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Close-up shot of a diamond-pattern knurling on a cylindric work piece. Knurling method: left/right with tips raised, spiral angle: 30°, pitch: 1 mm, profile angle: 90°. Object's diameter: 24 mm, material: aluminium (silver anodised)

Two examples of the use of knurling in hand tools.

Knurling is a manufacturing process, typically conducted on a lathe, whereby a visually-attractive diamond-shaped (criss-cross) pattern is cut or rolled into metal. This pattern allows human 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.^[1]

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.

A Diamond knurling tool with two of the interchangeable rollers shown above it

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.

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. 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 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.

Tool handles, mechanical pencils, barbell bars, and control knobs on electronic equipment are frequently knurled.

1. ^ Monroe, Tom. "Engine Rebuilder's Handbook". HPBooks, New York, 1996. Page 48.