Knurling is a manufacturing process, typically conducted on a lathe, whereby
a pattern of straight, angled or crossed lines is cut or rolled into the
material. 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-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 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,
barbell bars, 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. Aside from adding functionality to an
object, 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. 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 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
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. Cut knurling often employs automatic
feed, the tooling for cut knurling resembles that for form 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. Types
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, and is the most
common type used on everyday objects. References