抄録
Wood screws were withdrawn from side and end grains of solid red lauan to investigate the effects of crests of screws and pilot holes on driving torque and withdrawal resistance. After the withdrawal tests the test specimens were tangentially sliced into thin veneer of 0.23mm thickness to measure the area of distorted wood fiber (Fig. 2).
Crests of brass screws were ground off to prepare five different level heights of crest (Table II and Fig. 1), and driving torque and withdrawal resistance were measured. It was found that even a slight crest of a screw brought in a small increase in driving torque and a large increase in withdrawal resistance. (Figs. 3 and 4).
Pilot holes of various diameters were predrilled (Table III). With decreasing gradually the diameter ratio of a pilot hole in side grain from fifty percent, the driving torque increased but the withdrawal resistance decreased. Therefore, the diameter of a pilot hole should be five-tenths the outer diameter of a screw. Similarly, the most effective one was a pilot hole of which diameter was seven-tenths that of a screw when driven into end grain (Figs. 6 and 7). It should be noted that the withdrawal resistance decreased remarkably if the diameter was larger than that of the inner diameter of a screw.
In the case of withdrawing a screw from side grain without a pilot hole, the outline of the area where fiber was distorted resembled an ellipse of which major axis was along L-axis of wood and minor one along R-axis (Fig. 8).
