should not retreat in the face of such difficulties. In this article we will talk in detail about how to give wood material curved shape.

How to bend a tree?

No, our task is not at all to bend an innocent plant. We are talking about wood building materials. How to bend a tree so that it bends and does not break? Bending method wooden products known since ancient times: to give wood a shape, only heat and moisture are needed, under the influence of which the plasticity of the material increases with all the ensuing consequences. How to bend a tree? Hold it in hot water (the higher the temperature, the faster the processes occur) or steam ( a steam generator can be made from a kettle or use an iron). The higher the temperature, the faster the wood yields and you can start bending it. Moistened and heated wood can be bent under the influence of a load (the ends of the board are placed on supports), and a load is placed in the place of the future bend. Dried wood perfectly retains the minimum radius of curvature that was achieved during the bending process. Now we know how to bend wood, we can dwell on this issue in more detail.

Wood reaction to external influences

The fact is that wood reacts differently to bending. The convex part is subject to tension, the concave part is subject to compression. Moreover, the material also reacts differently to steaming. For example, the ability to compress increases by as much as a third, but the ability to stretch - just a couple of percent. That’s why you shouldn’t think about how to bend a board more than two centimeters thick at home. It is also necessary to take into account that different types wood reacts differently to bending. For example, species such as oak, larch, and maple bend poorly, but beech, ash, and walnut bend well. So before you think about how to bend the board, decide on the type of wood from which it is made.

How to bend plywood, fiberboard, MDF

At home, plywood is bent by increasing its humidity, then ironing it (an iron is required), and fixing it in a template. Any template can be used frame element and it is not at all necessary that its shape should be curvilinear. The product is attached to the template using tape. Can be clamped bent plywood between two spacers, give it a bent shape using ropes, tying them around the product in several places along the radius of curvature. Plywood can only be used after it has dried. It seems like we’ve figured out how to bend plywood – let’s move on.

How to bend fiberboard? The technique is the same as in the previous case! How to bend MDF? In this case, you can go two ways: either bend thin sheets(no more than 5 mm) and glue them together, or use flexible MDF, in which there are transverse slots on one side. The thickness of such sheets is usually 8 mm. When bending, they are placed on top of each other with their milled sides, and then glued together. That's all!

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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).

Process wood bending. 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 on 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 rectangular-section parts are processed 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.

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

Use saturated steam for steaming low pressure(0.02-0.05 MPa), 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 put on 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, before bending it is recommended to dry the workpieces to 20% humidity, keep them in the machine until the humidity is 12-15%, and the final drying of the workpieces removed from the machine to production humidity is carried out in drying chambers Oh.

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, they are used special devices, screws, pneumatic and hydraulic presses.

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 with outside the workpiece is pressed against the template through a tire using 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 ability of wood to bend, allows you to bend wood with large knots located on outside blanks. 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.