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Knurling is a manufacturing process, usually done on a lathe, in which a knurl pattern of straight, diagonal or intersecting lines is molded into the material. It is a process performed on screws or knobs that are designed to be turned by hand. The indented patterns are designed to eliminate slippage. The grid pattern of the workpiece can vary. With knurling, the workpiece material will slip less, and the simple layout of the grooves and grid knurled pattern will make the workpiece more visually appealing.

Currently, there are several types of knurling: 

  • A – straight (the knurling lines are parallel to the shaft axis),
  • B – right helical (inclined at 45° to the shaft axis),
  • C – helical left (inclined at 135° to the shaft axis),
  • D – helical-cross (convex),
  • E – helical-cross (concave),
  • F – cross-cut (convex),
  • G – cross-shaped (concave)

We write about the wider application and its advantages below!

Application of knurling

Why use knurling? For several reasons: 

  • Perform this operation to create a groove in a part of the workpiece. Knurling allows the hand to grip the knurled workpiece better than a smooth metal surface. Sometimes the knurling pattern is a series of straight spirals or ridges, rather than the more typical cross pattern.
  • Knurling can also be used as a repair method because the rolled knurled surface has convex areas around the pits, and these convex areas can compensate for wear on the part. In the era of cheap labor and expensive parts, this repair was possible for internal combustion engine pistons, and the worn piston shell was enlarged to nominal dimensions by knurling. As automotive parts are now becoming cheaper, knurling is not as common as it once was.
  • Knurling can also be used when components are to be assembled into low-precision products, such as metal pins in plastic molds. The outer surface of the metal pin is knurled so that the convex workpiece „bites” into the plastic, regardless of whether the size of the plastic hole exactly matches the diameter of the pin.
  • Tool handles, automatic pencils, gun handles, barbells, motorcycle handlebar mounting surfaces and electronic equipment knobs are often knurled. 
  • Knurling is also used in dart grips and BMX bicycle pedals. It is found in many surgical tools to identify the tools and facilitate cleaning with a brush.

How does the knurling process work? 

Knurling is usually done using one or more very rigid rollers that contain the opposite side of the pattern to be applied. „Straight” (non-cross) knurling can be extruded with a single roller, but the material must be properly supported to avoid distortion. The cross pattern can be obtained by any of the following methods:

  • Arming a single roll containing the complete reverse of the desired pattern. You can use them to create „male” or „female” patterns. 
  • Roll the piece straight to the left, then straight to the right (or vice versa), one or more left scrolls being used simultaneously with one or more right scrolls.
  • Use a flask with a pitch that is a multiple of the circumference or a pitch with a diameter greater than π. The diameter of the blanks is crucial for high-quality knurling. An improper blank diameter will cause double knurling, resulting in a finer pattern than the knurling was designed for. In order for the knurling to look correct without double tracking, you must match the diameter of the flask to the knurling stroke. This is very similar to having two matching gears with the same diameter. Each time a tooth is added, the diameter increases by a discrete amount. There is no intermediate diameter that will work. The same is true for knurling and knurling flasks, although fortunately knurling tolerates a certain amount of error before problems arise. The total number of knurls for a given diameter will usually be repeated three times from the bottom to the top of the pattern. Unlike knurling cuts, the spacing between cuts is not preset and can be adjusted to allow the total number of patterns to be created around the workpiece, regardless of the diameter of the workpiece.
  • The hand knurling tools provided are similar to pipe cutters, but contain knurling wheels instead of cutting wheels. Typically, the tool is carried on three wheels: two left and one right, or vice versa.
  • Cutting knurls often adopt automatic feeding. Cutting knurl tooling is similar to rolling knurl, except that the knurl has sharp edges and corners and is set at a certain angle to the workpiece to allow sharp edges and corners to cut the workpiece. Corner, diamond and linear knurling is not possible because lathe wheels cannot withstand such longitudinal speeds, and also because reasonable cutting speeds cannot be achieved.

Hand vs. machine knurling

There are two basic methods of knurling: manual or machine. The first method involves using a roller to create the desired pattern when pressed against the surface of the workpiece, while the second involves using a lathe to cut the desired pattern on the workpiece. Hand-rolling is the most basic method and requires only a small turning tool. When a worker moves the tool across the surface of the workpiece, it leaves a textured surface in the tool’s indentation pattern. Machine knurling, on the other hand, is a more complicated process that requires a lathe. Unlike hand knurling, machine knurling does not create a textured surface through pressure. Instead, it uses a drill bit to cut material from the workpiece.

Advantages of knurling

Knurling operations can enhance the aesthetics of a product by introducing an attractive pattern on its surface. Most people would agree that textured finishes are more attractive than smooth ones. However, manufacturing companies have several practical reasons for knurling beyond appearance. Products that require adhesion, such as hand tools, are often knurled to improve performance and usability. If a hammer has a smooth handle, the user’s hand may slip when holding and using it. Knurling prevents the hand from slipping and creates a more adhesive textured pattern.

What are the ways of knurling on a CNC lathe

To reduce the problems associated with traditional knurling processes, macros can be used to produce CNC knurled parts.

Multi- knurling head thread lathe for cross knurling

The principle of this method is to treat the moiré as a long line with positive and negative interlacing. The traditional processing technology involves generating programming other than knurling and a φ1.6 hole, and then performing knurling.Due to the high position tolerance requirements of CNC knurled parts, the extra clamping leads to a long build period and a high defect rate. The new processing technology completes all part processes at the same time through clamping, which is easy to ensure processing accuracy.

Use random cross knurling

During thread turning, thread disorder often occurs in the form of multiple strikes. This phenomenon can also be used for knurling. However, this method is more suitable for knurled parts with a small number of thread heads and a small angle of inclination of the screw line, which can increase the spindle speed, depending on the specific situation.

Simple knurling using the spindle lock function

The processing principle of this method is that when the spindle is locked, a longitudinal groove is drawn on the surface of the workpiece, and a continuous groove forms a straight knurl.

How to complete the knurling process on a CNC lathe?

The knurling process uses the tool to create patterns on the surface of the workpiece. These are straight lines and grid patterns, and the thickness of the patterns depends on the distance.

1. The form of knurling and the shape of the knurling pattern.

The thickness of the knurling pattern should be selected according to the diameter of the knurled surface of the workpiece. The larger the diameter, the larger the module pattern.

2. knurling machining

Clamp the workpiece as tightly as possible.

– When clamping the workpiece, the length of the protruding chuck should be as short as possible.

– Long workpieces are handled by the tip.

– When turning the outer circle of the knurled part, its diameter should be about 0.25 mm smaller than the final size.

Take care of the proper installation of the knurling attachment tool.

– Check that the knurling tool is free of dirt, clean it with a wire brush if necessary.

– When installing the knurling knife, the pivot pin should be aligned.

– Stably fix the tool.

Workpiece knurling.

– Choose a low knurl cutting speed and high feed rate.

– Start the machine spindle and apply enough coolant to the knurling tool.

– Slide the knurling tool into the workpiece and apply pressure until a full diamond pattern forms.

– Apply a longitudinal feed and then a plunge feed until the desired knurl length is achieved.

– Move the knurling machine to quickly lower the workpiece.

On the surface of the workpiece, deburr by cutting a 45° bevel at the bottom of the knurl groove.

– Visually adjust the knurl tool to deflect it by a small angle.

Knurling provides an opportunity to decorate surfaces with different patterns. The groove of the workpiece, the shape of the surface of the pattern, or the elements of turning a straight line are a few elements of the plastic masses that affect the entire process. In addition, proper lubrication of the workpiece surface is important, as improper lubrication will destroy the shape of the pattern surface. Damage to the pattern caused by improper knurling process is rather rare. Remember to minimize them by cross-grooving, grid pattern knurl wheels, or by installing a circular pitch.