Knurling is a manufacturing process, typically conducted on a lathe
Lathe (metal)
A metal lathe or metalworking lathe is a large class of lathes designed for precisely machining relatively hard materials. They were originally designed to machine metals; however, with the advent of plastics and other materials, and with their inherent versatility, they are used in a wide range of...

, whereby a visually attractive 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
A helix is a type of smooth space curve, i.e. a curve in three-dimensional space. It has the property that the tangent line at any point makes a constant angle with a fixed line called the axis. Examples of helixes are coil springs and the handrails of spiral staircases. A "filled-in" helix – for...

 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 piston
A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from...

s 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.

A tool is a device that can be used to produce an item or achieve a task, but that is not consumed in the process. Informally the word is also used to describe a procedure or process with a specific purpose. Tools that are used in particular fields or activities may have different designations such...

 handles, mechanical pencil
Mechanical pencil
A mechanical pencil or a propelling pencil is a pencil with a replaceable and mechanically extendable solid pigment core called a lead . It is designed such that the lead can be extended as its point is worn away...

s, barbell
A barbell is a piece of exercise equipment used in weight training, weightlifting and powerlifting. Barbells range in length from to although bars above 7' in length are used primarily by powerlifters and aren't commonplace...

 bars, and the control knobs on electronic equipment are frequently knurled.

Knurling is also used on the grips of darts and the pegs of most BMX
Bicycle motocross or BMX refers to the sport in which the main goal is extreme racing on bicycles in motocross style on tracks with inline start and expressive obstacles, and it is also the term that refers to the bicycle itself that is designed for dirt and motocross cycling.- History :BMX started...

 bicycles. Aside from adding functionality to an object (valve repairs and hand grips being some of the most common), knurling also adds an attractive 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. Brass ones are preferred over steel ones since the chances of getting rounded are less in brass ones.


On the lathe, knurl cutting is usually accomplished using the same automatic-feed mechanisms that are used to cut screw
A screw, or bolt, is a type of fastener characterized by a helical ridge, known as an external thread or just thread, wrapped around a cylinder. Some screw threads are designed to mate with a complementary thread, known as an internal thread, often in the form of a nut or an object that has the...

 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. 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
The integers are formed by the natural numbers together with the negatives of the non-zero natural numbers .They are known as Positive and Negative Integers respectively...

 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 pipecutter
A pipecutter is a type of tool used by plumbers to cut pipe. Besides producing a clean cut, the tool is often a faster, cleaner, and more convenient way of cutting pipe than using a hacksaw, although this depends on the metal of the pipe....

s 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.


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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