Bolts and Screws

In general, a screw is driven by a screwdriver, alien wrench, or other device whose tip fits into an opening in the head of the fastener. The shaft may be cylindrical or conical, but whatever its shape it has helical ribs called threads. A bolt, in contrast, is a threaded metal rod or pin with a wrench head (typically hexagonal or square) around which a wrench or other device is fitted for tightening or loosening; the other end is secured using a nut. Wood screws have pointed (or gimlet) tips; bolts are blunt-ended.

The basic varieties and screws and bolts are these:

Wood Screw. A wood screw, as its name suggests, is intended for use in fastening wood or other materials to wood. Wood screws have tapered shanks, and are often threaded for roughly two-thirds of their length. They may be purchased in flat-, round-, or dome-head profiles. Lengths vary greatly, from fractions of an inch to five inches or more.

Phillips-head, slotted, and other drives are all commonly available. Brass, steel, aluminum, stainless steel, plated steel, and corrosion-proof materials are all used in making wood screws.

Drywall Screw. In profile, the drywall screw has a bugle-shaped neck, its head usually a Phillips slot (although finer-headed varieties are also available with a square drive). The thread generally runs the full length of the screw. Various sizes are sold, including one inch, inch and five-eighths, and three inches. The drywall screw is most often used to fasten wallboard, but is also suited to fastening softwood and man-made woods.

Machine Screw. Used to assemble metal parts, machine screws are usually made of steel or brass, with flat, round, oval, or cylindrical (called fillister) heads. Though it may also be used with a nut, the machine screw most often threads directly into a tapped hole in a metal workpiece.

Like other screws, machine screws are sold in varying lengths and gauges, but also with differing thread pitches. Thus, in buying machine screws the length, gauge, and pitch must be specified, as in "I need a box of 8-32 x Vi screws, please." The 8 is the gauge; the 32 refers to the threads, namely that there are thirty-two threads per inch; the one-half to the length of the screw.

Sheetmetal Screw. Sheetmetal screws are designed to fasten together metal elements, like stovepipe, flashing, and other metal components. They are self-tapping screws, available in brass or steel, and with flat, round, oval, or fillister heads.