SHAPING AND BENDING OF TIMBER
Several methods may be used for producing the shaped wooden parts of jobs. The methods used for shaping the pieces will depend on the particular requirements of the article, taking into consideration its uses, the type of timber (hard or soft), the strength required, the degree of curvature, the type of finish required (painted or natural), and whether the method is practicable for the job.
Generally full size drawings of the curved shapes are required, accurately marked out on a board (working rod) from which full size templates cut out of plywood are prepared. This is particularly necessary when several members of similar shape have to be set out.
The timber used for curved shapes, especially for solid wood bends, must be care- fully selected. It must be straight grained and free from defects.
SAWN SHAPES from Solid Wood
Flat curves, such as those used for shaped rails, legs and anus for chairs, can be sawn out of solid straight grained wood, and provided the grain direction is along the general axis of the members they will possess reasonable strength.
Where more acute curves are required the strength of shapes sawn from solid stock is low because of "short grain", which may weaken the member in more than one place.
Shapes cut from plywood will be stronger, due to the crossing of the veneers and the glue bond, but the size of the pieces is limited by the thickness of the plywood.
Generally there is considerable waste of material when shapes are sawn out of solid wood or plywood. The shapes may be cut with curve cutting saws such as bow, keyhole, band and jig saws, and smoothed with spokeshaves, compass planes, surform shapers, files and glass paper.
BENT SHAPES
Bent shapes from (a) Solid Wood Bends and (b) Laminated Wood Bends.
The shapes produced by these methods are stronger and more reliable than sawn shapes because the grain runs along the length of the curve and there are no short-grained areas. They are more economical as there is little waste, smaller sections of timber can be used and the size of the material
to be seasoned is reduced.
(a) Solid Wood Bends
Solid wood bends are often used for bent wood shapes for caravan framework, trucks, boats, canoes, handles for garden tools, skis, and shaped members for furniture, such as the arms, rails and backs of chairs. Occasionally bent shapes are used in joinery for staircase handrails and shaped door and window heads.
When a piece of wood is BENT the cells and fibres are strained and deformed. This outside surface is said to have a "tensile" force or stress acting on it and is in tension.
The inside or concave surface is shortened in length or crushed due to a "compressive" stress and is in compression..
When bending, timber will only stretch a small amount, but it will compress considerably if treated with heat and moisture (wet steam). Thus the permissible amount of bend is largely governed by the tensile stress the piece will take before fracturing, and this will vary according to the thickness of the stock. Obviously thick timber will not bend as successfully as thin timber, as there will be more wood to compress on the inside of the curve. Hence the timber should be reduced to the smallest practical thickness before bending.
The permissible amount of bend, before fracturing, for good bending timbers can be determined roughly by considering the ratio of the thickness of the timber in mm to the radius of curvature in mm. The ratio of a dry wood bend is about 1:50 (25 mm stock will form a curve of about 1200 mm radius; and 13 mm about 600 mm radius); but the piece will not remain bent unless securely attached to a
frame.
Steam-treated pieces will bend and stay bent to about 1:30 without taking special precautions. Sharper bends can be made provided the wood is softened by steaming and the outside surface (the surface in tension) of the curve is supported by steel straps fitted with end stops to prevent the fibres stretching and fracturing while bending.
(i) Selection of Stock for Solid Wood Bends
To make satisfactory bends in solid wood it is necessary to select straight grained timber known to have good bending properties and which is free from all defects. Use the smallest section possible for the job, dressed-all-round; if saw marks are left on the wood they can lead to fracturing.
(ii) Softening or Plasticizing the Wood
Dry solid wood will bend to a certain extent without fracturing, but will not remain bent. If the
wood is subjected to heating, hot water or steam, the fibres and other cells are softened and made plastic or flexible enabling them to yield to the compressive forces during bending. Once a piece is bent and allowed to dry it will "set" in its bent shape.
Steam heating is the most common method used to soften the wood, requiring fairly simple apparatus consisting of a kettle or boiler to which a hose is attached to convey the steam to a chamber or box which holds the stock to be bent. The steam chamber can be made of wood or metal (e.g., 100-150 mm galvanised water piping), long enough to hold the job. Insert the job in the steam box while the steam is being generated, then plug up the ends with paper or rag. Steam the timber for about 1 hour for each 25 mm in thickness of the material.
(iii) Bending
To bend the softened wood it is pressed over a form or mould, which has been constructed to suit the required shape, and cramped to it until the bend dries and sets. The forms for solid wood bends need
to be heavy and strongly made.
For flat curves, pegs fixed firmly into a base board may be sufficiently strong, the softened wood being forced between the pegs and allowed to dry.
Alternatively the steamed timber may be cramped between two shaped formers, one
for the inside of the curve and one for the outside.
(b) Laminated Wood Bends
A laminated bend is produced by bending and gluing together several thin strips of wood (laminae or
laminations) over a form or mould and cramped to the form until the glue has dried. Unlike plywood, which has the grain of its adjacent layers running at right angles, the laminations have their grain running in the same direction, i.e., around the curve.
Laminated bends can be used in place of solid wood bends for skis, tennis racquets, curved portions of furniture, boats, canoes, caravans, etc. Wide span laminated arched ribs are frequently used for factory roofs, being much lighter and often cheaper than steel arches.
As mentioned above, the severity of the stresses in the piece being bent varies with its thickness; thus thin pieces will bend more readily than thick pieces with little distortion. If a member is built up of a number of thin strips, which are able to slide along each other as they bend, the tensile and compressive stresses are greatly reduced in the glued assembly.
Compared with solid wood bends, laminated bends can be made with a greater variety of timbers, and the bends can be wider and the radius of curvature more acute. The technique of bending is simple and requires no special equipment other than wooden forms similar to but lighter than, those used for steam bending.
(c) Kerf Bending or Kerfing
Kerfing enables solid wood, plywood or solid core board to be bent either along the grain or across the grain to produce curved faces or edges.
This method of bending consists of relieving the normal bending stresses in the piece by making a number of parallel saw kerfs part-way through the wood.
Naturally this method reduces the strength of the bend, but it can be used quite satisfactorily for some jobs where strength is not necessary, such as curved drawer fronts and small doors, rounded corners of cabinet carcases, plinths and thicknessing rails and edging for table tops. Kerfing is most suitable for stock over 6 mm thick.
The amount of bend depends on the width and spacing of the saw kerfs, and the bending characteristics of the remaining un-kerfed wood, which is governed by the depth of the saw kerfs. Deep kerfs will make the piece more flexible than shallow kerfs but will not increase the bend. Usually the kerfs are made about three quarters the thickness of the stock, or to leave about 3 mm but not less than 1.5 mm of wood.
The kerf spacing for a given radius of curve and width of kerf (the width of the kerf is fixed by the set on the saw teeth) can be determined by making one kerf in a test piece which is the same thickness as the job. Mark a point away from the kerf on the test piece equal to the radius of the bend required, then measure the distance the mark travels when the piece is bent to just close the kerf. This distance will be the spacing between the kerfs around the bend. For a curve which is an
arc of a circle all the kerfs should be of uniform spacing, width and depth.