Short lengths are a problem in both the producing and remanufacturing sections of the wood-using industry and generally are either discarded or sold at depressed prices.* Because of the limited demand for shorts, portions of logs that would produce them are often destroyed in quantity by the saw-millers. Further losses occur in docking sawn lengths to the nearest foot, and again, during remanufacture, shorts of high-grade timber are produced in docking specific lengths out of trade lengths.
With increasing costs of timber, the loss of material and reduction in returns by the discounting of short and unpopular lengths has become a serious matter and has stimulated the search for an economical means of end jointing. Gains in many sections of the timber and dependent industries will result if practical and efficient means of joining timber lengthwise are developed at costs that would allow: (a) recovery of short lengths, (b) pro-duction of timber in the exact length required by the user, and (c) docking to raise grade.
Finger jointing holds some promise of at least partly solving this problem.
Research and experimental work on finger joints is proceeding in Australia and commercial application has commenced.
Finger joints are produced by cutting several interlocking scarfs or wedges in the end section of pieces of the same width (see photo 17). Compared with a full scarf joint these smaller scarfs are easily machined and assembled, and by reducing the length of the piece over which the scarf extends, material saving is effected. Moreover, recent developments in glues and jointing equip-ment provide means of assembling random lengths into a continuous length that can be docked to the customer's exact requirements. The speed of jointing, and mechanisation of conveying the pieces to be jointed, have achieved economies in jointing costs so that many new fields of application seem possible.
The details of joints of this type have been subjected to intensive study overseas. Due to practical difficulties in preserving sharpness of cutters ending
* Condensed from a paper presented to the 1959 Australian T.I.S. Conference by M. W. Page, Div. of Forest Products, C.S.I.R.O.
in points, a compromise has been made between theoretical excellence and practical optimum, the toothed tenons, wedge dovetails (or fingers), being generally made with blunt ends.
Because of an increasing local interest in finger jointing, the Division of Forest Products decided to investigate the characteristics of this type of joint in a range of our commercially-important timbers. The results of these studies are available on application to the Division.
To date, finger joints have been produced and tested in radiata pine, mountain ash and alpine ash, jarrah and brush box, representing the density range of our commercial timbers. As a result of this work it is expected that in all these species finger joints could be made on a production basis that would have 60 per cent of the bending strength of matched, unjointed controls.
As an indication, the following average efficiencies were attained in the laboratory:
Radiata pine . . . 74 per cent.
Mountain ash . . . 78 per cent.
Jarrah . . . .. 78 per cent.
Brush box . . . 73 per cent.
Finger jointing has made rapid progress in the United States, and for some time jointed joinery stock, vat staves, wood-pipe stock, flooring, mouldings and railway wagon components have been an accepted part of their timber economy.
Finger Jointing
Parallel development can take place in this country and so increase the yield of high-grade timber and help alleviate the problem of disposing of short lengths. A number of plants are operating in Australia, one jointing radiata pine in South Australia, some in Queensland on hoop pine and other timbers, and at least one other working ash-type eucalypts in Victoria.
Finger Jointing Equipment
Production type finger jointing equipment is at present being manufactured in both North America and Germany. Machinery is available ranging in capacity from small units capable of joining six to eight pieces per minute up to large set-ups with a potential output of 80 to 100 linear feet per minute.
The items of equipment required for finger jointing are the following:
1. A docking saw or saws for removing defects.
2. A planing machine capable of handling lengths as short as eight inches.
3. Machinery for fashioning the finger profile on the ends of the individual pieces. These machines somewhat resemble a single end tenoner. Essentially they consist of a means of holding the timber rigidly while traversing it first past a trim saw to square the end, and then past a rotating vertical spindle carrying the profiled cutters. One interesting machine of German origin employs saws instead of cutters, the saws being mounted on two spindles
127
which are canted from the vertical by an amount equal to the slope angle of the fingers. The profiling machine also incorporates an automatic glue applicator mounted directly behind the cutter spindle.
4. An assembly machine for rapidly assembling the finger-jointed shorts into a continuous ribbon of timber, and for cutting this ribbon into pre-determined lengths. The most modern machine of this type is produced in the United States.