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Exploring the Knurling Process: Types, Tools, and Finishes
Manufacturers use knurling as a post-finishing operation to create textured finishes on machined parts. This article explains knurling, its types, and common defects and provides tips for a successful metal knurling operation.
What Is Knurling?
Knurling is a manufacturing technique usually conducted on a lathe involving intricate patterns on a component’s surface. These patterns can consist of straight lines, angles, or interlocking designs. The knurling process enhances the part’s durability and aesthetics while improving its grip. It’s important to note that knurled patterns can vary. They can feature straight ridges or a spiral arrangement of these “straight” ridges, deviating from the more conventional criss-cross design.
Knurling is an effective method for refurbishing parts because the raised portions of a knurled surface help reduce the effects of wear and tear on the part. It also helps assemble metal pins into plastic molds.
Knurling produces several products, such as tool handles, mechanical pencils, pistol grips, barbell bars, and more. Furthermore, knurling is common in the grips of darts and the footpegs of BMX bicycles. It is also commonly used in the production of surgical instruments.
Tooth Form
A knurled tooth is V-shaped, with a depth less than the depth of a theoretical V-form. The tooth has a rounded root and crest. The relationship between the actual depth of the tooth and the theoretical V varies with the pitch of the teeth. The tooth is a smaller proportion of the theoretical V-depth on finer pitches than on coarser pitches. Additionally, female diamond patterns have shallower tooth depth than male diamond patterns.
Purpose of Knurling
Knurling is a widely used machining technique that serves various purposes on a part or product. It is commonly applied to the handles of instruments, tools, and machinery controls to improve grip and prevent slipping by wet hands or oily. Additionally, knurling adds decorative elements to products, improving their appearance. It can also be used on surfaces to create images and branding. The patterns can act as reference points for aligning components, aiding assembly and positioning. Knurling is also used to prepare surfaces for threaded inserts for push fit and over-molding, providing adhesion between the insert and its location.
How Does Knurling Work?
In machine knurling, a hardened steel tool with the inverse of the desired surface form creates a pattern on a component through controlled plastic distortion. The tool rolls the pattern onto the part, plastically deforming the material’s surface into the recesses of the tool. This process can be performed on cylindrical parts on a lathe using relatively low force. Knurling can also involve a sliding cutting tool similar to broaching. Hand knurling can be achieved using various methods, with cut-based knurling resulting in linear scoring along the pattern grooves.
Types Of Knurling Techniques
There are various types of knurling that product manufacturers use to create the desired patterns on workpieces.
Straight Knurling
Straight knurling, also known as linear knurling, involves creating linear and parallel grooves along the axis of a cylindrical part. This type of knurling is commonly used for grip or decorative purposes. It is also extensively utilized to facilitate the push fitting of permanently engaged parts. This approach enables lower part precision with exceptional concentricity and a tight fit, delivering comparable performance to other methods, such as higher force smooth or cryogenic push fits, but at a lower operational cost.
Uses and Benefits of Straight Knurling
Anti-slip or Improved Grip Functionality
Straight knurling creates a rough texture on an object’s surface, making it easier to grip and manipulate. These patterns increase friction between your hand and the object, reducing the risk of slippage. This is particularly helpful in situations where a strong grip is important.
Decorative Purposes
Straight knurling is a decorative pattern that enhances the aesthetic appeal of a workpiece’s surface. It is ideal for cylindrical objects such as handles and knobs. Additionally, it is well-suited for high-end writing instruments, custom hardware, and jewelry applications, providing a stylish appearance.
Alignment and Positioning
The knurling texture is used as a reference for aligning and positioning components. The patterns serve as a visual indicator for aligning and adjusting components correctly. Straight knurling is excellent for equipment or mechanisms requiring precise alignment, such as fine-tuning, measurement tools, or optical devices.
Diamond Knurling
This design involves two straight-cut pattern wheels that rotate at an angle to the axis of the cylindrical parts. As a result, they create overlapping angled grooves, resulting in a diamond pattern cross-hatching. The angle between the grooves is typically set at 30°, but it can vary within a range of angles.
Uses and Benefits of Diamond Knurling
Durability
Diamond-shaped knurls improve the wear resistance and durability of a workpiece. They are often used on machine components, tools, or decorative pieces that undergo frequent contact or high friction levels. The diamond pattern helps to distribute pressure and wear evenly, reducing the risk of damage. Therefore, this knurling is well-suited for applications requiring high durability.
Decorative Application
The diamond-shaped indentation is distinctive and highly decorative, creating intricate and visually appealing knurling patterns on machined parts such as bicycle components, lighter casings, and personal accessories to achieve an attractive and sophisticated appearance.
Improved Grip
Diamond knurling improves grip on handheld workpieces by creating multiple contact points, providing better traction and control.
Helical Knurling
Uses and Benefits of Helical Knurling
The helical knurling technique involves creating diagonal lines in a spiral pattern on the surface of an object. This knurling pattern is typically used on cylindrical surfaces, similar to a screw thread.
Decorative Purposes
Helical knurling is similar to other kinds and is perfect for decorative purposes. It can be combined with techniques such as plating or polishing to enhance the product’s overall appearance.
Enhanced Functionality
Helical knurling improves the functionality of cylindrical parts, such as shafts, knobs, and handles, by providing a firm grip for rotating and operating the machined part.
Thread Formation
It involves creating threads that comply with specific thread standards on cylindrical items, reducing the necessity for additional threading operations and making it a cost-effective and efficient process for particular applications.
How to Operate the Knurling Process?
The knurling technique involves using specialized knurling wheels to create specific patterns on the surface of a workpiece. Here’s a breakdown of the process:
1: Choosing the Right Workpiece Material
The material being worked on must be malleable enough to be shaped by the knurling tools. Common machining materials include aluminum, brass, mild steel, and plastics. For harder materials, specific knurling tools and slower speeds may be necessary.
2: Selecting the Proper Knurling Method
Hand and machine knurling are the two primary methods. Hand knurling uses a small roller tool to press against the workpiece and create the desired pattern, while machine knurling involves using a lathe to cut the pattern into the workpiece. Hand knurling is best for softer metals, while machine knurling can achieve finer details in harder materials.
3: Setting Up the Knurling Wheels or Tools
Knurling wheels come in various tooth sizes, angles, and materials to accommodate different needs. The tooth profile determines the pattern and severity of the knurl. A knurl holder mounted to the work table holds these wheels in place. The intended knurl pattern is reverse-etched into the knurling tool.
4: Properly Securing the Workpiece
The workpiece is secured between centers or in a lathe chuck to prevent slipping under the knurling pressure.
5: Executing the Knurling Operation
The knurling wheels or tools gently feed the revolving workpiece. It’s crucial to have perpendicular alignment and apply constant pressure to achieve even knurling. The material’s surface deforms as the wheels or tools continuously feed into the workpiece with continuous pressure, resulting in bumps or depressions. In tougher materials, sharp, high knurls may require multiple passes.
Challenges in Knurling Operations
Knurled components have decorative and functional purposes in the jewelry, furniture, automotive, and aerospace industries. However, product designers face challenges in knurling operations:
1. Under or Over Sizing: Knurling patterns may be smaller (under-sizing) or larger (oversizing) than intended due to wrong setup or excessive pressure during knurling.
2. Surface Damage: Knurling can cause scratches or gouges on the workpiece due to excessive force or worn-out knurling tools.
3. Tool Wear: Knurling tools wear out over time, leading to less defined patterns and defects like flattened areas and burrs.
4. Poor Pattern Definition: Inadequate lubrication, incorrect tool geometry, or worn-out knurling wheels can result in a clearer pattern definition.
5. Chattering or Vibration Marks: During knurling, vibration can cause irregular workpiece patterns due to excessive tool pressure or improper alignment. Knurled parts serve decorative and functional purposes in various industries, such as furniture, jewelry, automotive, and aerospace. However, product designers encounter several challenges in knurling operations.
Applications of Knurling
Knurling is used in various applications:
1. Knurling creates decorative and wear-resistant surfaces, providing a technical and engineered appearance. For instance, It’s engineered to enhance grip and minimize wear on public seating through innovative patterning.
2. Hands may be wet or oily when handling machine handles, especially when a good grip is essential. This feature, such as lathe control handles, is crucial for engineering tools with long service lives and high wear resistance.
3. Knurling on a shaft (potentially inside a hole) can create a ductile and malleable zone, allowing for lower-cost, higher-strength push fits in shaft/hole applications. This reduces the overall precision and cost of the two components while ensuring a strong coupling, as seen in a driveshaft fitted to a machine flywheel.
4. When metal parts need to be coated with polymer or composite materials, a knurled pattern is often used to enhance adhesion and improve torsional and pull-out resilience in the coupling—for example, brass inserts for threading in plastic molding.
Conclusion
After learning about the knurling process in detail, you should be able to differentiate between various knurling patterns, tools, and common issues encountered during knurling operations. This acquired knowledge can help you make informed decisions regarding your project.
Enze is the ideal provider of machining services to consult for your knurling needs. Whether you need a knurled finish or other post-finishing for your projects, Enze is the right choice.
With over years of experience, our technicians have expertise in machining and finishing. You can trust us to provide excellent one-stop services to meet your requirements. Get in touch with us today and entrust your next project to the hands of professionals!
FAQs
Why is knurling crucial?
Knurling is critical because it enhances a part’s physical properties, repairs worn-out surfaces, and provides a better grip than smooth surfaces.
Can steel be knurled?
Yes, non-hardened steel of all types can be rolled or cut knurled before hardening. Hardened steels require cut knurling, as their material’s malleability is generally too low for roller knurling.
Does Knurling Increase Strength?
Knurling doesn’t significantly affect material strength but can cause slight weakening due to surface distortion. However, it can toughen the surface through work hardening and make surfaces more damage-resistant by reducing scratching and minimizing visible wear.
What Is the Difference Between Engraving and Knurling?
Knurling uses roller tools to deform surfaces while engraving cuts patterns using a single-point tool. Before precision machine tools, knurling was often done by engraving, blurring the difference between the two processes. Cut knurling enables knurled patterns on surfaces unsuitable for roller knurling.
