Knurling

Understanding What is Knurling and Its Comprehensive Guide

Knurled grips made from knurling are commonly seen on hammers, wrenches, and other hand tools. But what is Knurling? Knurling refers to a finishing process that makes patterns of concave and convex contours on the surface of the machine part. This rough textured pattern enhances a part’s aesthetics and increases friction on its surface to facilitate grip. Knurling is often used on cylindrical parts like shafts, bolts, wheels, and tool handles.

In this article, we will explore the different types of knurling patterns, the knurling process, its applications, advantages, disadvantages, etc. Read on and find more useful information about knurling through this article.

#1 What is Knurling?

Knurling is a manufacturing process that is usually performed on a lathe and involves rolling a pattern of straight, angled, or crossed lines into the part’s surface. The knurled part obtains added aesthetic appeal, increased durability, and better grip than the original smooth metal surface. Sometimes, the knurled pattern is a series of straight ridges or a helix of “straight” ridges rather than the more common criss-cross pattern.

In general, knurling operations can act as a repair method. This is because a rolled-in knurled surface has raised areas around the depressed areas, and these raised areas repair wear on the part. Using the knurling process also helps to assemble metal pins into the plastic molding.

Knurling is often seen in tool handles, mechanical pencils, pistol grips, barbell bars, the gripping surfaces of motorcycle handlebars, and control knobs on electronic devices. We can find knurling applications in the grips of darts and the footpegs of BMX bicycles. Knurling is also present on many surgical instruments for instrument identification.

#2 Hand vs. Machine Knurling

There are two methods for knurling: by hand and by machine. Let us take a closer look at each one.

#2.1 Hand Knurling

Hand knurling is performed with a hand-held knurling tool known as a hand knurler. Depending on the holding setup, it may feature one or more knurl wheels. A knurling wheel is often a small roller tool with patterns like diamonds or diagonals on its face. 

When the hand knurler is fastened to the material surface and spun, the knurling wheels press against it, deforming and altering the material surface. However, hand knurling has inherent inaccuracies, and its operation needs to be continuous and without a break to avoid overlapping knurl lines.

This method is typically limited to small soft workpieces that can be gripped and manipulated by hand. It produces simple knurl patterns. But complex patterns are very difficult to achieve manually.

hand knurling

#2.2 Machine knurling

Machine knurling methods are carried out on a machine, often a manual or CNC lathe. First, the workpiece is secured directly on a lathe. Then, you can fix the knurling wheels on a knurl holder and attach them to the worktable. Machine knurling makes contact with the workpiece to produce the desired knurl pattern. This knurling operation is continuous and typically error-free. 

Usually, the stock gets lubricated before turning to avoid overheating and to make cutting easier by smoothly interfacing the stock and tool. And the chips should be cleaned constantly to prevent pile-up, which can cause machine disruptions and work failure.

Machine knurling can accommodate workpieces of almost any size, length, and material. Precise tooling and feeds allow very fine pitch, complex knurls, and strict dimensional control.

Machine knurling

#3 The Knurling Process – How Does It Work?

The knurling process involves using knurling wheels to produce desired patterns on the surface of a workpiece. Here is how the knurling process is performed:

Step 1. Select the Workpiece Material

The material must be malleable enough to displace under pressure from the knurling wheels or tools. Common materials include aluminum, brass, mild steel, and some plastics. Harder materials may require special knurling wheels and slower speeds.

Step 2. Choose a Knurling Method

Hand and machine knurling are two primary ways to perform the knurling operations. The former uses a small roller tool that creates the desired pattern as it’s pressed against the surface of the workpiece, whereas the latter uses a lathe to cut the desired pattern into the workpiece. Hand knurling works best for softer metals, while machine knurling can achieve finer details in harder materials.

Step 3. Select and Secure the Knurling Wheels or Tools

Knurling wheels come in different tooth sizes, angles, and materials for specific applications. The tooth profile determines the pattern and harshness of the knurl. Knurling tools are secured on a knurl holder and attached to the worktable. Knurling tools contain the reverse image of the desired knurl pattern. 

Step 4. Mount the Workpiece 

The workpiece is mounted in a lathe chuck or between centers. Proper clamping of the workpiece is important to prevent slipping under the knurling forces.

Step 5. Feed the Wheels or Tool into the Workpiece

The knurling wheels or tools are slowly fed into the rotating workpiece. Perpendicular alignment and consistent pressure are critical to achieving even knurling.

Step 6. Produce the Required Knurl 

Then continuously feeding the wheels or tool into the workpiece while maintaining consistent pressure results in the deformation of the material surface. This produces bumps or depressions on the surface of a workpiece. Multiple passes may be required for sharp, high knurls in harder materials.

#4 What is the Purpose of Knurling?

Knurling can boost the aesthetics of products by providing an appealing pattern to their surface. Generally, textured finishes are more favored than smooth finishes. Aside from aesthetics, manufacturers conduct knurling for practical purposes.

Some products, like hand tools, are frequently knurled to increase performance and usability. A hammer with a smooth grip may cause hand slip while holding and using it. Knurling prevents hand slippage by forming a textured pattern that is easier to hold.

#5 Different Types of Knurl

Here are several common types of knurl patterns used in knurling:

#5.1 Straight/Standard Knurl

Generally, a straight knurling wheel generates a straight knurling pattern on the workpiece. For a straight knurl, the series of tooths on the knurling wheel is not bent at any angle.

Straight Knurl

#5.2 Right-handed Knurl

A right-handed knurl creates diagonal knurls that slope to the right. The teeth on these knurls have a 30 ° helix angle.

Right handed Knurl

#5.3 Left-handed Knurl

The opposite of a right-handed knurl is a left-handed knurl. It creates diagonal knurls that slope to the left. The teeth on left-handed knurls are also often retained at a 30 ° helix angle.

It should be noted that using left-handed and right-handed knurls together in a two-wheel holder can make diamond patterns.

Left handed Knurl

#5.4 Diamond Knurl

A diamond pattern may also be created using a single diamond knurling wheel. It makes diagonal knurls that slope to the left and right.

A female diamond knurl creates protruded diamond patterns. However, a male diamond knurl creates diamond patterns carved into the surface.

Diamond knurls provide maximum grip due to their pointed edges and pattern. It is the most common type used on everyday objects.

Diamond Knurl

#5.5 Concave and Convex Knurl

For a concave knurl, teeth on the knurling wheel are curved into the center of its surface. It is employed to feed one area of the workpiece at a time, axially.

A convex knurl is the finest choice for long traverse knurling processes. It features a round contour that allows it to move across the surface of stock easier.

concave and convex knurl

#5.6 Square and Beveled Knurl

Square profile knurls do well in single-location knurling operations, whereas beveled knurls achieve excellence at traversing knurling processes.

When a square profile knurl is moved left or right, the tooth edge has a heavy load. But the angled tooth of beveled knurls removes some load off the tooth edge, allowing it to move across the surface easier.

Square and Beveled Knurl

#6 Types of Knurl Tool Holders

The main types of knurl tool holders include bump, straddle, scissor, and swivel holders. Let’s look at their brief introduction.

Bump Holder

Bump holders come in single and double-die configurations. A single-die setup can accommodate one knurling wheel, whereas a double-die setup can accommodate two wheels.

Bump-style knurl holders are also available in a modular configuration. This type of knurl holder includes a tool head that can be swapped out, allowing it for a single or double-die setup without altering the holder.

On both manual and CNC lathes, bump holders are employed. As for Swiss-type lathes, compact bump-style holders are the ideal choice.

Bump Holder

Straddle Holder

Two knurl wheels can be housed in a straddle holder head. You may adjust the position of these wheels to operate with stocks of varied diameters. Often, the knurl wheels on a straddle holder must be aligned with the center of the workpiece.

When knurling with two wheels concurrently, the knurl lines can overlap if the wheels do not apply equal pressure to the workpiece. In addition, you should set each wheel perpendicular to the workpiece to prevent knurls from becoming heavier on one side.

straddle holder

Scissor Holder

A scissor holder resembles a straddle holder and can accommodate two knurl wheels. Rotating a nut on the holding handle may adjust the knurl distance.

When utilizing a scissor holder, the wheel should not be set at the center of the stock, as in straddle holders. The scissor holder should be somewhat before the center to apply a suitable pressing force.

Scissor Holder

Swivel Holder

The head of a swivel holder may carry up to six knurling wheels. You may mount several wheels on the head and quickly switch the wheel by twisting the head.

You may set it up with fine, medium, and coarse knurling wheels to produce different patterns in less time when you don’t have to spend as much time changing tools.

Swivel Holder

#7 Benefits of Knurling

Knurling offers several benefits across different industries and applications:

Enhanced Grip: Knurled surfaces increase friction, ensuring a secure grip even in slippery conditions. This allows for better control and safety.

Prevents Slippage: On components like bolts, knobs, wheels, and rollers, knurling helps prevent slippage during assembly or operation. The roughened surface has more friction against mating parts.

Aesthetic Appeal: Knurling adds visual interest and a unique texture to objects, making them visually appealing. 

Functional Usability: Knurling improves the usability of tools, handles, and components by enhancing their grip and ease of handling.

Paint Adhesion: Knurling creates more surface area and texture, which can help paint and coatings adhere better to smooth surfaces like plastics or polished metals.

Identification and Branding: Knurling can incorporate logos, markings, or identification features onto the surface of an object.

Safety: On staircases, walkways, ramps, and other foot traffic surfaces, knurling provides a non-slip pattern for safety. The raised tracks give shoes extra grip to prevent slipping.

#8 Challenges and Limitations of Knurling

While knurling offers numerous benefits, it also comes with some challenges and limitations:

Material Considerations: Knurling may not be suitable for all materials. Brittle or delicate materials may crack or deform under the pressure of knurling.

Weakened Surface: The knurling process hardens the surface but can also introduce stresses that lead to fatigue. Knurling may slightly weaken the surface or reduce impact resistance in some applications.

Surface Finish: Knurling can leave a rough surface finish, which may not be desirable in applications that require a smooth or polished appearance.

Precision Requirements: Achieving precise and consistent knurling patterns can be challenging, especially for complex geometries or small workpieces. 

Dimensional Inaccuracies: Aggressive knurling at high volumes can lead to slight variations in diameter or surface distortions. Close tolerances may be difficult to achieve. Secondary operations may be needed to re-size or re-surface knurled components.

Surface Defects: Improper speeds, feeds, tool condition, or pressure during knurling may lead to poor pattern definition, uneven tracks, or damage like cracks in more brittle materials. Defective knurls require re-working or scraping the component.

Knurling part

#9 Applications and Uses of Knurling

Knurling finds use in countless applications spanning every industry where non-slip, gripping, decorative, or locking surfaces are required. The followings are some of the typical applications of knurling:

Handles and Grips: Knurling provides extra traction on handles, grips, knobs, and levers so that hands can securely and comfortably grasp them. Knurled handles and grips are used on everything from hand tools and gym equipment to cabinets, pens, and appliances.

Shafts and Axles: On rotating shafts, axles, and rods, knurling helps mating parts like bearings, wheels, or gears grip securely without slipping. The knurled surface provides extra friction and a physical locking of parts.

Fasteners: Bolts, nuts, screws, and other fasteners frequently have knurled surfaces to prevent loosening or slipping during assembly and use. The knurl bites into mating surfaces, effectively locking the fastener in place. Knurled fasteners are popular for applications that demand maximum gripping power. 

Wheels and Rollers: On conveyor rollers, pulleys, and cylinder rollers, knurling provides extra traction to help materials feed and move efficiently under load without slippage. The raised knurl pattern aids in gripping and conveying items along the wheel or roller surface. Knurled wheels and rollers are used extensively in manufacturing, material handling, and processing industries.

Pumps and Valves: On plungers, pistons, sliders, and valve stems, knurling creates a non-slip seal to prevent seizing or leakage while allowing movement. The knurled surface holds pressure and maintains a seal between fluids. Knurled pump and valve components are common in hydraulic, pneumatic, and fuel handling systems where liquids must be contained and metered. 

Staircases and Walkways: On staircases, walkways, ramps, and other foot traffic areas, knurling provides a non-slip safety surface to prevent falls. The coarse, sharp knurl pattern maximizes grip for shoes and boots. Knurled staircases, catwalks, and ramps are utilized in industrial settings, public transit areas, and anywhere people may walk on uneven, wet, or slippery ground. 

Medical Devices: Some medical equipment like IV poles, walker handles, and orthopedic devices use knurling to create non-slip, gripping contact points, especially where fluids are involved. Knurled surfaces allow for manipulation without dropping or losing control of critical parts. Knurling on medical devices improves safety, handling, and precision during treatment and physical therapy applications. 

Aerospace: In aerospace, knurling is employed on fasteners, hoses, and components that require a non-slip gripping surface, especially in fuel systems or hydraulic lines. The knurl prevents leakage, slippage, or premature disconnection.

#10 Knurling Tips to Achieve the Best Results

Knurling creates a pattern of raised diamond-shaped or straight ridges on the surface of a workpiece. It improves grip and provides a textured surface for various applications. Here are some knurling tips to achieve the best results:

  1. Choose the appropriate knurling tool based on the workpiece’s desired pattern, pitch, and diameter. Knurling tools typically consist of two knurls mounted on a holder.
  2. Ensure the workpiece is properly cleaned and securely mounted in the lathe or milling machine. The surface should be smooth and free from any burrs or irregularities.
  3. Align the knurling tool perpendicular to the workpiece surface. This ensures that the knurls create an even and consistent pattern across the surface.
  4. After clamping the workpiece and tool, set proper machine parameters such as the speed (RPM) and feed. During the process, adequate support is provided to the workpiece to avoid deformation.
  5. The blank diameter must also match the pitch of the knurling tool to properly mesh the two surfaces to prevent jagged burs caused by wheel overlapping. 
  6. Because accurate tracking is frequently established after just one complete revolution of the part, the key to success is to “RAM” the die into the blank. By making a more profound, broader impression of the first revolution, the die teeth are likelier to return to the original grooves the second time.
  7. For straight knurling, engage the automatic cross-feed mechanism of the lathe or milling machine to produce uniform ridges along the workpiece. Adjust the cross-feed rate according to the desired pitch of the knurl pattern.
  8. If you’re knurling on a CNC and experiencing double tracking, you could pause or gently slow down the spindle for the initial contact. Return to regular speed when the tool has penetrated roughly 20-40% into the blank.
  9. Using a lot of lube is necessary. Knurling produces high pressures, and inadequately lubricated dies may bind up on or gall the pins. 
  10. Over-rolling knurls are not recommended. In general, only roll the pattern approximately 90% full. Roll up the pattern in as few revolutions as possible when rolling stainless steel to limit the work-hardening of the workpiece. The same may be stated about rolling brass and other soft materials, but in this instance, it’s to avoid “flanking” produced by rolling the part too many times after it’s been fully formed.
  11. After completing the knurling operation, carefully inspect the workpiece for defects or inconsistencies. Check the pattern, depth, and overall quality of the knurled surface. Make any necessary adjustments or refinements if required.

Summary

Knurling is the process that involves displacing or deforming the surface of a material to produce a pattern. It creates different patterns of straight or diagonal lines on round or cylindrical parts. This pattern increases the surface friction of the part and provides excellent aesthetic appeal. Knurling can be used in many hardware, mold, and mechanical components.

LEADRP is an experienced rapid prototyping and manufacturing service supplier. If you have any need for knurling or other prototyping services, we are here to welcome your inquiry. Contact us and get an instant quote!

References

Knurling – From Wikipedia

What Is Knurling? Here’s What You Should Know – From OneMonroe

What is Knurling? Explained with Examples – From Mellowpine

Knurling Explained: Beginners Guide – From Mellowpine

FAQs

Wood can be knurled, but the process is more complex than knurling metal. Knurling only works on hardwoods in the same way that it works on metal. Nevertheless, a simple pattern can be generated through a process known as checkering. 

The pattern is engraved on both sides of the tool's surface during knurled welding. Then, this tool is used to RF-weld fabrics together.

Plastics can also be knurled, but typically in a different method. Cut knurling is more commonly utilized than form knurling.

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