Finger-jointed products supply two main market segments. Structural wood products are produced with the joints designed to have high tensile strengths. Finger-jointing provides a greater degree of stability than single, large dimension lumber pieces which can in certain circumstances be prone to distortion.
Finger-joints have been developed because it is not possible to make strong butt joints by gluing the end grain of adjacent boards.
In finger-joints the glued surfaces are on the side grain rather than end grain, and the glueline is stressed in shear rather than in tension.
Finger-jointed timber is generally high quality timber obtained by cutting out defects from lower grade material before finger jointing.
Structural finger-joints are glued joints connecting timber members end-to-end capable of transferring appropriate loads.
The structural joints for New Zealand pine use finger lengths of 10mm through to 25mm, though shorter finger lengths of 10mm are preferred.
Adhesives used in structural jointing such as phenol resorcinol and melamine urea-based glues must meet strict exterior and exposure tests.
New Zealand finger-jointed pine meets the requirements of New Zealand, Australia , Japanese, USA and British structural testing standards.
Extensive qualification to recognised national standards and in-house quality control tests are conducted by finger-joint manufacturers to verify the on-going strength and reliability of the timber joints.
Finger-jointed lumber is used for a wide range of products where appearance is important. For this end use the 4mm 'micro joint' is offered by New Zealand manufacturers for Australasian markets as it is easily jointed, it provides a high quality finish and results in higher timber yields.
When clear adhesives are used, unblemished lengths of fingerjointed timber can be produced for high value end uses.
Appearance grade finger jointed products include mouldings. Fascia boards, handrails, balustrades, door and window components and weatherboard.
Finger jointed timber is used structurally in glue laminated beams, and sometimes in domestic construction as studs and trusses.
The purpose of structural finger joints is to maintain the strength of a piece of timber over its full length.
The strength of well made finger joints is significantly greater than the strength of the defects removed from the original timber.
For structural finger joints it is essential to dock the wood well away from knots, to avoid the strength-reducing effects of sloping grain around the knots.
Structural finger joints are governed by the requirements of AS 5068.
Finger jointed timber is used in many non-structural applications, generally where attractive defect-free wood is required for decorative or functional purposes.
Typical uses of finger jointed wood include fascia boards, weather boards, joinery, furniture, door jambs and mouldings. Finger jointed radiata pine is being used increasingly for purposes where native timbers were traditionally used.
Most finger jointed timber is of a high grade because defects are removed during the manufacturing process. Finger jointing is an excellent method of upgrading low grade material into valuable clear straight-grained timber.
1) Shape of fingers
Typical finger geometry is shown in the diagram below. Joints with square tips are not satisfactory for structural uses because the square tip section has little strength.
2) Size of fingers
Sizes of the fingers vary, depending on the finger jointing machines. Common sizes for structural joints range from 4 mm to 10 mm finger length. Non-structural joints may be 4 mm to 20 mm finger length.
3) Orientation of fingers
The fingers can be cut from edge-to-edge or from face-to-face, depending on the machine. The difference is mainly cosmetic, although bending strength increases if several fingers share the load. That means that a joist is slightly stronger with edge-to-edge finger joints and a plank is stronger with face-to-face finger joints.
Source: Courtesy Timber Design Guide, 2007