A knurling tool is used in conjunction with a lathe to emboss the ends of metal tubes and shafts. The embossed grooves may act as hand grips for the user or better traction for rubber or plastic covers. The knurling tool itself consists of multiple rotary cutters which are held against the metal shaft as it turns on the lathe at a relatively slow speed (500 rpm on average). Turning is a method by which cylindrical pieces of metal or wood are spun in place by a variable-speed electric motor. As the piece spins, various cutting tools can be placed against it to remove material or cut shapes. A knurling tool falls somewhere between an engraver and an embosser.
There are generally three shapes generated by most knurling tools - straight lines, diagonal lines and a diamond pattern. Knurling tools do come in a variety of sizes and cutting designs, depending on the purpose of the piece. The diamond pattern is most common with hand grips because it creates the most traction between a user's hand and the shaft. Diagonal and straight knurls are generally used to give extra traction to an external handle or other connective piece.
In order to create a knurl pattern, the lathe must hold the metal piece perfectly straight - a condition machinists call 'true'. As the lathe begins to turn, a special holder for the knurling tool is attached to the work table. The knurling tool itself is clamped into the holder and carefully directed to the turning piece with a small crank. Since knurling is an abrasive process, the machinist should use a generous supply of machine oil on the turning shaft. A knurling tool rarely makes a complete imprint the first time it is pressed against the shaft. Machinists usually make several passes with the knurling tool, allowing the individual cutters to make small bites into the metal.
A knurling tool is best suited for softer metals such as aluminum or standard grade steel. Hard metals such as titanium would most likely ruin the tool before any embossing could take place. In commercial tool and die shops, it is not unusual to see apprentices and entry-level workers assigned to the knurling lathe set-up. Knurling shafts for screwdrivers and other hand tools can be very repetitive and time-consuming, which makes the task ideal for workers with little seniority. But running a knurling operation successfully can lead to more advanced lathe work with more interesting cutting techniques.