This chapter discusses some of the most commonly used processes for manufacturing threads and gears. Screw threads are divided into classes to designate the fits between internal and external mating threads. For some applications, a nut may fit loosely on a screw; in other cases, they must fit together snugly. The different fits are obtained by assigning appropriate tolerances on the pitch, major, and minor diameters and allowances to the threads for each class. Screws may be cut or formed. They may be cut with single-point tools on a lathe or with multiple-tooth cutters that include dies, taps, and milling cutters on various types of machines. A gear is a machine element that transmits motion in a positive manner through teeth around its periphery. The spur gear is the simplest form with teeth parallel to its axis. A rack is a gear with an infinite radius; it moves in a straight line. Gears are cut from cast and forged blanks, barstock, sheet metal, laminated plastic, and molded shapes. Spur, helical, worm, and straight bevel gears may be cut on milling machines with standard dividing heads and arbors. The large majority of all gears are hobbed or shaped. Gear testing can be divided into two kinds: functional and analytical. Functional checking shows how errors affect the way the gears work together, as when they are rolled together to test for freedom or noise. It is usually the easiest way of testing for acceptable gears. Analytical checking actually measures the important elements.
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