Penetration End Joint

A new, high strength end joint for timber has been described. The ends to be joined are first slotted to the required depth of the joint to form parallel tongues. These are coated with adhesive and, with opposing tongues crossed, the ends are forced together so that the tongues cleave one another carrying the adhesive down into the slots. The force per unit of sectional area required to cause penetration generally lies between 1500 and 3000 lb./in.2 and depends on the contact area between opposing tongues and the density of the wood. The cutters required for slotting are simply a bank of circular sawblades. There is no question of fitting in the joint so that accurate indexing is not required. Joints have been made with slot widths of 0.031, 0.063, and 0.080 inch. The useful range of end areas left after slotting is from 50 percent to 70 percent. Tests on Douglas-fir beams have given joint strengths of from 60 percent to 80 percent of the control strengths. Strength depends on slot geometry, adhesive and depth of penetration. Exploratory tests have shown that a wide range of wood species (density range from 6 to 60 lb/ft3) can be joined by this method. The separation strengths of joints formed without adhesive were between 150 and 650 lb/in2. Joint grip increases with fineness of slots and density of wood. Unset joints are strong enough to machine confidently without waiting for the adhesive to set.

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