The layers are carefully lubricated with glue, placed in a template and pressed into place. Bent glued units made from veneer, from hardwood and softwood boards, from plywood. In bent-laminated veneer elements, the direction of the fibers in the veneer layers can be either mutually perpendicular or identical.
When manufacturing bent-profile units with longitudinal cuts, it is necessary to take into account the dependence of the thickness of the bent elements on the type of wood and the thickness of the bent part.
As the bending radius of the slab increases, the distance between the cuts decreases, as can be seen in the figure above. That is, the width of the cut directly depends on the bending radius of the slab and the number of cuts.
Now let's consider theoretical aspects bending
Curved solid wood parts can be made in two basic ways:
cutting out curved workpieces and giving a straight bar curved shape by bending it onto a template. Both methods are used in practice and have their advantages and disadvantages.
Sawing curved blanks The technology is simple and does not require special equipment. However, when sawing, the wood fibers are inevitably cut, and this weakens the strength so much that parts with large curvature and a closed contour have to be made up of several elements by gluing. On curved surfaces, half-end and end cut surfaces are obtained and, in connection with this, the processing conditions on milling machines and finishing. In addition, when cutting it turns out a large number of a large amount of waste. The production of curved parts using the bending method requires, compared to sawing, a more complex process. technological process and equipment. However, when bending, the strength of the parts is completely preserved and even in some cases increases; no end surfaces are created on their faces, and the modes of subsequent processing of bent parts do not differ from the modes of processing straight parts.
Element bending
A- nature of the workpiece deformation during bending;
6
- bending the workpiece with the tire according to the template:
1 - template; 2 - notches; 3 - pressing roller; 4 - tire
When the workpiece is bent within the limits of elastic deformations, normal to cross section stresses: tensile on the convex side and compressive on the concave side. Between the zones of tension and compression there is a neutral layer, the normal stresses in which are small. Since the value normal stress changes across the cross-section, shear stresses arise, tending to move some layers of the part relative to others. Since this shift is impossible, bending is accompanied by stretching of the material on the convex side of the part and compression on the concave side.
The magnitude of the resulting tensile and compressive deformations depends on the thickness of the bar and the bending radius. Let us assume that the block rectangular section bent in a circular arc and that the deformations in the bar are directly proportional to the stresses, and the neutral layer is located in the middle of the bar.
Let us denote the thickness of the bar H, its initial length through Lo, bend radius along the neutral line through R(Fig. 60, a). The length of the block along the neutral line when bending will remain unchanged and is equal to Lo = p
R ( j /180)
, (84) where p is the number pi(3, 14...), j - bend angle in degrees.
The outer stretched layer will receive elongation D L (delta L). The total length of the stretched part of the bar is determined from the expression Lo+ D L= p (R + H/2) j /180
(85)
Subtracting the previous one from this equation, we obtain the absolute elongation
D L= p (H/2)( j /180).
(86)
Relative extension Er will be equal to D L/Lo = H/2R, i.e. bending elongation D Ll/Lo depends on the ratio of the thickness of the bar to the bending radius; the thicker the block, the larger it is H and the smaller the bend radius R. A similar relationship for the value of relative compression during bending can be obtained in a similar way.
Let's assume that around the pattern R" bent block with initial length Lo and at the same time maximum compressive and tensile deformations are achieved. Designated by E szh the value of permissible compressive deformation of wood along the fibers, and through E grow the value of the permissible tensile strain along the fibers, we can write a relationship for the stretched side
L = Lo(1 + Erast)= p (R" + H) j /180
(87)
From here R" + H = / p ( j /180)
.
For the compressed (concave) side there will be L 2 = Lo (1 - Eczh) = p R"(j/180)
or R" = /
p ( j /180
). (88)
Subtracting the second from the first expression, we get
H = )