A knurl is a raised impression on the surface of the workpiece produced by two hardened rolls and is usually one of
two patterns: diamond or straight (Figure I-275). The diamond pattern is formed by a right-hand and a left-hand
helix mounted in a self-centering head.
The straight pattern is formed by two straight rolls. These common knurl patterns can be fine, medium, or coarse.
Diamond knurling is used to improve the appearance of a part and to provide a good gripping surface for levers and
tool handles. Straight knurling is used to increase the size of a part for press fits in light-duty applications. A disadvantage to this use of knurls is that the fit has less contact area than a standard fit.
Three basic types of knurling toolholders are used: the knuckle-joint holder (Figure I-276), the revolving head
holder (Figure I-277), and the straddle holder (Figure I-278).
The straddle holder allows small diameters to be knurled with less distortion. This principle is used for knurling on
Knurling works best on workpieces mounted between centers. When held in a chuck and supported by a center, the
workpiece tends to crawl back into the chuck and out of the supporting center with the high pressure of the knurl. This is especially true when the knurl is started at the tailstock end and the feed is toward the chuck. Long, slender pieces push away from the knurl and will stay bent if the knurl is left in the work after the lathe is stopped.
Knurls do not cut but displace the metal with high pressure.
Lubrication is more important than cooling, so a cutting oil or lubricating oil is satisfactory.
Low speeds (about the same as for threading) and a feed of about .010 to .015 in. are used for knurling.
The knurls should be centered on the workpiece vertically (Figure I-279), and the knurl toolholder should be
square with the work, unless the knurl pattern is difficult to establish, as it often is in tough materials. In that case, the toolholder should be angled about 5 degrees to the work so that the knurl can penetrate deeper (Figure I-280).
A knurl should be started in soft metal about half depth and the pattern checked. An even diamond pattern should
develop. If one roll is dull or placed too high or too low, a double impression will develop (Figure I-281) because the
rolls are not tracking evenly. If this happens, move the knurls to a new position along the workpiece, readjust up or down, and try again. If possible, the knurl should be made in one pass. However, this is not always possible with ordinary knurling toolholders because of the extreme pressure bearing on one side of the workpiece. Several passes may be required on a slender workpiece to complete a knurl because the tool tends to push it away from the knurl. The knurls should be cleaned with a wire brush between passes.
Material that hardens as it is worked, such as highcarbon or spring steel, should be knurled in one pass if at all
possible, and in not more than two passes. Even in ordinary steel, the surface will harden after a diamond pattern has developed to points. It is best to stop knurling just before the points are sharp (Figure I-282). Metal flaking off the knurled surface is evidence that work hardening has occurred. Avoid knurling too deeply, as it produces an inferior knurled finish.
Knurls are also produced with a type of cutting tool (Figure I-283) similar in appearance to a knurling tool. The
serrated rolls form a chip on the edge (Figure I-284).
Material difficult to knurl by pressure rolling, such as tubing and work-hardened metals, can be knurled by this cutting tool. Sulfurized cutting oil should be used when knurling steel with this kind of knurling tool.