Understanding Knurling in CNC Machining: Applications, Types, Design Standards, and Manufacturing Considerations

Knurling is a common surface treatment applied to CNC machined metal parts, primarily for enhancing grip and aesthetic appeal. Often seen on knobs, handles, and fasteners, knurled patterns improve tactile performance and can also serve functional purposes such as press-fit retention or torque transfer. This article outlines the main types of knurling, their applications, cost and processing difficulty comparison, technical drawing standards, selection of knurling pitch or modulus, and commonly used manufacturing methods.

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1. Purpose and Common Types of Knurling

Knurling is a cold-forming process that involves impressing a pattern—typically straight, diagonal, or cross-hatched—onto the surface of a workpiece. The primary purposes include:

- Anti-slip function for hand-operated components
- Improved grip for assembly or disassembly
- Aesthetic enhancement
- Mechanical engagement, especially in insert applications

 Common Knurling Types

 ● Straight Knurling

Straight knurls are parallel vertical lines used mainly for unidirectional grip. These are often found on:

- Thumbscrews
- Watch crowns
- Camera focusing rings

 ● Diamond (Cross) Knurling

Diamond knurls are formed by intersecting diagonal lines, offering multi-directional grip. They are commonly used on:

- Hand tools
- Control knobs (e.g., Tesla steering wheel rollers, camera dials)

 ● Diagonal and Custom Knurling

Diagonal patterns, typically at 30° or 45° angles, are used for specialized applications. Custom knurling includes wavy patterns, logos, or decorative elements and is often applied in:

- Premium consumer electronics
- Automotive interior controls
- Jewelry components

 ● Knurled Inserts

Knurled brass inserts are often embedded in plastic housings to increase retention force and prevent rotation. The knurling enhances the bonding between the insert and the plastic.

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2. Cost and Processing Difficulty Comparison

The complexity and cost of knurling vary based on the pattern, precision requirements, material hardness, and tool setup.

- Straight knurling is the easiest and most cost-effective, especially for soft metals like aluminum or brass.
- Diamond knurling requires precise tool synchronization to avoid mismatched or uneven patterns.
- Diagonal knurling adds angular complexity and requires stable machine settings.
- Custom knurling involves non-standard tooling and high-precision control, often resulting in the highest cost.

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3. Technical Drawing Standards for Knurling

To ensure proper communication with manufacturers, knurling must be correctly specified in technical drawings. Key parameters include:

 ● Knurl Type

Indicate whether the pattern is straight, diamond, or diagonal.

 ● Modulus (Pitch) Selection

The modulus (m) defines the spacing between knurl teeth. A larger modulus creates a coarser pattern; a smaller  modulus creates finer detail.

 International Standards

 DIN 82: Uses pitch (t) in mm (e.g., 0.3, 0.5, 0.6, 0.8, 1.0 mm)
 GB/T (China National Standard): Uses modulus (m) in mm (e.g., 0.2, 0.3, 0.4, 0.5 mm)

Conversion formula:
`t = π × m`

 Drawing Examples

 Straight knurl, modulus 0.3: `Straight m0.3 GB/T6430.3-2008`
 Diamond knurl, modulus 0.4: `Diamond m0.4 GB/T6403.3-2008`

 ● Surface Finish Requirements

Pre-knurling surface roughness: Ra ≤ 12.5 μm
Post-knurling diameter increase: approximately (0.8–1.6) × m

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4. How to Choose the Right Knurling Modulus

The selection of knurling pitch or modulus should consider:

 ● Function

 Decorative use: Use fine patterns (modulus 0.2–0.3 mm)
 Anti-slip or grip enhancement: Use coarser patterns (modulus 0.4–0.5 mm)

 ● Material

 Soft metals (aluminum, copper): Use smaller modulus to prevent burrs and deformation
 Harder metals (steel, stainless steel): Larger modulus ensures visible and consistent patterns

 ● Workpiece Size

 <20 mm diameter: 0.2–0.3 mm modulus
 20–50 mm: 0.4–0.5 mm modulus
 >50 mm: May require diagonal knurling or double knurling passes

 ● Knurling Precision Note

 To avoid mismatched patterns, the workpiece diameter (D) should be an integer multiple of the modulus (m):
 `D = m × Z (number of teeth)`

If not, the final tooth will overlap the first one, creating interference.

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5. Common Knurling Methods

Knurling can be performed using several machining methods depending on part geometry, batch size, and equipment availability.

 ● Manual Lathe Knurling

 Suitable for prototypes or low volumes
 Less precision, operator-dependent

 ● CNC Lathe Knurling

 Preferred method for high precision and consistency
 Capable of handling both straight and diamond knurls
 Can accommodate custom tools for diagonal or decorative patterns

 ● Rotary Knurling Tools

 Used in both manual and CNC machines
 Employ hardened wheels to roll the pattern into the material

 ● Form Knurling vs. Cut Knurling

 Form knurling compresses material using pressure; more common and suitable for soft metals
 Cut knurling removes material, more suitable for hard metals or detailed custom knurls

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Final Thoughts

Knurling is a small but vital aspect of CNC part design, especially for components that require grip, tactile feedback, or enhanced appearance. By understanding the function, cost, tooling, and technical specifications behind different knurling types, designers and engineers can make informed decisions that balance performance and manufacturability.

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FAQ: Frequently Asked Questions about Knurling in CNC Machining

Q1: Does knurling require a specific length tolerance on the drawing?
A1: Yes. Since knurling tools have a fixed width, it’s important to define the knurled length precisely to avoid overrun or under-coverage, especially on short features or stepped diameters.

Q2: Should chamfers be added before knurling?
A2: Adding a small chamfer at the knurling start point helps guide the tool, reduces edge deformation, and improves pattern consistency. It also prevents the knurl from flaring at the ends.

Q3: Can knurling be applied to hardened or heat-treated parts?
A3: Generally no. Knurling is best performed before heat treatment. Hardened materials are difficult to deform and may damage the tool or produce uneven patterns.