The size of screws is indicated by length and diameter of the shank. The length of a screw is determined by the part that actually goes into the wood, and is measured in inches. They are manufactured in lengths of 1/4" to 5". The diameter of the shank is designated in numbers of the American screw gage. The following table shows the gage sizes and their equivalents in inches.
Choice of sizes
When fastening two pieces of wood together with screws, the choice of the proper length screw is important to obtain the maximum holding power. The length has a direct bearing on the diameter of the shank. Screws are made in definite shank diameters for specified lengths.
A wood screw should penetrate the second member (the one to which the first member is being attached) to a depth 7 times the shank diameter. As shown, if a piece of lumber 1/2" thick is to be joined to a second member and a #5 screw is to be used, it should be 7 times longer than the diameter, plus 1/2" which is taken up by the thickness of the first member through which the screw must pass. A #6 screw measures .1368" and 7 times this would be .9576". To this is added the 1/2", bringing the total up to 1.457" or about 1 1/2".
Sometimes the construction of the piece is such that it is impossible to use a screw of the desired length. Then the difference can be made up by the use of several screws placed closer together, provided of course that they penetrate the second member sufficiently. It must be kept in mind that the screws should be so located as not to split the wood.
When using screws, the stock must be prepared by boring a hole through the first member large enough for the shank of the screw to pass through. This is known as a clearance hole. A small hole should be bored in the second member which is to receive the threaded portion of the screw. This is called a pilot hole. The size of the clearance hole is determined by the shank diameter or gage number of the screw, while the pilot-hole size is based on the root diameter.
The pilot holes in hardwoods should be larger in diameter than those bored in softwoods. Clearance holes that are to take a flathead or oval-head screw should be countersunk. This is done with a countersink.
When it is necessary to set the head of a screw below the surface (so as to increase the depth of penetration into the second member or to conceal the head), an operation known as counterboring is employed. This involves hole boring a hole larger than the diameter of the screw head. The location of the screwhole is established, and the counterbore bit is used to bore the large hole to the required depth.
The clearance hole is then bored through the stock, placing the point of the clearance drill in the center mark left by the counterbore bit. A screw that is driven into a counterbored hole can be concealed by setting a wood plug in the hole. The plug may be of the prefabricated type (A) or the flush type (B). The flush-type plug is made with a plug cutter. The plug cutter is used in a brace in the same manner as a bit.
The plugs, as they are cut, have tapered sides; when placing them in the holes, the smooth face should be down. The plug is glued in the hole, and dressed flush with the surface after the Blue has set. When setting in a plug, the grain of the plug should follow the grain of the stock into which it is set.
When driving screws into hardwood, it is advisable to lubricate the thread. Soap or wax, both good lubricants for the purpose, can be applied to the thread by placing the screw on the soap or wax and drawing it across the surface. Sufficient soap or wax will adhere to the thread to act as a lubricant. This will reduce friction, making the screw easier to drive, and eliminate the danger of the screw's breaking inside the wood. Never use oil, which may expand the wood fibers so greatly that the screw cannot turn.
Use of the ratchet brace and screwdriver bit
When it is necessary to drive a number of screws, the ratchet brace and screwdriver bit should be used. Because of the leverage that can be obtained with a brace, it is easier to use the screwdriver bit with it.
The tip of the screwdriver bit must fit the screw slot, as described on page 101. It is placed in the ratchet brace in the same manner as any other square-tanged bit. When using this combination of tools, be sure to place the screwdriver bit in the slot and hold the brace so as to form a straight line from the knob of the brace through the axis of the screw.
Pressure to hold the screwdriver bit in the screw slot is exerted by one hand placed on the knob of the brace. This hand must also steady the brace. The revolving of the brace, by the hand placed on the handle, does the driving or removing of the screw. The brace is revolved in a clockwise direction to drive the screw, and counterclockwise to remove it.
The ratchet attachment on the brace permits the movement of the chuck and screwdriver bit in one direction only. When the ratchet is engaged, it is possible to remove the bit if the handle of the brace is moved in one direction, but if the movement is reversed the screwdriver bit will not turn. This is useful when driving screws into locations that will not permit the full sweep of the brace, for a series of part-turns and reverses will drive the screw.
Lag screws are heavy-duty wood screws with square heads. They come in lengths ranging from 1" to 16" and have shank diameters ranging from 1/4" to 1". They are driven by a wrench rather than a screwdriver. The wood into which they are driven is prepared in the same manner as that described for wood screws. When using lag screws, always place a washer under the head.