In the manufacture of furniture, you can not do without curvilinear parts. You can get them in two ways - sawing and bending. Technologically, it would seem that it is easier to cut a curved part than to steam, bend and then withstand it for a certain time until it is fully ready. But sawing has a number of negative consequences.

Firstly, there is a high probability of cutting the fibers when working with a circular saw (it is used in this technology). The result of cutting the fibers will be the loss of strength of the part, and, as a result, of the entire product as a whole. Secondly, sawing technology involves a greater consumption of material than bending technology. This is obvious and no comment is required. Thirdly, all curved surfaces of sawn parts have end and half-end cut surfaces. This significantly affects the conditions for their further processing and finishing.

Bending avoids all these disadvantages. Of course, bending involves the presence of special equipment and fixtures, and this is not always possible. However, bending is possible in the home workshop. So, what is the technology of the bending process?

Technological process manufacturing of bent parts includes hydro heat treatment, bending blanks and drying them after bending.

Hydrothermal treatment improves the plastic properties of wood. Plasticity is understood as the property of a material to change its shape without destruction under the action of external forces and retain it after the action of forces is eliminated. Wood acquires the best plastic properties at a moisture content of 25 - 30% and a temperature in the center of the workpiece by the time of bending of about 100 ° C.

Hydrothermal treatment of wood is carried out by steaming in boilers with low-pressure saturated steam of 0.02 - 0.05 MPa at a temperature of 102 - 105°C.

Since the duration of steaming is determined by the time it takes to reach the set temperature in the center of the steamed workpiece, the steaming time increases with increasing thickness of the workpiece. For example, for steaming a workpiece (with an initial humidity of 30% and an initial temperature of 25°C) with a thickness of 25 mm to reach a temperature in the center of the workpiece of 100°C, 1 hour is needed, and with a thickness of 35 mm - 1 hour and 50 minutes.

When bending, the workpiece is placed on a tire with stops (Fig. 1), then in a mechanical or hydraulic press, the workpiece together with the tire is bent to a given contour; in presses, as a rule, several workpieces are bent simultaneously. At the end of bending, the ends of the tires are pulled together with a coupler. The bent blanks are sent for drying along with the tires.

The workpieces are dried for 6-8 hours. During drying, the shape of the workpieces stabilizes. After drying, the blanks are freed from templates and tires and kept for at least 24 hours. After holding, the deviation of the dimensions of the bent blanks from the original ones is usually ± 3 mm. Next, the blanks are processed.

For bent blanks, peeled veneer, urea-formaldehyde resins KF-BZh, KF-Zh, KF-MG, M-70, particle boards P-1 and P-2 are used. The thickness of the workpiece can be from 4 to 30 mm. Blanks can have a wide variety of profiles: angular, arc-shaped, spherical, U-shaped, trapezoidal and trough-shaped (see Fig. 2). Such blanks are obtained by simultaneous bending and gluing together veneer sheets lubricated with glue, which are formed into packages (Fig. 3). This technology makes it possible to obtain products of a wide variety architectural forms. In addition, the production of bent glued parts from veneer is economically feasible due to the low consumption of timber and relatively low labor costs.

Layers of plots are smeared with glue, laid in a template and pressed in (Fig. 4). After exposure under the press until the glue has completely set, the knot retains the shape given to it. Bent glued knots are made from veneer, from hardwood and coniferous plates, from plywood. In curved glued veneer elements, the direction of the fibers in the veneer layers can be either mutually perpendicular or the same. The bending of the veneer, in which the wood grains remain straight, is called a bending across the grains, and in which the fibers are bent - bending along the grains.

When constructing bent glued veneer units that carry significant loads during operation (legs of chairs, cabinet products), the most rational designs are those with a bend along the fibers in all layers. The rigidity of such knots is much higher than knots with mutually perpendicular direction of wood fibers. With a mutually perpendicular direction of the veneer fibers in the layers, curved glued knots up to 10 mm thick are designed that do not carry heavy loads during operation (walls of boxes, etc.). In this case, they are less susceptible to form change. The outer layer of such nodes should have a fractional direction of the fibers (bending along the fibers), since when bending across the fibers, small fractional cracks appear at the bending points, which exclude good finish products.

Permissible (curvature radii of curved veneer elements depend on the following design parameters: veneer thickness, number of veneer layers in a package, package design, billet bending angle, mold design.

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

In the tables, the elements remaining after the cuts are called extreme, the rest - intermediate. The minimum distance between cuts that can be obtained is about 1.5 mm.

With an increase in the bending radius of the plate, the distance between the cuts decreases (Fig. 5). The width of the cut depends on the bending radius of the board and the number of cuts. To obtain rounded nodes, in the slab, after veneering and grinding, a groove is selected in the place where the bend will be. The groove can be rectangular or dovetail. The thickness of the remaining plywood bridge (the bottom of the groove) should be equal to the thickness of the facing plywood with an allowance of 1-1.5 mm. A rounded bar is inserted into the rectangular groove with glue, and a strip of veneer is inserted into the dovetail groove. Then the plate is bent and kept in the template until the glue sets. To give the corner greater strength, a wooden square can be placed in it from the inside.

The layers are carefully lubricated with glue, laid in a template and pressed in. Bent glued knots produced from veneer, from plates of hardwood and coniferous species, from plywood. In curved 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 profile units with longitudinal cuts, it is necessary to take into account the dependence of the thickness of the bent elements on the type of wood and the thickness of the bent part.

With an increase in the bending radius of the plate, the distance between the cuts is reduced, as can be seen in the figure above. That is, the width of the cut directly depends on the bending radius of the plate and the number of cuts.

Now consider the theoretical aspects of bending

Curved solid wood parts can be produced in two fundamental ways:

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

Sawing curved blanks is simple technology and does not require special equipment. However, when sawing, the fibers of the wood are inevitably cut, and this weakens the strength so much that parts of large curvature and a closed contour have to be assembled from several elements by gluing. On curved surfaces, one-butt and end surfaces of cuts are obtained, and in connection with this, processing conditions worsen on milling machines and finishes. In addition, when cutting, a large amount of a large amount of waste is obtained. The manufacture of curved parts by bending requires a more complex technological process and equipment compared to sawing. However, when bending, the strength of the parts is completely preserved and even in some cases increases; end surfaces are not created on their faces, and the modes of subsequent processing of bent parts do not differ from the modes of processing of straight parts.

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

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

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

Denote the thickness of the bar H, its initial length through Lo, bending radius along the neutral line through R(Fig. 60, a). The length of the bar along the neutral line during bending will remain unchanged and is equal to Lo = p R( j /180) , (84) where p is the number pi(3, 14...), j - bend angle in degrees.
The outer stretched layer will receive an elongation D L (delta L). The total length of the stretched part of the bar is determined from the expression Lo+ D L= p (R+H/2) j /180 (85)
Subtracting the previous equation from this equation, we obtain the absolute elongation
D L= p (H/2)( j /180). (86)
Relative extension yer will be equal to D L/Lo=H/2R, i.e. relative extension when bending D Ll/Lo depends on the ratio of the thickness of the bar to the radius of the bend; it is the larger, the thicker the bar H and the smaller the bending radius R. A similar ratio for the amount of relative compression in bending can be obtained in a similar way.
Suppose around the pattern R" bent bar with initial length Lo and at the same time, maximum compressive and tensile deformations are achieved. Denoting through E compress the value of the allowable compressive deformation of wood along the fibers, and through E the value of the allowable tensile strain along the fibers grows, we can write the ratio for the stretched side
L = Lo(1 + Erast)= p (R"+H) j /180 (87)
From here R" + H = / p ( j /180) .
For the compressed (concave) side there will be L 2 = Lo (1 - Eczh) = p R"(j/180)
or R" = / p ( j /180 ). (88)
Subtracting the second from the first expression, we get
H = )