End Gluing Lumber

Three types of end joints of lumber are discussed. 1) Plain scarf joint yields average tensile strength of about 15,000 psi in uniform growth of southern pine, and 20,000 psi for ash when slope of scarf is l:9 or flatter. Only small additional strength is obtained by flattening the slope beyond l:9. Scarf joints, however, are wasteful of wood and difficult to fabricate, assemble, and cure under high-speed production. 2) Finger joints generally have fingers of between 5/8 to l-l/2 inches long, have rather flat scarfs on the sides of the fingers, and have relatively thick tips. They yield usually from 2,000 to 5,000 psi in Douglas-fir and southern pine in tension test. They are adapted for high speed production. 3) Fingered scarf joints are similar to finger joints, except they have fewer fingers and thinner tips. Three designs of this type of joint were shown which yield average tensile strengths between 9,000 and 12,000 psi in dense Douglas-fir and 14,000 to 16,000 for ash. Factors other than joint design which affect strength of the end glued lumbers are gluing technique, cross grain, and annual ring orientation. Moduli of rupture of southern pine beams made from fingered scarf joint lumbers are 10,900 psi and 12,600 psi which are 70 and 77 percent of controls. Production costs of finger joints range from 1 cent to 4 cents per joint for lumber of 1 by 4. If the average 1 by 4 short is about 3 feet, cost of efficient operation to produce end jointed lumber is approximately $15 per M. Needs of development for end gluing are: l) adequate joint, glue curing technique, and quality control program; 2) design of a machine to cure an average of 20 or more joints per minute; 3) improvement of cutterheads; 4) development of end-jointing machines which will cut from 2 to 4 end joints per minute and sells within $3,000; 5) development of a batch type hot press that will apply final end pressure on the joint and then cure it under about 150 psi of face pressure at about 45 second cycle.

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