Improved Tips for Finger Joints

A limiting factor in new end-joint design has been the weakening effect of the tips of a finger joint. The solution seems to lie in bringing the tips out to a feather end, eliminating damaging truncation. Tips as thin as 0.016 inches have been tried commercially but are still too thin for satisfactory production in cutting heavier woods. This study investigated the use of cold-forming techniques to obtain feather-ended tips. A sharp wedge with a slope of scarf somewhat flatter than that in the main finger-joint was driven into female tips which had already been conventionally cut to a fairly small dimension. The wood at the shoulder of the split tip was thus crushed back out of the way, leaving a feather-ended female tip. Two fingered scarf joints, the Auburn B-joint and the Auburn F-joint, were tested in white ash, yellow-poplar, medium southern pine, dense southern pine, Douglas-fir, and redwood. The new feather-tipped joints showed an increase in tensile strength of from 31 to 85 percent over the old truncated end joints. It is hoped that this report will lead to the development of practical methods for production of feather-tipped end joints.

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