Despite all their strength and durability, wooden parts can be easily and simply bent if suddenly during the construction process you need some special and original oval or round parts. Tree at correct processing can easily be changed in shape, and you can carry out this procedure with your own on our own without resorting to the help of professionals.
Types of work
There are two main ways to bend wood to the desired shape, one of them is cold, the other is hot. As the name implies, the methods differ only in the use of hot temperatures; in terms of effectiveness, both of these methods are exactly the same, just hot way fixing the tree is much faster. For each method you will need glue, pva or wallpaper, depending on what you have at hand, you don’t need to buy anything special. And with the help of metal parts you can create a kind of press or frame that will hold the tree in the desired shape. In order to bend the beam, you need to lubricate it with glue, firmly and thoroughly, without fear that the wood will become wet. In fact, under the influence of the adhesive solution, the entire excess moisture will leave the tree, and it will become even more durable and strong, 
which is extremely important. After the timber has been lubricated with glue, it must be secured using tools in the desired shape, and in the case of a cold work process, simply left fixed to dry. If you choose a hot one, then cover the timber with regular film so that it dries faster and all the moisture evaporates.
Durability
Whichever method is chosen, they are both effective and work the same way. The beam is completely fixed in its new form and will no longer return to the old one. You can bend a wet beam with glue as much as possible without fear that it will break. And as a result, you will receive an original and beautiful piece of furniture or detail to create an even more attractive interior of the house or its facade. The beam bent with glue will not even need to be treated with anything for durability, which is extremely convenient. Moisture will not penetrate through the adhesive solution, and insects will not attack wood that contains much more glue. That is why this method is the most optimal and practical if you urgently need to bend a wooden beam.
If there is a need to create a curved wooden element, then most likely you will encounter a number of difficulties. It may seem easier to cut the required component in a curved form, 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. | |
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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.
Tip: reliable and durable wood material for creating various products This can only happen 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 of wood, which allow 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 manufacturing technology bent wood, when steaming a workpiece, 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. Her the main task- 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 required form. 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 the wooden piece in a 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 ammonia solution, 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 in no way front 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 with inside bending 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. Perfect 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 for this effect. The sawing method is used when bending a metal corner or part of a box.
So, using these recommendations, you can bend a tree with your own hands without any problems.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. Steaming the same workpiece for a longer time 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 supposedly 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:
Usually, 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 that the workpiece is “hung” is to drill through holes in the side walls of the chamber and drive round hardwood wooden rods 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. You can put workpieces into the chamber only when the installation has reached full power 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 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 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 - with an inch oak board, 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 be the owner of 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.
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 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 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, as it is done manually and gives a 20-25% lower yield of blanks from the ridge than when sawing it.
After cutting (or splitting), blanks for parts with a round cross-section are processed on turning-copying or round-bar machines, and blanks for parts with a rectangular cross-section 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, wooden vats or metal baths and tanks are used. 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 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. Curved blanks 5 together with 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 of the workpieces removed from the machine to production humidity is carried out 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 compressed bag is used: electric contact, steam or high frequency currents (HFC). 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 you decide to decorate the room with wood or start creating beautiful furniture V classic style- then you will need to make curved parts. Fortunately, wood is a unique substance because it allows to an experienced master play with the shape a little. It's not as difficult as it seems, but not as easy as we would like.
Previously, there was already a publication on the site on bending plywood. In this article we will understand the principles of bending solid boards and timber, and find out how this is done in production. We will also give useful tips from professionals who will be useful to the home craftsman.
Why bending is better than sawing
Curvilinear wooden part can be obtained by two methods: by bending a flat workpiece, or by cutting out the required spatial shape. The so-called “sawing” method attracts users with its simplicity. For such production of parts and structures, you do not need to use complex devices, you do not have to spend a lot of time and effort. However, in order to cut out a curvilinear wooden product, it is necessary to use a workpiece that is obviously too large, and a lot of valuable material will be irretrievably lost as waste.
But the main problem is the performance characteristics of the resulting parts. When cutting a curved part from ordinary edged lumber, the wood fibers do not change their direction.
As a result, cross sections fall into the radius zone, which not only worsen appearance, but also significantly complicate the subsequent finishing of the product, for example, its milling or fine grinding. In addition, in the rounded areas that are most vulnerable to mechanical stress, the fibers run across the section, which makes the part prone to fracture in this place.
Whereas when bending, the opposite picture is usually observed, when the wood only becomes stronger. The edges of a curved beam or board do not have “end” cuts of fibers, so subsequently such workpieces can be processed without restrictions, using all standard operations.
What happens in wood when it bends?
Bending technology is based on the ability of wood, while maintaining its integrity, to change its shape within certain limits as force is applied, and then retain it after the mechanical stress is removed. However, we all know that without preparatory measures, lumber is elastic - that is, it returns to its original state. And if the applied forces are too great, then the beam or board simply breaks.
Layers of a wooden workpiece do not work equally when bent. Outside the radius, the material is stretched, inside it is compressed, and in the middle of the array, the fibers experience virtually no significant loads and have little resistance to the forces acting on the workpiece (this inner layer is called “neutral”). With critical deformation, the fibers at the outer radius break, and at the inner radius, “folds” usually form, which are a fairly common defect when bending soft wood. Compress fibers of plastic hardwood or coniferous species can be 20 percent or more, while the stretch limit is about one to one and a half percent.
That is, to determine the possibility of bending (without destruction), a more important indicator will be the limit of the relative elongation of the stretched layer. It directly depends on the thickness of the part and determines the radius that needs to be obtained. The thicker the workpiece and the smaller the radius, the greater will be the relative elongation along the fibers. Having data on the physical properties of popular wood species, it is possible for each of them to formulate the maximum possible ratio of thickness and radius of parts. In numbers it will look like this:
Bending using a steel bar
Bending without using a tire
These data indicate that softwood lumber, in comparison with dense hardwoods, is less suitable for free bending. To work with lumber at aggressive radii, it is necessary to use combined methods preliminary preparation parts and mechanical protection.
Tire as an effective way to avoid wood destruction during bending
Since the main problem is fiber breakage on the outer radius side, it is this surface of the workpiece that needs to be stabilized somehow. One of the most common methods is to use an overhead splint. The tire is a steel strip with a thickness of half a millimeter to two millimeters, which covers the beam or board along the outer radius and bends on the template along with the wood. The elastic strip absorbs part of the energy when stretched and at the same time redistributes the destructive load along the length of the workpiece. Thanks to this approach, coupled with humidification and heating, the permissible bending radius is reduced significantly.
In parallel with the use of steel bars in bending devices and machines, mechanical compaction of wood is achieved. This is done using a pressing roller, which presses on the workpiece along the outer bending radius. In addition, the template form in such a device is often equipped with 3 mm teeth (in increments of about 0.5 cm), oriented towards the movement of the workpiece.
The purpose of the jagged surface of the template is to prevent the workpiece from slipping, to prevent mutual displacement of fibers in the wooden massif, and also to create a small depressed corrugation in the concave radius of the part (the fibers are pressed inside the massif, therefore, problems with folds are solved).
Pressing with a tire allows you to bend bars and boards made of coniferous and soft deciduous wood with a minimum percentage of defects. Please note that items are from relatively hard rocks when bending with pressing, they become approximately ten to twelve percent thinner, and pine and spruce blanks become 20-30% thinner. But the positive aspects of this method include a significant increase in strength characteristics finished product, as well as a significant reduction in requirements for the presence of flaws and defects in wood blanks.
How to improve the plasticity of wood
In normal condition, lumber has elasticity, significant spatial rigidity and resistance to compression. Wood receives these valuable properties from lignin, a natural “mesh” polymer that gives plants a stable shape and strength. Lignin is located in the intercellular space and in cell walls, connecting cellulose fibers. Coniferous wood contains about 23-38 percent of it, in hardwoods- up to 25 percent.
Essentially, lignin is a kind of glue. We can soften it and turn it into a “colloidal solution” if we heat the lumber by steaming, boiling, treating with high frequency current (for small parts A household microwave is also applicable). After the lignin has melted, the workpiece is bent and fixed - as it cools, the molten lignin hardens and prevents the wood from returning to its original shape.
Practice shows that optimal temperature for bending solid wood (block, strip, board) it will be 100 degrees Celsius. This temperature must be obtained not on the surface, but inside the workpiece. Therefore, the time of temperature exposure will largely depend on how massive the part is. The thicker the part, the longer it will have to be heated. For example, if you use steaming to prepare for bending a 25 mm thick rail (with a humidity of about 28-32%), then on average it takes about 60 minutes. It is noteworthy that the steam exposure time for parts of similar dimensions for any species is approximately the same.
By the way, it is believed that it is also impossible to overheat the part, since lignin after hardening may lose its elasticity and become too brittle.
The boiling method is not often used, since the workpiece is heavily and unevenly moistened, and such water-saturated fibers and cells can tear when bent, at least with the formation of lint. After cooking, the parts have to dry for too long. But this method works well if you need to process only part of the workpiece for bending.
Steaming allows the workpiece to be heated evenly, and its output humidity tends to approach the optimum. The most suitable humidity for achieving maximum ductility of lumber is considered to be in the range of 26-35 percent (the moment of saturation of wood fibers).
To steam wood for bending at home, use homemade cylindrical chambers made of metal/polymer pipes or rectangular wooden boxes. Heating tanks act as a source of steam, electric kettles and other similar devices that can provide a temperature of about 105 degrees and low pressure. This is always followed by the stage of drying the part (+ holding the fixed shape) to about fifteen percent and finishing it.
Chemical methods for plasticizing wood
It is also known that it is possible to make lumber more pliable using impregnation various compositions. There are ready-made impregnations that make wood cells more plastic, for example, “Super-Soft 2”. Some practitioners soak wood in so-called textile conditioners, obtaining a similar result.
But quite primitive “recipes” can also be used containing ammonia and ethyl alcohol, glycerin, alkalis, hydrogen peroxide, dissolved alum... Many of them act extremely simply - they increase the ability of the workpiece to absorb water and help retain moisture in the fibers.
Thin products such as veneer are processed by spraying, but preparatory chemical impregnation of normal lumber is usually carried out using the full immersion method. It takes time for the working substances to get inside the bar or slats, usually from 3-5 hours to several days (although heating helps reduce the wait).
Largely because of the length of the processes, chemical plasticization is not often used, although there are other problems: the cost of chemicals, changes in colors, the need to provide protection from harmful fumes, the increased tendency of such curved parts to straighten...
Tips for bending lumber using hydrothermal preparation
- Select the quality of the blanks for bending very carefully. It is better not to use material with cracks, knots (even live and fused ones), or sloping fibers. If there are no options for this, then orient the part in the bending device (machine or template) so that the defects fall into the concave radius zone, and not into the tension zone at the outer radius. Give preference to the splint bending method.
- When selecting a workpiece, it is necessary to provide for a change in the size of the part after molding. For example, the thickness of a coniferous beam can be reduced by 30 percent if bending with pressing is performed.
- Even if you are planning an extensive finishing- do not leave too much material. The thinner the workpiece, the easier it bends without breaking.
- If the amount of work is small, then it is better not to cut out the workpieces, but to prick them from lumps. This way it is possible to avoid cutting the fibers and, as a result, defects during bending.
- For bending, it is advisable to use lumber with natural moisture. If you use dry workpieces, then preference should be given to those that have not been processed in drying chamber, and were dried under a canopy - in an atmospheric way.
- After steaming, work with softened wood very quickly, as lignin begins to harden almost immediately, especially in the most vulnerable outer layers of solid wood. Usually you need to focus on a time reserve of half an hour to 40 minutes, so there is no point in making large cameras if you simply do not have time to install all the material from them into templates.
- Place the material in the steaming chamber so that the surfaces facing the outer radius are freely exposed to the steam jets.
- To save time, many carpenters refuse to use templates with clamps. Instead, they use metal brackets and wedges or stop posts on the templates.
- Keep in mind that a curved bar or rail will still tend to straighten. And this straightening always occurs by a few percent. Therefore, when high precision in the manufacture of a part is required, it is necessary to conduct tests and, based on the results obtained, adjust the shape of the template (reduce the radius).
- After the part has cooled in the mold, let it stand some more. Some experienced furniture makers prefer to cure for 5-7 days. The tire, as a rule, is left attached to the part during this entire time.
