PALM WOOD PRODUCT AND DESIGN GUIDELINES

The machining of coconut palm wood is rendered difficult by the structure of the wood. For cost reasons these difficulties must aIready be taken into account during design (see also Table 4), otherwise processing may become uneconomic and the products do not have the required qualities. The effects of the properties of the wood on its processing during the machining operation are listed in Chapter 8.

Table 4: Design Guidelines for Industrially Manufactured Products

Problemetic:	To be Preferred
Designs and jointing techniques that require a large proportion of manual work	Designs and jointing techniques that can be carried out by the use of machinery
Mortise and tenon joints	Dowelled joints, if strength is sufficient
Profiles with a large material removal volume/large profile depth	Profiles with a small material removal volume/small profile depth
Dovetail joints/Finger joints	Dowelled joints
Frames with a counter profile	Frames with mitre joint
Edge radius < 5 mm	Edge radius > 5 mm
Highly profiled lathe-turned components with a profile base narrowing to a point	Lathe-turned components with small profile projections and a profile base narrowing to a rounded shape (circular rods, hollow cylinders)
Absolutely to be avoided:
Highly profiled cross-grain wood	Non profiled cross-grain wood with rounded edges
Finger joints	Longitudinal wood joints with dowels or tongue

The fact that the strength properties of coconut wood are lower compared to other wood species of the same density classes must be taken into account in the design and jointing techniques for a product. This is especially true for all wood joints in which there are glue bonds between the connecting longitudinal and transverse wood surfaces. The dowel led joint, which has distinct advantages in its manufacture, is especially critical here unless adequate precautions are taken to avoid the known negative factors. Even with coconut wood, however, the dowelled joint can achieve the strengths required in the construction of furniture, windows and doors provided that appropriate precautions are taken.

2.2.1 Preferred jointing and construction techniques

Dowelled joint

The dowelled joint is a simple and universally usable joint. It is used as a frame corner and box corner joint. When using coconut wood, adequate strength values are achieved if the glue is adjusted to a high viscosity and/or the glue is applied several times, especially in the cross-grain wood region, so that excessive absorption of the glue does not occur (starved glue line). This also applies to dowel holes, which have a high proportion of cross-grain wood, depending on the position of the drilled hole. In addition, in the case of jointing surfaces in the cross-grain wood region, special care must be taken to ensure that there are no raised vascular bundles that prevent an accurate, tight joint fit. Figures 5,6 and 7 show the principle of dowelled joints. The dowel spacing distances should not exceed 120 mm.

Fig. 5: Principle of a dowelled joint

Fig. 6: Butt-dowelled frame corner

Longitudinally profiled components are particularly suitable for mitred corner joints.

Source: Nutsch, 1983

Fig. 7: Dowelled mitred frame corner

Source: Nutsch, 1983

Fig. 8: Principle of a tongue and groove joint

Fig. 9: Tongue and groove joint with inserted tongue

Source: Nutsch, 1983

Laminated material

It is possible to design furniture of panel construction with coconut palm wood by means of solid wooden panels (glue-laminate wood panels). The panels are manufactured by gluing wood strips together in the width (see Figure 10). Gluing the strips in the length using finger joints is a problem with coconut palm wood (see Section 8.6.9). The dowelled or tongue and groove joint in the cross-grain wood region is recommended as a jointing principle for longitudinal joints. Strict adherence to the density class is necessary when selecting the wood. Otherwise the panels warp or dissimilar shrinkage and swelling effects occur between the bonded wooden strips. In addition to using the same density class in a single glue-bonded component (panel, squared timber, beam), attention should be paid to uniformity of colour among the individual elements (strips) that are to be glued together. It is also advisable to pay attention to the number of vascular bundles in a surface unit, since these affect the optical and strength characteristics

Laminated beams of the kind used to achieve load-bearing building structures and rather large spans or structural component lengths can be manufactured using coconut palm wood (see Photos 23 – 26). Because of their poor weathering resistance, they are usable only in a protected environment (in this connection see Chapter 7).

Fig. 10: Glue-bonded profile for full-width bonding

Photo 23: Laminated and bolted coconut boards

Hollow cylinder

A further design alternative is offered by sections of coconut palm trunks lathe-turned circular on the outside and centre-bored to an internal diameter of approx. 170 mm. These hollow cylinders (tubes) can be used structurally and decoratively, either as a whole or divided lengthways. Examples of their use include: cheeks and side-pieces for shelves or wall units, side-pieces for the supports of chairs and tables, posts, beams or trusses for wooden structures of all kinds (see also Photos 6,14, 15, 16). As a result of this tube principle, high strengths can be achieved, far exceeding the strengths of the massive material (the trunk as a solid cylinder). Another advantage is that drying cracks are prevented, which occur when drying material with different density areas.

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2.2.2 The wood structure as a design feature

In addition to its colour, the texture of coconut palm wood is also a conspicuous feature (see Figure 11). The intensity of the vascular bundles stands out to an extent that varies depending on the position in the trunk. They decrease towards the centre of the trunk and towards the top. In a longitudinal section the vascular bundles are as a rule seen as lines running more or less parallel. In a cross-section they appear as dark points in the light-coloured parenchyma tissue. In a diagonal section they appear as short, thick lines. Depending on the arrangement of the different graphical forms of the vascular bundles in the product, an aesthetic image is formed that contrasts with the inherent simplicity of the coconut palm wood.

Fig. 11: Vascular bundles in longitudinal, transverse and diagonal section

Source: Rossmann, 1994

Important
Cross-grain wood surfaces should not show any raised vascular bundles in the joint region; they must be absolutely flat and smooth! The adhesive should be adjusted to a high viscosity (especially when applying to cross-grain wood surfaces); alternatively the adhesive should be applied several times, in dowel-holes as well!

Sources FAO Report, Assessed on 22 February 2016