If there is a need for the manufacture of a curved wooden element, then at first glance it may seem that it is easier to cut the desired element in a curved form, but in this case the fibers of the wood material will be cut, thereby weakening the strength of the part, and as a result, of the whole product . In addition, when sawing, a large overspending of the material is obtained, which cannot be said about the method when the wooden billet is simply bent.
Wood is a cellulose fiber bound together by a chemical called lignin. The flexibility of wood depends on the location of the fibers.
Only well-dried wood will be a reliable and durable starting material for the production of various products. However, changing the shape of a dry wooden blank is a difficult process, because a dry tree can break, which is very undesirable.
Having studied the technology of how to bend a tree, as well as the basic physical properties of wood, which allow you to change its shape and subsequently save it, it is quite possible to do wood bending 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. It happens that tensile forces lead to rupture of the outer fibers. This can be prevented during preliminary hydrothermal treatment.
So, you can bend the blanks of a bar made of solid wood and glued wood. In addition, planed and peeled veneers are used for bending. The most plastic are hardwoods. Among them are beech, ash, birch, hornbeam, maple, oak, poplar, linden and alder. Bent glued blanks are best made from birch veneer. It should be noted that in the total volume of bent-glued blanks, birch veneer occupies approximately 60%.
When steaming a workpiece, the compressibility increases significantly, namely by a third, while the tensile ability increases by only a few percent. And, therefore, it is not worth a priori to think about whether it is possible to bend a tree thicker than 2 cm.
Steam box heating
First, prepare the steam box. It can be made by hand. Its main task is to keep the tree that needs to be bent. There should be an opening in it to allow steam pressure to escape. Otherwise, it will explode.
The steam outlet should be in the bottom of the box. In addition, a removable lid must be provided in the box through which it will be possible to pull out the bent tree after it has acquired the desired shape. Clamps should be used to hold the wooden bent part in the desired shape. They can be made independently from wood or bought in a specialized store.
Round trim should be made of wood - a few pieces. Drilled off-center holes are drilled into them. After that, it is necessary to insert the bolts through them, and then drill another hole through the sides to push them tightly. Such simple crafts can become excellent clips.
Now it is time to steam the tree, for this you should take care of the heat source and close the wood blank in the steam box. For every 2.5 cm of the thickness of the workpiece, steaming the product takes about an hour. After time, the tree needs to be removed from the box and give it the necessary shape. The process must run very fast. The workpiece bends neatly and gently.
Some types of wood bend easier than others due to different elasticities. Different methods require the application of forces of different sizes.
Once the desired result is achieved, the bent tree needs to be fixed in this position. You can fix the tree during its formation. This makes it easier to control the process.
By chemical impregnation
To destroy the lignin bonds between the fibers, it is possible to influence the tree with chemicals, and it is quite possible to do this at home. Ammonia is ideal for this. The workpiece is soaked in a 25% aqueous solution of ammonia. After which it becomes very obedient and elastic, which allows you to bend, twist it and squeeze relief forms in it under the press.
Ammonia is dangerous! Therefore, when working with it, all safety precautions should be observed. Soaking the workpiece should be carried out in a tightly closed container located in a room that is well ventilated.
The longer the wood is in the ammonia solution, the more plastic it becomes. After soaking the workpiece and shaping it, you need to leave it in such a curved form. This is necessary for fixing the form, and in order for the ammonia to evaporate. Again, leave a bent tree should be in a ventilated area. It is interesting that after the evaporation of ammonia, the wood fibers will gain their former strength, and this will allow the workpiece to retain its shape!
Layering method
First, you need to make a tree blank, which will be bent. The boards should be slightly longer than the length of the finished part. This is because the bend will shorten the lamellas. Before you start cutting, draw a diagonal line with a pencil. You need to do this across the bottom of the board. This will save the sequence of lamellas after moving them.
The boards are cut with a straight edge, in no case with the front side. So, they can be added together with the smallest change. A cork layer is applied to the mold. This will help to avoid unevenness in the form of a saw, which will allow for a sharper bend. In addition, the cork will hold the bundle in shape. Now glue is applied to the upper side of one of the wooden slats.
The glue is applied to the lamellas with a roller. Urea-formaldehyde adhesive consisting of 2 parts is best used. It has a high level of adhesion, but dries for a long time. You can also use epoxy resin, but such a composition is very expensive, and not everyone can afford it. Standard glue for wood cannot be used in this case. It dries quickly, but is very soft, which is not welcome in this situation.
A bent wood blank should be placed in the mold as soon as possible. So, one more is laid on the lamella smeared with glue. The process is repeated until the bent workpiece reaches the desired thickness. Boards are fastened together. After the glue has completely dried, it should be shortened to the desired length.
Propyl as a method
The prepared wooden cut needs to be sawn. Saws are made 2/3 of the thickness of the workpiece. They should be on the inside of the bend. You should be extremely careful, because rough cuts can break a tree.
The key to success when cutting cuts is to keep the distance between the cuts as even as possible. Ideally 1.25 cm.
Incisions are made across the tree pattern. Next, compress the edges of the workpiece so as to connect the resulting gaps together. This form will take a bend at the end of work. Then the bend is corrected. Most often, the outer side is treated with veneer, in some cases a laminate. This action allows you to correct the bend and hide any defects made during the manufacturing process. The gaps between the bent tree are hidden elementarily - for this glue and sawdust are mixed, and after this the gaps are filled.
Regardless of the folding method, after the tree has been taken out of shape, the bend will relax slightly. In view of this, it needs to be done a little more in order to subsequently compensate for this effect. The sawing method can be applied when bending a part of a box or a metal corner.
The layers are carefully lubricated with glue, laid in the template and pressed. Bent knots made from veneer, from deciduous and coniferous plates, from plywood. In bent glued veneer elements, the direction of the fibers in the veneer layers can be either mutually perpendicular or the same.
In the manufacture of bent knots 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.
With an increase in the radius of bending of the plate, 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 radius of bending of the plate and the number of cuts.
Now consider the theoretical aspects of bending.
Solid wood curved parts can be made in two fundamental ways:
cutting curved workpieces and giving a rectilinear bar a curved shape by bending it on a template. Both methods are applied in practice and have their advantages and disadvantages.
Sawing curved blanks differs in 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 the details of large curvature and closed loop have to be made up of several elements by gluing. On curved surfaces, one-and-a-half and end surfaces of the slices are obtained, and in this regard, the processing conditions on milling machines and finishes are worsened. In addition, when cutting, a large amount of a large amount of waste is obtained. The manufacture of curved parts by bending requires, in comparison with sawing, a more complex process and equipment. However, during bending, the strength of the parts is fully preserved and even in some cases; end faces are not created on their faces, and the modes of the subsequent processing of bent parts do not differ from the processing modes of straight-line parts.
Element bending
and - the nature of the deformation of the workpiece during bending;
6
- bending the workpiece with the tire according to the template:
1 - template; 2 - notches; 3 - pressing roller; 4 - bus
When bending a workpiece within elastic deformations, stresses normal to the cross section arise: tensile on the convex and compressive on the concave side. Between the zones of tension and compression there is a neutral layer, in which normal stresses are small. Since the magnitude of the normal stresses varies over the cross section, shear stresses arise, which tend to shift some layers of the part relative to others. Since this shift is not possible, bending is accompanied by tensile material on the convex side of the part and compression on the concave side.
The magnitude of the resulting tensile and compression strains depends on the thickness of the bar and the bending radius. Suppose that a bar of rectangular cross section is bent along an arc of a circle and that deformations in the bar are directly proportional to stresses, and the neutral layer is in the middle of the bar.
Denote the thickness of the bar Hits initial length through Lobending radius in a neutral line through R (Fig. 60, a). The length of the bar along the neutral line during bending will remain unchanged and equal to Lo \u003dp
R ( j /180)
, (84) where p is the number pi (3, 14 ...), j is the angle of the bend in degrees.
The outer stretched layer will receive an extension of D L (delta L). The total length of the stretched part of the bar is determined from the expression Lo +D L \u003dp (R + H / 2)j /180
(85)
Subtracting the previous from this equation, we obtain the absolute elongation
D L \u003dp (H / 2) (j /180).
(86)
Relative extension Yer will be equal to D L / Lo \u003d H / 2R, i.e. elongation at bending D Ll / lodepends on the ratio of bar thickness to bending radius; it is bigger the thicker the bar H and the smaller the bending radius R. A similar ratio for the relative compression in bending can be obtained in a similar way.
Suppose around the pattern R " curved block with initial length Lo while maximum compression and tensile strains are achieved. Marking through Ecc is the value of the allowable deformation of compression of wood along the fibers Ethe magnitude of the allowable tensile strain along the fibers, we can write the ratio for the stretched side
L \u003d Lo (1 + Erast) \u003dp (R "+ H)j /180
(87)
From here R "+ H \u003d /p (j /180)
.
For the compressed (concave) side, L 2 \u003d Lo (1 - Ecж) \u003d p R " (j / 180)
or R " = /
p (j /180
). (88)
Subtracting the second from the first expression, we obtain
H \u003d)