Knurling is a method used to cut or roll a pattern onto a material such as plastic or metal. This process is typically performed on a lathe, though in some cases a hand knurling tool will be used instead. A knurled object may have a diamond, criss-crossed, or straight line pattern imparted on it that adds both functionality and pleasing aesthetics. Knurling is often meant to provide a better gripping surface than offered by the bare material, and knurled patterns are often found on handles, knobs, and other similar items. Another use for the process can be to return a worn part into service, such as how internal combustion engine piston skirts were often knurled to increase performance.
The primary method used to knurl objects is a lathe process that uses a very hard roller to press the desired shape into the work material. A roller with a reverse imprint of the desired knurl is held in a knuckle or jig and then pressed into the piece being worked on. The main configurations used for this type of knurling contain either one or two rollers. A straight knurl can be pressed by one roller, but any type of a diamond or criss-crossed design will require rollers with opposing patterns. The drawback of this process is that the rollers need to be matched to the unique outer diameter of each workpiece, so it is best for the mass production of many identical components.
Another lathe process is similar to that employed in the cutting of screw threads. This method may be used to create a diamond pattern by essentially cutting both left and right hand threads into the same object. In this case, the knurling process is typically achieved using an automatic-feed lathe and a variety of cutting attachments. Unlike the rolling method, a single cutting attachment can typically be set up to knurl a wide variety of different workpieces.
A third way to knurl an object is with hand tools. These hand knurling tools work in much the same way as pipe cutters but are designed to press a pattern into the work piece, instead of cutting through it. Most hand knurling tools contain three wheels, one of which will have an opposing pattern to the other two. The benefit of this process is that the necessary tool can typically be carried to the location of the job, rather than bringing the work piece to a large, stationary lathe.