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Knurling is a manufacturing process, typically conducted on a lathe, whereby a diamond-shaped (criss-cross) pattern is cut or rolled into metal.
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-up 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 recommended against by performance engine builders.
Knurling can also be used when a high precision 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.
Knurling is also used on the grips of darts and the pegs of most BMX bicycles. 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. The knurled nut is one such application, which uses a knurled finish instead of hexagonal or square edges which helps 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.
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 single roller that contains the reverse of the complete desired pattern. These are available to form either "male" or "female" patterns,
- A left-handed straight roller followed by a right-handed straight roller (or vice-versa), or
- One or more left-handed rollers used simultaneously with one or more right-handed rollers.
Rolled knurls are somewhat more complicated to design than cut knurls because the outer diameter of the work piece must be chosen to allow the roller to roll an integral number of patterns around the workpiece. 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.
On the lathe, knurl cutting is usually accomplished using the same automatic-feed mechanisms that are used to cut screw threads; knurling can be thought of as simply a series of threads cut at extremely coarse pitch and in both the left-hand and right-hand directions.
- Annular rings
- Frequently used when the mating part is plastic. Rings allow for easy mating but ridges make it difficult to pull the components apart.
- Linear knurl
- Used with mating plastic pieces, the Linear Knurl allows greater torsion between components.
- Diamond knurl
- A hybrid of Annular Rings and Linear Knurling in which a diamond shape is formed. It is used to provide better grip on components.
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- Monroe, Tom. "Engine Rebuilder's Handbook". HPBooks, New York, 1996. Page 48.