In the realm of precision machining, whirling has emerged as a specialized and cost-effective process for manufacturing helical surface parts, particularly worm gears, ball screws, and extruder screws. Whirling has gained widespread recognition in industries ranging from automotive to aerospace. It offers unique advantages in efficiency and precision that set it apart from conventional machining techniques.​

Core Principles and Classification of Whirling​

Whirling is a high-speed machining process dedicated to producing helical or threaded surfaces, leveraging a rotating cutter head equipped with multiple forming inserts to remove material from a slowly rotating workpiece. A defining feature of whirling is that the surface finish achieved, even in its basic form, can match that of grinding processes.​

This exceptional surface quality stems from the relatively long contact time between the cutting inserts and the workpiece, which results in shorter and fewer polyhedral peaks on the machined surface. Selecting the appropriate tool system for whirling operations directly translates to faster machining cycles and superior surface finish, solidifying whirling’s status as the most economical and efficient solution for manufacturing external helical components today.​

Technologically, whirling is categorized into external whirling and internal whirling based on the relative position between the cutter head and the workpiece. Developed countries in the West have successfully applied whirling technology to the production of various threaded products, including ball screws. They have engineered specialized tooling, workpiece feeding systems, automatic positioning and clamping devices, and control systems tailored for threaded whirling, significantly boosting machining accuracy and production efficiency.​

External Whirling​

Suitable for machining both external and internal threads, this variant positions the cutter head outside the workpiece.​

 

Internal Whirling​

As illustrated in typical process diagrams, the cutting inserts are evenly arranged inside the cutter head. A key design element is that the axis of the cutter head is not coincident with that of the workpiece but forms an angle equal to the helix angle γ of the target thread.​

During internal whirling, the cutter head rotates at a high speed—with linear velocities reaching up to 400 m/min. CNC whirling machine tool heads from German manufacturers are capable of speeds as high as 40,000–60,000 rpm for small-module threads. Meanwhile, the workpiece rotates slowly in the same direction as the cutter head.​

For each full rotation of the workpiece, the cutter head advances along the axial direction by one thread lead, enabling the formation of the thread groove in a single pass. A notable characteristic of internal whirling is that only one insert engages in cutting at any given time, with the machining zone remaining nearly fixed throughout the process.​

The cutting thickness undergoes periodic variation (increasing from small to large and then decreasing), while the cutting width gradually expands until the operation is complete. The single-insert cutting mechanism provides ample time and space for heat dissipation between tool changes, extending tool life and enhancing workpiece surface quality.​

Additionally, most cutting heat is carried away by the chips, minimizing workpiece temperature rise and thermal deformation. Despite its merits, whirling has inherent limitations. The internal whirling structure, with inserts mounted inside the cutter head, restricts the number of tools and machining speeds.​

Moreover, constrained by the size of the cutter head, internal whirling cannot process workpieces with large thread leads, and chip evacuation remains a critical challenge that requires careful process design.​

 

Cutting Process, Chip Formation and Key Parameters of Whirling​

The whirling cutting process relies on the high-speed rotation of the whirling cutter head around the slowly rotating workpiece (controlled by the C-axis). The synchronized rotation of the workpiece and the axial movement of the cutter head (controlled by the Z-axis) determine the pitch of the machined thread.​

The cutting angle of the tool head is adjusted via the A-axis to match the thread helix angle, while the eccentricity of the X-axis sets the minor diameter of the thread. By simply reversing the Z-axis feed direction, whirling can produce either right-handed or left-handed threads.​

Notably, the whirling ring always rotates in the same direction as the workpiece; it is the feed direction that dictates the thread hand. In whirling operations, cutting inserts are installed within the cutter ring and move relative to the workpiece.​

Ideally, the cutting process generates comma-shaped chips, a hallmark of stable whirling performance. Although whirling is an interrupted cutting process, chip formation is remarkably smooth. This allows whirling to process hardened materials with hardness up to 65 HRC and brittle materials using cutting tools made of cemented carbide, cubic boron nitride (CBN), or ceramics.​

 

Key Whirling Parameters​

Key parameters governing whirling operations include:​

nW​: Rotational speed of the whirling cutter head​

nR​: Rotational speed of the workpiece​

SK: Cutting circle​

T: Depth adjustment (corresponding to thread height)​

D: Root diameter of the thread​

X: Eccentricity between the cutter head and workpiece axes​

K: Comma-shaped chips​

Whirling vs. Conventional Milling​

When machining external threads, it is essential to distinguish between whirling and conventional milling, as the two processes share superficial similarities but differ fundamentally in performance. Both whirling cutter rings and milling cutters feature multiple cutting teeth, and both remove material through interrupted cutting operations. However, the similarities end there.​

In whirling, the cutting edges engage and disengage from the workpiece gradually, resulting in a more favorable cutting action compared to conventional milling. This smooth engagement enables higher metal removal rates, reduces radial cutting forces, and minimizes stress on both the workpiece and the cutting inserts.​

Consequently, whirling significantly extends tool life and enhances overall cost-effectiveness, making it a superior choice for high-precision, high-efficiency thread machining.​

 

Typical Workpiece Types Processed by Whirling​

Whirling excels at machining various cylindrical helical surface components, with the following typical applications:​

Extruder Screws

A core component in plastic and rubber extrusion equipment, requiring precise helical profiles to ensure uniform material conveying.​

PC Rotary Pump Rotors

Critical for positive displacement pumps, where the helical rotor geometry directly impacts pump efficiency and flow stability.​

Automotive EPS Worms

Key parts in electric power steering systems, demanding high precision to guarantee smooth steering performance.​

Diamond Rolls

Used in metal rolling processes, with helical grooves that shape the final product.​

Oil Grooves

Helical oil grooves on shafts and components that facilitate lubrication and heat dissipation.​

Helical Gears

For power transmission systems requiring quiet operation and high load-bearing capacity.​

Ball Screws

Essential in precision machinery and CNC equipment, where whirling ensures the high accuracy of the ball track.​

In essence, most components with cylindrical helical surfaces are suitable for whirling machining.​

 

Leading Whirling Equipment Manufacturers​

The global whirling machine tool industry is dominated by renowned international manufacturers, while domestic Chinese enterprises have also made significant strides in this field.​

International Manufacturers​

Germany’s Leistritz, GWT, and Rexroth; the Netherlands’ Hembrug; Austria’s Linsinger; Japan’s Seiko; and the United States’ Toppson are among the market leaders. They offer high-precision CNC whirling machines for diverse industrial applications.​

Chinese Manufacturers​

Taiwan’s Haiwei is a prominent regional supplier. On the mainland, Beijing Precision Machine Tool Parts Factory was an early adopter of whirling technology.​

Subsequent entrants include Shandong Ball Screw Manufacturing Factory, Jiangsu Ball Screw Equipment Manufacturing Factory, Shanxi Fengyuan Machinery Factory, Jinan No.3 Machine Tool Factory, and Shaanxi Hanjiang Machine Tool Co., Ltd., among others, driving the localization of whirling equipment in China.​

 

Conclusion​

As a highly specialized and efficient machining technology, whirling has revolutionized the production of helical surface components, offering unparalleled advantages in surface finish, processing efficiency, and cost-effectiveness. From its core principles and process characteristics to its wide range of applicable workpieces and leading equipment providers, whirling continues to evolve with advancements in tool design and numerical control technology.​

The recent progress in internal whirling tool profile calculation further underscores the potential for optimizing this process to meet the increasingly stringent precision requirements of modern manufacturing. For machining enthusiasts and industry professionals alike, whirling represents a dynamic and promising field worthy of continued exploration and innovation.​

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