Knurling is a manufacturing process, typically conducted on a lathe, whereby a pattern of straight, angled, or crossed lines is rolled into the material. Knurling can also refer to material that has a knurled pattern, or the pattern itself.
Knurling is a post-finishing process wherein a textured pattern is created on the surface of a workpiece.
Early knurling processes were performed by hand, but the introduction of turning and milling machines has made it possible to create more precise and complex designs.
Let’s discuss how knurling works, its methods, and its applications.
What is a Knurling Process?
Knurling is a lathe machining process that creates a diamond-shaped pattern on a workpiece surface. It is done by using hardened metal wheels with special shapes to enhance the workpiece’s appearance and provide a better grip.
This process creates small ridges along the surface of the metal object so that it can be easily handled or lifted. These small ridges are a combination of horizontal, vertical, or crossing lines on the surface of the workpiece.
In addition, these knurled patterns improve grip, increase friction, or repair worn parts. The knurling process is usually done on a lathe machine. When a press fit is required between two parts, straight knurling is often used to increase the diameter of the workpiece.
Overview on Knurling
If you haven’t heard of knurling, you’ve probably seen a knurled product or item before. Knurled grips are often used on tools like wrenches, screwdrivers, and other hand tools. The knurling process creates a patterned texture that prevents tool slippage from the hand.
Rather than adding material to the grip, knurling creates a textured surface by cutting away some of the material or applying pressure. Tools used for knurling are designed with a specific pattern that produces the same pattern when pressed against a workpiece.
Purpose of Knurling Operation
Knurling is a widely used family of machining techniques that serves various and often multiple purposes on a part or product. It is applied to the handles of tools, instruments, and machinery controls to enhance grip and prevent slipping due to oily or wet hands.
Additionally, knurling is employed to add decorative elements to products, enhancing their engineered appearance. It can also be used selectively on surfaces to impose branding and images.
The patterns can serve as reference points for the alignment of components, aiding in assembly and positioning. It is used to prepare surfaces for threaded inserts for push fit and overmolding, providing security of adhesion between the insert and its location.
How Does Knurling Work?
Machine knurling uses a hardened steel tool that carries the inverse of the required surface form. The tool rolls the pattern onto the component in a tightly controlled plastic distortion process.
The material of the surface is pressed to plastically deform into the recesses of the tool. It makes a diapositive copy of the tool pattern as an imprint on the surface, with no material removal.
Such a process can be performed on cylindrical parts on a lathe, using a relatively low imposition of force and a rolling action. On non-cylindrical surfaces, the tool must carry the overall shape of the target surface.
It will generally be pressed onto the part to form the desired result. Where the tool (rather than the workpiece) moves, the knurling tool is either moved using a milling machine or on a lathe. Some tooling is adapted to clamp the stationary workpiece and use the machine axes to traverse and impress the tool.
Knurling is not always a plastic deformation process, but can involve a sliding cutting tool that uses a similar approach to broaching. Hand knurling can be achieved using a rotary hand tool (plastic deformation), a hammered pattern tool (plastic deformation), or a cutter/file format (material cutting).
The results from a cut-based rather than plastic deformation knurling process can generally be visually identified by linear scoring (or lay) along the sides of individual grooves of the pattern.
Types of Knurling
The following are the main types of knurling operations:
- Annular rings
- Linear knurl
- Diamond knurl
- Straight knurl
#1. Annular Ring.
This type of knurling is often used when the mating part is plastic. The annular rings allow for easy mating, but the ridges make it difficult to separate the components.
#2. Linear Knurl.
Linear knurling is really useful to use with mating plastic pieces since linear knurling allows for greater torsion between the components.
#3. Diamond Knurl.
This process is a combination of annular rings and linear knurls that create a diamond shape. It is typically used to provide a better grip on components and is the most common type used on everyday items.
#4. Straight Knurl.
Straight knurling is also a type of manufacturing process. This is usually conducted on a lathe, by which a pattern of straight rolls is rolled into the material to create a textured surface.
Tools for Knurling Operation
Knurling tools are available in a variety of forms, but all serve the same function. Knuckle joint tools and revolving head tools are two common types of knurling tools.
The knuckle joint tool features a pair of rollers that move with the workpiece as it is being knurled.
The revolving head tool has three pairs of rollers to change the pitch to a different noise without changing the setup. Knurling patterns are produced in two types: diamond and straight.
Rollers come in three pitches: coarse, medium, and fine. Usually, a diamond pattern is used with a medium pitch. When working with large diameters, a coarse pitch is used; when working with small diameters, a fine pitch is used.
Hand vs. Machine Knurling
The knurling operation can be done in two different ways: by hand or by machine. The former involves using a rolling tool that produces precise patterns as it is pressed against the workpiece surface.
In contrast, the latter involves using a lathe to cut the chosen pattern into the workpiece. Hand knurling is generally the most basic method, which requires a small roller tool to operate.
As the machinist moves this tool across the workpiece, it leaves a textured surface in the pattern of the tool’s indent.
On the other hand, machine knurling is a more complicated process as it requires a lathe. In contrast with hand knurling, machine knurling does not produce a textured surface through pressure. Instead, it employs a bit to cut material off from the workpiece.
Knurling Precautions
Following are some precautions considered while performing the knurling operation:
- Never stop the carriage when the tool is in contact with the work and the work is still moving, as this will cause wear on the work surface.
- Take a look at the knurling tool to make sure the tool isn’t forcing the work through the center hole.
- During operation, keep the workpiece and knurling tool well-oiled.
- Be sure not to let a brush or rag get between the rollers and the workpiece. Otherwise, the knurl will be wasted.
- Be careful not to allow the knurling tool to completely pass through the end of the workpiece, or it may damage the workpiece or lathe centers.
Application of Knurling
Knurling finds extensive use across various industries, providing significant benefits in both functional performance and aesthetic appeal. Key application areas include:
- Hand Tools. Hand tools, such as screwdrivers, wrenches, and pliers, often feature knurled grips to improve user handling. The enhanced grip reduces slippage, making tools safer and easier to use.
- Automotive Components. Knurling is commonly applied to parts like knobs, levers, and handles within vehicles, improving ergonomics and control for drivers and passengers.
- Medical Devices. In the medical field, knurling is utilized on devices such as surgical instruments and controls, ensuring a secure grip in critical situations and improving user safety.
- Electronics. Knurling can be found on knobs for electronic devices, providing tactile feedback and allowing precise control while maintaining an aesthetic appeal.
- Furniture and Fixtures. In the furniture industry, knurled surfaces are often seen on drawer pulls, cabinetry hardware, and lighting fixtures, contributing to both function and design.