Scaling only from the drawing, P = 2wRH/B
2 × 3.1416 × 1.6 × 1/1.27 = 7.74, say 7 ft. 9 in. pitch, whereas the intended pitch was 7 ft. 6 in.
A good illustration of the use of the screw may be seen in the carpenter's auger, used for making or boring holes in wood. These tools are provided with a small tapered screw on their points, and this is followed by cutting edges and a larger spiral. The larger spiral is for the purpose of drawing up the chips or shavings. Another tool is made having two blades attached to the bottom of an iron bar formed like the blades of a propeller, which is sometimes employed for boring or digging post holes in clayey or soft soil. The machine is turned by a cross handle on top, and is frequently drawn up to bring out the soil until the hole is deep enough. The ordinary wood screw is one of the most useful of contrivances for fastening wood together, and for attaching to surfaces, hardware, ornaments, or other materials. The adhesive strength of nails is already shown, and the adhesive strength of wood screws, according to Bevan, is set down as follows:
WOOD SCREWS
The following are the thicknesses or diameters corresponding to the list numbers. Other thicknesses can be interpolated, each size varying in succession 1⁄64 in.—
| No. | 00 | 0 | 1 | 5 | 10 | 14 | 18 | 22 | 27 | 32 | 40 |
| Thicknesses in parts of inches | 1⁄32 | 3⁄64 | 1⁄16 | 1⁄8 | 3⁄16 | 1⁄4 | 5⁄16 | 3⁄8 | 7⁄16 | 1⁄2 | 5⁄8 |
An ordinary 2-in. wood screw, driven through a 1⁄2-in. board into hard wood, was found to be 790 lbs., and a force of about 395 lbs. was required to extract it from soft wood.
When screws are hard to drive or screw in place, a long screw-driver should be used, as screw-drivers with long handles seem to have a much greater leverage than short handled ones in driving screws home. Screws, however, are often split at the head, if care is not taken when using a long driver.
If a screw is rusted, hard to move or withdraw, it can be loosened by applying a hot iron to the head and making it hot. The heat expands the screw and, of course, makes the hole larger, and when the screw cools it contracts a trifle so that it may be withdrawn quite easily.