In engineering, "thread" refers to a spiral groove machined into the surface of an external cylindrical surface or internal cylindrical hole. Its applications are extremely diverse, ranging from small fastening screws to custom components in large machinery. Cutting, tapping, and milling are the primary processes for producing threads, with thread milling being particularly well-suited for threading medium-to-large workpieces.
Although threads appear simple in mechanical structure, they perform a variety of functions, such as fastening parts, transmitting motion, bearing radial loads, and providing sealing. Consequently, their manufacturing process demands extremely high precision and is correspondingly complex.
What is thread milling?
Thread milling is a CNC precision machining process, a subtractive manufacturing technique, suitable for producing internal and external threads on workpieces. This process uses a specialized single-point milling cutter to perform a helical interpolation motion along the workpiece surface, gradually removing material to form the desired thread structure.
Compared to traditional tapping, thread milling offers greater flexibility in diameter control, enabling the production of threads of varying diameters on the same workpiece, even enabling variable diameter threads. Whether creating external threads on cylindrical surfaces or internal threads within holes, both can be completed in a single operation using a programmable tool path. The tool executes a helical circular motion along the workpiece's exterior or hole's interior, precisely cutting a continuous thread groove.
This process is particularly suitable for applications such as large holes and thin-walled components, where traditional tapping is difficult. The same thread milling cutter can produce threads of varying pitches and diameters, demonstrating excellent process adaptability and customization. Thread milling also offers excellent material compatibility, enabling efficient machining of hard materials such as tool steel (>45 HRC), titanium alloys, and Inconel, among other difficult-to-cut materials.
Mechanical Thread Terminology
To ensure clarity, we first clarify several key terms and their standard definitions related to design and production processes in engineering and manufacturing. Major Diameter: The diameter of the cylinder containing the external thread crest or internal thread root.
Minor Diameter: The diameter of the cylinder containing the external thread root or internal thread crest.
Crest: The surface connecting the top of the thread flank.
Root: The surface connecting the bottom of the adjacent thread flank.
Flange: The surface of the thread located between the crest and root.
Thread Angle: The angle between the flank and the perpendicular to the thread axis in an axial cross-section.
Pitch: The axial distance between corresponding points on adjacent threads.
Pitch Diameter: The diameter of the imaginary cylinder where the groove and projection on the thread profile are equal in width.
Lead: The distance traveled during one rotation of the nut and bolt when mated.
How does thread milling work?
CNC thread milling is an advanced thread processing method. Its key mechanism relies on the CNC program to precisely control the helical interpolation motion of the thread milling cutter along three axes (X, Y, and Z). Unlike tapping, thread geometry is not directly determined by the tool shape but is precisely controlled by CNC path programming: the circular motion of the X and Y axes determines the thread diameter, while the linear feed of the Z axis, synchronized with the rotation, controls the thread pitch.
The three core steps of CNC thread milling are as follows:
Step 1: Machine Setup and Tool Selection
First, select a suitable CNC milling machine or machining center. The workpiece must be pre-machined, with a pre-cut hole (for internal threads) or pre-machined external diameter (for external threads) at the target location. Tool selection is crucial for ensuring machining quality and efficiency:
Single-edge milling cutters: Suitable for small, high-precision thread machining.
Multi-tooth milling cutters: Suitable for high-speed, high-efficiency, high-volume production.
Indexable milling cutters: Suitable for large-scale thread machining and offer excellent cost-effectiveness.
The specific tool size is determined based on key parameters such as the major and minor diameters of the thread and the pitch.
Step 2: CNC Programming (G-Code and M-Code)
After clamping and aligning the workpiece, G-code and M-code are written to control the entire machining process. The program not only defines the thread type (e.g., M10x1.5), pitch, feed rate, and rotational speed, but also precisely plans the following:
The tool's entry and exit arc paths ensure thread quality.
The number of passes and depth of cut control the material removal rate.
Tool compensation and clearance ensure machining accuracy.
Step 3: Thread Cutting Execution
Run the CNC program, and the machine tool begins automatic machining. The tool smoothly enters the workpiece along a precalculated arc path and then cuts along a three-dimensional spiral path. After completing the predetermined number of passes, the tool finally retracts along a specific path, completing the thread.
Thread Milling Cutter Classifications
Integral thread milling cutters: Made entirely of carbide, these cutters offer excellent rigidity and precision, making them ideal for machining small and medium-sized, high-precision threads. Indexable thread milling cutters: Utilize replaceable carbide inserts mounted on the cutter bar for economical operation, primarily used for large-diameter, high-pitch threads.
Welded thread milling cutters: Carbide tips welded to a steel cutter body offer excellent cost-effectiveness and are suitable for threading common diameters and general precision.
Single-tooth thread milling cutters (forming cutters): The tool profile perfectly matches the thread profile, enabling the thread to be formed in a single pass. This offers high efficiency and specialized functionality.
Multi-tooth comb-type thread milling cutters: The cutter body features multiple rows of cutting teeth, resembling a comb, enabling efficient production of long threads. These cutters require high machine tool rigidity.
Single-tooth thread milling cutters (general-purpose): Consisting of only one cutting tooth, they can be programmed to produce threads of any diameter, hand direction, and pitch through CNC programming, offering exceptional flexibility.
Pipe thread milling cutters: Designed for sealed pipe threads such as NPT and BSPT, typically feature a 55° profile angle.
Taper thread milling cutters: The tool features a built-in taper and are used for producing tapered sealed threads such as NPT. Micro-diameter thread milling cutters: With extremely small diameters (<1mm), they are used for fine thread machining in precision instruments, medical devices, and other applications.
Advantages of Thread Milling
Extreme flexibility: One tool performs multiple tasks.
Core Advantage: Using a single-tooth thread milling cutter, you can produce internal and external threads with varying nominal diameters, pitches, and left-hand or right-hand threads by modifying the CNC program. This significantly reduces tool inventory and costs.
Wide Processing Range and Customization
Easily process large-diameter threads (e.g., M100 and above), which would be practical or extremely costly to manufacture taps of such large sizes.
Conquer Difficult-to-Machine Materials
Milling uses interrupted cutting, which facilitates chip evacuation and heat dissipation. It is ideal for machining difficult-to-machine materials such as high-temperature alloys, titanium alloys, and stainless steel, and avoids tap breakage caused by excessive torque during tapping.
High Thread Quality and High Precision
The milling process generates low cutting forces and heat, resulting in better thread surface quality and dimensional accuracy.
High safety and reduced risk of workpiece damage
Even if the tool breaks, it typically does not become stuck in the workpiece, making it easy to replace. Once a tap breaks in the hole, removal is extremely difficult, potentially rendering the entire workpiece useless.
Dry cutting is highly feasible.
Due to low cutting forces and excellent heat dissipation, dry cutting can be achieved in many cases without cutting fluid, which is more environmentally friendly and reduces costs.
Disadvantages of thread milling:
Complex programming and high technical requirements.
A three-axis CNC machine with helical interpolation is required. Programming is much more complex than tapping (G84/Tap Cycle), requiring higher operator and programmer skills.
High initial investment cost.
A high-performance CNC machine (at least three-axis) is required, while tapping can be performed on a standard drilling or tapping machine.
Required machine stability.
Thread milling requires high-quality servo response and motion control accuracy. Older or less rigid machines may not be able to achieve high-quality thread milling.
