Knurling: A Complete Guide for Process, Types & Patterns

Knurling is a mechanical process used to create a pattern of straight, angled, or crossed lines on the surface of a material, most commonly metal. This operation is almost always performed on a lathe, turning the smooth surface of a workpiece into a roughened texture. The primary purpose of knurling is to provide a gripping surface for hands or fingers, but it is also used for cosmetic enhancement, reducing glare, or securing a press-fit assembly by increasing the outer diameter of a component. While conceptually simple, achieving a high-quality, long-lasting knurled surface requires a detailed understanding of the process, the tooling, and the material science involved.

The Knurling Process: Understanding Deformation

Unlike cutting operations where material is removed by a sharp tool edge, knurling is fundamentally a cold forming or metal displacement process. This distinction is crucial to understanding the resulting surface finish and dimensions.

The tool used for this process is called a knurling tool, which typically consists of one or two hardened steel wheels, or knurls, featuring the inverse of the desired pattern. The knurling tool is pressed firmly against the rotating workpiece. Under extreme pressure, the tool wheels displace the material, pushing it up into the valleys of the knurl pattern and forming the corresponding peaks on the workpiece surface.

Because material is being displaced rather than removed, knurling results in an increase in the component's major diameter—the diameter measured across the highest points (peaks) of the knurled pattern. The amount of increase depends on several factors, including the pitch of the knurl, the material being worked, and the pressure applied. This dimensional change must be accounted for in the initial design; often, the workpiece blank diameter is undersized before the knurling process to compensate for the anticipated material growth.

There are two primary methods of knurling:

1. Form Knurling (or Bump Knurling): This is the most common method. It uses a single roller or two opposing rollers pressed directly into the workpiece. It is fast but requires significant force, which can sometimes deflect a long or slender workpiece.

2. Cut Knurling: This method uses specialized knurling wheels with sharper teeth designed to actually cut the material, rather than just displacing it. Cut knurling produces chips, requires less forming pressure, and is therefore better suited for thin-walled or small-diameter parts that might otherwise bend under the pressure of form knurling. This method offers greater control over the final diameter and pattern uniformity.

Knurling Patterns and Types

Knurling is defined by its pattern, which is determined by the design of the knurling wheels and how they are applied to the workpiece. The most common patterns are specified based on the angle and direction of the lines.

1. Straight Knurls: This pattern consists of parallel lines running along the axis of the workpiece. It is created using wheels that have straight ridges, and the resulting texture provides a grip that prevents the component from slipping rotationally. Straight knurling is often used on components that are meant to be turned or tightened manually.

2. Diamond Knurls (or Diagonal Knurls): This is the most popular and recognizable pattern. It is formed by two sets of helical (angled) lines that intersect each other, creating a diamond or pyramidal shape. Diamond knurls offer an excellent, multi-directional gripping surface, making them ideal for thumbscrews, tool handles, and adjustment knobs. This pattern can be achieved using either a single wheel with a helical pattern (applied multiple times) or, more commonly, a two-wheel holder where the wheels are angled to produce the intersecting lines in a single pass.

3. Angled Knurls: This pattern consists of parallel lines that run at an angle (helical) to the axis of the workpiece, but they do not intersect. This is typically achieved using a single-wheel tool holder. Angled knurls provide a distinctive look and are useful when the grip needs to be directional, though they are less common than the diamond pattern.

Pitch and TPI: The pitch of a knurl refers to the distance between two corresponding points on adjacent knurl teeth. It is often measured in TPI (Threads Per Inch) in the Imperial system or modulus (mm) in the Metric system. Knurling tools come in a variety of pitches, commonly designated as fine, medium, or coarse. The choice of pitch is crucial, as it affects both the appearance and the effectiveness of the grip. A finer pitch yields a smoother surface but a weaker grip, while a coarse pitch provides maximum grip but results in a rougher feel and appearance. Selecting the correct pitch is also essential for form knurling, as certain pitches may not properly mesh with the workpiece diameter, leading to double tracking or an uneven pattern.

Material Selection and Knurling Challenges

The machinability of the material significantly impacts the knurling process.

• Ductile Materials: Materials like brass, aluminum, and mild steel are generally easy to knurl using the form knurling method because they are highly malleable and readily deform under pressure. These materials allow the metal to easily flow into the desired pattern.

• Harder Materials: Hardened steels and certain stainless steels are much more difficult to form. These materials typically require the cut knurling method, which uses sharper tools and less pressure to chip away the material rather than pushing it aside. Attempting to form-knurl hard materials can lead to excessive tool wear, poor pattern definition, and potential breakage of the knurling tool or the workpiece.

Key Challenges:

1. Double Tracking: This occurs when the knurling wheel does not align perfectly with the pattern already started, leading to a blurry or double-image effect. This is often solved by proper alignment, ensuring the pitch of the tool is compatible with the workpiece diameter, or using a self-centering knurling tool holder.

2. Tool Wear: The immense pressure involved in form knurling causes significant stress on the knurling wheels. Choosing high-quality, high-speed steel or carbide-insert knurls is essential for long tool life, especially when working with tough materials.

3. Chatter: Vibrations during the process can cause an inconsistent pattern. To mitigate chatter, ensuring the lathe is rigid, using proper coolant (to reduce friction and heat), and maintaining correct spindle speed and feed rate are vital.

Applications and Specifications

Knurling is extensively used across numerous industries for both functional and aesthetic reasons.

• Ergonomics and Functionality: Knurled surfaces are found on microphone bodies, surgical instruments, flashlights, barbells, measuring tool knobs, and bicycle components (like pedal axles) where a secure, non-slip grip is paramount.

• Assembly: Knurling can be used to permanently secure two parts. When a shaft is press-fit into a hole, the raised knurl peaks bite into the mating material, creating a high-retention interference fit. This is often used for securing small pins or inserts into plastic or softer metal housings.

• Cosmetics: Knurling provides a high-quality, technical appearance, often seen in premium products or precision instruments. It can also be used to reduce light reflection or glare on optical equipment.

Proper knurling specification should always include the following details: the pattern type (straight, diamond), the pitch/TPI, the length of the knurled section, and the final major diameter (the diameter after the process). By carefully selecting the right process, pattern, and tooling, manufacturers can successfully turn a smooth surface into a functional, aesthetically pleasing, and dimensionally accurate knurled component.