Knurls and Serrations



Knurls are specified on many machined parts and are used for finger grips on cylindrical nuts and knobs, for holding an insert in a plastic or molded material, or for an interference fit within a mating piece.

Knurls are produced by rolling a knurling tool over the surface of the part. A typical knurling note specifies an approximate number of teeth per inch and describes the use for the knurl (finger grip, appearance, press fit, etc.) to guide the producer.

A typical knurl note includes:

Type: straight, diagonal, diamond
Size: coarse, medium, fine ¾ or approx. no. of teeth per inch (T.P.I.)
Finished Diameter: (when required) .xxx”
Tolerance Limits: (if required) .xxx”

Note: Teeth per inch (T.P.I.) refers to the knurling tool as well as the knurled part.

If a knurled surface is required to be a specific size, a maximum and minimum after-knurl dimension should be placed on the drawing as shown in Figure 2.

Adequate tolerances should be given for the length of knurl when specified, and standard knurl widths are recognized and used by the industry. Standard data on knurls can be obtained from handbooks published by many knurling tool producers and from ANSI/ASME B94.6, Knurling.


Straight spline type serrations, as shown in Figure 2, are usually required when pieces are to be mated or have a specific relationship to each other. Serrations are specified to an exact shape, size and number. Broaching or hobbing can be used to produce serrations. Some types of involute external splines can be rolled on standard thread rolling equipment, but tolerances are not as close as those obtained by broaching or hobbing.