Conventional wood screws commonly used are the Countersunk/flat head (252:2A), round head for attaching metal to wood (252:2B), and raised/oval head (252:2c), obtainable in brass, steel, japanned (round head), coppered, chrome, BMA finish, etc. according to the type. The length of a screw is taken as all that part of the screw concealed by the wood, thus the length of a countersunk screw will be the overall length (252:1), round head from under the head, and raised head from the junction of the raised head with the counter-sinking. The diameter of all screw-heads is twice the diameter of the shank, the angle of the countersink 45° to the central axis, and the length of the actual thread three-fifths (0.6) of the stated length. A useful formula for the determination of the stated gauge number is: diameter of head in 1/32 in — 3 = screw gauge number; thus diameter of head 13/32in — 3 = 10 which is the gauge number. The diameter of the shank can be calculated from the gauge number by taking the known diameter of a No. 1 screw as 0.066 in with an additional 0.014 in for each subsequent increase in the number of the gauge; thus a No. 8 screw will be 0.066 in (No. 1 screw) - 7(0.014) or 0.164 in diameter. This will be the size of the hole required for a No. 8 screw shank, but as drill sizes are in 1/64 in the nearest approximate size will have to be used, and the
formula commonly adopted is to add 3 to the gauge number to give the size of the drill in 1/64 in. Thus No. 8 screw plus 3 = 11/64 in which is 0.172 in in diameter. The table gives the usual drill sizes for the shank or clearance hole, and for the hole to receive the thread.
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