Knurling
Knurling is a manufacturing process, typically conducted on a lathe, whereby a pattern of straight, angled or crossed lines is rolled into the material. Etymology. The terms "knurl" and "knurled" are from an earlier "knur" ‘knot in wood’ and the diminutive "-le", from Middle English "knaur" or "knarre" ‘knot in wood; twisted rock; crag’. This descends from Old English "cnearra" but the vowel in Middle English may have been influenced by Old Norse "knǫrr" ‘merchant ship’ which was known as "cnearr" in Old English. The modern "gnarl" is a back-formation of "gnarled" which itself is first attested in  Shakespeare’s works and is apparently a variant of "knurled". Uses. The operation is performed for producing indentations on a part of a workpiece. Knurling allows hands or fingers to get a better grip on the knurled object than would be provided by the originally smooth metal surface. Occasionally, the knurled pattern is a series of straight ridges or a helix of "straight" ridges rather than the more-usual criss-cross pattern. Knurling may also be used as a repair method: because a rolled-in knurled surface has raised areas surrounding the depressed areas, these raised areas can make up  for wear on the part. In the days when labor was cheap and parts expensive, this repair method was feasible on pistons of internal combustion engines, where the skirt of a worn piston was expanded back to the nominal size using a knurling process. As auto parts have become less expensive, knurling has become less prevalent than it once was, and is specifically discouraged by performance engine builders. Knurling can also be used when a component will be assembled into a low precision component, for example a metal pin into a plastic molding. The outer surface of the metal pin  is knurled so that the raised detail "bites" into the plastic irrespective of whether the size of the hole in the plastic closely matches the diameter of the pin. Tool handles, mechanical pencils, the grips of pistols, barbell bars, the clamping surface of a motorcycle handlebar and the control knobs on electronic equipment are frequently knurled. Knurling is also used on the grips of darts and the footpegs of BMX bicycles. Found in many surgical instruments, with use in instrument identification, and the ease of brushing clean. The knurled nut is another application, having a knurled finish instead of hexagonal  or square edges to help in tightening or loosening the nut without the use of a tool. The knurled surfaces provide enough grip to hold the nut between the thumb and forefinger. Hence it is also called a thumb nut. It is mainly used on small-diameter bolts. Knurled nuts are typically used in electrical components, musical instruments and in automobiles. Aside from adding functionality to an object (valve repairs and hand grips being some of the most common), knurling also adds a decorative pattern to the material. Process. More common than knurl cutting, "knurl rolling" is usually accomplished using  one or more very hard rollers that contain the reverse of the pattern to be imposed. It is possible for a "straight" knurl (not criss-crossed) to be pressed with a single roller, however the material needs to be supported adequately to avoid deformation. A criss-cross pattern can be accomplished using any of: A popular myth is that rolled knurls are somewhat more complicated to design than cut knurls because the outer diameter of the workpiece must be chosen to allow the roller to roll an integer number of patterns around the workpiece; in practice if the knurl is  applied positively, it will engage with its own impression and create a proper knurl on any diameter of work. The integer number of knurls for any given diameter typically varies by three repetitions from the bottom to the top of the pattern. By comparison, for cut knurls, the spacing of the cuts is not preset and can be adjusted to allow an integral number of patterns around the workpiece no matter what the diameter of the workpiece. Hand knurling tools are available. These resemble pipecutters but contain knurling wheels rather than cutting wheels. Usually, three wheels are carried by the  tool: two left-handed wheels and one right-handed wheel or vice versa. Cut knurling often employs automatic feed, the tooling for cut knurling resembles that for rolled knurling with the exception that the knurls have sharp edges and are presented to the work at an angle allowing the sharp edges to cut the work, angled, diamond and straight knurling are all supported by cut knurling. It is impossible to cut knurling "Like extremely coarse pitch threads" both because lathe gear trains will not support such longitudinal speeds and because reasonable cutting speeds would be impossible to achieve.