The Technique of FURNITURE MAKING

Metal fittings/fasteners and their application 26 Screws, nails and pins

WOOD SCREWS

Conventional wood screws commonly used are the Countersunk/flat head (252:2 A), 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.

RECESSED HEAD SCREWS

These are available in countersunk/flat, round head and raised/oval head forms, and have a cross slot in lieu of the single slot which gives less tendency for the screwdriver to ride up axially as torque is applied, and therefore less danger of marring the work if the driver slips. Special shaped screwdrivers are required and advantages claimed are more positive engage­ment of the driver in the slot, easier alignment of the screw and the work, particularly at difficult angles, increased speed of driving and more power. This type of screw has almost universal application in production-work with power screwdrivers for rapid assembly, but the conservative craftsman still clings faithfully to the conventional screw, and on balance the single-slot screw would appear to be more decorative, although this may well be the prejudice of ingrained habit.

As every workshop inevitably collects an assortment of odd screws over the years, a useful screw size gauge can be made by inserting a large-diameter screw in a piece of scrap wood, nipping two short lengths of brass strip round

the shank with panel-pins either side, and soldering on a brass cross-piece. Screws of known gauge number can then be slid between the strips, the exact position dotted with a centre-punch and the gauge number and drill sizes scratched on the brass. Figure 252:4 shows the gauge with a No. 12 screw so marked.

Metric values for all the above quoted inch sizes will depend on the units ultimately adopted by the International Standards Organization, as there are many different units quite unrelated to each other in use throughout the metric world at the present time.