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Knurling is a manufacturing process, typically conducted on a lathe, whereby an visually-attractive diamond-shaped (criss-cross) pattern is cut or rolled into metal allowing human hands or fingers to get a better "grip" on the knurled object than would be provided by the originally-smooth metal surface. It may also be 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. Occasionally, the knurled pattern is a series of straight ridges or a helix of "straight" ridges rather than the more-usual criss-cross pattern.
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.
Knurl rolling is usually accomplished using one or more very hard rollers that contain the reverse of the pattern to be imposed. A "straight" knurl (not criss-crossed) can obviously be accomplished with a single roller. 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 isn't 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 pipe cutters 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.
Tool handles, mechanical pencils, barbell bars, and control knobs on electronic equipment are frequently knurled.
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