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A toothed tool is used in conjunction with a lathe to engrave the ends of metal tubes and shafts. Raised grooves can be used as user handles or as better traction for rubber or plastic covers. The toothed tool itself consists of multiple rotating cutters that are held against the metal shaft while the lathe rotates at a relatively low speed (500 rpm on average). Turning is a method by which cylindrical pieces of metal or wood are rotated in place by a variable speed electric motor. As the part rotates, various cutting tools can be placed against it to remove material or cut shapes. A toothed tool falls somewhere between an engraver and an engraving machine.
In general, most serrated tools generate three shapes: straight lines, diagonal lines, and a diamond pattern. Serrated tools come in a variety of sizes and cutting designs depending on the purpose of the part. The diamond pattern is more common with handles because it creates more traction between the user’s hand and the shaft. Diagonal and straight knurls are often used to give extra traction to an outer shaft or other connecting piece.
To create a serration pattern, the lathe must hold the piece of metal perfectly straight, a condition machinists call “true.” When the lathe starts to turn, a special holder for the toothed tool is attached to the worktable. The toothed tool itself is fixed in the holder and is carefully directed to the rotating part with a small crank. Since knurling is an abrasive process, the machinist should use a liberal amount of machine oil on the rotating shaft. A toothed tool rarely leaves a complete impression the first time it is pressed against the shaft. Machinists often make multiple passes with the toothed tool, allowing individual cutters to make small cuts in the metal.
A serrated tool is more suitable for softer metals such as aluminum or standard grade steel. Hard metals like titanium would probably ruin the tool before the engraving could be done. In commercial die and tool shops, it is not uncommon to see apprentices and entry-level workers assigned to the toothed lathe setup. Serrated shafts for screwdrivers and other hand tools can be very repetitive and time consuming, making the task ideal for workers with little seniority. But running a successful knurling operation can lead to more advanced turning work with more interesting cutting techniques.